Chapter 2 Sensors_3rd Edition_Feb2017.pdf

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2
Sensors
Millions of sensors are in use across diverse applications in the manufacturing plants like those for
automobiles, food and beverages, cