Festo sensors for handling and copy

093046 EN
Sensors for
handling and
processing
technology
Proximity sensors
Textbook FP 1110

Order No.: 093046
Description: NAEH-SCH.LHRBCH
Designation: D:LB-FP1110-EN
Edition: 09/2003
Author: Frank Ebel, Siegfried Nestel
Graphics: Barbara Böhland, Frank Ebel
Layout: 04.09.2003, Beatrice Huber
© Festo Didactic GmbH & Co. KG, 73770 Denkendorf/Germany, 2003
Internet: www.festo.com/didactic
E-mail: did@festo.com
The copying, distribution and utilization of this document as well as the
communication of its contents to others without expressed authorization is
prohibited. Offenders will be held liable for the payment of damages. All rights
reserved, in particular the right to carry out patent, utility model or ornamental
design registration.

© Festo Didactic GmbH & Co. KG • FP 1110 3
Notes on the layout of this book _________________________________________ 7
1. General notes ________________________________________________ 11
1.1 The importance of sensor technology _____________________________ 11
1.2 Terms _______________________________________________________ 11
1.2.1 Sensor ______________________________________________________ 11
1.2.2 Sensor component_____________________________________________ 12
1.2.3 Sensor system ________________________________________________ 12
1.2.4 Multi-sensor system ___________________________________________ 13
1.3 Typical output signals of sensors _________________________________ 13
1.4 Binary and analogue sensors ____________________________________ 15
1.4.1 Binary sensors ________________________________________________ 15
1.4.2 Analogue sensors _____________________________________________ 15
1.5 Proximity sensors _____________________________________________ 16
1.5.1 Overview of position sensors ____________________________________ 17
1.5.2 Operating voltages ____________________________________________ 18
1.6 Fields of application for proximity sensors__________________________ 18
2. Mechanical position switches ___________________________________ 25
2.1 Electro-mechanical position switches _____________________________ 25
2.1.1 Function description ___________________________________________ 25
2.1.2 Technical characteristics________________________________________ 26
2.1.3 Notes on installation ___________________________________________ 28
2.1.4 Examples of application ________________________________________ 30
2.2 Mechanical-pneumatic position switches __________________________ 31
2.2.3 Notes on application ___________________________________________ 32
2.3 Exercises ____________________________________________________ 33
Contents

Contents
4 © Festo Didactic GmbH & Co. KG • FP 1110
3. Magnetic proximity sensors_____________________________________ 35
3.1 Reed proximity sensors_________________________________________ 35
3.2 Contactless magnetic proximity sensor ____________________________ 42
3.3 Magnetic-pneumatic proximity sensors ____________________________ 45
3.3.4 Example of application _________________________________________ 46
3.4 Exercises ____________________________________________________ 47
4. Inductive proximity sensors_____________________________________ 49
4.1 Function description ___________________________________________ 49
4.2 Technical characteristics________________________________________ 52
4.3 Notes on application ___________________________________________ 53
4.4 Examples of application ________________________________________ 55
4.5 Exercises ____________________________________________________ 58
5. Capacitive proximity sensors____________________________________ 61
5.1 Function description ___________________________________________ 61
5.2 Technical characteristics________________________________________ 64
5.3 Notes on application ___________________________________________ 64
5.3.1 Considerations for application ___________________________________ 65
5.4 Examples of application ________________________________________ 65
5.5 Exercises ____________________________________________________ 69

Contents
6. Optical proximity sensors ______________________________________ 71
6.1 General characteristics _________________________________________ 71
6.1.1 Emitter and receiver elements in optical proximity sensors ____________ 72
6.1.2 Construction of an optical proximity sensor_________________________ 73
6.1.3 Operating margin for optical proximity sensors______________________ 74
6.1.4 Variants of optical proximity sensors ______________________________ 77
6.2 Through-beam sensors _________________________________________ 78
6.3 Retro-reflective sensors ________________________________________ 81
6.4 Diffuse sensors _______________________________________________ 85
6.5 Optical proximity sensors with fibre-optic cables ____________________ 92
6.6 Exercises ___________________________________________________ 100
7. Ultrasonic proximity sensors___________________________________ 107
7.1 Function description __________________________________________ 107
7.2 Technical characteristics_______________________________________ 110
7.3 Notes on application __________________________________________ 111
7.3.1 Minimum distances ___________________________________________ 111
7.3.2 Required minimum size of the object _____________________________ 112
7.3.3 Type of object _______________________________________________ 113
7.3.4 Minimum possible distance of object_____________________________ 113
7.3.5 Position of object_____________________________________________ 114
7.3.6 Effect of ambient temperature, humidity, air pressure _______________ 115
7.3.7 Diverting the ultrasonic beam___________________________________ 115
7.3.8 Effect of temperature of the object_______________________________ 115
7.3.9 Effect of ambient noise ________________________________________ 115
7.4 Examples of application _______________________________________ 116
7.5 Exercises ___________________________________________________ 117

Contents
8. Pneumatic proximity sensors___________________________________ 119
8.1 General characteristics ________________________________________ 119
8.2 Back pressure sensors (Back pressure nozzles) ____________________ 121
8.3 Reflex sensors _______________________________________________ 122
8.4 Air barriers __________________________________________________ 123
8.5 Notes on application __________________________________________ 124
8.6 Characteristic curves of pneumatic proximity sensors _______________ 125
8.6.1 Characteristic curves of back pressure sensors_____________________ 125
8.6.2 Characteristic curves of reflex sensors____________________________ 127
8.6.3 Characteristic curves of air barriers ______________________________ 129
8.7 Examples of application _______________________________________ 130
8.8 Exercises ___________________________________________________ 135
9. Selection criteria for proximity sensors __________________________ 137
9.1 Object material ______________________________________________ 137
9.2 Conditions for the detection of objects ___________________________ 138
9.3 Installation conditions_________________________________________ 139
9.4 Environmental considerations __________________________________ 139
9.5 Safety applications ___________________________________________ 139
9.6 Options/features of proximity sensors____________________________ 140
10. Connection and circuit technology ______________________________ 141
10.1 Types of connection___________________________________________ 141
10.1.1 Two-wire DC and AC technology _________________________________ 141
10.1.2 Three-wire DC technology ______________________________________ 143
10.1.3 Four- and five-wire DC technology _______________________________ 144
10.1.4 Terminal designation__________________________________________ 145
10.2 Positive and negative switching outputs __________________________ 145
10.2.1 PNP-output__________________________________________________ 146
10.2.2 NPN-output _________________________________________________ 147
10.3 Circuit technology ____________________________________________ 148
10.3.1 Parallel and series connection of proximity sensors _________________ 148
10.3.2 Parallel connection of proximity sensors using two-wire technology____ 149
10.3.3 Parallel connection of proximity sensors using three-wire technology __ 150
10.3.4 Series connection of proximity sensors using two-wire technology_____ 151
10.3.5 Series connection of proximity sensors using three-wire technology ___ 152
10.4 Connection technology under conditions of
strong electro-magnetic influence _______________________________ 153
10.5 Connection of controllers, relay and display elements _______________ 153
10.6 Required current supply _______________________________________ 154

Contents
11. Physical fundamentals________________________________________ 155
11.1 Fundamentals of inductive and capacitive proximity sensors__________ 155
11.1.1 Inductive proximity sensors ____________________________________ 155
11.1.2 Capacitive proximity sensors ___________________________________ 164
11.2 Fundamentals of magnetic proximity sensors ______________________ 167
11.2.1 Permanent magnetism ________________________________________ 167
11.2.2 Electromagnetism ____________________________________________ 169
11.2.3 Detecting a magnetic field______________________________________ 169
11.3 Fundamentals of ultrasonic-proximity sensors _____________________ 175
11.3.1 Generation of ultrasound ______________________________________ 179
11.3.2 Attenuation of ultrasound in air _________________________________ 182
11.3.3 Ultrasonic proximity sensors____________________________________ 184
11.4 Fundamentals of optical proximity sensors ________________________ 186
11.4.1 Reflection ___________________________________________________ 187
11.4.2 Refraction___________________________________________________ 188
11.4.3 Total reflection_______________________________________________ 189
11.4.4 Photoelectric components _____________________________________ 189
11.4.5 Fibre-optic cables ____________________________________________ 193
12. Circuit symbols for proximity sensors____________________________ 199
12.1 Circuit symbols to standard DIN 40 900___________________________ 199
12.2 Examples of circuit symbols ____________________________________ 200
13. Technical terms relating to proximity sensors_____________________ 201
13.1 General terms________________________________________________ 201
13.2 Terms for dimensional characteristic values _______________________ 204
13.3 Terms of electrical characteristic values __________________________ 207
13.4 Terms for time and function characteristics________________________ 208
13.5 Actuating characteristics of mechanical-electrical position switches____ 210
13.6 Terms relating to environmental conditions _______________________ 211
14. Standards and protection classes _______________________________ 213
14.1 Standards___________________________________________________ 213
14.2 Protection classes ____________________________________________ 214
14.3 Colour coding________________________________________________ 217
14.3.1 Colour symbols to DIN IEC 757 __________________________________ 217
14.3.2 Colour coding to EN 50 044_____________________________________ 217
14.3.3 Numerical designation to EN 50 044 _____________________________ 218
14.4 Designs of proximity sensors ___________________________________ 218

Contents
15. Special designs and variants of proximity sensors ________________ 225
15.1 Variants of inductive proximity sensors __________________________ 225
15.1.1 Example of a universal two-wire design: Quadronorm by IFM ________ 226
15.1.2 Proximity sensors for use in installations with explosion hazard ______ 227
15.1.3 Magnetic field proof (welding plant) inductive proximity sensors _____ 229
15.1.4 Inductive proximity sensors for higher temperature range___________ 231
15.1.5 Inductive proximity sensors for higher pressure range______________ 231
15.1.6 Inductive proximity sensors with large switching distance___________ 231
15.1.7 Inductive proximity sensors with high switching frequency __________ 231
15.1.8 Inductive proximity sensors with idle return function _______________ 232
15.1.9 Self-monitoring proximity sensors ______________________________ 232
15.1.10 Inductive proximity sensors for specific material detection __________ 235
15.1.11 Inductive proximity sensors with material independent
switching distance___________________________________________ 236
15.1.12 Ring type inductive proximity sensors ___________________________ 237
15.1.13 Slot type inductive proximity sensors ___________________________ 238
15.1.14 Inductive proximity sensors for broken drill monitoring _____________ 239
15.2 Variants of optical proximity sensors ____________________________ 240
15.2.1 Slotted light barrier sensors ___________________________________ 241
15.2.2 Frame barrier sensors ________________________________________ 242
15.2.3 Laser barrier sensors_________________________________________ 243
15.2.4 Polarised retro-reflective sensors_______________________________ 243
15.2.5 Printing mark sensors ________________________________________ 245
15.2.6 Luminescence sensors _______________________________________ 246
15.2.7 Angled light barrier sensors ___________________________________ 247
15.2.8 Sensors for accident prevention________________________________ 248
15.2.9 Dynamic sensors ____________________________________________ 250
16. Solutions ___________________________________________________ 251
16.1 Solutions to exercises from Chapter 2 ____________________________ 251
17. Bibliography of illustrations _______________________________________ 271
18. Index ______________________________________________________ 273

This textbook forms part of the Function Package FP 1110 "Proximity Sensors" and
belongs to the Learning System for Automation and technology by Festo Didactic
GmbH & Co. KG.
In this book the trainee becomes familiarised with the subject of proximity sensors.
The function package serves both, as a support for vocational and further training
programs as well as for self-instruction. The function package consists of an
equipment set and training documentation.
Chapter 1 to 10 introduce the area of proximity sensors with notes on application,
mode of operation and characteristics. The fundamental basics are taught and with
the help of exercises the trainee is guided towards independent problem solving of
the various applications of proximity sensors. Solutions to these exercises are
contained in chapter 16.
Chapter 11 to 15 deal with the physical and technical fundamentals of individual
types of proximity sensors and contains a list of technical terms as well as an
overview of the applicable standards. In addition, examples of special variants of
proximity sensors are described in detail.
The index at the end of the book makes it possible to look up information with the
help of key words.
When conducting practical exercises with the equipment sets of Function Package
FP 1110, an additional workbook (Order no. 529 939) with exercises and a collection
of component data sheets are available as a supplement.
Notes on the layout of this book

The ever increasing automation of complex production systems necessitates the use
of components which are capable of acquiring and transmitting information relating
to the production process.
Sensors fulfil these requirements and have therefore in the last few years become
increasingly important components in measuring and in open and closed loop
control technology. Sensors provide information to a controller in the form of
individual process variables.
Process status variables, for instance, are physical variables such as temperature,
pressure, force, length, rotation angle, container level, flow etc.
There are sensors for most of these physical variables which react to one of these
variables and pass on the relevant signals.
1.2.1 Sensor
A sensor is a technical converter, which converts a physical variable (e.g.
temperature, distance, pressure) into a different, more easily evaluated variable
(usually an electrical signal).
Additional terms for sensors are:
Encoders, effectors, converters, detectors, transducers.
The designation "measuring sensor" should be avoided. In sensing terms, a
"displacement encoder" does not cause displacement, but rather records the
"displacement" variable.
A sensor does not necessarily have to generate an electrical signal.
Example
– Pneumatic limit valves generate a pneumatic output signal (in the form of a
pressure change).
Sensors are devices which can operate both by means of contact, e.g. limit switches,
force sensors, or without contact, e.g. light barriers, air barriers, infrared detectors,
ultrasonic reflective sensors, magnetic sensors etc.
Even a simple limit switch can be interpreted as a sensor.
1. General notes
1.1
The importance of
sensor technology
1.2
Terms

1. General notes
Within a controlled process, sensors represent the "perceivers" which monitor a
process by signalling faults and logging statuses and transmitting such information
to other process components.
To quote a human example:
Eye brain (visual faculty) limbs
A sensor becomes useful only with regard to processing or evaluating.
e.g. Eye + visual faculty outline recognition, colour, 3D-vision, motion sequences
1.2.2 Sensor component
Apart from the word "sensor", the following terms are also used:
By a sensor component we are talking about the part of a sensor or sensor system,
which records a measured variable, but does not permit an independent utilization,
because additional signal processing and pre-assembling (housing, connections) are
required.
1.2.3 Sensor system
A sensor system consists of several measuring and evaluating components, often
with a significant proportion of signal processing functions.
The components are often modular and can be interchanged within a product family.
Apart from sensors, signal processors, micro computers and data compatible
interfaces are also available for signal conditioning.
Example – Image processing systems with CCD image sensor,
– Laser measuring systems, identification systems.
In the case of signal processing capabilities, one speaks of intelligent sensors or
"smart sensors".

1. General notes
1.2.4 Multi-sensor system
Sensor system with several similar or different types of sensors.
Example
– A temperature and humidity sensor or a pressure and temperature sensor, each
forming part of the same device.
– A combination of several proximity sensors to distinguish shape and material of
workpieces.
– A combination of several chemical sensors for gases, whereby sensors have
overlapping response ranges and by means of intelligent evaluation provide
more information as a whole than an individual sensor.
– Use of several human sense organs (smell, taste, optical perception, feeling by
tongue) during the intake of food.
When using sensors, it is important to know the different types of electrical output
signals.
Sensors with switching signal output (binary signal output).
Examples – Proximity sensors
– Pressure sensors
– Filling level sensor
– Bimetal sensor
As a rule, these sensors can be connected directly to programmable logical
controllers (PLC).
Sensors with pulse rate output.
Examples – Incremental length and rotary angle sensors.
Generally, PLC-compatible interfaces are available. PLC requirements:
Hardware and software counters with the possibility of greater word length.
1.3
Typical output signals
of sensors
Type A
Type B

1. General notes
Sensor components with analogue output and without integrated amplifier and
conversion electronics, which provide very small analogue output signals not for
immediate evaluation (e.g. in the millivolt range) or a signal which is to be evaluated
only by using additional circuitry.
Examples – Piezoresistive or piezoelectric sensor components
– Pt-100- or thermoelectric cells
– Magnetoresistor and Hall sensor components
– pH- and conductivity measuring probes
– Linear potentiometer
These are often applications where, in the case of high production, the user chooses
his own electronic solutions.
Sensors with analogue output and integrated amplifier and conversion electronics
providing output signals which can be immediately evaluated.
Typical example of output signals – 0 to 10 V
– 1 to 5 V
– -5 to +5 V
– 0 to 20 mA
– 4 to 20 mA
– -10 to +10 mA
Sensors and sensor systems with standardised signal output, e.g. RS-232-C,
RS-422-A, RS-485 or with data bus interfaces such as field bus (Profibus, sensor-
actuator-bus).
Type C
Type D
Type E

1. General notes
1.4.1 Binary sensors
Binary sensors are sensors which convert a physical quantity into a binary signal,
mostly an electrical switching signal with the status "ON" or "OFF".
Examples of binary sensors – Limit valve
– Examples of binary sensors
– Proximity sensor
– Pressure sensor
– Filling level sensor
– Temperature sensor
1.4.2 Analogue sensors
Analogue sensors are sensors which convert a physical quantity into an analogue
signal, mostly an electrical analogue signal such as voltage or current.
Examples of analogue sensors – Sensors for length, distance, displacement
– Examples of analogue sensors
– Sensors for linear and rotational movement
– Sensors for surface, form, geometry
– Force sensors
– Weight sensors
– Pressure sensors
– Sensors for torque
– Flow sensors (for gases and fluids)
– Throughput sensors (for solid materials)
– Filling level sensors
– Sensors for temperature/other thermal values
– Sensors for optical values
– Sensors for acoustic values
– Sensors for electromagnetic values
– Sensors for physical radiation
– Sensors for chemical substances
– Sensors for physical matter characteristics
1.4
Binary and analogue
sensors

1. General notes
In this textbook, sensors dealing with "discrete position" form the main topic, i.e.
sensors which detect whether or not an object is located at a certain position. These
sensors are known as proximity sensors. Sensors of this type provide a "Yes" or
"No" statement depending on whether or not the position, to be defined, has been
taken up by the object. These sensors, which only signal two status, are also known
as binary sensors or in rare cases as initiators.
With many production systems, mechanical position switches are used to
acknowledge movements which have been executed. Additional terms used are
microswitches, limit switches or limit valves. Because movements are detected by
means of contact sensing, relevant constructive requirements must be fulfilled. Also,
these components are subject to wear. In contrast, proximity sensors operate
electronically and without contact.
The advantages of contactless proximity sensors are:
• Precise and automatic sensing of geometric positions
• Contactless sensing of objects and processes; no contact between sensor and
workpiece is required with electronic proximity sensors
• Fast switching characteristics; because the output signals are generated
electronically, the sensors are bounce-free and do not create error pulses.
• Wear-resistant function; electronic sensors do not include moving parts which
can wear out
• Unlimited number of switching cycles
• Suitable versions are also available for use in hazardous conditions (e.g. areas
with explosion hazard).
Today, proximity sensors are used in many areas of industry for the reasons
mentioned above. They are used for sequence control in technical installations and
as such for monitoring and safeguarding processes. In this context sensors are used
for early, quick and safe detection of faults in the production process. The
prevention of damage to man and machine is another important factor to be
considered. A reduction in downtime of machinery can also be achieved by means of
sensors, because failure is quickly detected and signalled.
1.5
Proximity sensors

1. General notes
1.5.1 Overview of position sensors
Fig. 1.5.1 illustrates the different types of contactless position sensors in separate
groups according to physical principles and type, whereby basically each sensor
type can be either an analogue or binary sensor. In this instance, we are only
concerned with the binary type.
Magnetic
position sensors
Ultrasonic
position sensors
Pneumatic
position sensors
Inductive
position sensors
Capacitive
position sensors
Optical
position sensors
analogue: ...
analogue: ...
analogue: ...
analogue: ...
analogue: ...
binary:
magnetic
proximity sensors
binary:
ultrasonic
proximity sensors
binary:
pneumatic
proximity sensors
binary:
inductive
proximity sensors
binary:
capacitive
proximity sensors
binary:
optical
proximity
sensors
with contacts
contactless
pneumatic output
Ultrasonic barriers
Back pressure sensors
Through-beam with/
without FOC*
Light
barriers
Diffuse
sensors
with FOC*
*FOC = Fibre optic cable
Reflexsensors
Retro-reflective with/
without FOC*
without FOC*
Ultrasonic sensors
Air barriers
Fig. 1.5.1: Classification of sensors for position detection (FOC = Fibre optic cable)

1. General notes
1.5.2 Operating voltages
In European countries, proximity sensors are primarily operated with nominal
24 V DC, whereby sensors are generally designed for a range between 10 – 30 V or
10 – 55 V.
In South East Asia, North and South America as well as Australia and South Africa an
estimated share of 30 % of inductive and optical proximity sensors are operated via
AC supply.
Inductive, capacitive and optical proximity sensors are often available not only for
DC but also for AC voltage, whereby the AC voltage is usually 24 V, 110 V, 120 V or
220 V. Inductive, capacitive and optical proximity sensors are also available in
universal voltage designs, which can be connected to both DC and AC voltage, e.g.
within a range of 12 – 240 V DC or 24 – 240 V AC. Other manufacturers, for instance,
offer designs for 20 – 250 V DC AC voltage (e.g. 45 – 65 Hz). An alternative term used
is universal current design (UC).
Typical fields of application for proximity sensors are in the areas of:
– Automotive industry
– Mechanical engineering
– Packaging industry
– Timber industry
– Printing and paper industry
– Drinks and beverages industry
– Ceramics and brick industry
The possibilities of application of proximity sensors in automation technology are so
diverse and vast that it is impossible to provide a comprehensive description of
these. This book therefore lists a selection of typical examples of possible
applications.
1.6
Fields of application for
proximity sensors

1. General notes
In applications to detect whether an object is available at a specific position; e.g. for
the operation of pneumatic cylinders, electrical drives, grippers, protective guards,
winding systems and doors.
Fig. 1.6.1: Non-contacting actuation
In workpiece positioning applications, e.g. in machining centres, workpiece transfer
slides and pneumatic cylinders.
Fig. 1.6.2: Positioning
Detecting objects
Positioning

1. General notes
Counting application for parts and motion sequences, e.g. conveyor belts, sorting
devices.
Fig. 1.6.3: Counting items
Application for measuring the speed of rotation, e.g. of gear wheels or for detecting
zero-speed.
Fig. 1.6.4: Detection of rotational movements
Counting
Measuring
rotational speed

1. General notes
Application for material detection, e.g. for providing or sorting material (re-cycling).
Fig. 1.6.5: Distinguishing materials
Application for defining the direction of linear or rotary movement, e.g. defining
direction for parts sorting.
Fig. 1.6.6: Directional sensing
There are inductive sensors, which only detect the movement of an object in one
direction, but not the opposite direction ("Idle return function", see chapter 15).
Detecting materials
Defining direction

1. General notes
Tool monitoring applications.
Fig. 1.6.7: Checking for drill breakage
Application for monitoring filling levels by means of optical, capacitive or ultrasonic
proximity sensors.
Fig. 1.6.8: Filling level limit switch
Monitoring tools
Monitoring filling levels

1. General notes
Application for approximate distance measuring (distance x).
Fig. 1.6.9: Measuring distances
Application for measuring speed (speed v).
Fig. 1.6.10: Measuring the speed of a moving object
Measuring distance
Measuring speed

1. General notes
Application for protecting machinery against dangerous contact.
Fig. 1.6.11: Accident prevention, e.g. by means of sensors
Light barriers used for accident prevention often have to satisfy certain conditions,
which are laid down in specific regulations as required by the individual countries.
Applications for the detection of the shape of an object by means of several
proximity sensors arranged to sense the contours.
Fig. 1.6.12: Detecting the shape of an object
Accident protection
Note
Contour recognition

2.1.1 Function description
With mechanical limit switches an electrical contact is established or interrupted by
means of an external force. The contact service life would be a maximum of
approximately 10 million switching cycles. Depending on design, relatively high
electrical voltages and currents can be transmitted. In the case of a mechanical limit
switch, the gap which separates two open contacts of different polarity is described
as the contact gap. Switch-over times of mechanical micro limit switches are in the
range of 1 – 15 ms. When electromechanical position switches are used for counting
operations, contact bounce should be taken into consideration.
Compression spring (1) Normally open contacts (4) Contact pressure spring (7)
Housing (2) Normally closed contacts (5) Contact blade (8)
Detent lever (3) Arched spring (6) Guide bolt (9)
Fig. 2.1.1: Limit switch (unactuated and actuated position)
2.1
Electro-mechanical
position switches
2. Mechanical position switches

2.1.2 Technical characteristics
The following types of electro-mechanical position switches can be differentiated:
Miniature position switches, miniature and subminiature micro switches
– Control switches, limit switches
– Snap-action or slow make-and-break switches
– Unenclosed position switches
– Plastic-clad position switches
– Metal-clad position switches
– Safety position switches
– Precision position switches
The most important components of a mechanical micro limit switch are the contacts.
The most widely used contact materials are: gold-nickel, fine gold, silver, silver-
cadmium oxide, silver-palladium and silver-nickel. By making an appropriate choice
of contact material, it is possible to achieve favourable operating conditions in any
field of operation of limit switches.
By fitting actuators, limit switches can be used for a wide range of application
possibilities. Typical types of such actuators are shown in the illustration.

a) b) c)
a) Roller lever
b) Roller lever with idle return
c) Whisker actuator
Fig. 2.1.2: Actuators for mechanical limit switches
The table below lists the key technical data relating to micro switches. The figures
listed in this table are typical examples and merely provide an overview.
Parameter Value
Switching capacity (resistive load) 24 V DC, 6 A
250 V AC, 6 A
Switching point accuracy 0.01 – 0.1 mm (Precision switch up to 0.001 mm)
Switching frequency Approx. 60 – 400 switching operations/min.
Service life 10 Million switching cycles
Protection class (IEC 529, DIN40050) IP00 – IP67
Table 2.1.1: Technical data of a micro switch

2.1.3 Notes on installation
Because limit switches are components of mechanical precision, the following must
be observed with regard to installation:
• Accuracy with regard to assembly, (precise gap between switch actuating
component and object)
• Rigidity of switch/mounting support connection
• Careful observation of the activating devices (approach from side or front)
Care must be taken when making the electrical connections. In the case of clamp or
screw connections, connections must be insulated. If the cables are soldered on,
care should be taken to avoid any heat damage to the switch housing during
soldering. A distorted housing can lead to faulty functioning of the switch. The
connecting lines to the limit switch are to be kept free of tension.
If the limit switch is to be approached directly, it should be noted that it cannot be
used as a mechanical end stop (in normal cases).
There are many applications, where the disadvantages of mechanical limit switches,
such as actuation through touch operation, contact bounce or wear, do not matter.
In these cases, it is possible to take advantage of these moderately priced
components.
Typical areas of application for mechanical limit switches include, for example.
instances where there is noisy electrical environment as a result of electro-magnetic
fields, such as in the case of welding facilities, where electronic proximity sensors
can fail.
There are precision control switches with a very high switching point accuracy of e.g.
0.001 mm, which are suitable for accurate positioning tasks.
With electro-mechanical position switches, maximum current must be restricted as
this can otherwise lead to arc discharge during switching on and off and therefore
burning out of the contacts. A series resistor serves as a current limiter thus
prolonging the service life of the contacts.
When switching inductive loads, a high voltage spike is created at the moment of
cut-off. For this reason, a protective circuit must be provided for the position switch.

The protective circuit can either be a suitable RC element or a corresponding diode
or Varistor (see circuit diagram). The electrical values of these components depend
on the following power component (e.g. relay, contactor etc.).
If a relay or contactor is activated, it is essential that the technical data of the switch
and the relay or contactor be observed.
The pull-in power of a relay or contactor is several times higher (8- to 10-fold) than
the holding power. Therefore it is important that the pull-in power is used as a main
reference.
+24 V DC
+24 V DC
0 V
0 V
V
LR
D
V
LR
R
C
L
L
Load resistance (RL) Protective capacitor (C)
Inductance of load (L) Protective diode or varistor (D)
Protective resistor (R)
Fig. 2.1.3: Protective circuits for electro-mechanical position sensors

2.1.4 Examples of application
Fig. 2.1.4: Door monitoring
Fig. 2.1.5: Braking light switch

Fig. 2.1.6: End position checking of transfer unit
With this type of proximity sensor, a pneumatic circuit is directly effected by means
of the mechanical effect of an approaching object. A plunger, for example, actuates a
pneumatic valve. As far as the design principles are concerned, this type of valve is
similar to the previously described electro-mechanical position switches. However,
they have the advantage that in view of the absence of electrical switching contacts,
contact burn-out cannot occur.
2
1
Supply port (1) Working or output lines (2) Exhaust (3)
Fig. 2.2.1: Pneumatic position sensor (micro-stem valve)
2.2
Mechanical-pneumatic
position switches

The table below lists the key technical data relating to mechanical-pneumatic
position sensors. The figures listed in this table are typical examples and merely
provide an overview.
Parameter Value
Working pressure -95 – +800 kPa (-0.95 – 8.0 bar)
Temperature range -10 – +60 °C
Actuating force at 6 bar operating pressure 6 – 10 N
Switching point pressure-dependent, varies max. 0.8 mm within
pressure range of 0 – 800 kPa (0 – 8 bar)
Table 2.2.1: Technical characteristics of a mechanical-pneumatic position sensor
2.2.3 Notes on application
These limit switches are preferably for use in areas of application where pneumatic
components are already in use. In this case, the supply of compressed air required
for the switches is already available and a conversion of the switch output into an
electrical value is not necessary.
stroke
Fig. 2.2.2: Reversing of a double-acting cylinder by means of adjustable position sensors

Fig. 2.2.3: Auxiliary function for lifting of thin workpieces
Protective circuits for electro-mechanical limit switches
Describe the different types of load which can occur with the connection of a limit
switch. You do not need to take into account mixed types of load. Indicate the
different options of protective circuits.
Switching with low electrical power
A limit switch is to be used for switching very low power. The voltage is approx.
5 V DC, the current is less than 1 mA. At this level even the smallest amounts of dirt
on the contacts can to lead to faults. Suggest a circuit, which overcomes this
problem.
2.3
Exercises
Exercise 2.1
Exercise 2.2

Magnetic proximity sensors react to the magnetic fields of permanent magnets and
electro magnets.
In the case of a reed sensor, contact blades made of ferromagnetic material (Fe-Ni
alloy, Fe = iron, Ni = nickel) are sealed in a small glass tube.
The tube is filled with an inert gas i.e. nitrogen (inert gas meaning a non active, non
combustible gas).
S
Fig. 3.1.1: Magnetic reed proximity sensors
If a magnetic field approaches the reed proximity sensor, the blades are drawn
together by magnetism, and an electrical contact is made.
3. Magnetic proximity sensors
3.1
Reed proximity sensors

The table below lists some of the most important technical data relating to
contacting proximity sensors.
Parameter Value
Switching voltage 12 – 27 V DC or AC
Switching accuracy ±0.1 mm
Maximum contact rating 40 W
Maximum magnetic interference induction 0.16 mT
Maximum switching current 2 A
Maximum switching frequency 500 Hz
Switching time ≤2 ms
Conductance 0.1 Ω
Contact service life (with protective circuit) 5 Million switching cycles
Protection class (IEC 529, DIN 40050) IP66
Ambient operating temperature -20 – +60 °C
Table 3.1.1: Technical characteristics of reed proximity sensor
Reed proximity sensors often have a built-in light emitting diode to indicate
operating status. Fig. 3.1.2 illustrates the internal and external connections. The
light emitting diodes in conjunction with the series resistor assume the function of a
protective circuit for an inductive load.
+24 VDC
0 V
BN(1)
BU(3)
BK(4)
R
L
R
Load resistance (RL) Light emitting diodes (L1, L2) Protective resistor (R)
Fig. 3.1.2: Block circuit diagram of a reed proximity sensor with light emitting diode

When a permanent magnet is moved past a reed proximity sensor, several switching
ranges are possible (see Fig. 3.1.3). The switching ranges depend on the orientation
of the pole axis of the magnet.
Fig. 3.1.3: Response characteristics of a reed proximity sensor

Fig. 3.1.4: Examples of magnetic reed switches for detection of cylinder positions ("cylinder sensors")
When installing reed type proximity sensors, it is important to ensure that there are
no interfering magnetic fields near the sensor exceeding a field strength of more
than 0.16 mT (T = Tesla). Should this be the case, then the proximity sensor must be
shielded accordingly.
If several pneumatic cylinders are fitted with proximity sensors, a minimum distance
of 60 mm is required between the proximity sensors and the adjoining external
cylinder walls. If these distances are reduced, a shift in switching points will occur.

With reed sensors, maximum current flow must be reduced. Otherwise this can lead
to arc discharge during switching on or off and therefore burning of the contact
blades. A resistor fitted in series serves as a current limiter and leads to extended
service life of the contacts.
When switching inductive loads, a high voltage peak is created at the moment of
switch-off. For this reason a protective circuit must be provided for the proximity
sensor unless one is already built in.
The protective circuit can either be a suitable RC element or a corresponding diode
or varistor (see circuit diagram Fig. 3.1.5). The electrical values of these components
depend on the following power component (e.g. relay, contactor etc).
If a relay or contactor is to be actuated, the technical data of both the proximity
sensor and the relay or contactor must be observed.
The pull-in power of a relay or contactor is considerably higher (8- to 10-fold) than
that of the holding power. Therefore, it is important to take the pull-in power as a
reference.
+24 V DC
+24 V DC
0 V
0 V
V
LR
D
V
LR
R
C
L
L
Load resistor (RL) Protective resistor (R) Protective diode or Varistor (D)
Inductance of load (L) Protective capacitor (C)
Fig. 3.1.5: Protective circuits for reed contacts

Fig. 3.1.6: Pneumatic cylinder with magnetic proximity sensors
• Most widely known and used application: Cylinder switches
• With the use of magnetic proximity sensors a wide range of other sensor
problems can be solved if the object to be detected is fitted with a magnet, e.g.:
– Measuring the rotational speed of parts made of any material
– Selective sensing of individual workpieces from a similar series.
– Incremental displacement encoding systems
– Counting devices
– Door switches
– Material positioning

a)
b)
1
1
Permanent magnet on cylinder piston (1)
a) The proximity sensor is unactuated; the switching contacts are open.
b) With the approach of a magnetic field the switching contacts
Fig. 3.1.7: Principle of application of magnetic proximity sensors for the detection of cylinder positions

These proximity sensors, similar to inductive proximity sensors, have a built-in
oscillator (LC oscillating circuit). In contrast to inductive proximity sensors, however,
the oscillating coil is not of a half-shell core design creating a magnetic field directed
outwards, but a coil with a closed-shell core design, e.g. a coil with a shielded ferrite
core. With the approach of a permanent magnet, the core material of the oscillator
coil is saturated, thereby causing a variation in the oscillator current of the proximity
sensor. A trigger stage evaluates the change and converts it into a defined output
signal. These proximity sensors only react to magnetic fields, but not to any metallic
objects.
With these proximity sensors, the direction of the magnetic polar axis in comparison
with the proximity sensor axis must be taken into consideration.
1
2
3
LED display on the reverse side (1) Cable or plug-type connection (2) Active surface (3)
Fig. 3.2.1: Inductive-magnetic proximity sensor
Resistor strips (e.g. Wi- or InSb, Wi=Wismut, In=Indium, Sb=Antimon) change their
electrical resistance in magnetic fields. This effect, i.e. magnetoresistive, can be
used for various sensor types.
If a semiconductor (e.g. InSb) is exposed to a magnetic field, a voltage is created
perpendicular to the direction of the current, i.e. the so-called Hall voltage. Certain
physical dimensions apply in this particular case, i.e. the thickness of the plate must
be small in comparison with the dimensions of length and width. Voltages of up to
1.5 V can be created.
The underlying physical effect is described as the Hall effect after the American
physicist, E. Hall.
3.2
Contactless magnetic
proximity sensor
Inductive-magnetic
proximity sensors
Magnetoresistive
proximity sensors
Hall proximity sensors

The Wiegand sensors consist of a wire which is made from a ferromagnetic alloy of
vanadium, cobalt and iron. The direction of magnetisation of this wire changes
spontaneously when an approaching magnetic field exceeds a certain value. If a coil
is wound around this Wiegand wire, a voltage pulse of up to 3 V is induced.
In principle, Wiegand sensors do not require any external voltage supply.
Only the inductive type of magnetic proximity sensor should be considered from
hereon.
Parameter Value
Operating voltage 10 – 30 V
Maximum switching current 200 mA
Minimum response induction 2 – 35 mT
Maximum magnetic interference induction 1 mT
Response travel (Dependent on field strength and cylinder) 7 – 17 mm
Hysteresis 0.1 – 1.5 mm
Switching point accuracy ±0.1 mm
Voltage drop (at maximum switching current) 3 V
Maximum current consumption (idling) 6.5 mA
Switching frequency 1000 Hz
Protective circuit for inductive load integrated
Protection to (IEC 529, DIN 40050) IP67
Operating temperature -20 – +70 °C
Table 3.2.1: Technical data of an inductive-magnetic proximity sensor
Inductive-magnetic proximity sensors have the following basic advantages
compared with reed proximity sensors:
• No problem with contact bounce
• Wear-free, no moving parts
• Only one single switching area is created, if the magnetic pole axis is suitably
aligned, see Fig. 3.2.2.
Wiegand proximity sensors

Fig. 3.2.2: Response characteristics of an inductive-magnetic proximity sensor
It should be noted that with the application of inductive-magnetic proximity sensors
the proximity sensor may show an asymmetrical switching behaviour. Therefore it
should be checked that the sensor switches reliably in the actual circumstances.
Ferromagnetic materials near a magnetic proximity sensor may lead to changes in
characteristics or to interference, the same as when these sensors are used under
strong external magnetic field influence such as in welding plants or aluminium
smelting works, for instance.
When several pneumatic cylinders are fitted with magnetic proximity sensors, a
minimum distance of 60 mm is required between the proximity sensors and the
nearby external wall of the cylinder.
Inductive-magnetic proximity sensors generally have a built-in protective circuit for
connecting inductive loads as well as against voltage spikes. An additional
protective circuit is therefore superfluous.

One of the most common fields of application for contactless magnetic proximity
sensors is, as in the case of reed proximity sensors, position sensing with pneumatic
cylinders. They can however be used for many other applications, similar to reed
proximity sensors, refer to 3.1.4.
A pneumatic valve is switched by means of a permanent magnet, thereby generating
a control signal.
a)
b)
Switching reed (1) Permanent magnet on piston (2) Flexible conduit (3)
a) The proximity sensor is unactuated, a switching reed interrupts the air flow from 1.
b) The switching reed is actuated by a magnetic field, the air flows from 1 to 2.
Fig. 3.3.1: Principle of application of a magnetic-pneumatic proximity sensor for detection of cylinder positions
3.3
Magnetic-pneumatic
proximity sensors

Parameter Value
Operating pressure range 400 – 600 kPa (4 – 6 bar)
Signal pressure (supply pressure 500 kPa) 8 kPa (80 mbar)
Switching accuracy ±0.2 mm
Maximum magnetic interference induction 0.2 mT
Switching frequency approx. 50 Hz
Table 3.3: Technical characteristics of a magnetic-pneumatic proximity sensor (example)
The proximity sensor corresponds in principle to an air barrier, whereby a switching
blade continually interrupts the air flow of an impending signal. As the magnetic
field approaches (e.g. permanent magnet on the piston of a cylinder) the switching
blade is attracted and releases the air flow, thus creating a signal at the outlet.
Some sensor types are operated in combination with a pressure amplifier.
The distance between two magnetic-pneumatic proximity sensors should be at least
50 mm. It should be checked that the available magnetic field is sufficient for the
reliable operation of the proximity sensor.
If the low pressure output signal is to be used for further processing, then it is
recommended to fit a pressure amplifier in series.
3.3.4 Example of application
Magnetic-pneumatic proximity sensors are primarily used for position sensing of
pneumatic cylinders.
They are particularly suitable for purely pneumatic solutions, i.e. if compressed air is
the only source of auxiliary energy.

Maximum passing speed
Calculate the maximum passing speed of a cylinder piston, the position of which is
to be sensed by means of a reed contact. To do this, assume that the switching time
of the proximity sensor used is 2 ms and take the response travel from table 3.4.1.
Calculate the value for a Festo cylinder, type DNNZ with a diameter of 32 mm as an
example.
What is the change in maximum speed if, for instance, a valve is to be switched with
a switching time of 15 ms?
Piston
diameter [mm]
Type Hysteresis Hmax [mm] Response travel Smin [mm]
SME SMP SME SMP
8 ESN, DSN 2 1.5 7 9
10 ESN, DSN 2 1.5 5 9
12 ESN, DSN 2 2 8 11
16 ESN, DSN 2 2 6 9
ESN, DSN 2 2.5 7 920
DGS
ESN, DSN 1.5 2 6 1725
DGS 2 1.5 7 10
ESW, DSW 2 1.5 10 12
DN, DNZ 2.5 4 7 15
32
DNNZ 2.5 4 7 15
ESW, DSW 2 2 9.5 12
DN, DNZ 2.5 4.5 8 15
40
DNNZ 2.5 4.5 8 15
ESW, DSW 2 2 10.5 12
DN, DNZ 3 5 8 17
50
DNNZ 3 5 8 17
Table 3.4.1: Hysteresis and response travel of various cylinders (example)
3.4
Exercises
Exercise 3.1

Permanent magnet (1) Sensor off (4) Hysteresis (H)
Cylinder barrel (2) Centre of sensor (5) Response range (S)
Sensor on (3)
Fig. 3.4.1: Schematic representation of hysteresis and response travel of a magnetic proximity sensors
Electrical connection of a reed proximity sensor
Describe the behaviour of a reed sensor as shown in Fig. 3.1.2, with the supply
voltage being reversed, i.e. polarity reversal of the proximity sensor.
Can this damage the reed sensor?
Resolution of a reed proximity sensor
What is the smallest possible cylinder stroke that can be detected by two reed
proximity sensors?
Use the technical data in table 3.2.1 and 3.4.1 for your answer.
Exercise 3.2
Exercise 3.3

The most important components of an inductive proximity sensor are an oscillator
(LC resonant circuit), a demodulator rectifier, a bistable amplifier and an output
stage.
6
Oscillator (1) Switching status display (4) Internal constant voltage supply (7)
Demodulator (2) Output stage with protective circuit (5) Active zone (coil) (8)
Triggering stage (3) External voltage (6) Sensor output (9)
Fig. 4.1.1: Block circuit diagram of an inductive proximity sensor
The magnetic field which is directed towards the outside, is generated via a half-
open ferrite core shell of an oscillator coil and additional screening. This creates a
limited area across the active surface of the inductive proximity sensor, which is
known as the active switching zone.
When a voltage is applied to the sensor, the oscillator starts and a defined quiescent
current flows. If an electrically conductive object is introduced into the active
switching zone, eddy currents are created, which draw energy from the oscillator.
Oscillation is attenuated and this leads to a change in current consumption of the
proximity sensor. The two statuses – oscillation attenuated or oscillation
unattenuated – are electronically evaluated.
4. Inductive proximity sensors
4.1
Function description

1
2
3
4
5
b)
a)
Actuating element (1) Resonant circuit coil (4)
High-frequency magnetic field (2) Energy is drawn from the high-frequency magnetic field (5)
Active surface (3)
a) Proximity sensor unactuated (oscillator unattenuated)
b) Proximity sensor actuated (oscillator attenuated)
Fig. 4.1.2: Method of operation of an inductive proximity sensor
Only electrically conductive materials can be detected by means of inductive
proximity sensors.
Depending on switch type (normally open contact or normally closed contact), the
final stage is switched through or inhibited if a metallic object is present in the active
switching zone. The distance to the active area, where a signal change of the output
signal occurs, is described as the switching distance. The important criteria for
inductive proximity sensors is therefore the size of the coil incorporated in the
switching head. The bigger the coil, the greater the active switching distance.
Distances of up to 250 mm can be achieved.

A standardised calibrating plate is used to determine the switching distance of
inductive proximity sensors. Only in this way can useful comparisons of switching
distances of different inductive proximity sensors be made. The standard measuring
plate is made of steel S 235 JR and is 1 mm thick. It is square and the length of a side
is equal to
• the diameter of the active surface of the sensor,
or
• three times the nominal switching distance.
The higher of the two values is to be used as the lateral length of the standard
calibrating plate. Using plates with larger areas does not lead to any significant
changes in the switching distance measured. However, if smaller plates are used
this leads to a reduction of the switching distance derived.
Also, the use of different materials leads to a reduction of the effective switching
distance. The reduction factors for different materials are listed in the table below.
Material Reduction factor
Steel S 235 JR (old: St37) 1.0
Chrome nickel 0.70 – 0.90
Brass 0.35 – 0.50
Aluminium 0.35 – 0.50
Copper 0.25 – 0.40
Table 4.1.1: Guide values for the reduction factor
The above table shows that the largest switching distances achieved are for
magnetic materials. The switching distances achieved for non-magnetic materials
(brass, aluminium, copper) are clearly smaller.

+ 24 V DC
Load (L)
Fig 4.1.3: Connection symbol of an inductive proximity sensor in direct voltage three-wire technology
The connection designations of inductive proximity sensors are standardised, see
chapter 10 and 14. For further notes on circuit layout see chapter 10.
The table below lists the key technical data relating to inductive proximity sensors.
The figures listed in this table are typical examples and merely provide an overview.
Parameter Value
Object material Metals
Operating voltage 10 – 30 V
Nominal switching distance 0.8 – 10 mm, maximal 250 mm
Maximum switching current 75 – 400 mA
Vibration 10 – 50 Hz, 1 mm amplitude
Sensitivity to dirt insensitive
Service life very long
Switching frequency 10 – 5000 Hz, maximal 20 kHz
Design cylindrical, block-shaped
Size (examples) M8x1, M12x1, M18x1, M30x1,
Ø 4 – 30 mm,
25 mm x 40 mm x 80 mm
Protection class to IEC 529 (DIN 40050) up to IP67
Table 4.2.1: Technical data of DC inductive proximity sensors
4.2
Technical characteristics

Many of the inductive proximity sensors which are available on the market have the
following built-in precautions to guarantee simple handling and safe operation:
• Reverse polarity protection (against damage as a result of reversing connections)
• Short circuit protection (against short circuiting of output against earth)
• Protection against voltage peaks (against transient overvoltages)
• Protection against wire breakage (The output is blocked if a supply line is
disconnected)
1
3
2
Active surface (1) LED (2) Cable or plug-in connection (3)
Fig. 4.2.1: Inductive proximity sensor in threaded design
If inductive proximity sensors are fitted in metal fixtures, care should be taken that
the characteristics of the proximity sensor are not be altered. Differentiation should
be made here between the two different types of proximity sensors, i.e. flush-fitting
and non-flush fitting proximity sensors.
d d d
F
Diameter of proximity sensor (d) Free zone ≥3 x sn (F) Nominal switching distance (sn)
Fig. 4.3.1: Flush-fitting inductive proximity sensors
4.3
Notes on application

Where proximity sensors are to be flush-fitted in metal, they must be installed in
such a way as to ensure that the electromagnetic field is directed from the active
zone forwards. In this way, the characteristics of the proximity sensor cannot be
influenced by the method of assembly. In the case of series assembly of proximity
sensors, a minimum gap corresponding to their respective diameter must be
provided. This is essential in order to prevent the proximity sensors from influencing
one another. The free zone in front of the proximity sensor should be at least three
times the nominal switching distance of the proximity sensor used. The free zone is
the area between the proximity sensor and a background object.
The advantage of flush-fitting proximity sensors is that these are very easy to install
and space saving. Their disadvantage compared to non-flush-fitting proximity
sensors is that although the external diameter of the proximity sensor housing is
identical, the switching distance is smaller.
d
F2F3
F1
Diameter of proximity sensor (d) Nominal switching distance (sn)
Free zone 1 = 3 x sn (F1) Free zone 2 ≥ 3 x sn (F2) Free zone 3 ≥ 2 x sn (F3)
Fig 4.3.2: Non-flush fitting inductive proximity sensors
Recessed proximity sensors which are mounted in a material which influences their
characteristics (metal) require a free zone which surrounds the entire active area.
However, these proximity sensors can be embedded in plastics, wood or other non-
metallic materials without the characteristics of the proximity sensor being affected.
This type of sensor can often be recognised by the coil head protruding from the
housing of the proximity sensor.

Fig. 4.4.1: Sensing the piston rod on a pneumatic or hydraulic cylinder
1 2
3
Band conveyor (1) Workpiece carrier (2) Proximity sensor inductive (3)
Fig. 4.4.2: Detection of metallic workpiece carriers on a band conveyor
4.4
Examples of application

Fig. 4.4.3: Sensing a cam controller by means of inductive proximity sensors (Source: Turck)
Fig. 4.4.4: Measurement of speed and direction of rotation (Source: Turck)

1
2
Pneumatic swivel drive (1) Inductive proximity sensor (2)
Fig. 4.4.5: Two inductive proximity sensors check the end positions of a semi-rotary drive
1
Inductive proximity sensor (1)
Fig. 4.4.6: Detecting end position of a press ram

1
Fig. 4.4.7:
Two inductive proximity sensors check whether the slide of a feeding device is in one of two normal end positions
Application of an inductive proximity sensor
The number, distance and direction of transport of material containers are to be
checked on a conveyor belt. For the purpose of marking, the transport containers are
provided with an aluminium marking plate. What do you need to consider when
selecting an inductive proximity sensor for this task?
How do you achieve the largest possible switching distance for a given sensor
diameter?
What do you need to pay particular attention to in this instance?
What is the positive influence of the hysteresis on the switching behaviour of an
inductive proximity sensor? Consider what you would need to observe in practice if
switch-on and switch-off point were exactly the same distance from the proximity
sensor.
4.5
Exercises
Exercise 4.1

Detection of vibrating steel cylinders
Steel cylinders are transported on a conveyor belt, see fig. 4.5.1 and 4.5.2. The steel
cylinders are to be counted by means of an inductive proximity sensor, which is to
be connected to a programmable logic controller. Due to conveyor vibrations, the
steel cylinders also effect a slight vibration movement with amplitude "a".
An inductive proximity sensor is to be used.
What problems can occur with the counting of the steel cylinders?
The proximity sensor has a nominal switching distance of 8 mm. The hysteresis can
be 1 % to 5 % of the switching distance. This is on the assumption that these
hysteresis values also apply for lateral approach of the proximity sensor, as in this
case. What is the maximum vibration amplitude "a" permitted without causing the
problems which occur in paragraph 1?
Fig. 4.5.1: Counting of steel cylinders on a conveyor belt by means of an inductive proximity sensor
1
2
Vibration amplitude (1) Cylinder (2)
Fig. 4.5.2: Vibratory movement of the steel cylinders
Exercise 4.2

The operational principle of a capacitive proximity sensor is based on the
measurement of the change of electrical capacitance of a capacitator in a RC
resonant circuit with the approach of any material.
An electrostatic stray field of a capacitive proximity sensor is created between an
"active" electrode and an earth electrode. Usually, a compensating electrode is also
present which compensates for any influence of the proximity sensor through
humidity.
6
Demodulator (2) Output stage with protective circuit (5) Active zone (capacitor) (8)
Triggering stage (3) External voltage (6) Switching output (9)
Fig. 5.1.1: Block circuit diagram of a capacitive proximity sensor
If an object or medium (metal, plastic, glass, wood, water) is introduced into the
active switching zone, then the capacitance of the resonant circuit is altered.
This change in capacitance essentially depends on the following parameters:
• The distance of the medium from the active surface,
• the dimensions of the medium and
• the dielectric constant of the medium.
5. Capacitive proximity sensors
5.1

The sensitivity (switching distance) of most capacitive proximity sensors can be
adjusted by means of a potentiometer. In this way it is possible to suppress the
detection of certain media. For instance, it is possible to determine the fluid level of
hydrous solutions through the wall of a bottle.
The switching distance of a capacitive proximity sensor is determined by means of
an earthed metal plate. The table below lists the variation in switching point
distances in respect of different materials. The maximum obtainable switching
distance of industrial capacitive sensors is approximately 60 mm.
Material thickness [mm] Switching distance [mm]
1.5 –
3.0 0.2
4.5 1.0
6.0 2.0
7.5 2.3
9.0 2.5
10.5 2.5
12.0 2.5
Table 5.1.1:
Variation of switching distance as a function of the material thickness using a cardboard strip (width = 30 mm)

With capacitive proximity sensors it should be noted that the switching distance is a
function resulting from the type, lateral length and thickness of the material used.
Most metals produce roughly the same value and a number of different values are
listed in respect of other materials.
Material Reduction factor
All metals 1.0
Water 1.0
Glass 0.3 – 0.5
Plastic 0.3 – 0.6
Cardboard 0.5 – 0.5
Wood (dependent on humidity) 0.2 – 0.7
Oil 0.1 – 0.3
Table 5.1.2: Guide values for reduction factor

The table below lists the key technical data relating to capacitive proximity sensors.
The figures listed in this table are typical examples and merely provide an overview.
Parameter Value
Object material all materials with dielectric constant >1
Operating voltage 10 – 30 V DC or
20 – 250 V AC
Nominal switching distance 5 – 20 mm, max. 60 mm
(usually variable, adjustable via potentiometer)
Maximum switching current 500 mA
Sensitivity to dirt sensitive
Service life very long
Switching frequency up to 300 kHz
Size (examples) M12x1, M18x1, M30x1,
up to Ø 30 mm,
25 mm x 40 mm x 80 mm
Protection (IEC 529, DIN 40050) up to IP67
Table 5.2.1: Technical data of capacitive proximity sensors
As with inductive position sensors, flush and non-flush fitting capacitive proximity
sensors are to be distinguished. Furthermore, it should be noted that these sensors
can be easily contaminated. Also, their sensitivity with regard to humidity is very
high due to the high dielectric constant of water (ε = 81). On the other hand, they
can be used for the detection of objects through a non-metallic wall. The wall
thickness in this case should be less than 4 mm and the dielectric constant of the
material to be detected should be higher by a factor of 4 than that of the wall.
5.2
5.3

Due to its ability to react to a wide range of materials, the capacitive proximity
sensor can be used more universally as an inductive proximity sensor. On the other
hand, capacitive proximity sensors are sensitive to the effects of humidity in the
active zone. Many manufacturers, for instance, use an auxiliary electrode to reduce
the effects of moisture, dew or ice thus compensating these disturbances.
5.3.1 Considerations for application
• For cost reasons, the use of inductive as opposed to capacitive proximity sensors
is generally preferred to detect metallic objects.
• For the detection of non-metallic objects, optical proximity sensors compete as a
viable alternative.
• There is a particular field of application where the use of capacitive sensors
provides a distinct advantage.
Capacitive proximity sensors for instance are suitable for monitoring filling levels of
storage containers. Other areas of application include the detection of non-metallic
materials.
These objects can be made of rubber, leather, plastic and other materials, which are
not detected by diffuse optical sensors and where ultrasonic proximity sensors are
too expensive.
Fig. 5.4.1: Detection of black rubber soles
5.4
Detection of matt,
black objects

In the case of detecting filling levels of fluids through thin walls of plastic containers,
inspection glass etc., the wall thickness must be limited such as to enable the
capacitive proximity sensor to respond to the contents alone.
a)
b)
a) Capacitive proximity sensor encapsulated in plastic or quartz glass
b) Detection of liquid level through plastic or glass tube
Fig. 5.4.2: Detection of filling level inside a steel container
Detecting filling levels
of fluids

Capacitive proximity sensors are suitable for the detection of powder, grain or
granular type bulk goods through containers or silos.
For example, it is possible to check the filling volume inside food containers through
the sealed packaging by means of capacitive proximity sensors.
The illustration below shows four capacitive proximity sensors at the base of a
cardboard box to check that four soft drinks bottles have been inserted.
Fig. 5.4.3: Checking of packaging contents through cardboard
Detecting filling levels
of granular material

Capacitive proximity sensors react to copper containing electrical wires or cables of
relatively small diameter, whereas inductive proximity sensors react at a smaller
switching distance or not at all. Optical proximity sensors too may fail in this
instance.
Fig. 5.4.4: Monitoring for cable breakage by means of a capacitive proximity sensor
A capacitive proximity sensor checks whether each box travelling past contains a
light bulb.
Fig. 5.4.5: Checking the presence of bulbs inside cardboard boxes (Source: Turck)
Monitoring the winding of
electrical wires and cables
Checking the presence of
bulbs inside assembled
cardboard boxes

Measuring the filling level in a grain silo
You intend to use a capacitive proximity sensor to detect the filling level in a grain
silo.
What do you have to remember?
Environmental effects on capacitive proximity sensors
You are using a capacitive proximity sensor on an outdoor installation.
What do you need to remember, particularly in the spring and autumn?
Detection of cardboard boxes
You intend to use a capacitive proximity sensor for the detection of cardboard boxes
of varying material thickness.
What do you have to remember?
Detection of a transparent panel
In a factory producing food products, the presence of a panel made of transparent
film is to be checked on empty cardboard packaging (see fig. 5.5.1). You are not sure
whether to use a capacitive, an optical or an ultrasonic proximity sensor.
What are your arguments for this?
1
2
Transparent panels 50 x 30 mm, Cling film 0.1 mm thick (1) Cardboard packaging (2)
Fig. 5.5.1: Packaging with transparent panel
5.5
Exercises
Exercise 5.1
Exercise 5.2
Exercise 5.3
Exercise 5.4

Optical proximity sensors employ optical and electronic means for the detection of
objects. Red or infrared light is used for this purpose. Semiconductor light emitting
diodes (LEDs) are a particularly reliable source of red and infrared light. They are
small and robust, have a long service life and can be easily modulated. Photodiodes
or phototransistors are used as receiver elements. When adjusting optical proximity
sensors, red light has the advantage that it is visible in contrast to infrared light.
Besides, polymer optic cables can easily be used in the red wavelength range
because of their reduced light attenuation.
Infrared (non visible) light is used in instances, where increased light performance is
required in order to span greater distances for example. Furthermore, infrared light
is less susceptible to interference (ambient light).
With both types of optical proximity sensor, additional suppression of external light
influences is achieved by means of modulating the optical signal. The receiver (with
the exception of through-beam sensors) is tuned to the pulse of the emitter. With
through-beam sensors an electrical band-pass is used in the receiver. Particularly in
the case of infrared light, the use of daylight filters further improves insensitivity to
ambient light.
9
Oscillator (1) Switching status display (7)
Photoelectric emitter (2) Output stage with protective circuit (8)
Photoelectric receiver (3) External voltage (9)
Preamplifier (4) Internal constant voltage supply (10)
Logic operation (5) Optical switching distance (11)
Pulse/level converter (6) Switch output (12)
Fig. 6.1.1:
Block circuit diagram of an optical proximity sensor (Emitter and receiver are installed in the same housing)
6. Optical proximity sensors
6.1
General characteristics

6.1.1 Emitter and receiver elements in optical proximity sensors
For versions without fibre-optic connection:
• GaAIAs – IRED
• Wavelength 880 nm (non visible, infrared)
For versions with fibre-optic connection:
• GaAIAs – IRED
• Wavelength 660 nm (visible, red)
Silicon-phototransistor
(Versions with in series connected daylight filters are used for proximity sensors
operating at 880 nm.)
Optical proximity sensors usually have already built-in protective measures:
– Reverse polarity protection
– Short-circuit protection of outputs
– Protection against voltage peaks
With through-beam sensors and retro-reflective sensors, switching functions are
distinguished as follows:
• Light switching method
The output is switched through when the light beam is undisturbed by an object
(Normally open output, N/O = Normally Open). In the case of a light switching
through-beam sensor, the receiver output is switched through if no object is in
the light beam.
• Dark switching method
The output is open (not switching) when the light beam is undisturbed by an
object (Normally closed output, N/C = Normally Closed). In the case of a dark
switching through-beam sensor, the receiver output is switched through if there
is an object in the light beam.
The switching function of optical diffuse sensors is as follows:
• Light switching method
The output closes, if an object to be detected enters the light beam.
(Normally open output, N/O = Normally Open)
• Dark switching method
The output opens, if an object to be detected enters the light beam.
(Normally closed output, N/C = Normally Closed)
Emitter
Receiver

6.1.2 Construction of an optical proximity sensor
Optical proximity sensors basically consist of two main units: the emitter and the
receiver. Depending on type and application, reflectors and fibre-optic cables are
required in addition.
Emitter and receiver are either installed in a common housing (diffuse sensors and
retro-reflective sensors), or housed separately (through-beam sensors).
The emitter houses the source of red or infrared light emission, which according to
the laws of optics extends in a straight line and can be diverted, focussed,
interrupted, reflected and directed. It is accepted by the receiver, separated from
external light and electronically evaluated.
1
4
2
5 6
3
7
Transparent cover (1) Electronics (SMD-technology) (5)
Shield (2) Brass sleeve (6)
Potentiometer (3) Cable (7)
Photoelectric modules (4)
Fig. 6.1.2: Construction of an optical proximity sensor with cylindrical design
The proximity sensor is fitted with an internal shield, which is insulated from the
housing. The electronic components are encapsulated and a potentiometer is fitted
at the output end for the adjustment of sensitivity.
Usually, proximity sensors include a light emitting diode (LED), which lights up when
the output is switched through. The LED display serves as a means of adjustment
and functional testing.

6.1.3 Operating margin for optical proximity sensors
Optical proximity sensors may be exposed to contamination such as dust, splinters
or lubricants during operation. Contamination can cause interference with proximity
sensors. Both contamination of the lens forming part of the proximity sensor optics
as well as contamination of the reflector with retro-reflective sensors and of the
object to be detected in the case of diffuse sensors can cause failure.
Heavy contamination in the light beam of through-beam sensors and retro-reflective
sensors can cause an interruption of the light beam. This then continually feigns the
presence of an object. In the case of diffuse sensors, heavy contamination of the
lens system can be evaluated as an object present, if the light emission is reflected
back to the receiver as a result of the contamination of the lens. Heavy
contamination of the object itself can lead to the evaluation of an object not present,
if less light is reflected as a result of contamination.
In order to achieve reliable operation, the following measures should be taken:
1. Operating the optical proximity sensor with sufficient operating margin.
– Carrying out pre-trials.
– Selecting a suitable proximity sensor with sufficient operating margin.
2. Using proximity sensors with setting aids, e.g. flashing LED function in marginal
areas.
3. Using proximity sensors with an automatic contamination warning signal.
Optical proximity sensors have a certain operating margin (also known as function
reserve) β, being the quotient of the actual optical signal power on the receiver input
PR divided by the just detectable optical signal power at the switching threshold PT:
T
R
P
P
=β
If the received optical emission is at the switching threshold level, this means β = 1,
i.e. there is no operating margin. If the factor is for instance β = 1.5, then an
operating margin of 50 % is available.

Factor β on the one hand depends on the distance between the emitter and the
receiver in the case of the through-beam sensor, between the emitter and reflector
in the case of retro-reflective sensors or between the proximity sensor and object in
the case of a diffuse sensor.
On the other hand, the pattern of the operating margin factor is dependent on
distance s with regard to the individual proximity sensor. Figs. 6.1.3 to 6.1.5
illustrate a number of schematic operating margin curves.
Operatingmarginfactorß
Distance s
40
60
20
10
6
4
2
1
100
400
600
1000
200
0.01 1042 m10.40.20.10.04
Fig. 6.1.3: Example showing the pattern of the operating reserve factor using a through-beam sensor
Distance s
40
60
20
10
6
4
2
1
100
400
600
1000
200
0.1 0.2 10020 m104 6210.4
Fig. 6.1.4: Example showing the pattern of the operating reserve factor using a retro-reflective sensor

Distance s
40
60
20
10
6
4
2
1
100
400
600
1000
200
1 2 1000200 mm1004020104 6
Fig. 6.1.5: Example showing the pattern of the operating reserve factor using a diffuse sensor
The higher the risk of contamination, the higher the required operating margin
factor. If the manufacturer specifies operating margin curves, then a specific value
can be defined when dimensioning the layout of a proximity sensor application. The
anticipated contamination can be estimated considering the transmission factor τ.
If one takes τ = 1 for transmission without contamination then τ = 0.1 means that
with contamination, only 1/10 of the optical signal capacity reaches the receiver. In
this case, an operating margin factor of β >10 is required.
In the absence of manufacturer's specifications, the operating margin can be tested
by means of simulating contaminated conditions.
A flashing indicator on the proximity sensor is useful for checking the operating
margin. This is actuated if the sensor falls below the minimum operating margin.
Designs are available, which start to flash if the operating margin factor of β = 1.5 is
reached, thereby signalling that 50 % operating margin is still available.
A flashing indicator can also be used as a setting aid during the assembly and
adjustment of a proximity sensor layout and at the same time serve as an indicator
of contamination during the subsequent operational process if the operating margin
gradually reduces.

A different type of contamination indicator operates dynamically by checking with
each actuation of the proximity sensor whether, on reaching the switching
threshold, the optical signal capacity has increased to a level which still leaves
sufficient operating margin. For this mode of operation, switching operations are
presumed to take place. An LED flashes, if there is insufficient operating margin or
an electrical warning signal is provided at an additional output.
Other reasons, apart from contamination, can be the cause for falling below the
operating margin, e.g.:
– Exceeding of safe sensing range
– Changes in the material surface of objects detected
– Incorrect assembly (maladjustment)
– Ageing of emitter diode
– Fracture in fibre-optic cable
6.1.4 Variants of optical proximity sensors
Schematically, the variants can be divided as follows:
Designs with
fibre optic
cable
Optical proximity sensors
Through-beam
sensors
Light
barriers
Designs with
fibre optic
cable
Designs with
fibre optic
cable
Retro-reflective
sensors
Diffuse
sensors
Fig. 6.1.6: Variants of optical proximity sensors

Through-beam sensors consist of separately assembled emitter and receiver
components whereby wide sensing ranges can be achieved. For the interruption of
the light beam to be evaluated, the cross-section of the active beam must be
covered. The object should permit only minimum penetration of light, but may
reflect any amount of light.
Failure of the emitter is evaluated as "object present".
Fig. 6.2.1: The principle of the through-beam sensor
The table below lists the key technical data relating to through-beam sensors. The
figures listed in this table are typical examples and merely provide an overview.
Parameter Value
Object material any, problems with highly transparent objects
20 – 250 V AC
Range 1 – 100 m (usually adjustable)
Switching current (transistor output) 100 – 500 mA
Service life long (approx. 100 000 h)
Switching frequency 20 – 10 000 Hz
Designs generally block-shaped but also cylindrical designs
Ambient operating temperature 0 – 60 °C or
-25 – +80 °C
Table 6.2.1: Technical data of through-beam sensors
6.2
Through-beam sensors

Receivers have PNP or NPN transistor outputs and partly additional relay outputs.
1 2
34
5
Emitter (1) Emission range (3) Response range (5)
Receiver (2) Reception range (4)
Fig. 6.2.2: Response range of through-beam sensors
The response range is precisely defined by the size of the optical aperture of the
emitter and the receiver. In this way, precise lateral position sensing is given.
• Enhanced reliability because of permanent light during non-operation.
• Wide range.
• Small objects can be detected even at large distances.
• Suitable for aggressive environment.
• Objects can be diffuse reflecting, mirroring or low transluscent.
• Good positioning accuracy.
• Two separate proximity sensor modules (emitter and receiver) and separate
electrical connections are required.
• Cannot be used for completely transparent objects.
Advantages of a through-
beam sensor
Disadvantages of a through-
beam sensor

• In the case of transparent objects, it is possible to reduce the emitter power by
means of the built-in potentiometer to the extent where the receiver is
deactivated if the object enters the light beam.
• Failure of the emitter is evaluated as "object present" (important with accident
prevention applications).
Fig. 6.2.3: Checking for broken drills by means of a through-beam sensor
Notes

1
2
Through-beam sensor, Emitter (1) Through-beam sensor, Receiver (2)
Fig. 6.2.4: Accident prevention on a press by means of a through-beam sensor
Safety barriers must comply with the accident prevention regulations of the
employer's liability insurance associations. Equipment must be constantly self-
monitoring and tested by the technical control boards and passed in relation to the
design. Access to presses and cutting machines in particular must be monitored
because of their high accident risk rate.
Light emitter and light receiver are installed in one single housing. An additional
reflector is required. Interruption of the light beam is evaluated.
Interruption of the light beam must not be compensated by direct or diffuse
reflection of an object. Transparent, bright or shiny objects may in some cases
remain undetected.
Mirroring objects must be positioned in such a manner that the reflecting beam does
not impinge on the receiver.
6.3
Retro-reflective sensors

Compared to a diffuse sensor, the retro-reflective sensor has a greater range.
Fig. 6.3.1: The principle of the retro-reflective sensor
The table below lists the key technical data relating to retro-reflective sensors. The
Parameter Value
Object material any, problems with reflecting objects
20 – 250 V AC
Range up to 10 m (usually adjustable)
Service life long (approx. 100 000 h)
Switching frequency 20 – 1000 Hz
-25 – +80 °C
Table 6.3.1: Technical data of retro-reflective sensors

1 2 3
4 5
Emitter/receiver (1) Reception range (3) Retro-reflector (5)
Response range (2) Emission range (4)
Fig. 6.3.2: Response range of retro-reflective sensors
The response range is within the lines which form the limit of the aperture edge of
the emitter/receiver optics and the edge of the reflector. As a rule, the response
range near the reflector is smaller than the reflector cross section, depending on the
distance of the proximity sensor and the potentiometer setting.
• Enhanced reliability because of permanent light during non-operation.
• Simple installation and adjustment.
• Object can be diffuse reflecting, mirroring or transparent as long as a sufficiently
high percentage of the light is definitely absorbed.
• In most cases, a greater range in comparison with diffuse sensors.
• Transparent, very bright or shiny objects may remain undetected.
Advantages of a retro-
reflective sensor
Disadvantages of retro-
reflective sensors

• In the case of transparent objects, the light beam passes the object twice and as
a result is attenuated. It is possible to detect objects of this type by means of an
appropriate potentiometer setting.
• Reflecting objects must be arranged in such a manner to ensure that the
reflection does not hit the receiver.
• With particularly small objects, an orifice in the light beam can improve the
effectiveness.
• Failure of the emitter is evaluated as "object present".
• Reflectors can deteriorate with age and dirt; At temperatures of over 80 °C plastic
can be affected permanently, unsuitable reflectors can limit the range and
effectiveness considerably.
Fig. 6.3.3: Monitoring build-up and counting of objects by means of retro-reflective sensors
Only the passive reflector is required on one side of the conveyor without the need
for electrical cabling for the receiver of a through-beam sensor.
Notes
Advantage

Fig. 6.3.4: Slack control by means of retro-reflective sensors
Reflective foil or individual triple reflectors
The solution shown in Fig. 6.3.4 is not applicable in the case of transparent material.
The emitter and receiver are fitted in the same housing. The object diffusely reflects
a percentage of the emitted light thereby activating the receiver. Depending on the
design of the receiver, the output is then switched through (normally open function)
or switched off (normally closed function). The switching distance largely depends
on the reflectivity of the object. The size, surface, shape, density and colour of the
object as well as the angle of impact determine the intensity of the diffused light so
that as a rule only small distances within a range of a few decimeters can be
scanned. The background must absorb or deflect the light emission, i.e. when an
object is not present, the reflected light beam must be clearly below the response
threshold of the receiving circuit.
Fig. 6.4.1: The principle of diffuse sensors
Reflector
6.4
Diffuse sensors

The table below lists the key technical data relating to diffuse sensors. The figures
listed in this table are typical examples and merely provide an overview.
Parameter Value
Object material any
20 – 250 V AC
Sensing range 50 mm – 2 m (usually adjustable)
Life cycle long (approx. 100 000 h)
-25 – +80 °C
Table 6.4.1: Technical data of diffuse sensors
As a rule, the sensing width specified in data sheets refers to white cardboard,
whereby the white reverse side of a Kodak grey card CAT 152 7795 is generally used.
The white side of this test card has a constant reflection of 90 % within a spectral
range of approximately 450 – 700 nm. The grey side reflects 18 %.

1 2
3
Emitter/Receiver (1) Emitting range (2) Response curve (3)
For small distances: Small diffuse reflecting surface required.
For large distances: Large back-reflection surface required.
Fig. 6.4.2: Response curves of diffuse sensors
• Because the reflection on the object activates the receiver, an additional reflector
is not required.
• The object can be diffuse reflecting, mirroring or transparent to translucent as
long as a sufficiently high percentage of the light beam is definitely reflected.
• Whereas with through-beam sensors objects can only be detected laterally to the
light beam, diffuse sensors allow frontal detection, i.e. in the direction of the
light beam.
• Depending on the setting of the diffuse sensor, objects can be detected
selectively in front of a background.
• The response curves according to Fig. 6.4.2 are not completely straight.
Therefore, diffuse sensors are not as suitable as through-beam sensors, if
accurate lateral response is crucial.
• The size, surface, shape, density and colour of the object determine the intensity
of the diffused light emission and hence the actual sensing range. The nominal
sensing range given in data sheets is measured using the white side of the
standard Kodak test card. The background must absorb or deflect the light
emission, i.e. in the absence of an object, the reflected light emission must be
clearly below the response threshold of the receiving circuit.
• Failure of the emitter is evaluated as "no object present".
Advantages of the
diffuse sensor
Disadvantages of a
diffuse sensor
Notes

Correction factors to take into account different object surfaces
The switching distance must be multiplied by the correction factor.
Material Factor
Cardboard, white
1)
1.0
Expanded polystyrene, white 1.0 – 1.2
Metal, shiny 1.2 – 2.0
Wood, coarse 0.4 – 0.8
Cotton material, white 0.5 – 0.8
Cardboard, black matt 0.1
Cardboard, black shiny 0.3
PVC, grey 0.4 – 0.8
1) Matt white reverse side of Kodak grey card CAT 152 7795
Table 6.4.2: Correction factors for the switching distance of retro-reflective sensors
Background masking with diffuse sensors
a
b1
2 3
Setting potentiometer (1) Object (2) Background (3)
Distance between proximity switch and object (a),
Distance between proximity switch and background (b)
Fig. 6.4.3: Background fade-out with diffuse sensor

The effect of the diffuse sensor depends on the difference in reflection of the object
and the background. With only a slight contrast, the response threshold can, if
necessary, be selected via the sensitivity setting on the proximity sensor (1-turn
potentiometer or multiturn potentiometer) in a way that the object is reliably
detected even under these difficult conditions.
However, a tolerance range must be taken into consideration in respect of ageing,
voltage and temperature fluctuations and dirt. For this reason, the setting range
must not be taken up completely when making the adjustment.
When carefully setting the diffuse sensor with the potentiometer, a certain margin
must be made to take into account changes in the condition of the object such as
contamination of the proximity sensor, dust in the atmosphere etc. Close, barely
functional adjustments can lead to problems.
Some diffuse sensor haves a built-in flashing LED display to facilitate reliable
setting, which flashes if the sensing object is not clearly detected. The adjustment of
a proximity sensor with normally open output should be made in such a way that the
light emitting diode is on in the active status without flashing.
Behaviour of a diffuse sensor with a mirroring object
1 2
Emitter/Receiver (1) Reflecting surface (2)
Fig. 6.4.4: Object is detected
Adjustable sensitivity

1 2
Emitter/Receiver (1) Reflecting surface (2)
Fig. 6.4.5: Object is not detected
– Light glass
– Light plexiglass
– Transparent cling film
These materials usually have smooth, reflecting surfaces and a diffuse sensor can
therefore be used.
Condition: The surface of the object must be vertically aligned to the direction of the
light beam.
– Matt black plastic
– Black rubber
– Dark materials with a rough surface
– Dark textiles
– Burnished steel
Diffuse sensors do not react to this type of material or only at a very small distance.
• Through-beam sensors or retro-reflective sensors for lateral approach
• Capacitive proximity sensor or ultrasonic proximity sensor for frontal approach.
Transparent objects
Objects with reduced
reflection
Alternative solutions

1 2
correct (1) wrong (2)
Fig. 6.4.6: Monitoring the position of a workpiece by means of a diffuse sensor
Careful adjustment of sensitivity on the potentiometer is required, whereby
tolerances with regard to differences in material, dirt, etc. must be taken into
account.
Fig. 6.4.7: Shape and position checking using diffuse sensors
A connected controller checks whether all sensors respond (the proximity sensor
outputs are connected according to AND-logic). For high accuracy and small
distances, diffuse sensors with fibre-optic cables should be considered.

Optical proximity sensors with fibre-optic cable adaptors are used if conventional
devices take up too much room. Another application, where the use of fibre-optic
cable adaptors is of advantage, is in areas with explosion hazard. With the use of
fibre-optic cables the position of small objects can be detected with high accuracy.
Fig. 6.5.1: Through-beam sensor with fibre-optic cables (principle)
By using two separate fibre-optic cables it is possible to construct a through-beam
sensor. Because of their handling flexibility, these can be used universally.
Fig. 6.5.2: Diffuse sensor with fibre-optic cables (principle)
Emitter and receiver fibre-optic cables are incorporated in sensor head.
6.5
Optical proximity sensors
with fibre-optic cables

1
2
3 4 5
Emitter (1) Reception range (3) Emission range (5)
Receiver (2) Response range (4)
Fig. 6.5.3: Response range of through-beam sensors with fibre-optic cables
The response range is accurately determined by the aperture of the fibre-optic cable
ends. This makes possible an accurate lateral approach, even with small objects.
1
2
3
4
Optoelectronic proximity sensor (1) Emitter fibre-optic cable (3)
LED display and adjusting screw (2) Receiver fibre-optic cable (4)
Fig. 6.5.4: Through-beam sensor with fibre-optic cables (design example)

1
2
3
4
5
Optoelectronic proximity sensor (1) Emitter fibre-optic cable (3) Object (5)
LED display and adjusting screw (2) Receiver fibre-optic cable (4)
Fig. 6.5.5: Diffuse sensor with fibre-optic cables (design example)
• Detection of objects in areas of restricted access, e.g. through holes.
• Possibility of remote installation of proximity sensor housing (e.g. hazardous
environment: heat, water, radiation, explosion risk).
• Accurate detection of small objects.
• Sensing elements can be moved.
• Mechanically stronger than fibre-glass.
• Length can be reduced easily by cutting the ends on the proximity sensor with a
sharp knife.
• Cost saving.
• Suitable for higher temperatures.
• Reduced optical attenuation with large distances as well as at close infrared
range.
• Longer lasting.
Advantages of optical
proximity sensors adapted
for use with fibre-optic
cables
Advantages of polymer
fibre-optic cables
Advantages of glass
fibre-optic cables

If emitter/receiver units with separate housing (as with through-beam sensors) are
used, it should be noted that if several sensors are similarly orientated, mutual
interference can occur.
a)
b)
E1
R2
E1
E2
R1
R2
R1
E2
a) Problem: Mutual interference of emitter and receiver
b) Solution: Alternate arrangement of emitter and receiver
Fig. 6.5.6: Avoiding mutual interference

Although optical proximity sensors are to a certain extent protected against external
light influences, excessive external light (e.g. filming lights, flash lights, strong
sunlight) can cause interference.
Problem: Interference from extraneous light
Solution: Turn away optical axis from the external source or install an orifice in the light beam
Fig. 6.5.7: Avoiding interfering light

A reflecting surface in the vicinity of some types of optical proximity sensors can
lead to interference, if stray light from the emitter reaches the receiver via a
reflecting surface, see Fig. 6.5.8. If diffuse sensors are used, then a reflecting
background (e.g. light anodised aluminium parts) can create problems.
a)
b)
a) Problem: Reflecting surfaces in surrounding area.
b) Solution: Cover reflecting surfaces or reflection by means of orifices.
Further possibilities are:
– To set the optical axis at an angle in order to "deflect away" the interfering reflection.
– To reduce the sensitivity of the receiver.
Fig. 6.5.8: Avoidance of reflective interference
The lenses of optical proximity sensors must be screened against dirt or regularly
cleaned (e.g. with jets of compressed air). If dirt could cause interference, basic
consideration should be given to whether alternative proximity sensors less affected
by dirt should be used.

Fig. 6.5.9: Detecting small objects by means of a diffuse sensor with fibre-optic cables

Fig. 6.5.10:
Distinguishing between one or two layers of fabrics by means of a through-beam sensor with fibre-optic cable
One layer of fabric lets through more light than two, which leads to switching with an
appropriate proximity sensor setting.
Fig. 6.5.11: Checking of threads
Threaded screws reflect sufficient diffused light to make the receiver switch. If the
surface is smooth, the emitted light beam is deflected away from the sensor.

1
2
a b
Workpiece (1) Workpiece carrier (2) Through-beam sensor with fibre-optic cable (a – b)
Fig. 6.5.12: Detection of workpieces on a workpiece carrier
Environmental effects on optical proximity sensors
What do you need to consider when using an optical proximity sensor in a dusty
environment?
Suggest options for solving this problem.
Selection of optical proximity sensors
Objects are to be detected on processing equipment in a highly inaccessible place
where ambient temperature may increase up to 100 °C. The use of optical proximity
sensors is intended.
Which solution is particularly suitable in this case?
What is to be considered when selecting the means of detection?
Operational reliability of optical proximity sensors
What effect does the modulation of light emission have on the operational reliability
of optical proximity sensors?
6.6
Exercises
Exercise 6.1
Exercise 6.2
Exercise 6.3

Detection of burnished steel
A diffuse sensor is installed in a production plant. When installed, it responds
without being actuated by the object, i.e. its switching output switches through and
the light emitting diode responds. In the presence of an object, it switches off. The
object in question is a burnished steel part.
How can this behaviour be explained?
Electrical connection of proximity sensors
In a factory, a number of optical proximity sensors have failed for unknown reasons
when installed. The engineer has no experience in dealing with proximity sensors,
therefore wrong connection cannot be ruled out. On the other hand, short-circuit
proof and reverse polarity protected proximity sensors have been used. The
engineer confirms that the proximity sensors have been connected to a power
supply of 24 V DC. The power supply unit which has been fitted has a filter circuit
(inductance and filter capacitor), but without electronic control.
What, in your opinion, are the reasons causing the failure?
Exercise 6.4
Exercise 6.5

Measurement of filling level by means of optical proximity sensors
This illustration shows an application where an optical proximity sensor is used for
liquid level measurement.
1. Which types of optical proximity sensor are to be considered for this application?
2. Does this solution permit accurate liquid level monitoring? Why?
3. Under which conditions could this solution fail?
4. Is this solution suitable for measuring the liquid level in a container of melted
candle wax?
5. What other solutions do you know for liquid level measurement?
Fig. 6.6.1: Liquid level measurement by means of a through-beam sensor
On reaching a defined liquid level, the light emitted by the emitter will be reflected
on the surface and will reach the receiver.
Exercise 6.6

Detection of workpieces
Figure 6.6.2 shows a workpiece on a conveying slide. The workpiece is positioned
in a recess of the workpiece carrier. The edge of the workpiece is to be detected
through holes.
1. Is it possible to solve this problem by using a through-beam sensor? Or is too
much light lost when the light passes through the hole?
2. With other conveying slides, sufficient space is available on one side only or
above the slide for a proximity sensor or fibre-optic cable to be mounted.
The workpiece is made of plastic and has a matt lateral sawn edge as well as a
smooth reflecting surface. The slide is made of matt aluminium.
Which solution can you recommend?
1
2
a b
Workpiece (1) Workpiece carrier (2) Through-beam sensor with fibre-optic cable (a – b)
Fig. 6.6.2: Interrogation by means of a through-beam sensor with fibre-optic
Exercise 6.7

Use of optical proximity sensors in car washes
A decision is to be made as to whether optical proximity sensors can be used for car
washes to control movement of the drying nozzle as shown in the illustration below.
After the car has been washed, a gantry bearing the drying nozzle which extends
across the width of the car, traces the contours of the car. The task of the proximity
sensors is to ensure that the drying nozzle constantly follows the contour of the car
at a certain distance. The proximity sensors may be splashed by water during the
preceding wash cycle.
Which type of proximity sensor would you recommend?
How many proximity sensors would you suggest for each car wash unit and in which
order?
Fig. 6.6.3: Drying in a car wash
Exercise 6.8

Use of optical proximity sensors equipped with fibre-optic cables
An enterprising designer would like to use an optical diffuse sensor equipped with
fibre-optic cables as a retro-reflective sensor and thereby employing a reflector as
shown in the illustration. With this solution, he hopes to achieve a greater range of
detection for dark, matt workpieces, which can only be approached via restricted
access.
Does this solution work?
What, in your opinion, are the characteristics of this solution?
1 2
3
Fibre-optic diffuse sensor (1) Triple mirror reflector (2) Object (3)
Fig. 6.6.4: Application of a diffuse sensor with fibre-optic cable as a retro-reflective sensor
Exercise 6.9

Checking of bottles
A drinks manufacturer would like to use a proximity sensor to detect automatically
which empty bottles returns are fitted with light metal screw caps. The bottles are to
pass below a proximity sensor on a conveyor belt (see illustration). Because of the
variation in bottle height and the different screw caps fitted a maximum tolerance in
height H of 8 mm should be calculated.
1. Which solution of optical proximity sensors is to be recommended?
2. Is it also possible to use inductive proximity sensors (e.g. proximity sensors with
a nominal switching distance of 8 mm)?
Fig. 6.6.5: Separating bottles with or without sealing caps
Exercise 6.10

The operational principle of an ultrasonic proximity sensor is based on the emission
and reflection of acoustic waves between an object and a receiver. Normally, the
carrier of these sound waves is air. The travelling time of the sound is measured and
evaluated.
6
Oscillator (1) External voltage (6)
Evaluation unit (2) Internal constant voltage supply (7)
Triggering stage (3) Active zone (ultrasonic transducer) (8)
Switching status display (4) Switch output (9)
Output stage with protective circuit (5)
Fig. 7.1.1: Block circuit diagram of an ultrasonic proximity sensor
The proximity sensor can be divided into three main modules, the ultrasonic
transducer, the evaluation unit and the output stage. A short pulse briefly triggers
the ultrasonic transmitter. This is usually a piezo-electric module, e.g. on the basis
of piezo-oxides.
7. Ultrasonic proximity sensors
7.1

The ultrasonic transmitter emits sound waves in the non-audible range at any
frequency usually between 30 – 300 kHz. In most cases, the ultrasonic transmitter
changes from emission to reception, i.e. now operating in the sense of a
microphone. Filters inside the ultrasonic proximity sensor check whether the sound
received is actually the echo of the emitted ultrasonic waves.
1 2
td to
te
tp
V
Emission pulse (1) Pulse duration (td) Echo transmission time (te)
Echo (2) Oscillation decay time (to) Pulse interval (tp)
Fig. 7.1.2: Principle of distance measurement by evaluating the transmission time of ultrasonic pulses
The speed of operation of ultrasonic proximity sensors is limited by the maximum
pulse repetition frequency which, depending on design, can range between 1 Hz and
100 Hz.
A major advantage of ultrasonic proximity sensors lies in the fact that these can
detect a wide range of different materials. Detection is independent of shape, colour
and material, whereby the material can be solid, fluid or in powder form. Testing is
not affected by dusty, steamy or smoky atmospheres.

Ultrasonic proximity sensors are generally available in the form of diffuse sensors,
where the emitter and received are in one housing. In addition, ultrasonic barriers
are available, which have separate emitters and receivers.
Preferred areas of application for ultrasonic proximity sensors are:
• Storage facilities
• Transport systems
• Food industry
• Metal, glass and plastics processing
• Monitoring of bulk material
Ultrasonic proximity sensors have the following advantages:
• Relatively large range (up to several meters)
• Object detection irrespective of colour and material
• Safe detection of transparent objects (e.g. glass bottles)
• Relatively dust and dirt insensitive
• Fading out of background possible
• Outdoor application possible
• Feasibility of contactless sensors with accurate variable switching points. The
area of detection can be flexibly divided into zones. Programmable versions are
available.
Ultrasonic proximity sensors have the following disadvantages:
• If ultrasonic proximity sensors are used for slanting object surfaces, the sound is
deflected. It is therefore important that the object surface to be reflected is at a
right angle to the axis of the sound propagation or to use ultrasonic barriers
instead.
• Ultrasonic proximity sensors react relatively slowly. Maximum switching
frequency is between 1 Hz and 125 Hz.
• Ultrasonic proximity sensors are generally more expensive than optical proximity
sensors (e.g. factor 2).

The table below lists the key technical data relating to ultrasonic sensors. The
Parameter Value
Object material any, with the exception of sound-absorbing materials
Operating voltage typ.24 V DC
Nominal switching distance 100 mm – 1 m, max. up to 10 m, usually adjustable
Sensitivity to dirt moderate
Service life long
Ultrasonic frequency 40 – 220 kHz
Protection (IEC 529, DIN 40050) typ. IP65, max. up to IP67
Ambient operating temperature 0 – +70 °C, partly as low as -10 °C
Table 7.2.1: Technical data of ultrasonic sensors
Ultrasonic proximity sensors as a rule are equipped with a light emitting diode for
status indication and very often with a potentiometer for setting of the operating
range. There are also designs with two potentiometers for setting a switching
window as well as special programmable designs, with which different operating
ranges can be selected via an electronic interface.
Some ultrasonic proximity sensors are equipped with synchronised inputs, whereby
trouble free and alternating operation is possible if several adjacent proximity
sensors are used.
7.2

7.3.1 Minimum distances
With the installation of ultrasonic proximity sensors, as with that of inductive and
capacitive sensors, different minimum distances must be observed between
adjacent proximity sensors.
When assembling ultrasonic proximity sensors without the option for
synchronisation, make sure that mutual influence of the proximity sensors does not
occur. Observe the following listed minimum distances in relation to the detection
range of the proximity sensors used. These values apply if the object to be tested is
moved in front and vertically to the proximity sensor. The values indicated merely
provide examples. Deviations may occur depending on type and manufacturer's
instructions.
Detection range [cm] Typical minimum distance [cm]
6 – 30 >15
20 – 100 >60
80 – 600 >250
Table 7.3.1: Lateral minimum distance between two parallel ultrasonic proximity sensors
Under other operating conditions, the minimum distances are established
experimentally for the respective assembly.
If two ultrasonic proximity sensors are opposite one another, then the values given
in the table below are to be observed.
6 – 30 >120
20 – 100 >400
80 – 600 >2500
Table 7.3.2: Minimum distances between opposing ultrasonic proximity sensors
7.3

In cases where a wall or other reflecting objects are adjacent to an ultrasonic sensor,
the following values apply:
6 – 30 >3
20 – 100 >15
80 – 600 >40
Table 7.3.3: Minimum distances between ultrasonic proximity sensors and a lateral, reflecting wall
7.3.2 Required minimum size of the object
The required object size depends on the acceptance angle of the ultrasonic beam. If
the ultrasonic sound waves travel past an object which is too small, then any objects
which may be alongside or in the background may interfere. As there is often
insufficient data provided by the manufacturer, a preliminary test is recommended
by moving the test plate from the side towards the object to be detected whilst
observing the switching distance.
1 2 3 4
Ultrasonic sensor (1) Sound cone (3)
Acceptance angle of sound cone (2) Object (4)
Fig. 7.3.1: Detection area of an ultrasonic sensor

7.3.3 Type of object
Suitable are solid, fluid, pulverised or granulated materials. Unsuitable for
ultrasonic sensors are ultrasound absorbing materials such as coarse clothing
material, cotton wool, terry cloth, foam rubber, rock wool. On the other hand, it is
possible to detect these materials by means of ultrasonic barriers.
Similarly it is possible to detect transparent, reflecting or jet black objects, where
optical proximity sensors may fail. Even very thin transparent film of a thickness of
approx. 0.01 mm can be detected head-on by means of ultrasonic proximity sensors.
7.3.4 Minimum possible distance of object
As a proximity sensor requires a minimum processing time to detect the ultrasonic
echo, it cannot operate within a certain blind area. In the case of short distances,
"secondary lobes" of the ultrasonic sound cone can lead to error pulses. With
designs consisting of a single ultrasonic transducer, completion of oscillation must
be achieved after emission (see Fig. 7.1.2), before the echo pulse can be registered.

7.3.5 Position of object
Similarly as with light, ultrasound is deflected on flat surfaces. In this case, an
ultrasonic sensor does not receive an echo signal. Objects with smooth, even
surfaces, can no longer be detected if the deviation is for instance more than
±3° – ±5° of the vertical alignment to the proximity sensor. With objects of a rough or
irregular surface a wider angle is possible, whereby the ultrasonic wave length, the
surface finish and distance are also relevant.
3° - 5° 3° - 5°
45°
1
2
3°
Sand (1) Liquid (2)
Fig. 7.3.2: Effect of the object surface when using ultrasonic sensors

7.3.6 Effect of ambient temperature, humidity, air pressure
Ultrasonic speed depends on air temperature by approximately 1,8 ‰ per °C.
Because ultrasonic proximity sensors are invariably not temperature compensated,
a slight change in switching point may occur as a result of ambient temperature. The
humidity content of air at a temperature range below 40 °C effects a maximum
change in the speed of sound by 1.4 % between a relative air humidity of 0 % and
100 %. Natural changes in atmospheric air pressure do not cause any significant
changes in the speed of sound. Only at high altitudes does the speed of sound
decrease slightly.
7.3.7 Diverting the ultrasonic beam
The ultrasonic sound wave beam can be diverted by means of even or slightly
concave reflectors, whereby objects can be detected "around the corner".
7.3.8 Effect of temperature of the object
Very hot objects, such as melting baths or red hot metal leads to strong air striation
and can interfere with ultrasonic propagation. Preliminary experiments are therefore
recommended.
7.3.9 Effect of ambient noise
As the transmission frequencies are in the range of 30 – 250 kHz and due to the
limited receiver bandwidth, ultrasonic proximity sensors are generally little affected
by external noise. In exceptional cases they may react to intensive, selective
interference.

Ultrasonic proximity sensors are used for monitoring filling levels in silos.
Ultrasonic proximity sensors have also proved reliable for the control of automatic
trolleys in warehouses.
The following illustrations show a few additional examples:
a)
b)
c)
a) Monitoring slack between web feed rollers
b) Sorting according to different height
c) Monitoring of batch thickness
Fig. 7.4.1: Examples of application for ultrasonic proximity sensors
7.4

Smallest measurable distance
When distance measuring with ultrasonic proximity sensors, the smallest
measurable distance has to be taken into account.
Why?
Deflection of ultra-sonic sound waves
Is it possible to deflect sound waves similar to light with a mirror, by 90° for
instance?
What do you need to observe?
Detection of boxes on a conveyor belt
A conveyor belt for metal boxes is to be interrogated as to whether boxes are
available, filled or empty. The proximity sensors to be used must not only detect
whether boxes are present, but also "look inside" the box from above and check
whether they have been filled. The use of optical proximity sensors was questioned
due to the different colours of containers and the contents as well as the risk of
contamination.
Explain the advantages and disadvantages of ultrasonic proximity sensors as
opposed to diffuse optical sensors for an application of this type.
7.5
Exercises
Exercise 7.1
Exercise 7.2
Exercise 7.3

With pneumatic proximity sensors the presence or absence of an object is detected
by means of contactless sensing with air jets. When an object is present a signal
pressure change occurs, which can be further processed.
The advantages of these proximity sensors are:
• Operational safety in dusty environments
• Operational safety with high ambient temperatures
• Can be used in areas of explosion hazard
• Insensitive to magnetic influences and sound waves
• Reliable even in extreme ambient brightness and for sensing of light transparent
objects, where optical proximity sensors may not be suitable.
Pneumatic proximity sensors can be differentiated between back pressure sensors,
reflex sensors and air barriers. Detectable distances range from 0 to 100 mm, see
Fig. 8.1.1
A common requirement for the application of pneumatic sensors is to reduce the
system air pressure to a low pressure range by means of pressure regulators. A
supply of filtered, oil-free air is essential.
As the pneumatic signal is generally too weak for further evaluation, a pressure
amplifier needs to be connected downstream. A pneumatic proximity sensor with
binary electrical output signals is created with the help of pneumatic-electric
converters (pressure switches).
When obstructing the exhaust nozzles, it is important to ensure that the amplifier is
designed for any necessary increased pressure.
When replacing pneumatic sensors, it is generally necessary to adjust the amplifier
or threshold setting, due to discrepancies as a result of production tolerances.
8. Pneumatic proximity sensors
8.1
General characteristics

s [mm]
s [mm]
s [mm]
0
4
0
5
0,5
15
5
a)
1 1 12 2
1 2
1 2 1 2 1 2
1 2
1 1
1 2
2
b)
c)
10040
Supply pressure (1) Output pressure (signal pressure) (2) Sensing distance (s)
a) Back pressure sensors
b) Reflex sensors
c) Air barriers
Fig. 8.1.1: Typical sensing distances of various pneumatic proximity sensors
Supply pressure can vary, but is generally in the region of 0 – 800 kPa (0 – 8 bar).
The signal pressure generated depends on the supply pressure and the distance
between the nozzle and the object.

The obstructing of an air jet drilling by means of an object to be detected leads to a
signal pressure build-up in the control port to the level of the supply pressure.
Alternative designation: Back pressure nozzle
11 22
Supply pressure (1) Output pressure (signal pressure) (2)
Fig. 8.2.1: Method of operation of back pressure nozzle
8.2
Back pressure sensors
(Back pressure nozzles)

The reflex type of sensor consists of an annular ring jet nozzle and a central receiver
nozzle.
If an object is moved towards the air escaping from the ring jet nozzle (sender), an
excess pressure builds up in the central nozzle (receiver nozzle) when the object is
at a certain distance from the ring jet. Fig. 8.3.1 provides a schematic representation
of the air flow.
11 22
Fig. 8.3.1: Method of operation of a reflex sensor
The reflection of the air jet on an object to be detected creates a signal pressure
build-up in the control port relative to the sensing distance and supply pressure.
The reflex sensor is typical of this design. A reflex sensor generally consists of a
sender and receiver nozzle arranged concentrically. A constant air jet is emitted by
the sender.
The approach of an object towards the reflex sensor influences this air jet and a back
pressure (reflex) builds up in the receiver nozzle, which can be evaluated as a signal
(output 2).
Alternative designation: Reflex nozzle
8.3
Reflex sensors

By placing a ring jet nozzle directly opposite a receiver nozzle, it is possible to
construct an air barrier (analogous to a through-beam sensor) which is interrupted
by an object. With this type, it is also possible for an air jet to be interrupted by an
opposite air jet instead of an object. This is known as an interference jet barrier.
Distances of up to 100 mm can be spanned by air barriers.
Simple air barriers, where the air escapes from the sender only, are subject to dirt
collecting in the receiver nozzle, because the flow of air collects dirt particles from
the surrounding area. This can lead to a malfunction or a total breakdown due to
blockage.
a)
b)
2 1 1
1 1
a) Receiver nozzle
b) Emitter nozzle
Fig. 8.4.1: Method of operation of an air barrier
8.4
Air barriers

Most air barriers on the market operate on the principle of the deflecting jet,
whereby air escapes on both sides of the barrier. The function of the receiver side
mode of operation can be compared to that of a reflex sensor. In this way, it is
possible to greatly reduce susceptibility to contamination.
Since the price of a complete pneumatic proximity sensor (nozzle and pressure
amplifier/pressure switch) is generally higher than that of a standard inductive,
capacitive or even optical proximity sensor, pneumatic proximity sensors are used
preferably for special applications in new developments, where other proximity
sensors are unsuitable.
Advantageous applications for pneumatic proximity sensors:
• Use in areas with explosion hazard.
• Use in welding installations, where AC and DC fields are generated.
• Use in damp and dirt and dust laden environment.
• Use in high ambient temperatures.
• Used in measuring filling levels of foaming liquids.
8.5

The following illustrations show the characteristic curves relating to the performance
of pneumatic proximity sensors, using Festo products as an example. The data
indicated is in respect of back pressure sensors, reflex sensors and air barriers.
8.6.1 Characteristic curves of back pressure sensors
-100
0
100
200
300
400
500
700
600
Signalpressure
Signalpressure
Nozzle distanceNozzle distance
kPa
1
2
3
4
5
6
7
8
0.02 0.021.00.4 1.00.40.2 0.20.1 0.10.04 0.04mm mm
1
2
3
4
5
6
7
8
-4
0
4
8
12
16
20
28
24
kPa
Supply pressure: 700 kPa (1), 600 kPa (2), 500 kPa (3), 400 kPa (4),
300 kPa (5), 200 kPa (6), 100 kPa (7), 10 kPa (8)
Fig. 8.6.1:
Signal pressure as a function of nozzle distance and supply pressure with a Festo SD-3 back pressure sensor
8.6
Characteristic curves of
pneumatic proximity
sensors

Airconsumption
Supply pressure
l/min
kPa
14
12
10
8
6
4
2
0
0 600400300200100
Fig. 8.6.2: Air consumption as a function of supply pressure with a Festo SD-3 back pressure sensor

8.6.2 Characteristic curves of reflex sensors
Signalpressure
Signalpressure
Axial distance
0 7mm54321
-0.1
1.0
-0.2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
kPa
a a
Lateral distance
Reference edge
approx 1 mm
s = 1.5 mm
a
-2 5mm3210-1
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
kPa
1.0
Supply pressure = 15 kPa (0.15 bar)
Fig. 8.6.3: Signal pressure as a function of nozzle distance and supply pressure with a Festo RML-5 reflex sensor

supply pressure
70
0
10
20
30
40
50
60
l / min
0 100kPa604020
Fig. 8.6.4:
Air consumption as a function of supply pressure with uninhibited air outlet on a Festo RML-5 reflex sensor

8.6.3 Characteristic curves of air barriers
Distance s
Signalpressurex
0 160mm12010080604020
s
Emitter Receiver
1.0
kPa
0.05
0.01
0.5
0.1
pE1 pE2 pE3
pR
x
1
2
pE
1
pR = 10 kPa = const. pE1 = 10 kPa pE2 = 20 kPa pE3 = 50 kPa
Fig. 8.6.5: Signal pressure as a function of supply pressure and distance of the Festo SFL-100 air barrier

• Measuring the speed of transport of screens (plastic screens) for silk screen
printing. These screens are easily contaminated and optical proximity sensors
are therefore unsuitable. A possible solution is to provide holes at specified
intervals along the edge of the screens and to use pneumatic sensor nozzles for
sensing.
• Monitoring of tools (e.g. checking for broken drill) in environments where for
instance optical proximity sensors are unsuitable because of contamination due
to oil or cooling agents.
• Checking holes after drilling operation.
• Testing ceramic plates for smoothness after burning.
Fig. 8.7.1: Checking for broken drills by means of an air barrier
By using an air barrier, it is possible to check that a drill is in position immediately
before drilling the workpiece.
If the drill is broken, the air jet from the sender nozzle hits the receiver thus creating
a signal. With an air barrier a signal is created only if an object is not present.
A major advantage of this solution (as opposed to an optical proximity sensor for
instance) is in that contamination such as drilling fluid does not interfere with
operation.
8.7

20 mm
381 mm
Fig. 8.7.2: Use of an air barrier for counting plastic sheets
Clear plastic sheets pass on a transport device in gaps of 20 mm.
The gaps between the sheets are used for the purpose of counting.
A pressure amplifier is fitted downstream of the air barrier receiver.
• Supply pressure of sender 25 kPa (0.25 bar)
• Response time 16 ms
• Maximum speed of transport device 37 m/min.
The use of optical or capacitive proximity sensors would present problems in this
instance; ultrasonic proximity sensors would be a possible alternative.
Key data

1
2
3
p = 10 to 30 kPa
1
2
Back pressure nozzle (1) Immersion tube (2) to the pump (3)
Fig. 8.7.3: Filling level monitoring by means of a back pressure nozzle
The threaded end of the back pressure nozzle enables the attachment of an
immersion tube. Once the level of fluid in the immersion tube has reached a certain
height, the back pressure nozzle and the pressure amplifier fitted downstream
respond.
The pressure of the output signal is proportional to the height of the fluid level. The
maximum pressure of output signal 2 corresponds with the supply pressure.
This solution is primarily suitable for foaming liquids, as pneumatic proximity
sensors only react to the fluid, but not to the foam.

1
x1
x1
1
x2
x2
Fig. 8.7.4: Band edge control
Fig. 8.7.5: Sensing of instrument pointers

Fig. 8.7.6: Camshaft control using air barrier sensor
Fig. 8.7.7: Counting of glass bottles

Range of air barrier sensors
Parts of a width of 90 mm are to be detected in an area subject to explosion hazard.
Check whether on the basis of the characteristics of air barrier sensors of type Festo
SFL listed in chapter 8.6, these can be used in this instance. Specify the value of the
output signal in kPa (mbar).
Checking lids by means of a reflex sensor
A reflex sensor is to be used to check that lids have been fitted. Specify a practicable
value for the distance between the sensor and the lid. The respective characteristic
curves can be found in chapter 8.6.
Also determine the air consumption for this configuration in accordance with the
characteristic curves in chapter 8.6.
Fig. 8.8.1: Lid monitoring by means of a reflex sensor
8.8
Exercises
Exercise 8.1
Exercise 8.2

In the first instance, proximity sensors can be selected according to the material
which they are to detect. Metals of any kind can be detected easily and economically
with inductive proximity sensors if short switching distances only are required (e.g.
0.4 – 10 mm). For greater distances, optical proximity sensors of varying designs are
available. The greatest distances can be spanned by means of through-beam
sensors.
Capacitive proximity sensors are suitable for the detection of a wide range of
materials, but again only for relatively small distances, similar to inductive proximity
sensors. Objects to be detected by a capacitive proximity sensor must be of a certain
minimum volume. Ultrasonic and optical diffuse reflective proximity sensors are able
to detect a wide range of different materials over greater distances. However, the
detection of reflecting objects with tilted surfaces may create problems.
Further criteria for the selection of proximity sensors are the conditions under which
the object is to be detected, what the installation requirements for the proximity
sensor are and the environmental factors to be taken into account. Once all these
requirements have been established, a suitable proximity sensor can be selected
from the various alternative products on offer.
A systematic listing of the above mentioned criteria is set out overleaf.
Electrically conductive material
– Steel
– Stainless steel
– Brass
– Copper
– Aluminium
– Nickel
– Chromium
– Metal-coated, electrically non-conductive materials, depending on specific
coating thickness
– Graphite
9. Selection criteria for proximity sensors
9.1
Object material

Electrically non-conductive material
– Plastics
– Cardboard, paper
– Wood
– Textiles
– Glass
Nature of non-conductive materials
– Optically transparent or non-transparent
– Optical reflex ability of surface (absorbent to reflecting)
– Homogenous, non-homogenous (e.g. composite material)
– Porous, fibrous
– Solid, liquid, loose material
– Dielectric constant
Size and shape
– Dimension of structure to be detected and possibly classification to standard
shapes, e.g. block, cylinder, sphere, cone inter alia.
• Contacting or non-contacting
• Required distance between proximity sensor and object, possibly taking into
account any tolerances which may occur in respect of distance, e.g. in the case of
moving objects.
• Speed of a moving object or time during which the object is present or down
time.
• Constant or changing sensing requirements, e.g. different position of object.
• Distance to adjacent objects, required resolution of interrogation.
• Type of background or area below
9.2
Conditions for the
detection of objects

• Free space available (distance/volume) around sensing area. The need to use
miniature designs or remotely positioned proximity sensors when using fibre
optic attachments or pneumatic sensor heads. The necessity for detecting
"around the corner", in crevisses or through holes.
• Necessity of flush mounted installation.
• Required minimum distance between several adjacent proximity sensors.
• Ambient temperature
• Effect of dust, dirt, particles, humidity, splashing water, water jets inter alia, see
IP protection classes.
• Influence of magnetic or electric fields, e.g. in a welding environment.
• Influence of external light emissions (peculiarities of ambient lighting).
• Area with explosion hazard
• Clean room environment
• Requirements for hygiene or sterilisation for use with food packaging or in a
medical environment.
• Application in high pressure or vacuum conditions.
• Application in areas with explosion hazard
• Application for the purpose of accident prevention
• Application where increased safety measures are required against breakdown
9.3
Installation conditions
9.4
Environmental
considerations
9.5
Safety applications

• Design/type with specification of dimensions
• Voltage supply (direct current, alternative current)
• Type of switch output and type of protective circuits:
– Positive switching (PNP output)
– Negative switching (NPN output)
– Short circuit protection
– Reverse polarity protection
• Connection: Cable or plug
• Protection class to IEC 529, DIN 40050
• Permissible ambient temperature during operation
• Available special designs e.g. to DIN 19234 (NAMUR) or intrinsically safe design
("explosion protection"), or accident protection design
• Extent of switching distance or range, fixed value or adjustable value
• Nominal switching distance or nominal range
• Switching hysteresis
• Reproducibility
• Maximum operating frequency (switching frequency)
• Maximum load current
• Flush mounted or non-flush mounted option
• Minimum required distance between adjacent proximity sensors of the same
type
• Operating reserve factor for optical proximity sensors
• Fibre optic design available for optical proximity sensors. The following technical
data apply in respect of fibre optic designs, e.g.:
– Range
– Dimensions of fibre optic head
– Fibre optic cable length
– Detection angle, response ranges
– Permissible ambient temperature
• Available accessories for retro-reflective sensors (reflectors, dimensions)
• Prices or price categories of proximity sensors
9.6
Options/features of
proximity sensors

10.1.1 Two-wire DC and AC technology
Proximity sensors in two-wire technology have only two connecting wires. They are
connected in series to the load to be switched and thus receive their supply voltage
via the load. This has the effect of a certain amount of residual current flowing via
the load even if the output is closed, and that of a voltage drop over the proximity
sensor in the switched through status.
Proximity sensors are designed with either "normally closed" contacts (N/C) or
"normally open" (N/O) contacts, but designs are also available which incorporate
the two functions.
BN(1)
BU(3)
BN(1)
BU(3)
BN(1)
BU(3)
BN(1)
BU(3)
V
V
V
V
Operating voltage (V) Load (L)
Fig. 10.1.1: Connection diagrams for two-wire technology (DC, AC and DC/AC (universal current) – designs)
10. Connection and circuit technology
10.1
Types of connection

A potential protective grounding terminal is identified by green-yellow.
In the case of designs for AC or AC/DC (universal current), the connection cables may
be identified in any colour other than green-yellow. Generally, however brown or
blue is selected, as for direct current designs.
Voltage supply e.g.: 15 – 250 V DC
20 – 250 V AC
In the case of two-wire sensors it should be noted that in the unactuated status, a
residual current must flow to provide a current supply for the proximity sensor. The
residual current also flows via the load. In the acknowledged status, a minimum load
current must flow to guarantee the reliable operation of the proximity sensor.

10.1.2 Three-wire DC technology
Proximity sensors in three-wire technology have three connecting wires. As a rule,
the colours of the connecting wires comply with European standard EN 50 044. Two
wires are for the purpose of voltage supply (brown +, blue -). The third wire (black)
represents the signal output of the proximity sensor.
L
L
L
L
a)
b)
c)
d)
Load (L)
a) PNP normally open contact
b) PNP normally closed contact
c) NPN normally open contact
d) NPN normally closed contact
Fig. 10.1.2: Connection diagrams for three-wire technology (DC)

10.1.3 Four- and five-wire DC technology
Proximity sensors designed in four- or five-wire technology are further divided into
proximity sensors with PNP outputs (positive switching) and NPN outputs (negative
switching).
Unlike proximity sensors in three-wire technology, proximity sensors in four-wire
technology are equipped with antivalent switching function, i.e. they possess both a
normally open as well as a normally closed output.
Devices in five-wire technology feature electrical isolation between the control
voltage circuit and the supply voltage (relay output).
L
L
a)
b)
Load (L)
a) PNP normally open/normally closed contacts
b) NPN normally open/normally closed contacts
Fig. 10.1.3: Connection diagram of four-wire technology (DC)

10.1.4 Terminal designation
Function Colour Designation
Positive supply voltage (+) brown BN
Negative supply voltage (-) blue BU
Switch output black BK
Antivalent switch output white WH
Table 10.1.1: Terminal designation of proximity sensors
Terminal designation is in accordance with European standard EN 50 044. The
colour short code is laid down in the international standard IEC 757.
Generally, two proximity sensor designs are distinguished, PNP (positive switching)
and NPN (negative switching). Other designations are P-switching or positive
switching as well as N-switching or negative switching. Positive switching proximity
sensors usually have a PNP transistor output. However, positive switching proximity
sensors with an NPN transistor output are also possible. The designations PNP and
NPN output are nevertheless widely used.
10.2
Positive and negative
switching outputs

10.2.1 PNP-output
In the case of direct current proximity sensors with PNP output, the output is
connected to positive potential in the switched state. This means that if a load is
connected (display, relay, ...), one connection must be connected to the proximity
sensor output and the other connection to 0 V. PNP proximity sensors are positive
switching.
BN(1)
BK(4)
BU(3)
L
+24 V DC
0 V
Load (L)
Fig. 10.2.1: PNP output (The purpose of the diodes is to provide a protective circuit)
PNP-proximity sensors can be differentiated as being "normally closed" or
"normally open".
L
Load (L)
Fig. 10.2.2: PNP normally open contact

L
Load (L)
Fig. 10.2.3: PNP normally closed contact
10.2.2 NPN-output
In the case of proximity sensors with NPN-output, the output is connected to the
negative potential in the switched state. This means that if a load is connected
(display, relay, ...), one connection is connected to the proximity sensor output and
the other connection to the positive potential.
BN(1)
BK(4)
BU(3)
L
+24 V DC
0 V
Load (L)
Fig. 10.2.4: NPN output (The purpose of the diodes is to provide a protective circuit)
In the same way, one differentiates between "normally closed" and "normally
open" with NPN proximity sensors.

L
Load (L)
Fig. 10.2.5: NPN normally open contact
L
Load (L)
Fig. 10.2.6: NPN normally closed contact
Usually, logic operations of the proximity sensor are carried out by the controller. By
means of series or parallel connections it is possible to achieve the logic operation
of several sensors.
10.3.1 Parallel and series connection of proximity sensors
With parallel connection, it is possible to effect a logic (Boolean) OR-connection and
with series connection, a logic AND-connection.
The advantages of this type of connections are:
• Logic operations can be achieved without using an electrical controller.
• With the use of electrical controllers, logic operations can be carried out
immediately on the spot so that only the logic operation result is signalled to the
controller using a minimum amount of cabling.
10.3
Circuit technology

The disadvantages are:
• The design and construction of logic operations requires experience, as the
mutual influences of proximity sensors, increases response and drop-off times
and a limit in the number of proximity sensors connected must be taken into
account.
• Maintenance becomes more difficult.
If however an electrical controller is used for signal processing, then it is more
straightforward to carry out all logic operations in the controller.
10.3.2 Parallel connection of proximity sensors using two-wire technology
With parallel connection of proximity sensors in two-wire technology, the following
points must be observed:
• Because the sum of all possible quiescent currents of parallel connected
proximity sensors flows via the load in the unswitched status, steps must be
taken to ensure that this does not lead to a malfunction of controllers connected
downstream.
• If a proximity sensor has switched through, then it "withdraws" the supply
voltage from the other parallel connected proximity sensors. This has the effect,
that the remaining proximity sensors can no longer indicate their actual
switching status. If the first proximity sensor now returns to its unswitched
status, then a second already activated proximity sensor can only indicate its
switching status correctly after the ready delay time of the actual proximity
sensor. This can lead to incorrect signals.
• Parallel connection is not possible with NAMUR-technology.
BN(1)
BU(3)
BN(1)
BU(3)
L
0 V
+ 24 V DC
Load (L)
Fig. 10.3.1: Parallel connection in two-wire technology

10.3.3 Parallel connection of proximity sensors using three-wire technology
Parallel connection of proximity sensors in three-wire technology can be achieved
without any problems. The following points must be observed:
• In the unswitched status, the low residual currents of the parallel connected
proximity sensors accumulate (simultaneous use of mechanical contacts and
proximity sensors is possible).
• If proximity sensors with an output stage in the form of an open-collector circuit
are used, then there is no mutual effect. In the case of proximity sensors with
different switch outputs, decoupling diodes are necessary (see Fig. 10.3.2). The
diodes are usually integrated in the sensor for the purpose of reverse polarity
protection.
BN(1)
BK(4)
BU(3)
BN(1)
BK(4)
BU(3)
L
+ 24 V DC
0 V
Load (L)
Fig. 10.3.2: Parallel connection in three-wire technology (DC)
Direct current three-wire proximity sensors can be parallel connected without major
limitations, if the residual currents of the signal outputs are sufficiently small in the
non switched status. This is the case with most proximity sensors so that for
instance up to 20 or 30 proximity sensors can be parallel connected. Also, a
combination of proximity sensors and mechanical switches is possible.
The decoupling diodes illustrated in the sketch are provided in order to prevent the
activated sensor from being loaded with the output operating resistances of other
parallel connected sensors. Moreover, this avoids all LEDs illuminating in the case of
sensors with LED displays. If the diodes are an integral part of the sensor protection
circuitry, no additional external diodes are necessary.
Parallel connection of AC sensors is not recommended, as malfunction can occur
during oscillator start-up.

10.3.4 Series connection of proximity sensors using two-wire technology
As a rule, series connection of proximity sensors using two-wire technology is to be
avoided. If it is unavoidable, the following points must be observed.
• The supply voltage is distributed to each series connected sensor. If identical
proximity sensors are used, the following applies in respect of the voltage for
each proximity sensor (in activated status):
n
V
V
voltageSupply
sensoroximityPr = (n = Number of proximity sensors)
• In the switched through status, a voltage drop occurs through each proximity
sensor (approximately 0.7 – 2.5 V per sensor). When calculating the load, it
should also be taken into account that the voltage through the load is full supply
voltage reduced by the individual voltage drops through the in series connected
proximity sensors.
V
V1
V2
BN(1)
BU(3)
BN(1)
BU(3)
L
0 V
+ 24 V DC
Load (L)
Fig. 10.3.3: Series connection in two-wire technology

10.3.5 Series connection of proximity sensors using three-wire technology
Series connection of proximity sensors using three-wire technology is possible, as
shown in Fig. 10.3.4, whereby the following points must be observed:
• The outputs of the individual series connected proximity sensors are loaded
additionally: Added to the current consumed by the load is the current
consumption of each individual proximity sensor connected in series.
• In the switched through status, a voltage drop occurs with each proximity sensor
(approximately 0.7 – 2.5 V per sensor). As a result of this, the supply voltage
available for the load is reduced by the sum total of the individual voltage drops.
• As in the case of series connected three-wire sensors, it is always the supply
voltage of the proximity sensor connected downstream which is switched, the
actual time delay before availability must be taken into account. If a "detection
process" falls within the period of the time delay before availability, this can lead
to malfunction. In the case of proximity sensors with operating status display
(LED,...), correct indication of the operating status cannot be guaranteed.
V
BN(1)
BK(4)
BU(3)
BN(1)
BK(4)
BU(3)
V1
V2
L
0 V
+ 24 V DC
Load (L)
Fig. 10.3.4: Series connection in three-wire technology

As far as connection is concerned, it should be ensured that proximity sensor cables
are installed separately from supply lines to motors, switching valves etc.
If proximity sensor connection cables run over long distances in cable ducting or
cable trays parallel to other cables which conduct alternating currents or strong
current pulses, this can lead to interference with the proximity sensor via the
connection cable.
If the proximity sensors are used in areas of high interference (welding equipment,
motors, magnetic couplings, ...), the following steps are to be taken:
• Keep the connection cables of proximity sensors short
• Screen the sensor connection cables
• If possible, error signal to be limited at source
• Install interference voltage filter into the voltage supply
If the output of a proximity sensor is loaded as a result of a downstream connected
device, the following must be observed:
• Current consumption of the connected load should not exceed the permissible
load current of the proximity sensor. Typical values for proximity sensor load
currents range between 50 – 500 mA.
• In order to guarantee reliable operation of the proximity sensor in the switched
state, the resistance of the connected load should not be too high such as to
impair the flow of the minimum load current.
• Proximity sensors can emit irregular switching signals if the supply voltage is
switched on or off, depending on whether the proximity sensor is attenuated or
unattenuated. These stray pulses can lead to malfunctions in controllers
downstream and must therefore be suppressed by using additional hardware or
taken into account in the software programming of the controller.
• If lamps are used by way of display elements it should be noted that the switch-
on current of lamps with a cold spiral-wound filament is considerably higher than
the nominal current. It is therefore possible for the switch-on current to be
reduced as a result of preheating the spiral-wound filament by means of a by-
pass resistor which is connected in parallel to the proximity sensor.
• If a relay (a valve or an other high-inductance device) is to be actuated by
proximity sensors, they should be checked for built-in protection against voltage
peaks. If not, additional protection diode circuitry is to be provided.
10.4
Connection technology
under conditions of strong
electro-magnetic influence
10.5
Connection of controllers,
relay and display elements

When switching on and off power supply units, care should be taken to ensure that
there are no voltage peaks which may jeopardise the function of the connected
proximity sensors. Power supply units with insufficient electronic control can create
voltage spikes during switch-on, which can be above the permissible voltage supply
of the proximity sensor and which, depending on the time constant, fade away
relatively slowly. In the case of unfamiliar power supply units, it is recommended to
check the voltage switch-on behaviour by means of a storage oscilloscope.
Depending on the specification given in the data sheets for proximity sensors, the
supply voltage ripple must not exceed a certain limit value.
10.6
Required current supply

Inductive and capacitive proximity sensors are based on the use of oscillators, their
oscillating amplitude being affected by an approaching object.
In order to generate sinusoidal oscillation, LC-oscillators (consisting of a coil and an
capacitor), quartz oscillators and RC-oscillators (consisting of a resistance, a
condenser and an amplifier, e.g. Wien bridge oscillators) are used.
The following denote:
L = Inductance Unit: Henry (H) 1 H = 1 Vs/A
C = Capacitance Unit: Farad (F) 1 F = 1 As/V
R = Resistance Unit: Ohm Ω 1 Ω = 1 V/A.
11.1.1 Inductive proximity sensors
Let us now consider the LC resonant circuit as applied in an inductive proximity
sensor.
The coil of an LC oscillator is inside a unilaterally magnetic half shell core. This
oscillator oscillates typically at a frequency in the range of approx. 100 – 1000 kHz.
The LC oscillator generates a high frequency electromagnetic alternating field
(HF field), which is emitted on the active surface of the proximity sensor.
The amplitude of oscillation decreases as a metallic conductor approaches the half
shell core or oscillation stops completely.
The cause is the withdrawal of energy as a result of a loss in eddy currents as the
object approaches.
If a piece of metal in a constant magnetic field is moved, this induces eddy currents
in this piece of metal. The same happens if stationary metal parts are exposed to
magnetically alternating fields.
An inductive proximity sensor operates with a low power consumption of several
microwatts and this has several advantages:
• No significant magnetising effect
• The HF field does not cause any interference
• No temperature rise in the object to be sensed
11. Physical fundamentals
11.1
Fundamentals of inductive
and capacitive proximity
sensors
Eddy currents

Electrical oscillations can be clearly illustrated by means of mechanical oscillations.
In the case of mechanical spring oscillation, a periodical change takes place
between potential and kinetic energy (potential energy and motive energy).
Analogous to this, electrical and magnetic field energy changes in the case of
electromagnetic oscillation.
A comparison of mechanical and electrical values is provided by:
• Deflection x Charge q
• Load m Inductance L
• Friction constant k Resistance R
• Spring constant D Reciprocal of capacitance 1/C
Fig. 11.1.1: Comparison of mechanical and electrical oscillations
Oscillations

Electromagnetic oscillations are created in a so-called LC resonant circuit consisting
of a coil and a capacitor. Once the capacitor is loaded, it discharges via the coil.
During this process, current intensity and voltage change periodically.
R1
C L R2
R3
R4
Fig. 11.1.2: LC resonant circuit
Unattenuated oscillation in this instance can however only be obtained if the
resonant circuit does not have any ohmic resistance. In practice, it is therefore
necessary to use an amplifier, which compensates for the attenuation resulting from
the resistance. In Fig. 11.1.2, an operational amplifier is used to illustrate the
principles of the circuit.
In order to obtain a value for the frequency of an LC resonant circuit, the time
varying charge Q on the capacitor is examined. The following applies in the case of a
plate capacitor with capacity C and voltage V:
Q = C · V
At any given time t, a variable charge q(t) is obtained, which provides a variable
voltage v(t).
The LC resonant circuit

The derivation of this charge according to time, dq/dt, determines the current i(t),
which flows through the coil with inductance L. The voltage obtained on the
capacitor is
vC(t) = q(t)/C
and the voltage on the coil
vL = L di/dt = L d2
q/dt2
The equation for oscillation is
vC + vL = L d2
q/dt2
+ q/C = 0
If this equation is divided by L, the result for unattenuated oscillation is:
d2
q/dt2
+ q/LC = 0
The result for the resonant frequency of the resonant circuit without attenuation is:
ω2
= 1/LC
For example, if one assumes
L = 100 µH, and C = 10 nF,
then the resonant frequency is
ω = 1/ (100 · 10-6
· 10 · 10-9
)1/2
= 1 · 106
Hz = 1 MHz
Example

6
Demodulator (2) Output stage with protective circuit (5) Active zone (coil) (8)
Triggering stage (3) External voltage (6) Switch output (9)
Fig. 11.1.3: Block circuit diagram of an inductive proximity sensor
V V
V V
t t
t t
Fig. 11.1.4: Oscillator amplitude and switching threshold of the triggering stage
A demodulator is connected to the oscillator for evaluating changes in amplitude.
This is where the output signal for the actuation of the triggering stage is created. In
the triggering stage, the analogue signal is converted into a digital signal. The
triggering stage does not supply an output signal unless the input signal is above a
certain threshold.
Basic circuit of an inductive
proximity sensor

With the signal provided by the triggering stage, the output stage is switched.
Depending on the switching status, the threshold of the triggering stage is also
slightly changed. Thus, the hysteresis of the proximity sensor is created. An output
signal is created if, with the increasing attenuation of the proximity sensor, the
rectified amplitude signal falls below the triggering threshold. With the decreasing
attenuation, a higher amplitude of oscillation is required to switch off the output
signal. In this case the triggering threshold is slightly higher than in the former case
and the proximity sensor indicates hysteresis.
Amongst other things, the switching distance depends on the electrical conductivity
of the metal to be detected. The following table lists the values in respect of the
conductivity of different metals and alloys. The third column indicates the reduction
factor for the switching distance of an inductive proximity sensor. This simple
dependence does not apply in the case of ferromagnetic metals and alloys. With
ferromagnetic material, considerably higher losses are created by the eddy currents
in the attenuated material than with non-ferromagnetic material.
Conductor
Conductivity 





⋅Ω 2
mm
m Reduction factor
Copper 56.0 0.25 – 0.40
Aluminium 33.0 0.35 – 0.50
Brass 15.0 0.35 – 0.50
Chrome-nickel 1.0 0.70 – 0.90
Table 11.1.1: Conductivity and reduction factors of various materials
Switching distance and
conductivity

1 2
3
Magnetic stray field HS (1)
Eddy currents through magnetic field HW (2)
Attenuating material (3)
Fig. 11.1.5: Schematic field pattern of inductive proximity sensors
The field HW created as a result of the eddy currents acts against the generating field
HS. This effect is described as field displacement. The skin effect has another,
however less powerful effect on the various switching distances of different
materials with the typical oscillator frequencies used. As a rule, the oscillator
frequency for inductive proximity sensors is in the range of 300 – 800 kHz.
Up to now, the dependence of the switching distance of the material to be
attenuated could not be calculated explicitly.
Losses are created as a result of eddy currents in a metal plate. Assuming that the
depth of penetration of the field is small and that the approaching field does not
penetrate the metal plate, the following applies:
κ
µ⋅⋅π
=
f
H
area
ndissipatiopower 2
0
H0 = r.m.s value of the magnetic field strength of the stray field on the plate
surface
µ = µ0 · µr = Magnetic permeability,
µ0 = 1.257 · 10-6
Vs/Am = Magnetic field constant,
µr = Relative permeability
κ = Electrical conductivity
f = Frequency
Power dissipation in the
attenuating material

The value of H0 depends on the distance of the plate from the proximity sensor and
from the field distribution. Power dissipation increases with the square root of the
permeability. With the increase in conductivity, power dissipation on the other hand
decreases. Power dissipation is decisive as regards attenuation of the oscillator. In
the case of materials with high power dissipation, attenuation already occurs at
greater distances leading to switching, but in the case of lower power dissipation at
small distances only.
Materials, which reduce the magnetic field of a measuring coil, are described as
diamagnetic, i.e. permeability is less than 1. The reduction is, however, very small.
With paramagnetic materials, a slight strengthening of the field occurs, i.e.
permeability is higher than 1. Ferromagnetic materials considerably strengthen the
magnetic field and as such are given a separate name. Their permeability is
considerably higher than 1 and apart from that they depend heavily on pre-
treatment of materials.
Paramagnetic materials Diamagnetic materials Ferromagnetic materials
Manganese Zinc Iron
Chromium Lead Cobalt
Aluminium Copper Nickel
Platinum Silver
Table 11.1.2: Paramagnetic, diamagnetic and ferromagnetic materials
With a linear conductor carrying a direct current, current density has the same value
at all points of the conductor cross section. With alternating currents, however, the
current is forced towards the surface. In the case of very high frequencies, the
current is practically restricted to a thin layer on the surface of the conductor, hence
the name skin effect. Skin effect means that a wire for a high-frequency alternating
current has a higher resistance than that for direct current.
If one assumes that the wire is made up of several conductors of lesser cross-
section, then the mutual induction of such a conductor in the centre is greater than
that of one at the outer edge. The passing alternating current is thus forced to the
surface, i.e. the area with least alternating current resistance.
Diamagnetism,
paramagnetism and
ferromagnetism
Skin effect

The skin thickness, within which the current amplitude has decreased by the amount
1/e (= 1/2.718), is described as the penetration depth d. The following formula
applies:
f
1
d
0r ⋅κ⋅µ⋅µ⋅π
=
Whereby µ0 = 1.257 ⋅ 10-6
Vs/Am = Magnetic field constant
µr = Relative permeability
κ = Conductivity
f = Frequency
It can be seen that the greater the permeability and the conductivity of the material,
the smaller the penetration depth. If the material thickness of the object being
detected by an inductive proximity sensor is less than the penetration depth of the
field, then a part of the field is outside the plate, thus resulting in greater switching
distances.
Materials Penetration depth [mm]
Copper 0.073
Aluminium 0.094
Brass 0.16
Table 11.1.3: Field penetration depths at a frequency of f = 800 kHz
Penetration depth

11.1.2 Capacitive proximity sensors
The active element of a capacitive proximity sensor consists of a capacitor, which is
made up of a disc-shaped metallic electrode and a beaker-shaped half-open metallic
shield. If a non-conductive or conductive material is introduced into the active zone
in front of the sensor, capacity C of the capacitor changes.
With capacitive proximity sensors, an RC resonant circuit is tuned in such way that a
sensor in the unactuated state produces a stray field in front of the active surface.
Only if an object enters into this zone, is it possible for the RC-oscillator to respond.
The change in capacitance leads to this response.
The capacitance change depends on the following factors:
• Distance and position of the object in front of the electrode
• Dielectric constant of the object
• Dimensions of the object
If a non-conductive object is introduced into the active zone, then capacitance
increases with the dielectric constant εr of the material and vice versa in proportion
with the distance from the disc-shaped capacitor electrode. The greatest switching
distance is achieved with either a water surface or with earthed, electrically
conductive materials. The smaller the relative dielectric constant of a non-
conductive material, the smaller the switching distance.
As with inductive proximity sensors, it is possible to detect moving or stationary
objects.

Material Relative dielectric constants
Ethyl alcohol 25.1
Polyvinyle chloride 2.9
Methyl alcohol 33.5
Polyethylene 2.3
Glass 3 – 15
Polystyrene 3.0
Water 81
Transformer oil 2.2 – 2.5
Ice 4
Slate 6 – 10
Air 1
Brick 2.3
Hard rubber 3 – 4
Vaseline 2.1 – 2.3
Paper 1.2 – 3.0
Cable sealing compound 2.5
Paraffin 2.2
Oiled paper 5
Table 11.1.4: Relative dielectric constant of various materials

An RC circuit is not capable of oscillation of its own accord. An active element is
required in order to make up an oscillator.
Circuits are often used which are similar to a Wien-Robinson oscillator.
R2
C2
R1
C1
R3
R4
Fig. 11.1.6: RC resonant circuit
For R1 = R2 = R and C1 = C2 = C, the resonant frequency of the RC-oscillator is
ω = 1/RC
RC-resonant circuit

With magnetism, differentiation is made between permanent magnetism and
electromagnetism. A simple illustration of magnetic field lines in space can be
achieved by means of iron filings. To give an example, iron filings placed on a sheet
of paper with a magnet underneath will be activated into the lines of the magnetic
field. Even small compass needles can be used for the detection of field lines.
11.2.1 Permanent magnetism
If the dynamic effects of the poles of two separate permanent magnets are
compared when they are brought together they can either attract or repel. Similar
poles (e.g. two north poles) repel one another, whereas opposite poles (e.g. a north
and a south pole) attract one another.
NS SN
NN
Fig. 11.2.1: Illustration of field line pattern with similar and opposing magnetic poles
11.2
Fundamentals of magnetic
proximity sensors

The field lines of a magnet are closed lines which run from the north pole to the
south pole. South or north poles never occur individually; every magnet always has
two poles.
Permanent magnets are made of various materials:
• Hard ferrite magnets
• Metallic alloy magnets
• Magnets made of rare minerals: samarium-cobalt or neodymium-iron-boron.
If a proportion of magnetism remains after the effects of a strong magnetic field, this
is called remanence Br. A reverse magnetic field with the coercive field strength -Hk is
required to cancel this magnetisation completely.
B
H
-Hk
Br
1
2
Magnetic field strength (H) Soft magnet (1)
Magnetic induction (B) Hard magnet (2)
Fig. 11.2.2: Magnetisation curve

11.2.2 Electromagnetism
The areas surrounding current-carrying conductors always have a magnetic field.
The magnetic field lines around a straight conductor are always in concentric circles.
The direction of field lines surrounding a current-carrying conductor is determined
by the corkscrew rule. If a corkscrew is screwed using the right hand in the direction
of the flowing current, then this direction of rotation indicates the direction of the
field lines.
I
I
N S
Fig. 11.2.3: Magnetic flux pattern of a conductor and a coil
11.2.3 Detecting a magnetic field
The most simple and usual method of detecting a magnetic field is to use a reed
switch. Two soft magnetic metal reeds are brought into contact by means of an
external magnetic field and an electrical contact is established. Closing of this
contact is however not bounce-free. Fig. 11.2.4 illustrates the switching behaviour of
this contact.
OFF
ON
tt1 t2
Fig. 11.2.4: Switching characteristic of bouncing mechanical contacts
Reed switch

Furthermore, it should be noted that this switch has two or three switching zones
depending on the direction of the magnetic pole axis. If the pole axis points
vertically in the plane of the switching reeds, then two switching zones will always
be obtained (Fig. 11.2.5). This is due to the shape of the magnetic flux pattern. When
the magnet passes, the field strength which is required to trigger the switch is
obtained twice. If the pole axis is parallel to the switching reeds, then three
switching zones are created for small switching distances, a main switching range
plus two minor switching ranges. Minor switching ranges occur due to the magnetic
reversal effects of the switching reeds when entering the magnetic field (Fig. 11.2.6).
Polar axis vertical to switching reed plane
Fig. 11.2.5: Switching ranges of a reed switch in relation to the magnetic pole axis
S
N
Polar axis parallel to switching reed plane
Fig. 11.2.6: Switching ranges of a reed switch in relation to the magnetic pole axis

Similar to inductive proximity sensors for the detection of metals, the oscillation
status of an electronic oscillator is evaluated as a binary signal. The difference as
opposed to a "purely" inductive proximity sensor lies in the fact that the coil of the
oscillator is shielded so that no electromagnetic field is emitted. However, an
externally active magnetic field leads to additional magnetisation of the core
material. This causes the proximity sensor to switch through.
There are designs, where the coil is pre-attenuated by means of a small soft
magnetic plate. An externally active magnetic field induces the magnetisation of this
small plate. The oscillator then oscillates and the proximity sensor switches through.
With this type of proximity sensor too, the number of switching zones depends on
the orientation of the magnetic pole axis. One advantage compared with a reed
switch is that only one single switching range occurs if the pole axis of the magnet
runs parallel to the active surface.
Fig. 11.2.7: Switching ranges of a inductive-magnetic proximity sensors
Inductive-magnetic
proximity sensors

The Hall effect was discovered in the last century by E. Hall. He discovered that a
voltage difference is created on the opposite sides of a small thin gold plate,
through which a current passes, if a magnetic field operates vertically to this.
Subsequently, it was discovered that this effect also occurs with many semi-
conductors. Certain physical characteristics are required for this. The thickness of
the small plate must be less than the dimensions of length and width. Voltages of up
to 1.5 V can be created.
t
I
A
B
VH
Fig. 11.2.8: Schematic representation of the Hall effect
The formula for Hall voltage is
t
B
IRV HH ⋅⋅=
VH = Hall voltage
RH = Hall constant
I = Current
B = Magnetic induction
t = Plate thickness
The reciprocal value in the equation of occurring Hall constants is the density of the
charge carrier in the material.
Hall sensor elements are used for the measurement of current and magnetic field or
in combination with moving magnets for angle and position.
Hall sensors

Magneto-resistive sensors operate on the principle of a change in the electrical
resistance of ferromagnetic materials under the influence of a magnetic field.
Sensors of this type consist of thin foil in strips or a meander structure. The
resistance layers consist of nickel-iron alloys (Permalloy), which are arranged in
Wheatstone bridge circuits. The bridge voltage changes under the influence of an
external magnetic field. The effect is not linear; saturation occurs at a change in
resistance of approximately 1 – 2 %. In polycrystalline, ferromagnetic materials the
change in resistance also depends on the direction of incidence of the magnetic
field.
Magnetoresistive sensors often consist of semi-conductive materials such as
indium-antimonide (InSb) or indium-antimonide/nickel-antimonide (InSb-NiSb),
which change their electrical resistance in the presence of a magnetic field.
The output resistance of a magnetoresistor without an external magnetic field and at
room temperature depends on the dimensions and the conductivity of the material
used. The conductivity of the semi-conductor is determined by its dotation. By
dotation one understands the deliberate introduction of impurity atoms into a semi-
conductor in order to increase its conductivity. In that sense one speaks of extrinsic
conduction instead of intrinsic conduction, because the introduced impurity atom
(impurities) decisively influence conductivity.
The change in resistance in the case of small magnetic fields is very small, due to the
fact that the sensor resistance is a square function of the magnetic field. Increased
sensitivity can be achieved through a magnetic bias in the magnetoresistor by
means of a permanent magnet. The working point of the magnetoresistor is now in
the steeper range of the squared characteristic curve thus creating a greater change
in resistance. Sensors of this type are therefore constructed from magnetoresistive
semi-conductor material in conjuction with a permanent magnet and flux-guiding
soft iron materials.
Magnetoresistive sensors can be excited by means of externally approaching
permanent magnets or – in the case of magnetically biased sensors – by means of
ferromagnetic materials. The latter design is also described as a ferro-sensor. With
the approach of a ferromagnetic material, the magnetic field of the permanent
magnet contained in the sensor is changed. This field change is detected by the
magnetoresistor and converted into an output signal. The sensor responds only to
ferromagnetic materials.
Magnetoresistive effect
Magnetoresistive sensor
Ferro-sensors

A wire-shaped ferromagnetic material with one single magnetic domain is used as a
sensor medium. Magnetic polarisation can only take up one of the two directions
parallel to the wire. The soft magnetic core is enclosed by a hard magnetic shell. In
the presence of an external magnetic field, a magnetic reversal takes place along the
entire length of the wire. A voltage signal is created in a coil which is wound around
the wire. Voltage signals of 2 – 8 V amplitude are provided with a sensor length of
15 – 30 mm.
One characteristic feature of Wiegand sensors is that no external voltage supply
is required to operate the sensor. These operate at a temperature range of
-196 – +175 °C.
1
2
3
15 - 30 mm
~Ø1mm
4
Single or multiple layer coil with concentrated windings per unit length (1)
Hard magnetic shell (2)
Soft magnet core (3)
Directions of magnetisation (4)
Fig. 11.2.9: Wiegand sensor
Wiegand effect

Sound frequency which is above the limit of human hearing is described as
ultrasound. The lower limit is at approximately 20 kHz. The particular characteristics
of ultrasound applied to proximity sensors are the result of the high frequency and
the correspondingly short wavelength.
10 100 1 k 10 k 100 k 1 M 10 M 100 M 1 G (Hz)
1 2 3 4
Infra (1) Audible (2) Ultra (3) Hypersound (4)
Fig. 11.3.1: Sound frequency range
The propagation of sound is the result of propagation of mechanical long waves,
which manifests itself in a periodic density variation in the carrier medium, leading
to alternating compressions and dilutions. The propagation of sound waves is
dependent on a transmitting medium, it is not possible in a vacuum.
For solid objects, the propagation speed of sound waves equals:
ρ
=
E
v
E = Modulus of elasticity
ρ = Density
The modulus of elasticity of a material is determined by Hooke's law:
lA
lF
E
∆⋅
⋅
=
Here, F is the force which lengthens or shortens a body of length l by length ∆l, and A
is the cross-sectional area of the body.
11.3
Fundamentals of ultrasonic-
proximity sensors
Speed of sound in
solid objects

The speed of sound in fluids equals:
ρ
=
K
v
K = Compression modulus
ρ = Density
For the speed of sound in gases the following applies:
TR
p
v ⋅⋅κ=
ρ
⋅κ
=
κ = Adiabatic exponent
p = Pressure
ρ = Density
T = Temperature
R = Gas constant
The adiabatic exponent κ describes the quotient of the specific heat capacity at
constant pressure cp, and the specific heat capacity at constant volume cv.
This equation demonstrates that the speed of propagation of sound waves in gas
depends to a large extent solely on the temperature and not on the pressure of the
gas.
The following formula applies for the speed of sound in dry air at temperature T:
2.273
C/T
1vv 0
°
+=
v0 = 331.6 m/s
or
C
T
s
m
58.0vv 0
°
⋅⋅+=
Speed of sound in fluids
Speed of sound in gases
Speed of sound in air

Solids (at 20 °C) v [m/s]
Aluminium 5110
Iron 5180
Gold 2000
Cork 500
Copper 3800
Brass 3500
Steel 5100
Fluids (at 20 °C) v [m/s]
Benzene 1320
Chloroform 1000
Glycerine 1923
Petroleum 1320
Mercury 1415
Water, distilled 1483
Gases (at 0 °C and 101.3 kPa) v [m/s]
Argon 308
Helium 971
Carbon dioxide 258
Carbon monoxide 337
Air 332
Hydrogen 1286
Table 11.3.1: Speed of sound in various materials

Because of the short wavelength, ultrasonic waves behave in a similar way as light
waves. Also, the law of optical geometry (angle of incidence = angle of reflection)
applies to ultrasonic waves.
The surface structure is of great significance as far as the directed reflection is
concerned. If surface roughness is within 1/4 to 1/6 of the sound wavelength, the
waves are reflected diffusely, whereas smooth surfaces have a maximum angle of
approx. ±5° to the sound cone, roughly structured substances, e.g. bulk goods, can
be detected up to an angle of approx. ±45°.
The wavelength λ equals:
ν
=λ
v
λ = Wavelength
ν = Frequency
v = Speed of sound
At a frequency of 200 kHz and a speed of sound in air of approx. 340 m/s, the
following value is obtained in respect of the wavelength
mm7.1m107.1
Hz10200
s
m
340
v 3
3
=⋅=
⋅
=
ν
=λ −
Example

11.3.1 Generation of ultrasound
There are three different methods of generating ultrasound: mechanical, magnetic
and electrical. In this context, mechanical generation of ultrasound is only of minor
importance.
With the help of magnetorestriction, it is possible to generate ultrasound of up to
approx. 50 kHz. Ferromagnetic substances change their length in a magnetic field.
The relative change in length is within a maximum range of 4 · 10-5
.
x 10
-6
40
10
0
-40
-30
-20
-10
l/l
20
80 240160 kA/m
H
1
2
3
4
5
6 % Nickel, 94 % Iron (1) Iron (3) Nickel (5)
29 % Nickel, 71 % Iron (2) Cobalt (annealed) (4)
Fig. 11.3.2: Magnetostrictive strain curves of various materials in relation to field strength H
Magnetic generation

With electrorestriction (inverse piezoelectrical effect) an alternating voltage of high
frequency is connected to a crystal plate. This plate then carries out the mechanical
oscillations of the corresponding frequency, which become particularly strong with
resonance. Frequencies of up to approximately 10 000 kHz are achieved.
F
F
a)
b)
1
4
3
6
2
5
7
Unloaded body (1)
Compressive stress (2)
Tensile stress (3)
DC voltage, opposed to polarisation (4)
DC voltage, parallel to polarisation (5)
AC voltage, leads to alternate lengthening and shortening (6)
Polarisation axis (7)
a) Conversion of force into voltage
b) Conversion of voltage into linear change
Fig. 11.3.3: The piezoelectric effect (source: Philips Components)
Nowadays, instead of crystals, piezoelectrical materials, which are widely
distributed under the trade name Piezoxide (e.g. by Valvo), are used to generate
ultrasound. These materials are made of lead-zirconate-titanate.
Electrical generation

Figs. 11.3.4 and 11.3.5 illustrate the dependence of speed of sound in air on the
temperature and relative air humidity.
360
0
330
335
340
345
350
m/s
v
t
0 40°C30252015105
Fig. 11.3.4: Speed of sound in dry air as a function of temperature
C
T
s
m
58.0vv 0
°
⋅+=
s
m
6.331v0 =
0
0.5
1.5
1.0
%
Percentage
increaseinv
Relative air humidity
0 % 10050
40 °C
-10 °C
0 °C
20 °C
30 °C
Fig. 11.3.5: Percentage change in speed of sound as a function of relative air humidity

11.3.2 Attenuation of ultrasound in air
When selecting ultrasonic proximity sensors, the frequency of the emitter should be
taken into account. The attenuation of ultrasound in air depends on the ultrasonic
frequency and as such also the range of an ultrasonic sensor.
With the propagation of sound in air, the sound pressure amplitude decreases
exponentially with the distance
d
0 epp α−
∧∧
⋅=
∧
p = Peak value of the sinusoidal sound pressure wave on the output of the
emitter (d = 0)
0p
∧
= Peak value of sound pressure wave at distance d from emitter (assuming that
the sound ray does not diverge)
α = Attenuation coefficient (Unit m-1
)
Correspondingly, the following applies in respect of accoustic power:
d'
0
d2
0 ePePP α−α−
⋅=⋅=
This formulation is often used with 2α = α' representing the attenuation coefficient.
Instead of the (linear) attenuation coefficient α or α', a logarithmic attenuation ratio
αL is also used, which on examination of the sound pressure amplitude is defined by
the relation
20
d
0
L
10pp
α
−∧∧
⋅=
or on examination of the accoustic power by the relation
10
d'
0
L
10PP
α
−
⋅= with α’L = 2α
αL or α'L are indicated in dB/m (1 dB = 1 decibel).
Physical law
Logarithmic
attenuation ratio

10
2
10
-5
10
-4
10
-3
10
-2
10
-1
1
10
dB/m
AttenuationcoefficientαL
a
b
c
10 10
4
10
6
10
5
10
3
10
2
Hz
Frequency
a
b
c d
a) Relative air humidity 10 %
b) Relative air humidity 40 %
c) Relative air humidity 80 %
d) Theoretic attenuation based on standard absorption: Linear attenuation is proportional to the square
of the frequency of sound.
Fig. 11.3.6: Dependence of attenuation coefficient on ultrasonic frequency (air temperature 20 °C)

11.3.3 Ultrasonic proximity sensors
6
Evaluation unit (2) Output stage with protective circuit (5) Ultrasonic transducer (8)
Triggering stage (3) Switch output (6) External voltage (9)
Fig. 11.3.7: Block circuit diagram of an ultrasonic proximity sensor
A high frequency alternating voltage is generated to excite the piezoceramic module
into oscillation. This AC voltage is switched through to the ceramic module by means
of a pulse generator, when the transmitting pulse is to be emitted. Distance
measurement is calculated according to the ultrasound propagation time. A ramp
generator is triggered at the time of transmission, which generates a time-
dependent voltage. Thereupon, the piezoceramic module is switched over to
receiving. The ultrasonic signal is reflected if an object is present in the active range
of the proximity sensor. The proximity sensor receives the signal and the ramp
generator is stopped. The voltage level is evaluated at this point and an output
signal emitted.

1 2
D
α
Near field (~D
2
/λ) (1) Far field (2)
Fig. 11.3.8: Sound emission characteristic of an ultrasonic transducer
An object must not be present in the sound field of the proximity sensor within the
so-called near field, as this can lead to error pulses at the proximity sensor output.
For an ultrasonic proximity sensor with a transducer diameter of 15 mm and an
emitting frequency of 200 kHz, the range of the near field is approximately 130 mm.

Optical proximity sensors are devices which convert signals generated by light
emission into electrical signals. The response of optical receivers varies according to
different ranges of wavelength. Fig. 11.4.1 indicates the spectral ranges of
electromagnetic emission.
10
-10
10
8
10
14
µm10
10
10
6
10
4
10
2
110
-2
10
-4
10
-6
10
-8
1 2 3 4 5
10 380 455
492
577
597
622 nm780 10
6
Ultraviolet Visible light Infrared
Close DistantExtreme Close Violet Blue Green Red
Yellow Orange
Cosmic rays (1) X-rays (3) Radio waves (5)
Gamma rays (2) Radar waves (4)
Fig. 11.4.1: Spectral ranges of electromagnetic light emissions
The range of visible light is just a small section of the overall spectral range reaching
from violet (approx. 380 nm) to red (approx. 780 nm). The frequencies of light are in
the range of 1015
Hz.
Light spreads in a straight line. A consequence of this statement lies in the formation
of a shadow. A pin sized light source produces a core shadow. In the case of
extended (or several pin sized) light sources, core and half-shadows are
superimposed.
Light beams, which radiate from one point, are divergent (the beam cross-section
increases as the distance grows). Beams which focus on one point are convergent
(the beam cross-section decreases towards the crossover point). Beams without a
common output or point of direction are known as diffuse.
11.4
Fundamentals of optical
proximity sensors

The speed of light in a vacuum is roughly 300,000 km/s. The table below lists some
values for the speed of light in respect of different materials.
Medium v [km/s] Refractive index
Vacuum 300,000 1
Air 300,000 1.0003
Water 225,000 1.33
Crown glass (Dependent on type) 198,000 1.51
Flint glass (Dependent on type) 186,000 1.61
Diamond 124,000 2.42
Polymethylmethacrylate (PMMA) 200,000 1.49
Table 11.4.1: Speed of light and refractive index
11.4.1 Reflection
The following principle applies with regard to reflection of light:
Angle of incidence = Angle of reflection
Here the angles are measured between the vertical and the angle of incidence.
α α
Fig. 11.4.2: Reflection of light beams

11.4.2 Refraction
At the interface of two transparent media a light beam is not only reflected, but part
of its energy overspills in a different direction in the new medium, i.e. it is refracted.
Here, a medium with reduced speed of light propagation is known as optically
denser and that which is greater as optically thinner.
With the transition from an optically thinner to an optically denser medium, the
angle of refraction is smaller than the angle of incidence, the beam is refracted
towards the vertical.
With the transition from an optically denser to an optically thinner medium, the
angle of refraction is greater than the angle of incidence, the beam is refracted away
from the vertical.
ϑ1
ϑ2
ϑ2
ϑ3
ϑ3
ϑ4
ϑ4 ϑ5
1 2 3 4 5
n = 1.0003 n1
n = 1.000 n2
n = 1.0003
Air (1) Substance 1 (2) Vacuum (3) Substance 2 (4) Air (5)
Fig. 11.4.3: Refraction of light beams in various media

11.4.3 Total reflection
With the transition from an optically dense medium into an optically thin medium,
the angle of incidence cannot exceed a certain limit value. In the case of angles
greater than this, total reflection occurs, i.e. the entire light energy is reflected into
the optically dense medium.
αg
n1
n1
n2
n2
n › n1 2
α
α› g α
Fig. 11.4.4: Total reflection
11.4.4 Photoelectric components
In optoelectronic proximity sensors, photoelectronic emitting components are used
to create light emission and photoelectronic receiving components for receiving light
emission.
The most commonly used emitter elements are luminescent diodes, which are also
known as LEDs (light emitting diode). For special applications, laser diodes are also
used.

For receiving elements, photodiodes or phototransistors are generally used. In
addition, photoresistors are also of some signficance, e.g. in photoelectric exposure
meters.
Luminescent diodes (LED) are semiconductor diodes which emit light beams when
an electrical current passes through. Depending on the composition of the semi-
conductor material, light beams of varying wavelength are created, see Table 11.4.2.
Material Colour Wave length [nm]
Gallium arsenide infrared 950
Gallium aluminium arsenide infrared 880
Gallium aluminium arsenide red 660
Gallium arsenide phosphide red 660
Gallium arsenide phosphide red 635
Gallium arsenide phosphide yellow 590
Gallium phosphide green 565
Gallium nitride blue 480
Table 11.4.2: Typical materials and wavelengths of luminscent diodes
Luminescent diodes in the infrared and red spectral range are mainly used in
sensors, because this produces good adaption to the sensitivity of photodiodes
when receiving light emissions.
Luminescent diodes

Luminescent diodes represent a relatively small spectral width of the emitted light,
which is generally between 30 – 140 nm (spectral halfwidth), see Fig. 11.4.5.
%
0
20
40
60
100
80
Rel.radiantintensity
Wavelength
850 nm 10501000950900
1
Spectral halfwidth (1)
Fig. 11.4.5: Emission spectrum of a GaAs-LED
Photodiodes are semi-conductor components which are based on the principle of
single-crystaline silicone or germanium. They are constructed in the same way as
ordinary semiconductor diodes and have a barrier layer which is however very
closely arranged underneath the crystal surface. If the diode is exposed to light
emission, then the photons penetrating the crystal (quantum of the optical
radiation) are absorbed and electrical charge carrier pairs are created. This effect is
known as the photoelectric effect. The charge carrier pairs are separated in the
barrier layer and an electrical current is created, i.e. the photocurrent.
Photodiodes are basically divided into the following types:
• PN photodiodes
• PIN photodiodes
• Schottky photodiodes
• Avalanche photodiodes
Photodiodes

PN photodiodes have two differently doped areas in the crystal material, the so-
called P-area and N-area, which are separated by a thin barrier layer. (Dotation
refers to the process of integrating atoms from other materials, e.g. of boron or
gallium into the crystal material. By means of dotation it is possible to influence the
conductivity of a semiconductor).
With PIN photodiodes the P-area and the N-area is separated by a relatively wide
layer of intrinsically conductive semiconductor material (I = intrinsic). This creates a
layer of low insulating capacity and a fast switching time of the PIN photodiode.
PN silicone photodiodes and PIN silicone photodiodes are the most widely used
types of photodiodes.
Schottky photodiodes are named after the Schottky effect and renowned for their
excellent sensitivity in the ultraviolet spectral range.
Silicone avalanche diodes are based on the avalanche effect in barrier layer
semiconductors. They operate at a high reverse voltage and are suitable for the
detection of very small light output with reduced reaction times.
A typical characteristic curve of spectral sensitivity within a silicone photodiode is
shown in Fig. 11.4.6. One important property is the maximum value of spectral
sensitivity, which for silicone photodiodes can range between approx. 600 nm and
1000 nm, depending on type.
100
0
10
20
30
40
50
60
70
80
%
R/Rmax
wavelength
400 1000nm800700600500
Fig. 11.4.6: Relative spectral sensitivity R/Rmax of a silicon photodiode

The sensitivity of silicone photodiodes in the spectral maximum is typically at
0.5 A/W, i.e. at a received light emission power of 1 mW for instance, a photocurrent
of 0.5 mA is created.
Responsivity R of a photodiode is the quotient of the photocurrent I and the optical
radiant power P, which impinges on the photodiode:
P
I
R =
11.4.5 Fibre-optic cables
Fibre-optic cables (glass or plastic fibre cables) are used in sensor technology for the
purpose of conveying light to inaccessible or particularly exposed areas, where there
is no room for an emitter and/or receiver or where difficult environmental conditions
prevail.
The operation of an optical fibre is based on the total reflection of incoming radiated
light inside the fibre.
Fig. 11.4.7: Total reflection of light beams in the core of an optical fibre

In order to achieve total reflection, the high-refracting core is surrounded by a low-
refracting cladding.
1 2
dCore (1) dCladding (2)
Fig. 11.4.8: Principle of a fibre-optic cable
There are three different types of fibre:
– Step index, Multimode
– Step index, Monomode
– Gradient index, Multimode
"Modes" refers to the particular forms of propagation of a light beam inside the
fibre optic cable, which differ according to their individual direction of propagation.

a)
b)
c)
AO
AO
AO
AI
AI
AI
t
t
t
t
t
t
Output pulse
Output pulse
Output pulse
Input pulse
Input pulse
Input pulse
dCdCdC
dCldCldCl
r
r
r
n
n
n
n = constCl
nCl
nCl
nCl
n = constCl
n = constC
nC
nC
n(r)
n = constC
n = n(r)C
n = constCl
a) Profile section of step index (Multimode)
b) Profile section of step index (Monomode)
c) Profile section of gradient index
Fig. 11.4.9: Types of optical fibres
A step index fibre has a sharp limit between the core and cladding refraction index,
the light beams can pass through the fibre in several ways (Multimode). A small
input pulse is widened on passing through this fibre, because different acceptance
angles produce different distances. In the case of the step index monomode fibre,
only one path is possible for the light beam. The pulse retains its form to a large
degree.
Step index fibre

With the gradient index fibre a continuous transition of the refraction index is
achieved. The pulse width is not particularly strongly widened.
Polymer fibre-optic cables are used preferably in the red range (660 nm) and glass
fibre-optic cables predominantly in the infrared range. Glass fibre-optic cables
absorb considerably less light in this wavelength range than polymer fibre-optic
cables. In contrast, polymer fibre-optic cables are particularly flexible and can be cut
to a required length.
Gradient index
Polymer and glass
fibre-optic cables

TransmissionTransmission
Wavelength
Wavelength
Glass
Polymer
0
20
60
40
%
400 900nm700600500
0
20
60
40
%
400 2200nm14001000800600
1
2
3
5
4
6
7
8
polymer fibre, length: 1 m (1), 2 m (2), 3 m (3), 4 m (4), 5 m (5)
glass fibre, length: 500 mm (6), 1000 mm (7), 3600 mm (8)
Fig. 11.4.10: Optical transmission of polymer and glass fibre as a function of the wavelength

The following are possible fibre materials:
• Multicomponent glass with a silicone-dioxide content of approx. 70 %
• Glass with a very high silicone-dioxide content of nearly 100 %
• Plastics
• Fluids
Basically, two cables are used in conjunction with proximity sensors. One cable
transmits the light emitted by the light source, while the other cable conducts the
light to the receiver of the proximity sensor. Diffuse sensors as well as through-
beam sensors can be realized using optical fibre. In order to increase the relatively
short sensing range of diffuse sensors with fibre-optic cables, they may be used in
conjunction with reflectors to form a retro-reflective sensor.
For sensor applications, fibre optic cables include bundles of individual fibres. The
arrangement of the optical fibre in emitter and receiver cables can be done in a wide
variety of manners. The chosen arrangement depends on the individual case of
application.
a)
d)
c)
f)
b)
e)
a) arbitrary c) uniform e) semi-circular
b) segmented d) concentric f) in-line
Fig. 11.4.11: Schematic design forms of fibre-optic cables (Source: Schott)

Circuit symbols Description
Proximity sensor
Approach-sensitive device, block symbol
Note:
Method of operation to be specified
Example:
Approach-sensitive device, capacitive, reacts to approach of a
solid object
Contact sensor
Table 12.1.1: Circuit symbols for sensors to standard DIN40 900, Part 7
Circuit symbols Description
Contact-sensitive sensor (normally open contact)
Proximity-sensitive sensor (normally open contact)
Proximity-sensitive sensor (normally open contact), actuated by
approach of a magnet
Fe
Proximity-sensitive sensor (normally closed contact), actuated
by approach of ferrous object
Table 12.1.2: Circuit symbols for sensors to standard DIN 40 900, Part 7
12. Circuit symbols for proximity sensors
12.1
Circuit symbols to
standard DIN 40900

12. Circuit symbols for proximity sensors
BN(1)
BN(1)
BN(1)
BN(1)
BN(1)
BN(1)
BN(1)
BN(1)
BK(4)
BK(4) BK(4)
BK(4)
BK(4)
BK(4)
WH(2)
WH(2)
BK(4)
BU(3)
BU(3)
BU(3)
BU(3)
BU(3)
BU(3)
BU(3)
BU(3)
1 2
3
6
5
4
7
Magnetic proximity sensor (1)
Capacitive proximity sensor (3)
Ultrasonic proximity sensor (4)
Through-beam optical sensor, Emitter and receiver in separate housing, Receiver
with 2 switching outputs (5)
Optical proximity sensor, Emitter and receiver in one housing, 2 switching outputs (6)
Optical proximity sensor, Receiver and emitter in one housing, 1 switching output (7)
Fig. 12.2.1: Examples of circuit symbols for proximity sensors
12.2
Examples of
circuit symbols

Active surface
The surface which emits the electrical field and on which a contactless proximity
sensor reacts to an approaching object.
Constant light operation
The light beam is not modulated and is evaluated only in respect of the intensity of
constant light.
Diffuse sensor
An optical proximity sensor whose light is scattered by the surface of an object
(diffusion).
Diffusion
Diffuse reflection of light from the surface of an object.
Directed reflection
Directed reflection of light emission by means of reflecting surfaces.
Fibre-optic cables
Material, through which light can be conducted other than in a straight line and with
minimum losses.
Flush fitting proximity sensors
The proximity sensor can be surrounded by metal or other materials up to the point
of its active surface, without the characteristic values of the sensor being affected.
Free zone
The area surrounding the proximity sensor, which must be kept free of materials
affecting the characteristic values of the proximity sensor.
Inductive proximity sensors
A device which creates a high frequency electro-magnetic field by means of an LC
resonant circuit and emits a signal at the output in the event of certain attenuating
conditions being fulfilled.
IR-Light
Infrared light is an invisible light form which has a greater wavelength than visible
light (780 nm to approx. 100 µm).
Modulated light operation
The utilisation of a modulated light beam.
13. Technical terms relating to proximity sensors
13.1
General terms

Non-attenuating material
Any material which does not significantly affect the characteristic values of an
inductive proximity sensor.
Non-flush fitting proximity sensors
Sensors that require a free zone when fitted in metal or other materials in order to
maintain the characteristic values of the proximity sensor.
Operating reserve factor
With optical proximity sensors the operating reserve factor β is derived from the
quotient of the actual received optical signal power PE in relation to the necessary
optical signal power PS at the switching level: β = PE/PS
Photoelectronic sensor, optoelectronic sensor
A term generally used for all devices which detect objects via a light source, i.e.
ranging from infrared emissions and visible emission (wavelength range of
380 – 780 nm) to ultraviolet emission (UV).
Photoreceiver
The light receiving part of a light barrier or of a diffuse sensor.
Phototransmitter
Light emitting part of a light barrier or a diffuse sensor.
Reference axis
The axis vertically through the centre of the active surface of a proximity sensor.
Reflection
Deflection and reflection of light emissions on the boundary surfaces of various
media.
Reflector
Optical aid for reflecting optical emissions, often in the form of triple reflectors.
Retro-reflection
Directed reflection of light emission to the source of the emission.
Retro-reflective sensor
The light of an optical emitter is reflected by means of a reflector (retro-reflection).

Standard test plate
A mild steel test plate, of square shape and 1 mm thick used for the purpose of
carrying out comparative measurements of the switching distance of inductive
sensors.
The lateral length equals:
• Diameter of the inscribed circle of the active surface.
or
• three times the value of the nominal switching gap.
The higher of the two values is to be applied.
Through-beam sensor
An optical sensor arrangement with separate emitter and receiver, which reacts to
an interruption of the light beam directed between the emitter and the receiver.
Triple reflector
Optical aid, whereby retro-reflection is created by means of multiple reflection on its
pyramid shaped inner surfaces.
UV light
Ultraviolet light in the wavelength range of 380 – 10 nm.
Visible light
Light ranging from red to violet (approx. 780 – 380 nm wavelength).

Axial approach
Approaching of calibrating plate centrally to the reference axis.
Nominal range
Standard specified range of light barriers. This range is established in a dry and
clean environment and includes a reserve range to cover sundry tolerances. In the
case of retro-reflective sensors this range refers to the reflector specified for the
sensor.
Nominal switching distance
Standard specified sensing range of a diffuse optical proximity sensor.
Nominal sensing range
The switching distance of a proximity sensor at nominal supply voltage and nominal
temperature without compensation for production tolerances.
Radial approach
Approach of the calibrating plate at a right angle and in the direction of the reference
axis of the active surface of the proximity sensor.
a)
15 15mm505mm
sx sx
sn
350
400
150
mm
300
250
200
100
50
1
2
b)
1
2
3
Switch-on point (1) Switch-off point (2) Hysteresis (3)
a) Optical proximity sensor, object approached from side
b) Inductive proximity sensor
Fig. 13.2.1: Response characteristic
13.2
Terms for dimensional
characteristic values

Range
Maximum distance between the emitter and receiver of a through-beam sensor or
between the emitting and receiving device and the reflector of a retro-reflective
sensor.
Real switching distance
The switching distance of an inductive proximity sensor measured at nominal
voltage and nominal temperature, taking into account manufacturing tolerance.
Maximum deviation from the nominal switching distance is ±10 %.
Reproducibility
Switching point difference which occurs within 8 hrs at a temperature of 15 – 30 °C
and a nominal voltage deviation of ±5 %.
Sensing range
Distance between a diffuse sensor and a reference surface of specified dimensions
(matt white paper) as it approaches the device in the direction of the axis until a
signal change takes place.
15 15mm505mm
sx sx
sn
350
400
150
mm
300
250
200
100
50
1
2
Switch-on point (1) Switch-off point (2)
Fig. 13.2.2: Response characteristics of diffuse sensors

Switching distance
The distance at which a standard target approaching the active surface of a
proximity sensor generates a signal change.
1
2
3
4
5
d
sa
0,81 sn
sn
1,1 sn
0,9 sn
1,21 sn
su
sr
Actuating element (1) sn = Nominal switching distance
Total tolerance range (2) sr = Real switching distance
Manufacturing tolerance (3) su = Useful switching distance
Reliable operating range (4) sw = Working distance
Proximity sensor (5)
Fig. 13.2.3: Switching distances
The real switching distance is specified by 0.9 sn < sr < 1.1 sn.
The useful switching distance is generally specified by 0.9 sr < su < 1.1 sr,
or partly as above by 0.81 sn < su < 1.21 sn, which is the same.

Switching hysteresis
The difference between the switch-on point and switch-off point during the axial or
radial approach of the calibrating plate to the active surface of a proximity sensor.
Useful switching distance
The switching distance of an inductive proximity sensor within the full rated supply
voltage and temperature ranges. Maximum deviation from the real switching
distance is ±10 %.
Working switching distance
Switching distance of an inductive proximity sensor within which reliable operation
is guaranteed, independent of manufacturing tolerances or environmental factors.
The values are between 0 and the lowest value of the useful switching distance.
Nominal voltage Vn
A value within the operating voltage range, to which technical data refer.
Operating voltage Vs
Range of supply voltage which must not be exceeded or fallen below.
Permanent current Ia
Current flowing during continuous operation.
Residual current Ir
Current which flows when output is switched off.
Residual ripple
Alternating current superimposed on direct current. The residual ripple from peak to
peak must not exceed the operating voltage limits.
Residual voltage Vr
Voltage, which is measured via load when the proximity sensor is not actuated.
Short-time current Ik
Short-time current which flows for a specified period and frequency.
Voltage drop Vd
Voltage measured between the switch output and the supply voltage (pnp type) or
between the switch output and ground (npn type) at maximum current load and
when the proximity sensor is actuated.
13.3
Terms of electrical
characteristic values

Adjustable switching (N/O – N/C)
Converting the device from normally open to normally closed operation.
Analogue output
The change in the physical quantity detected causes a continual change in the
output signal.
Changeover function (anti-valent switching function)
An output with N/O function and an output with N/C function are available
simultaneously.
Dark switching
The output is switched through if the photoreceiver is unilluminated.
Digital output
A digital output occurs if a change of a detected physical quantity results in a step
response of the output signal.
Light switching
The output is switched through if the photoreceiver is illuminated.
Normally closed function (N/C)
The output is open if an object is detected and switched through if an object is not
detected.
Normally open function (N/O)
The output is switched through if an object is detected and open if an object is not
detected.
Reset time
Delay time between the actuating element leaving the active zone and the signal
change at the output. The minimum required distance between two elements is
determined by this time taking into account the travel time.
Response time
Delay time between the actuating element entering into the active zone of a
proximity sensor and the signal change at the output. The speed at which the
actuating element could pass through the active zone is limited in relation to the
width of the actuating element.
Signal duration
Duration of the output signal with dynamic actuation. This must correspond to the
input delay of the connected load.
13.4
Terms for time and
function characteristics

Switching frequency
According to European Standard EN 50 010, the maximum switching frequency of an
inductive proximity sensor is measured as shown in Fig. 13.4.1.
d
m = d
2m
m
m
s / 2n
1
2
Proximity sensor (1) Test plate (2)
Fig. 13.4.1: Measurement of switching frequency
Switch-on delay
Time between switching on the operating voltage and the ready status of the device

Actuating force (AF)
The final stage of the actuating force which triggers the switchover of the contacts.
Changeover displacement (COD)
The displacement of the actuator between the switching and reset point.
Final position (FP)
The position taken up by the actuator when it reaches the final position.
Forward displacement (FD)
The travel of the stem or actuator from its free position to the switching point.
Free position (FP)
The position taken up by the actuator when it is not contacted by a drive element.
Overtravel displacement (OD)
The displacement beyond the switching point up to the final position; the minimum
permissible overtravel displacement is specified. Exceeding of this value reduces the
specified mechanical service life of the switch.
Positive opening displacement (POD)
The travel of the actuator from its free position to the position, where the
mechanical forced opening of the contacts is effected.
Reset force (RF)
The remaining actuator spring force, which effects the automatic reset of the spring
contact.
Reset position (RSP)
The position of the actuator, in which the released spring contact returns to its
normal position.
Switching point (SP)
The position of the actuator, in which the switch-over of the loaded spring contact
takes place.
Total displacement (TD)
The displacement of the actuator from its free position to the final position.
Total force (TF)
The force to be applied to the actuator in order to get from the release to the final
position.
13.5
Actuating characteristics of
mechanical-electrical
position switches

Chemical resistance
Behaviour in aggressive environment.
Climatic resistance
Behaviour in specified climatic conditions.
Nominal ambient temperature
Ambient temperature to which the technical operating data refers
Operating temperature (Ambient)
Temperature range in which the device operates reliably.
Protection class IP
Protection against contact and penetration by foreign matter (dust) as well as water
under specified conditions to IEC 529 (DIN 40 050).
Shock stress
Behaviour under conditions pertaining to IEC 68-2-6.
Storage temperature
Temperature range of device when not in use.
Vibration stress
Behaviour under specified conditions pertaining to IEC 68-2-27.
13.6
Terms relating to
environmental conditions

EN 50 008
"Inductive proximity sensors Form A for direct current, 3 or 4 terminals"
(European standard for three- and four-wire proximity sensors in cylindrical
housings for direct current.)
EN 50 010
"Inductive proximity sensors. Methods for measuring the operating distance
(switching distance) and the operating frequency (switching frequency)"
(European standard for measuring techniques to establish the switching distance
and the switching frequency for proximity sensors in DC or AC design.)
EN 50 025
"Inductive proximity sensors Form C, for direct current, 3 or 4 terminals"
(European standard for three and four wire proximity sensors for direct current in
block shape with rectangular section.)
EN 50 026
"Inductive proximity sensors Form D, for direct current, 3 or 4 terminals"
(European standard for three- and four-wire proximity sensors for direct current in
block shape with rectangular section.)
EN 50 032
"Inductive proximity sensors. Definitions, classification, designation."
(European standard for terms and designations used in European standards for
proximity sensors.)
EN 50 036
"Inductive proximity sensors Form A, for alternative current, 2 terminals"
(European standard for two-wire proximity sensors in cylindrical housings for
alternating current.)
EN 50 037
"Inductive proximity sensors Form C, for alternating current, 2 terminals"
(European standard for two-wire proximity sensors in block shape with square cross
section.)
EN 50 038
"Inductive proximity sensors Form D, for alternating current, 2 terminals"
(European standard for two-wire proximity sensors for alternating current in block
shape with rectangular cross section.)
14. Standards and protection classes
14.1
Standards

EN 50 040
"Inductive proximity sensors Form A, for direct current, 2 terminals"
(European standards for two-wire proximity sensors in cylindrical housings for direct
current.)
EN 50 044
"Inductive proximity sensors. Identification of terminals."
(European standard for designations of terminals for proximity sensors with cable
connection, plug connection or terminal wiring facility.)
DIN 40 050
"IP protection classes"
(Standard for the definition of protection classes for protection against contact,
foreign matter and water for electrical equipment.)
IEC 529
"Classification of degrees of protection provided by enclosures"
(equivalent to DIN 40 050.)
DIN IEC 757
"Code for designation of colours"
(Definition of a colour code for the identification of specific colours in electrical
engineering.)
DIN 44 030
"Light barriers and sensors"
(Definition of terms.)
The protective classes are indicated by a symbol, which is made up of the two code
letters IP (= International Protection) and two codes for the degree of protection.
Example IP 67
The first code (0-6) specifies the degree of protection against contact and
penetration of foreign matter, the second code (0-8) the degree of protection against
penetration of water. The protection class is stated on the housing or the rating
plate.
14.2
Protection classes

First code Degree of protection (contact and foreign matter protection)
0 No specified protection
1 Protection against penetration of solid foreign bodies with a diameter greater than
50 mm (large foreign bodies)
1)
No protection against intentional access, e.g. of a hand, but protection against large-
area contact
12 mm (medium-sized foreign bodies)
1)
Protection against finger contact or similar
2.5 mm (small foreign bodies)
1)2)
Protection against tools, wires et al. with a diameter grater than 2.5 mm
1 mm (granular material)
1)2)
Protection against tools, wires et al. with a diameter grater than 1 mm
5 Protection against harmful dust deposits. The penetration of dust is not totally
prevented: but dust is not able to penetrate in sufficient quantities to impair
operation (protected against dust)
3)
Complete protection against contact
6 Protection against penetration of dust (dust-proof)
Complete protection against contact
1)
With equipment of protection classes 1 to 4, foreign bodies of even or uneven shape of three
vertically aligned dimensions greater than the corresponding numerical value of the diameter
are prevented from penetrating.
2)
For protection classes 3 and 4, the implementation of this table with regard to equipment with
drain holes or cooling air apertures falls within the responsibility of the individual technical
committee responsible.
3)
For protection class 5, the implementation of this table with regard to equipment with drain
holes falls within the responsibility of the individual technical committee responsible.
Table 14.2.1: Classes of protection against contact and foreign bodies

Second code Protection class (water protection)
0 No particular protection
1 Protection against dripping water falling vertically.
Drops of water must not have any harmful effects.
2 Protection against dripping water falling vertically. Water drops falling at any angle
up to 15° from the normal position of tilted equipment (housing) must not have any
harmful effects. (water drops falling diagonally).
3 Protection against water falling at any angle up to 60° from the vertical.
Spraying water must not have any harmful effects.
4 Protection against water splashing against equipment (housing) from all directions.
Splashing water must not have any harmful effects.
5 Protection against jets of water from a nozzle directed against the equipment
(housing) from all directions.
Jets of water must not have any harmful effects.
6 Protection against heavy seas or strong jets of water.
Water must not penetrate the equipment (housing) in harmful quantities (flooding).
7 Protection against water when the equipment (housing) is immersed in water under
the specified pressure and time conditions.
Water must not penetrate in harmful quantities (immersion).
8 The equipment (housing) is suitable for permanent submersion under conditions to
be described by the manufacturer (submersion)
1)
1) This protection class normally refers to equipment which is sealed hermetically. With certain
types of equipment it is however possible for water to penetrate insofar as this has no harmful
effect.
Table 14.2.2: Classes of protection against water

14.3.1 Colour symbols to DIN IEC 757
This standard defines the standard colour coding in electrical engineering.
Abbreviation English Deutsch
BK black schwarz
BN brown braun
RD red rot
OG orange orange
YE yellow gelb
GN green grün
BU blue blau
VT violet violett
GY grey grau
WH white weiß
PK pink rosa
GD gold gold
TQ turquoise türkis
SR silver silber
GNYE greenyellow grüngelb
Table 14.3.1: Colour abbreviations
14.3.2 Colour coding to EN 50 044
This standard covers all inductive proximity sensors to standards EN 50 008,
EN 50 025, EN 50 026, EN 50 036, EN 50 037, EN 50 038 and EN 50 040.
The standard differentiates between polarised and non-polarised proximity sensors.
In the case of non-polarised proximity sensors with two connecting wires for DC or
AC operation, the wires can be any colour except green/yellow.
14.3
Colour coding

In the case of polarised proximity sensors for direct current and two connecting
wires, the connecting wire for the positive terminal must be brown and blue for the
negative terminal.
Where proximity sensors have three or four connecting wires, the wires must be
identified as follows
Positive terminal Brown
Negative terminal Blue
for three connecting wires Black independent of function;
for four connecting wires Black for the normally open contact function,
White for normally closed contact operation.
14.3.3 Numerical designation to EN 50 044
This differentiates between polarised and non-polarised proximity sensors.
For non-polarised proximity sensors, terminals 3 and 4 have the normally open
contact function and terminals 1 and 2 the normally closed function. For polarised
proximity sensors for direct current with two terminals, the positive terminal must be
identified with 1, the negative terminal with number 3. Number 4 is for the normally
open contact function and number 2 for the normally closed function.
The designs for inductive proximity sensors are laid down in European standards.
Many manufacturers offer all of these design types as well as their own designs
which differ from these standards. Standard EN 50 008 specifies the dimensions for
cylindrical proximity sensors (design A). In addition, the minimum values for nominal
switching distances and switching frequencies which must be achieved are indicated
below.
Operating voltage
Load output
14.4
Designs of
proximity sensors

d3
m ml3
l1
l2
sn
d1
d2
1 2
Calibrating plate (1) Width across flates sw (2)
Fig. 14.4.1: Cylindrical, inductive proximity sensors (design A)
Design Dimension
A1 •
flush
fitting
A2 •
non-
flush
fitting
Body Nut
Size Size d1 l1
min.
l2
min.
sw
h12
m
0.15
d3
max.
1)
• • 1 – M 8 x 1 40 60 13 4 15
• • 2 • • 2 M12 x 1 40 80 17 4 20
• • 3 • • 3 M18 x 1 50 100 24 4 28
• • 4 • • 4 M30 x 1.5 50 100 36 5 42
1)
d3 = 1.13 sw
Table 14.4.1: Dimensions for cylindrical, inductive proximity sensors (design A) in millimetres

Design A1 •
flush mounted
Design A2 •
non-flush mounted
Size Nominal switching-distance sn [mm] Size Nominal switching-distance sn [mm]
• • 1 1 – –
• • 2 2 • • 2 4
• • 3 5 • • 3 8
• • 4 10 • • 4 15
Design Minimum Switching frequency f [Hz]
A11 1000
A12 800
A13 500
A14 300
A22 400
A23 200
A24 100
Table 14.4.2: Nominal switching distances in millimetres and minimum switching frequencies

The relevant data for inductive proximity sensors of form C (block-shaped, with
square cross section) and D (block-shaped, with rectangular cross section) is
specified in standards EN 50 025 and EN 50 026.
2
1
3
16min.
40±1.5
20 ± 1
45 ± 1.5
120 max.
60 ± 0.5
30±0.5
40±1.5
5.3
+0.3
5.3
+0.3
7.3 ± 0.3
Active surface with design form C 21.1 (1)
Active surface with design form C 21.2 (2)
Cable entry (3)
Bild 14.4.2: Dimensions of inductive proximity sensors (Design C) in millimeters
The nominal switching distance is 15 mm, the switching frequency must be at least
100 Hz.

As far as inductive proximity sensors of form D (block-shaped, with rectangular
cross section) are concerned, these cannot be flush mounted in metal. Standard
EN 50 026 specifies the data in respect of dimensions, nominal switching distances
and switching frequencies.
16min.
40±1.5
1 2
l2
b1
b2
b /22
b /21
l1
5.3
+ 0.3
2
Active surface (1) Cable entry (2)
Fig. 14.4.3: Block-shaped inductive proximity sensors (design D)
Size l1max l2 = b2 b1max
• • 1 120 45 ± 0.5 60
• • 2 135 65 ± 0.5 80
l1 ≥ b1
Table 14.4.3: Dimensions for block-shaped inductive proximity sensors (design D) in millimetres

Design Nominal switching distance sn [mm]
D 21 25
D 22 40
Design Switching frequency fmin [Hz]
D 21 50
D 22 10
Table 14.4.4:
Nominal switching distances in millimetres and minimum attainable switching frequencies (design D)

• Universal two-wire design
• Designs for welding environment
• Designs for higher temperature range
• Designs for higher pressure range
• Designs with large switching gaps
• Designs with high switching frequency
• Designs with directional orientation (idle return function)
• Designs with safety technology
– Self-monitoring safety switches
– NAMUR-Switch for use in areas with explosion hazard
• Designs with selective action according to material
• Designs with switching distance independent of material
• Ring and slot shaped designs
• Special designs for checking of broken drills
15. Special designs and variants of proximity sensors
15.1
Variants of inductive
proximity sensors

15.1.1 Example of a universal two-wire design: Quadronorm by IFM
With the QUADRONORM inductive two-wire DC proximity sensor, 4 output functions
can be accomplished in one proximity sensor:
WH
BK
V
BK
WH
V
WH
BK
V
BK
WH
V
a)
d)
c)
b)
a) Normally open contact, negative switching
b) Normally open contact, positive switching
c) Normally closed contact, negative switching
d) Normally closed contact, positive switching
Fig. 15.1.1: Two-wire proximity sensor

15.1.2 Proximity sensors for use in installations with explosion hazard
Special proximity sensors are available for use in areas with explosion hazard, which
conform to DIN Standard 19 234. This type of proximity sensor is also known as a
NAMUR switch (NAMUR is an abbreviation for the German Standards Committee for
Measuring and Control Technology in the Chemical Industry, Working Group for
Contactless Controllers).
a) b)
I
R
V
1 2 3
4
5
a) Area with explosion hazard b) Area without explosion hazard
Object (1)
Proximity sensor (Two-wire DC sensor consisting essentially of an oscillator circuit) (2)
Circuit amplifier (3)
Supply voltage (4)
Binary output signal (5)
Fig. 15.1.2: Circuit principle of NAMUR proximity sensors

The following requirements are characteristics of NAMUR switches (in simplified
terms):
• The current-voltage characteristic curve V(I) must be within the specified range to
DIN 19 234. This guarantees that there is no sparking to trigger off explosion.
The characteristic curve is effected during the transition between the switching
statuses "inhibiting" and "conducting".
V
I
0.15 mA 1670.6
V
4
7
12
9
1
Permissible range of characteristic curve V(I) (1)
Fig. 15.1.3: Current voltage characteristic curve

• The response range for changing the switching status is between 1.2 mA and
2.1 mA.
• The safe switching status "inhibiting" is between 0.4 mA and 1.0 mA.
• The safe switching status "conducting" is above 2.2 mA.
• Monitoring and response ranges are defined for the interruption of the circuit
(line break monitoring).
• A short circuit response is defined within the circuit (short circuit monitoring).
• Certain test conditions and data sheet specifications must be adhered to.
For protection type "Intrinsically safe" the following additional standards apply (for
instance in Germany):
• DIN VDE 0165
• DIN 50014-1977 / VDE 0170 / 0171 Part 1 / 5.78
• DIN 50020-1977 / VDE 0170 / 0171 Part 7 / 5.78
Furthermore, the DIN standard 57 165 defines three zones with explosion hazard for
flammable gases, fumes and vapours (zone 0, zone 1 and zone 2) as well as two
zones (zone 10, zone 11) for flammable dust. For each of these zones certain
requirements have been defined for electrical installations, whereby the "intrinsic
safety" requirement represents just one of several requirements. It is for example
also possible to achieve explosion protection by means of encapsulation.
The above details merely serve as a rough guide; definitive information is available
through relevant standards. NAMUR proximity sensors (inductive, capacitive and
magnetic) and NAMUR circuit amplifiers are available from a large number of
manufacturers.
15.1.3 Magnetic field proof (welding plant) inductive proximity sensors
Inductive proximity sensors resistant to magnetic fields are used in the vicinity of
welding equipment. Their mechanical and electrical properties by far exceed those
of ordinary proximity sensors.
The overall surface of the proximity sensor must be resistant against any occuring
welding sparks.
Particularly high demands are made on the electronics due to the fact that in the
vicinity of such welding equipment currents flow in the kA range. These currents
cause a very strong magnetic field and would interfere with the function of an
ordinary proximity sensor because the proximity sensor coil represents a good
antenna for such strong magnetic fields and saturates the resonant oscillator circuit.
Note

By using a special core material for the oscillator coil and an electronic circuit which
recognises the presence of a welding field and blocks the switch output during the
short welding pulse, it is possible to use these sensors in welding lines such as in
the automotive industry. Just how large these magnetic fields are, can for instance
be seen by the fact that a steel wrist watch at a distance of approximately 30 cm
from the current-carrying conductor is easily attracted by this.
• Sizes M 12 x 1, M 18 x 1 and M 30 x 1.5 with switching distances of 2 mm, 5 mm
and 10 mm. These proximity sensors are magnetic field proof in continuous and
alternating fields with magnetic currents of up to 25 kA. A Teflon protective
screwed cover is available for the protection of active surface against welding
splashes.
• Magnetic field proof inductive proximity sensors in stainless steel design with a
ceramic front surface. A version in PBTP housing material is available for welding
currents of up to 100 kA.
Welding currents Distance [mm]
I [kA] 12.5 25 50 100
5 80 mT 40 mT 20 mT 10 mT
10 160 mT 80 mT 40 mT 20 mT
20 320 mT 160 mT 80 mT 40 mT
50 800 mT 400 mT 200 mT 100 mT
100 1600 mT 800 mT 400 mT 200 mT
Table 15.1.1: Reference values for magnetic induction
The proximity-related calculation of magnetic induction B in mT can be made using
the following formula:
mm/a
A/I
2.0
mT
B
⋅≈
I = Current in amps
a = Distance in mm
Examples

15.1.4 Inductive proximity sensors for higher temperature range
While normal inductive sensors cover a temperature range of -25 – 70 °C, there are
sensors to cover requirements at the higher temperature limit of 100 – 250 °C.
15.1.5 Inductive proximity sensors for higher pressure range
For use in hydraulic systems and for underwater research at deep sea levels, sensors
are required which can withstand high pressure.
Sensors are available to a pressure level of up to 80 MPa (800 bar). Standard type
sensors are used for a range of approx. 500 kPa – 1 MPa (5 – 10 bar).
15.1.6 Inductive proximity sensors with large switching distance
The potential switching distance is determined primarily by the size of the resonant
circuit coil. Large switching distances therefore require larger coils.
Proximity sensors with large switching distances are for instance of advantage in
cases where alternating distances occur between the object to be detected and the
proximity sensor, e.g. as a result of position tolerances or different object sizes.
Inductive proximity sensors in plastic housings of 80 mm, 90 mm or 100 mm dia.
with switching distances of 50 mm, 70 mm or 100 mm. Switching distances of
45 mm and 90 mm can be achieved with metal housings of 100 mm and 200 mm dia.
for flush mounting.
• In conformance with Standard EN 50 010, large switching distances require
correspondingly large standard calibrating plates or correspondingly large object
surfaces.
15.1.7 Inductive proximity sensors with high switching frequency
Inductive proximity sensors generally have a maximum switching frequency which is
in the range between 500 Hz and 5 kHz, whereby the smaller designs operate at the
highest frequencies. Large designs with switching distances in excess of 20 mm can
have switching frequencies of less than 50 Hz. High switching frequencies are for
instance required for the sensing of fast rotating parts. Products are available with
switching frequencies of up to 20 kHz.
Example
Note

15.1.8 Inductive proximity sensors with idle return function
Directional inductive sensors have two adjacent active zones. If an object passes
these two zones, then the proximity sensor only registers the object moving in a
certain direction, but not in the opposite direction. The basic requirement for this is
that the object to be sensed fully traverses the active zones both in the counting and
idle return directions.
1
3
2
4
Active zones (1, 2) Counting direction (3) Idle return direction (4)
Fig. 15.1.4: Inductive proximity sensor with idle return function
15.1.9 Self-monitoring proximity sensors
The self-monitoring proximity sensor (safety switch) is used in all those instances
where a high degree of reliability is required. A fault occurring with the switch is
detected by the evaluation unit and triggers the required actions. Generally the
entire installation is switched to safe status.
With some safety systems, not only the switch itself, but also the voltage supply
lines, the voltage supply and the evaluation electronics are continually checked.

F400
selbstüberwachend
2
3
4
5
1
1
6
Production process (1) Error message (3) Intervention in production process (5)
Fault (2) Error signal (4) Control (6)
Fig. 15.1.5: Design example: Self-monitoring inductive sensor system (source: IFM)
The connected sensors, including the connection cables are constantly monitored
for correct function.

7
8
16
15
14
13
12
11
10
9
24 V DC
I1
C
I2
C
I3
I4
Input
voltage
supply
Clock
Output
error
5
6
4
3
2
1
Evaluationlogic
External testTest key
Input
4. efector
Input
1. efector
Input
2. efector
Input
3. efector
Output
4. efector
Output
3. efector
Output
2. efector
Output
1. efector
Fig. 15.1.6: Block diagram of a function monitoring circuit (Source: IFM)
blueblue
blackblack
brownbrown
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16
O1 O2 O3 O4 F
I1 C I2 I3 C
External
rest
I4
Power supply
24 V DC
Fig. 15.1.7: Connection of a function monitor, Example using 2 sensors (Source: IFM)

The function monitor generates sensing pulses to monitor the sensors which are
connected to it. The pulses reach the black signal connections of the sensors via a
common timing circuit. Special sensor designs are used, where the black connection
is not, as in the case of standard sensors, for the output of the switching signal, but
in this case for receiving the testing clock. The sensors are supplied with voltage by
the function monitor via the brown connections. The blue connections serve as the
outputs of the sensors to the function monitor.
The sensors are continually reprogrammed from normally open to normally closed
function in accordance with the rhythm of the clock frequency. The pulses from the
pulse line (BK) and the signal line (BU) are connected in the function monitor in such
a way that the clock pulses are filtered out logically and the appropriate switching
status of the sensors is available at the signal outputs.
The fault-free status is signalled via a positive output signal on a common error
message output. In the case of a fault, e.g. line break, short circuit or damage to a
sensor, the error message output and the signal output of the sensor concerned are
closed and the error can be located by means of a test key.
15.1.10 Inductive proximity sensors for specific material detection
For certain applications, it is desirable that an inductive proximity sensor should
react to specific materials only. Ordinary inductive sensors respond to all metallic
objects. The largest switching distance is achieved with steel.
On the other hand, there are proximity sensors which respond to specific materials
achieving the greatest switching distance using iron-free materials ("proximity
sensors for non-ferrous metal"). Ferrous metals have a reduced effect and therefore
flush-fitting installation in steel is possible.
• Selective proximity sensors of cylindrical M 30 design, as well as block shaped
with nominal switching distances in relation to aluminium of 10 mm and 20 mm.
These proximity sensors are suitable for objects made of copper, aluminium, tin,
brass, bronze, zinc, silver, gold, manganese and lead.
• Selective proximity sensors with switching distances of 8 mm, 10 mm and 20 mm
of types M 30 x 1.5, block shaped 34 mm x 50 mm x 65 mm and 40 mm x 40 mm x
114 mm.
Examples

15.1.11 Inductive proximity sensors with material independent switching distance
Proximity sensors with constant switching distances, irrespective of material, have
the advantage that in the case of changing material no re-adjustment is required,
and a single switching distance is continually maintained, as in the case of standard
proximity sensors with the standard steel plate in steel S 235 JR.
Inductive proximity sensors with switching distances of 5 mm, 10 mm and 15 mm,
each independent of type of metal.
S 235 JR Pb Al CuMsV2A
Material
100
10
20
30
40
50
60
70
80
%
sn
a) a)a)a)a)a)b)
b)
b)
b)
b)
b)
a) Material independent proximity sensor b) Standard sensor
Fig. 15.1.8:
Comparison of switching distance between material independent proximity sensors and standard sensors
Example

15.1.12 Ring type inductive proximity sensors
The oscillator coil consists of a ferrite ring core with internal coil. The oscillator is
attenuated as soon as an electrically conductive object enters the ring.
Ring proximity sensors are suitable for instance for contactless sensing of small
metal parts, which are transported via a conveyor tube, whereby the conveyor tube
passes through the ring sensor.
Fig. 15.1.9: Ring type proximity sensor
Proximity sensors with an internal diameter of 10 mm, 15 mm, 21 mm and 43 mm.Example

15.1.13 Slot type inductive proximity sensors
Slot proximity sensors are in the shape of a fork, where two oscillator coils are
placed opposite one another. The proximity sensor responds to metallic objects in
the space between the fork, similar to a light barrier sensor. Proximity sensors of
this type are used in applications where constant accurate reproducibility of the
switching point is required even if the line of movement of the object varies slightly.
Fig. 15.1.10: Slot type proximity sensors
Proximity sensors with slot widths of 2 – 30 mmExample

15.1.14 Inductive proximity sensors for broken drill monitoring
Inductive sensors are also available for monitoring drill breakage, whereby drills,
taps or reamers are monitored for fracture during the working process. The principle
is based on the induced linkage of the sensor coils when a tool is introduced.
Diameters from 1 – 25 mm can be monitored. Tools are checked for availability on
actuation of the upper or lower sensing level.
6
5
4
2
3
1
Incorporated limit valve (1) Upper sensing point (3) Mounting plate (5)
Lower sensing point (2) Sensors (4) Workpiece (6)
Fig. 15.1.11: Proximity sensors to monitor drill breakage (Source: Euchner)

The following briefly describes a number of variants:
• Slot type barrier sensors
• Frame sensors
• Laser sensors
• Retro-reflective sensors with polarisation filter
• Printing mark sensors
• Luminescence sensors
• Angled light barrier sensors
• Sensors for accident prevention
• Dynamic sensors
There are many more variants in addition to the above, for example:
• Colour distinguishing sensors
• Sensors with integrated contamination signal
• Light grid sensor (using several through-beam sensors)
• Light curtain sensor with glass fibre optics
• Wide beam diffuse sensor for the detection of cling film or glass
• Special sensors for monitoring drill breakage
(starting from a drill diameter of 1.5 mm)
• Sensors for data transmission
• Diffuse sensors for reading bar codes
• Explosion-proof designs, NAMUR versions
• Designs for connecting up to 2 or 3 fibre optic adaptors to a sensor module
15.2
Variants of optical
proximity sensors

15.2.1 Slotted light barrier sensors
Slotted barrier sensors are through-beam sensors, in which the emitter and the
receiver are mounted opposite each other in a single U-shaped housing. They are
often available in low cost versions in plastic housings.
1
2
Emitter (1) Receiver (2)
Fig. 15.2.1: Slotted light barrier sensor
The interruption of the light beam within the fork is evaluated as the switching
signal.
Slotted barrier sensors are available in a range of slot widths between 3 mm and
50 mm.
These sensors are for instance used for measuring rotary or linear movements,
whereby a slotted disc or a linear scale is sensed. In this way, it is possible to
achieve a digital potentiometer without sliding contact. Relatively high switching
frequencies are possible, for example up to 1 MHz. Also light barrier sensors are
available, which can detect the direction of movement of an object.

15.2.2 Frame barrier sensors
Frame barrier sensors operate according to the principle of a light curtain. On two
opposite sides of the frame a large number of emitters and receivers are fitted in
close alignment, completely covering the inside of the frame with a light curtain.
Fig. 15.2.2: Frame light barrier
Frame sensors are used preferably to detect small parts falling through the frame,
for example for ejection monitoring of punched or pressed parts. Because of their
application in dynamic processes, frame sensors generally only have a dynamic
switching behaviour. Permanently existing parts, such as a transparent conveyor
tube which may be contaminated by dust and oil are therefore not detected. The
response time of frame sensors, for example, can be 150 µs and parts of up to
2 mm in diameter can be resolved.

15.2.3 Laser barrier sensors
Light emitting diodes (LED) are mostly used as the light source for optical sensors.
However, by using laser diodes, it is possible to construct laser sensors, which have
the following advantages:
• Extremely wide range by means of concentrating the laser beams
• Very narrow and precise response range over great distances
With laser beams having a cross section of 18 mm x 10 mm, it is possible to detect
objects at ranges of more than 200 m for instance. With shorter ranges of for
example 2 m, it is possible to detect an object of only 0.3 mm diameter. Such
extremely small response areas are particularly useful for accurate approaching and
setting of tools and workpieces.
15.2.4 Polarised retro-reflective sensors
Where retro-reflective sensors are used to detect highly reflective objects, the
proximity sensor is unable to distinguish whether the reflection originates from the
reflector or the object, i.e. it does not recognise the object. One solution to this
problem is to use polarisation filters.
1 2 3
5
4
No object in lightbeam
Lenses (1) Front cover (3) Analyzer (5)
Polarisor (2) Reflector (4)
Fig. 15.2.3: Polarised retro-reflective sensors (source: Sick)
Operating principle

1 2 3
5
4
The reflecting object does not produce the same polarised light beam as the reflector and is detected.
Lenses (1) Front cover (3) Analyzer (5)
Polarisor (2) Reflecting objector (4)
Fig. 15.2.4: Polarised retro-reflective sensors (Source: Sick)
The two polarisation filters for emitter and receiver are built-in between the lens of
the proximity sensor and an additional glass cover on the front of the proximity
sensor. A feature of the polarisation filter is that it only lets through light waves
which oscillate at a certain level. The light generated by the optical sensor (e.g. red
light LED) oscillates on several levels of polarisation.
The polarisation filter of the emitter lets through only that part of light which
oscillates at a specific polarisation level. In this way only the polarised light beam
reaches the reflector (there is no need for the ambient light level to be taken into
account, because this will be suppressed in the receiver anyway). The reflector
which is in the shape of a triple mirror then rotates the polarisation level by 90°. In
order that the light reflected by the triple mirror can be received by the receiver, the
series-connected polarisation filter is rotated by 90° opposite to the emitter
polarisation filter.
If there is a reflecting object in the lightbeam then in contrast to the triple mirror,
polarisation is maintained. In this way, the light from the object which hits the
receiver polarisation filter is not allowed to pass through to the receiver and the
receiver evaluates the absence of the light signal as "object available".

15.2.5 Printing mark sensors
Printing mark sensors are used for the detection of printed contrast markings, e.g. of
printed black-white or coloured marks on packaging materials, identification codes
on storage containers. Other examples include positioning with printing, for
applying glue or for cutting material widths according to patterns, or for cutting
labels or bags.
Printing mark sensors are also described as marking readers/scanners.
Printing mark sensors operate similarly to diffuse sensors, except for one difference
in that the emitting beam is focussed on a specific sensing distance. Printing mark
sensors are able to detect very slight contrast differences, whereby differences in
colour can also be interpreted as contrast differences.
The object must be within certain tolerances of the switching distance of the
proximity sensor. The strength of radiation reflected by the object is compared in the
receiver with an adjustable critical value. The threshold corresponds to a specific
grey-scale value on the object. If the threshold of the grey-scale value is fallen below
of or exceeded, the printing mark sensor changes its switching status.
Depending on the different types of application (varying reflection with different
colour contrasts), printing mark sensors are often equipped with optical sensors
whose wavelengths can be changed by using different light emitting diodes. Even
bulbs in conjunction with selectable colour filters are used.
Printing mark sensors can be used with fibre optic adaptors. However, in such cases,
luminous radiation is as a rule unfocussed and the sensing width may vary; however
contrast sensitivity quickly diminishes as the distance increases.
Printing mark sensors are also able to detect very small marks. Designs
incorporating an LED light source are for instance able to detect printing marks of a
dimension of 0.5 mm at switching distances of 20 mm, whereas designs using laser
radiation source are able to detect far smaller marks.

15.2.6 Luminescence sensors
Markings can no longer be reliably detected by means of printing mark sensors, if
the markings are amongst other similar textures, e.g. printed labels. Also, in many
cases the printing mark is not meant to be seen. In such cases, luminescence
sensors, which respond to invisible, luminescent markings are suitable.
The emitter of a luminescence sensor emits ultraviolet light at a wavelength of
365 nm for example. The ultraviolet light excites a fluorescent marking substance,
which emits at a higher wave length range (e.g. between blue and red).
The luminescence radiation excited is detected by the receiver, which generates a
switching signal. The emitter and receiver are modulated so that the receiver only
responds to the modulated light of the emitter. In addition, optical filters are used to
prevent the influence of external light effects.
Luminescent sensors also work perfectly with reflecting surfaces. Devices with
sensing ranges of up to 500 mm are available. Luminescent sensors can also be
used in conjunction with fibre optic cables.

15.2.7 Angled light barrier sensors
An angled light barrier sensor is a through-beam sensor with an angled light
emission. The optical emitter and receiver are focussed on a common point. If an
object appears at this focal point, the receiver detects the light reflected by the
object and generates a switching signal.
Angled light barrier sensors are used for the accurate detection of objects at smaller
distances whereby in contrast to retro-reflective sensors the switching distance is
independent of the degree of reflection.
1
2
3
Emitter (1) Receiver (2) Object (3)
Fig. 15.2.5: Angled light barrier sensor

15.2.8 Sensors for accident prevention
Sensors for accident prevention are used to protect access to danger zones where
power driven equipment is used, e.g. presses, automatic metal-cutting and shaping
tools, cutters, winding machines, foundries, robots, rollers and stirrers. Sensors
which are used for the purpose of accident prevention must meet the national safety
regulations as laid down by individual regulatory bodies.
Depending on these regulations, sensors for accident protection come under the
category of contactless protective devices, which can mean through-beam sensors
or systems connected to these, such as light curtains or light grids. Protection
devices must give a switching command if parts of the body enter the protected
area. The purpose of the switching command is to prevent or interrupt a potentially
dangerous movement.
The following requirements must be fulfilled (we do not claim completeness,
appropriate local regulations take precedence):
• Specification of response time and size of obstacle on rating plate.
• Indication of at least two operating statuses.
• Prevention of any risk in case of interrupted operation of the protective device.
• Sufficient protection against external influences such as vibration, dirt, stray
fields, mains interference, short circuit, line break.
• Self test, start-up test, self monitoring. Operative malfunctions in the protective
device must be detected and signalled to the potentially dangerous equipment in
the form of a cut-off command.
• Inhibit re-start following the interruption of a hazardous movement.
• Observation and identification of a specified safety distance between the
protective area and the danger zone as well as identification of overtravel time.
• Protection against encroachment of or reaching into the protective area from
below or above as well as against remaining between the protective area and the
danger zone.
• Tests (prior to initial commissioning and additional regular routine testing also
after retooling and repairs).

Industrial designs of accident protection sensors for example have the following
features compared to ordinary sensors:
• Several indicator lights for operating and function display, e.g. for "emitter
switched on", "light path free", "Light path interrupted", "light reception",
"Light reception good" and "Light reception poor".
• For sensors with relay output, two positive action relay contacts for the
connection of both outputs to the machine control.
• Front lens heating, lens contamination indicator.
• Permanent self-monitoring.
• High optical and electrical noise immunity.
Accident protection grid sensors consist of a system of parallel acting sensors, which
create a dense grid of parallel infrared emission zones. The individual sensors of the
grid are actuated in quick succession according to the multiplex method. A beam
from the receiver to the emitter serves the purpose of synchronisation. Resolution is
for example 35 mm between parallel beams (Minimum obstacle size).
Various designs are available to achieve different height levels,
e.g. from 400 – 1000 mm.
Fig. 15.2.6: Safety screen of through-beam sensors

15.2.9 Dynamic sensors
Standard sensors react to static build-up in the light beam. The switching signal
reacts as long as the build-up is present.
Dynamic sensors, in contrast, react to rapid changes in the emission strength
received. Slow changes such as for instance as a result of gradual contamination or
slowly occurring objects are not registered because the switching threshold in the
receiver is continually adjusted.
Dynamic sensors are often used as thread breakage monitors in the textile industry.
The thread passes through the sensor. A thread breakage creates a slight, brief
change in the light being passed and is detected. Breakages in the finest threads
(e.g. up to 0.05 mm) can be detected. The minimum brightness variation can be
adjusted.

Protective circuits for electro-mechanical limit switches
Differentiation must be made between ohmic, inductive and capacitive loads.
Depending on the type of load, a suitable protective circuit is to be designed in order
to achieve a longer service life for the switching contacts.
If the load is purely ohmic, no additional protective measures need to be taken to
observe the limit values of the respective data sheet.
A great amount of current flows briefly if a capacitive load is switched on. If this
current exceeds the value specified in the data sheets, external measures must be
taken to restrict it. A protective resistor is connected in series with the switch. The
design of the protective circuit is in accordance with the formula
R = V / Imax
with switching voltage V and maximum switching current Imax. The resistance is to be
selected so that it can accept the required electrical power.
If there is a coil in the circuit, it stores magnetic energy while the circuit is closed.
When switched off, this magnetic energy is reduced suddenly thus producing an arc
across the two switching contacts which causes them damage. Different protective
measures are required depending on voltage type.
24 V
0 V
D L
Fig. 16.1.1: Protective circuits for DC
16. Solutions
16.1
Solutions to exercises
from Chapter 2
Exercise 2.1
Ohmic load
Capacitive load
Inductive load

16. Solutions
C
C
R
R
L L
Fig. 16.1.2: Protective circuits for DC and AC
A
I
100
nF
C
≈
coilofcetanresisOhmic
R
≈
Ω
L
Fig. 16.1.3: Protective circuit for AC current using a varistor

16. Solutions
Switching of low electrical capacity
The switching reliability of a limit switch can be considerably improved by fitting a
resistor to the load.
RL RP
Load resistance (RL) Parallel resistance (RP)
Fig. 16.1.4: Circuit for low contact rating
Maximum passing speed of a cylinder piston across a reed proximity sensor
The maximum passing speed for a piston is calculated using formula:
vmax = Smin / TS
Smin is the smallest possible response range of the proximity switch when
overtravelled by the cylinder piston. TS is the switching time of the proximity sensor
or of another affected part, e.g. a valve. In this instance, the result obtained from the
data sheet of the reed proximity sensor (SME) is the value TS = 2 ms for the response
time of this component.
A value of 7 mm is obtained from table 16.2.1 in respect of the Festo cylinder DNNZ
with a diameter of 32 mm for the response travel. For vmax a value of 3.5 m/s is
obtained.
Exercise 2.2
16.2
from Chapter 3
Exercise 3.1

16. Solutions
Piston
diameter
[mm]
Typ Hysteresis
Hmax [mm]
Response travel
Smin [mm]
SME SMP SME SMP
8 ESN, DSN 2 1.5 7 9
10 ESN, DSN 2 1.5 5 9
12 ESN, DSN 2 2 8 11
16 ESN, DSN 2 2 6 9
ESN, DSN 2 2.5 7 920
DGS
ESN, DSN 1.5 2 6 1725
DGS 2 1.5 7 10
ESW, DSW 2 1.5 10 12
DN, DNZ 2.5 4 7 15
32
DNNZ 2.5 4 7 15
ESW, DSW 2 2 9.5 12
DN, DNZ 2.5 4.5 8 15
40
DNNZ 2.5 4.5 8 15
ESW, DSW 2 2 10.5 12
DN, DNZ 3 5 8 17
50
DNNZ 3 5 8 17
Table 16.2.1: Hysteris and response range of various cylinders (example)
Again for the Festo cylinder DNNZ with a diameter of 32 mm, a value for
vmax of 0.467 m/s is obtained for a valve with a response time of 15 ms.

16. Solutions
Electrical connection of a reed proximity sensor
+24 VDC
0 V
BN(1)
BU(3)
BK(4)
R
L
R
Series resistor (R) Load (RL) Light emitting diodes (L1, L2)
Fig. 16.2.1: Circuit diagram of reed proximity sensor
In the case of a proximity sensor with reed contacts, a circuit is built-in for protection
against inductive switch-off peaks, which at the same time acts as a bipolar
switching status display.
The protection diodes are connected parallel to load L via the series resistance,
similar to the protective circuit shown in Fig. 16.1.1. The protective circuit also works
with an alternative supply voltage.
When a load is connected, care should be taken that the load resistance is
sufficiently great so that the maximum permissible switching current of the
proximity sensor is not exceeded. Provided this requirement is met, the polarity of
the supply voltage can be exchanged without causing any damage.
It should be noted that particularly during testing, a sensor can easily be damaged if
the load output BK (4) is accidentally short-circuited to terminal BU (3).
Exercise 3.2

16. Solutions
Resolution of a reed proximity sensor
The minimum possible stroke that can be detected for a cylinder fitted with two reed
proximity sensors is calculated by:
Hmin = 2 ⋅ Hmax
Hmax is the maximum hysteresis of the cylinder switch combination.
The relevant values can be taken from table 16.2.1. The value for a Festo cylinder of
type DNNZ with a diameter of 32 mm, fitted with a reed switch (SME) is
Hmax = 2.5 mm. This results in a minimum possible stroke of 5 mm.
Application of an inductive proximity sensor
The number of parts containers is established by means of counting the output
pulses of proximity sensors.
If one assumes that the transport speed is constant, the time difference between
two consecutive proximity sensor pulses can be converted into the distance between
the parts containers.
A second proximity sensor is required for direction detection. It is necessary to
establish the sequence in which the two proximity sensors emit an output signal to
obtain the information in respect of direction.
Exercise 3.3
16.3
from Chapter 4
Exercise 4.1

16. Solutions
Fig. 16.3.1: Schematic assembly of transport device
With inductive proximity sensors the switching distance is dependent on the
material to be detected. In this case, the nominal switching distance specified in the
data sheets must be multiplied by the value 0.5 (reduction factor for aluminium).
This results in a value which is only half as great as the nominal switching distance
specified in the data sheet. Because the distance between the aluminium container
and the proximity sensor can fluctuate, it is important to select a proximity sensor
with a switching distance which is not too small. In addition, a greater nominal
switching distance facilitates the adjustment of the proximity sensor on the
transporting device.
In the case of specified built-in diameters, the greatest switching distance is
achieved if a non-flush fitting type of proximity sensor is used. In this case, however,
care must be taken to ensure that the active zone of the proximity sensor is free of
metal.

16. Solutions
d d d
F2
d
F2F3
F1
a)
b)
a) Flush mounted
b) Non-flush mounted
Diameter of proximity sensor (d) Nomial switching distance (sn)
Free zone 1 = 3 x sn (F1) Free zone 2 ≥ 3 x sn (F2) Free zone 3 ≥ 2 x sn (F3)
Fig. 16.3.2: Installation specifications for proximity sensors
Hysteresis is the term used to describe the difference between the switch-on point
and the switch-off point of a proximity sensor. This is essential to guarantee the safe
switching of the proximity sensor. Should the two switching points coincide, this
would result in fluttering of the output signal when the object is passed in front of
the proximity sensor precisely at the switching distance.
Detection of vibrating steel cylinders
1. Movement of the steel cylinders may lead to several counting pulses being
triggered off per steel cylinder if the reaction time of the control is less than the
vibration period and if steps have not been taken to suppress the multiple pulses
by means of software.
2. 1 % of 8 mm = 0.08 mm
Exercise 4.2

16. Solutions
Filling level measurement in a grain silo
To detect the filling mounds of granular materials, material-specific characteristics
must be taken into account. When the silo is filled, a mound of bulk material is
created. The angle of settlement is a characteristic which depends on the material
used. When being emptied a depression is created. These two characteristics must
be taken into consideration when selecting the place of installation of the proximity
sensor. If this is not done, it can lead to error measurements.
Fig. 16.4.1: Level sensing by detecting the filling mound and emptying depression in granular material
Furthermore, it should be noted that the switching distance to be attained with
capacitive proximity sensors heavily depends on the water content of bulk materials.
Damp bulk materials result in a greater switching distance than dry materials.
Environmental effects on capacitive proximity sensors
A capacitive proximity sensor measures a change of capacitance in the active zone
and evaluates this change. If humidity settles on the proximity sensor housing (dew,
fog), this can lead to an error signal. Because water has a high dielectricity constant
(ε = 81), small droplets of moisture are sufficient to interfere with the proximity
sensor. Capacitive proximity sensors are available which can compensate the effects
of humidity by means of an auxiliary electrode.
16.4
from Chapter 5
Exercise 5.1
Exercise 5.2

16. Solutions
Detection of cardboard boxes
Because the capacitance change caused by a thin cardboard box is relatively small,
it may be that the capacitive sensor is unable to detect the boxes. In this instance,
each individual case must be checked as to whether the proximity sensor responds
to all objects which it is to detect. A change in sensitivity can usually be made by
adjusting the potentiometer screw on the capacitive proximity sensor. Please take
into consideration that the humidity content of cardboard may have an influence on
the switching distance.
Detection of a transparent panel
A capacitive proximity sensor reacts to capacitance changes. The capacitance
change which is caused by 0.1 mm thick plastic film is insufficient to actuate the
sensor. Wall thicknesses of more than 1 mm are generally required for materials
made of plastic in order to actuate a capacitive proximity sensor.
A diffuse sensor is suitable for use as an optical solution. Sensitivity can be adjusted
by means of the setting potentiometer in such a way that the diffuse sensor reacts to
the plastic film and not to the inside of the packaging on the other side. The diffuse
sensor must be aimed vertically at the reflecting transparent panel.
This solution requires a concentration of ultrasonic emission on to the transparent
panel. A test is recommended without a transparent panel to check that the
ultrasonic proximity sensor does not respond to the packaging itself. This may
happen in the case of a large distance and proximity sensors with an ultrasonic cone
which is opened too wide. In certain circumstances, the use of a sound absorbing
aperture plate is required. This proximity sensor too must be directed vertically at
the transparent panel.
Exercise 5.3
Exercise 5.4
Capacitive
proximity sensor
Optical
proximity sensor
Ultrasonic
proximity sensor

16. Solutions
Fig. 16.4.2: Ultrasonic proximity sensor with a sound-absorbing shield (material: e.g. felt)
Environmental effects on optical proximity sensors
In a dusty environment, it is to be expected that the lenses of optical proximity
sensors and reflectors may become contaminated.
By means of the following example using a retro-reflective sensor, you will discover
how much the function of a optical proximity sensor depends on whether its lens
and reflecting device are clean. Let us assume that the lens and the reflector are
dimmed by deposits of dirt by 10 %. This is a value which is easily achieved. This
level of pollution is barely detectable by visual means. Because the light beam of a
retro-reflective sensor has to penetrate this contamination four times, the irradiated
light is weakened from 100 % to approx. 66 %. Almost a third of the effective
emission capacity is used up as a result of this slight contamination.
With optical systems, it is always important to ensure that the lenses and/or
reflectors are clean. If required, additional measures must be taken to prevent rapid
or high build-up of contamination (e.g. blowing by compressed air, installing a dust
trap). The maximum contamination permissible depends on the capacity margin of
the proximity sensor; see chapter 6.1 for further details.
16.5
from Chapter 6
Exercise 6.1
Example

16. Solutions
Selection of optical proximity sensors
If insufficient mounting space is available at the point where the proximity sensor is
to be employed for object detection, optical proximity sensors with fibre-optic cables
are particularly suitable. Because of the small dimensions of the sensor heads, fibre-
optic cables can be used in inaccessible places.
The choice of fibre-optic cable material has to be made on the basis of
environmental conditions. Whilst polymer fibre-optic cables generally can only be
used in a temperature range of -25 – +70 °C, these values range between
-20 –+200 °C for glass fibre-optic cables. Special designs are available for different
temperature ranges. Resistance to chemicals also has to be taken into account when
selecting fibre-optic cables.
An important advantage of this arrangement is that the actual proximity sensor with
its electrical connections does not have to be installed near the point of detection
and can be mounted outside possible danger areas.
Operating reliability of optical proximity sensors
By means of modulating light emission, it is possible to improve the protection of
optical proximity sensors against the influence of surrounding light. This means that
their sensitivity to interference as a result of ambient light is reduced.
The light emitter pulses the emissions at a specified frequency actuated by a signal
generator. The generator signal is transmitted to the logic module of the signal
receiver. The signals are checked as to their compatibility and an output signal is
generated only if this condition is met.
Another possibility is to suppress the ambient light by means of a bandpass, which
only allows the emission frequency of the emitter to be passed.
In the case of optical proximity sensors operating in the infrared zone, additional
daylight filters are installed. This further reduces the effects of the surrounding light.
Exercise 6.2
Exercise 6.3

16. Solutions
Detection of burnished steel
The response of this optical sensor is determined by the environment. The
background or wall towards which it is directed, reflects sufficient light to trigger a
response. Anodised aluminium, for instance, reflects very strongly. The reason why
the sensor switches off is because burnished steel has a low degree of reflection.
a
b1
2 3
Distance between proximity sensor and object (a)
Distance between proximity sensor and background (b)
Setting potentiometer (1) Object (2) Background (3)
Fig. 16.5.1: Background fade-out
To begin with, efforts should be made to adjust the diffuse sensor by means of a
setting potentiometer so that it responds to the burnished steel part at distance "a"
only and not to the background at distance "b". If this is not possible, then the
background must be covered with less reflecting material.
Electrical connection of proximity sensors
In the case of unregulated power supplies with filter, it is possible for voltage peaks
to occur during switching on. These may be above the permissible operating voltage
of the proximity sensor used and can lead to its failure.
A simultaneous "connecting to ground" of the normally open and normally closed
contact can also lead to failure. To provide short-circuit protection, the output of the
proximity sensor is pulsed. With simultaneous "connecting to ground" of the two
inverted outputs, the short circuit is constantly cancelled on one output and created
again on the other. This causes overloading and thus failure of the proximity sensor.
Exercise 6.4
Exercise 6.5

16. Solutions
Filling level measurement by means of optical proximity sensors
1. Through-beam sensors, retro-reflective sensors, through-beam sensors with
fibre-optic cables.
2. yes
α
∆h
Fig. 16.5.2: Response accuracy
The proximity sensor responds when the height of the filling level is within a certain
range. The width of the response range ∆h is dependent on the diameter of the
active surface "a" of the proximity sensor and on the angle α:
αα⋅=∆ 2sinsinah
With a = 1 mm (using a polymer fibre-optic cable) and an angle of α = 10° – 45°:
∆h = 0.5 – 0.7 mm
3. If the liquid in the container moves, for example, if foam is on the liquid or if the
proximity sensors are splashed during filling.
4. Molten candle wax is prone to hardening on the outer edges, if the container is
emptied quickly and heated from the base only. For this reason, filling level
measurement cannot be carried out at the edge of the container. If set for the
centre, this solution is suitable if it can be guaranteed that no unmelted
remnants are floating in the melted wax.
5. Vertically onto the surface of the liquid or through the container wall (for
example, with correspondingly thin container side made of plastic) by means of
capacitive proximity sensors.
By means of float switches, potentiometer sensors, hydrostatic pressure
measurement at the base of the container, resting the container on load cells,
microwave filling level sensors, vibration filling level sensors.
Exercise 6.6

16. Solutions
Detection of workpieces
1. Yes, however the values specified in the data sheets regarding maximum range
must be observed.
2. From above by means of retro-reflective sensors (using fibre-optic cables if space
is restricted or if it is difficult to fade out the background).
Use of optical proximity sensors in car washes
Protection class IP 65 is sufficient (protection against penetration of dust and
splashing water). Care must be taken that the lenses do not become dirty (blowing
with compressed air, installing a dust trap). A minimum of two lines of light barriers
is required, which are staggered so that the gantry does not touch the body of the
car when moving back and forth.
Use of optical proximity sensors with fibre-optic cables
This solution works. In this way, the response range can for example be increased
from 10 – 60 mm, whereby it should be noted that white or reflecting objects cannot
be detected reliably at a small distance via the fibre-optic cable. This solution is
suitable for the detection of matt, dark (black) objects. Also, it should be noted that
compared to operating without a reflector, the switch output (and the LED) of the
proximity sensor is inverted compared to operating without a reflector.
Checking of bottles
The nominal switching distance of an inductive proximity sensor is 8 mm (for steel
S 235 JR). For aluminium, the switching distance is reduced to 4 mm. Due to variable
height h, an inductive proximity sensor cannot be considered as a solution. The
sealing caps can be detected by means of an optical diffuse sensor, whereby the
sensitivity of the proximity sensor must be set in such a way that it does not react to
the bottle necks. It is an essential requirement that the bottle positions on the
conveyor belt always remain within the sensing range of the proximity sensor.
Exercise 6.7
Exercise 6.8
Through-beam sensor
Exercise 6.9
Exercise 6.10

16. Solutions
Smallest measurable distance
Ultrasonic proximity sensors which have only one ultrasonic transducer, operate
alternatively as an emitter and as a receiver. The ultrasonic transducer creates
oscillations by means of a connected alternating voltage and emits ultrasonic waves.
If the voltage is switched off, then the oscillation of the transducer dies out
exponentially. The transducer must stop oscillating before a reflecting signal can be
received. The final oscillation time is dependent on the size of the transducer. This
does not occur with designs which have separate emitter and receiver transducers.
However, neither type of proximity sensor should be used to detect objects at small
distances for another reason. Characteristically, the ultrasonic emission from these
proximity sensors produces secondary lobes in the near field adjacent to the main
emitting zone. If an object approaches laterally within the range of the near field,
sensing becomes highly irregular so that no predictable response is possible.
1 2
D
α
Near field (~D
2
/λ) (1) Far field (2)
Fig. 16.6.1: Sound emission characteristic of an ultrasonic proximity sensor
16.6
from Chapter 7
Exercise 7.1

16. Solutions
Deflection of ultra-sonic sound waves
Because the same principle applies for ultrasonic waves as for light beams, i.e. the
angle of incidence equals the angle of reflection, a deflection of ultrasonic waves by
90° is possible.
The reflector must be carefully adjusted. As deflection causes dissipation, multiple
deflection should be avoided.
Sensing of boxes on a conveyor belt
The device used (range 20 – 100 cm) is adjusted in such a way that it just fails to
detect the base of an empty box. In this way, a signal is generated when a filled box
passes. The signal is independent of the height of the box or the filling level. The
presence of a box is signalled by means of a short signal as the sound cone passes
through the side of the box.
Range of air barrier sensors
The components to be detected have a width of 90 mm. From the characteristic
curve of a Festo through-beam sensor SFL-100 can be seen that a signal pressure of
0.7 mbar is reached by applying a sender pressure of 20 kPa (0.2 bar) at a distance
of 100 mm and 5 mm either side of the object). If the supply pressure is increased to
50 kPa (0.5 bar), a signal pressure of nearly 0.3 kPa (3 mbar) is attained under
otherwise identical conditions. This output signal can be amplified with the help of
suitable pressure amplifiers. An air barrier is suitable for detection of the parts.
Exercise 7.2
Exercise 7.3
16.7
from Chapter 8
Exercise 8.1

16. Solutions
Distance s
Signalpressurex
0 160mm12010080604020
s
Emitter Receiver
1.0
kPa
0.05
0.01
0.5
0.1
pE1 pE2 pE3
pR
x
1
2
pE
1
pR = 10 kPa = const. pE1 = 10 kPa pE2 = 20 kPa pE3 = 50 kPa
Fig. 16.7.1: Characteristic curves of the Festo SFL-100 air barrier

16. Solutions
Checking lids by means of a reflex sensor
The characteristic curve of a reflex sensor specifies values which apply at a supply
pressure of 15 kPa (150 mbar). One possible value for the distance between sensor
and lid is between 2 – 4 mm. At this distance, a signal pressure of 0,3 – 0,4 kPa
(3 – 4 mbar) is produced. This output signal can be amplified with the help of
suitable pressure amplifiers.
Signalpressure
Axial distance s
0 7mm54321
-0.1
1.0
-0.2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
kPa
Airconsumption
Supply pressure
70
0
10
20
30
40
50
60
l / min
0 100kPa604020
Fig. 16.7.2: Characteristic curves of the Festo RML-5 reflex sensor
At a pressure of 15 kPa (150 mbar), the air consumption of this sensor is
approximately 16 l/min if exhausted directly to atmosphere.
Exercise 8.2

Fig. 4.4.3 Sensing of a cam switching mechanism by means of inductive
proximity sensors excerpt
from: "Sensor technology", Hans Turck GmbH & Co KG, Mülheim an
der Ruhr, catalogue, 1st edition, 1989
Fig. 4.4.4 Measurement of speed and direction of rotation excerpt
Fig. 5.4.5 Checking the presence of bulbs inside cardboard boxes excerpt
Fig 11.3.3 The piezoelectric effect excerpt
from: "Piezoxide (PXE) Characteristics and applications", Philips
Components, Hamburg, by Hüthig Verlag, Heidelberg, 1st edition,
1988
Fig. 11.4.11 Illustrations of fibre-optic cables excerpt
from: "Fibre-optic components by Schott", Schott Glaswerke,
Wiesbaden, catalogue, 1990
Fig. 15.1.5 Design example: Self monitoring inductive sensor system excerpt
from: "Product range catalogue", ifm electronic GmbH, Essen,
catalogue, 1990
Fig. 15.1.6 Block diagram of a function monitoring system excerpt
catalogue, 1990
Fig. 15.1.7 Connection of a function monitoring system, example of 2 sensors
excerpt
catalogue, 1990
Fig. 15.1.11 Proximity sensor to check for broken drills excerpt
from: "Program overview", Euchner & Co., Leinfelden,
catalogue, 1989
Fig. 15.14 No object present in the lightbeam excerpt
from: "Light barriers", Erwin Sick GmbH, Freiburg, catalogue, 1989
17. Bibliography of illustrations

17. Bibliography of illustrations
Fig. 5.15 The reflecting object does not create the same polarisation effect as
the reflector and is detected excerpt
from: "Lichtschranken", Erwin Sick GmbH, Freiburg, catalogue, 1989

A Accident protection ________________________________________ 81, 247
Actuators ____________________________________________________ 26
Air barriers __________________________________________________ 123
Analogue sensors _____________________________________________ 15
B Back pressure sensors ________________________________________ 121
Background fade-out____________________________________________ 8
Binary sensors ________________________________________________ 15
Block circuit diagram for
inductive proximity sensors ___________________________________ 49
capacitive proximity sensors __________________________________ 61
optical proximity sensors _____________________________________ 71
reed proximity sensors _______________________________________ 36
ultrasonic proximity sensors _________________________________ 107
C Characteristic curves of pneumatic proximity sensors _______________ 125
Circuit symbols ______________________________________________ 189
Colour coding (abbreviated codes for electrical engineering)__________ 217
Conductivity_________________________________________________ 160
Connection designation________________________________________ 145
Connection of controllers, relays and display elements ______________ 153
Contact bounce ___________________________________________ 25, 169
Contact materials______________________________________________ 26
Correction factors, diffuse sensors________________________________ 88
Current supply _______________________________________________ 154
Cylinder switches______________________________________________ 40
D Dark switching method _________________________________________ 72
Deflecting jet ________________________________________________ 124
Depth of penetration (electromagnetic field)_______________________ 161
Designs (of proximity sensors) __________________________________ 218
18. Index

18. Index
E Eddy currents________________________________________________ 155
Effect
Hall- __________________________________________________ 42, 172
Skin-_____________________________________________________ 162
magnetoresistive________________________________________ 42, 173
Wiegand- _________________________________________________ 174
Electromagnetic influences_____________________________________ 153
Emitting characteristic, ultrasonic proximity sensors ________________ 185
Exercise 2.1: Protective circuits for mechanical-electrical limit switches__ 33
Exercise 2.2: Switching with low electrical power ____________________ 33
Exercise 3.1: Maximum passing speed of a pneumatic cylinder piston
over a reed proximity sensor_____________________________________ 47
Exercise 3.2:Electrical connection of a reed proximity sensor __________ 48
Exercise 3.3:Resolution of a reed proximity sensor___________________ 48
Exercise 4.1:Application of an inductive proximity sensor _____________ 58
Exercise 4.2:Detection of vibrating steel cylinders ___________________ 59
Exercise 5.1:Measuring the filling level in a grain silo_________________ 69
Exercise 5.2:Environmental effects on capacitive proximity sensors _____ 69
Exercise 5.3:Detection of cardboard boxes _________________________ 69
Exercise 5.4:Detection of a transparent panel_______________________ 69
Exercise 6.1:Environmental effects on optical proximity sensors_______ 100
Exercise 6.2:Selection of optical proximity sensors__________________ 100
Exercise 6.3:Operational reliability of optical proximity sensors _______ 100
Exercise 6.4:Detection of burnished steel _________________________ 101
Exercise 6.5:Electrical connection of proximity sensors ______________ 101
Exercise 6.6:Measurement of filling level by means
of optical proximity sensors ____________________________________ 102
Exercise 6.7:Detection of workpieces_____________________________ 103
Exercise 6.8:Use of optical proximity sensors in car washes __________ 104
Exercise 6.9:Use of optical proximity sensors equipped
with fibre-optic cables_________________________________________ 105
Exercise 6.10:Checking of bottles________________________________ 106
Exercise 7.1:Smallest measurable distance________________________ 117
Exercise 7.2:Deflection of ultra-sonic sound waves _________________ 117
Exercise 7.3:Detection of boxes on a conveyor belt _________________ 117
Exercise 8.1:Range of air barrier sensors__________________________ 135
Exercise 8.2:Checking lids by means of a reflex sensor_______________ 135

18. Index
F Ferro sensors ________________________________________________ 173
Fibre-optic cables ____________________________________________ 193
Glass fibre-________________________________________________ 196
Polymer-__________________________________________________ 196
Flush fitted sensors ____________________________________________ 53
non-flush fitted _____________________________________________ 54
Fotodiodes __________________________________________________ 191
I Idle return function ___________________________________________ 232
L Light barriers, dynamic ________________________________________ 250
Through-beam- _____________________________________________ 78
Laser- ____________________________________________________ 243
Retro-reflective _____________________________________________ 81
Light emitting diodes (LEDs) _________________________________ 71, 189
Light switching method_________________________________________ 72
Light, infrared_________________________________________________ 71
visible____________________________________________________ 186
M Magnetism
Dia-______________________________________________________ 162
Electro-___________________________________________________ 169
Ferro- ____________________________________________________ 162
Para-_____________________________________________________ 162
Permanent- _______________________________________________ 167
Magnetoresistive sensors ______________________________________ 173
Multi sensor system ___________________________________________ 13
N NAMUR-switches _____________________________________________ 227
Normally closed contacts ______________________________________ 141
Normally open contacts________________________________________ 141

18. Index
O Operating reserve _____________________________________________ 74
Operating voltages ____________________________________________ 18
Oscillations, electrical _________________________________________ 156
Oscillator _____________________________________________ 42, 49, 155
Output signals ________________________________________________ 13
Output
NPN-_____________________________________________________ 147
PNP- _____________________________________________________ 146
P Parallel switching of proximity sensors _______________________ 149, 150
Polarisation _________________________________________________ 243
Position switches,
electro-mechanical __________________________________________ 25
mechanical-pneumatic _______________________________________ 31
Power dissipation ____________________________________________ 161
Protection classes ____________________________________________ 214
Protective circuits,
electrical-mechanical position switches _________________________ 29
reed proximity sensors _______________________________________ 39
Proximity sensors _____________________________________________ 16
Hall- ______________________________________________________ 42
inductive ______________________________________________ 49, 155
inductive, magnetic field-proof _______________________________ 227
inductive, variants__________________________________________ 225
capacitive______________________________________________ 61, 164
magnetic-contactless ________________________________________ 42
magnetic-pneumatic _________________________________________ 45
magnetoresistive____________________________________________ 42
optical ____________________________________________________ 71
optical, variants____________________________________________ 240
optical, with fibre-optic cables _________________________________ 92
pneumatic ________________________________________________ 119
reed-______________________________________________________ 35
self-monitoring ____________________________________________ 232
ultrasonic- ________________________________________________ 107
Wiegand- __________________________________________________ 43

18. Index
R Reduction factors,
inductive proximity sensors ___________________________________ 63
Reflection ___________________________________________________ 187
Total- ____________________________________________________ 189
Reflex sensors (pneumatic)_____________________________________ 122
Refraction___________________________________________________ 188
Resonant circuit,
LC- ___________________________________________________ 49, 157
RC- ___________________________________________________ 61, 164
Response characteristics,
inductive-magnetic proximity sensors ___________________________ 44
Reed proximity sensors_______________________________________ 37
Response curves, Diffuse sensors ________________________________ 87
Response range,
Retro-reflective sensors ______________________________________ 79
Through-beam sensors_______________________________________ 83
Through-beam sensors with fibre-optic cables ____________________ 93
S Sensing distances of pneumatic proximity sensors__________________ 120
Sensor ______________________________________________________ 11
Sensor component_____________________________________________ 12
Sensor selection criteria _______________________________________ 137
Sensor system ________________________________________________ 12
Sensors,
Angled light- ______________________________________________ 245
Luminescence-_____________________________________________ 246
printing mark______________________________________________ 246
Sensors, diffuse_______________________________________________ 85
Series connection of proximity sensors _______________________ 151, 152
Speed of light________________________________________________ 186
Speed of sound ______________________________________________ 175
Standards___________________________________________________ 213
Switching distance,
inductive proximity sensors _______________________________ 52, 160
Switching range(s),
inductive-magnetic proximity sensors ___________________________ 44
reed proximity sensors ___________________________________ 37, 170

18. Index
T Technology
Four and five-wire- _________________________________________ 144
Three-wire- _______________________________________________ 143
Two-wire-_________________________________________________ 141
Triggering stage______________________________________________ 159
U Ultrasonic_______________________________________________ 108, 175
attenuation _______________________________________________ 182
generation ________________________________________________ 179
W Wiegand wire _________________________________________________ 43

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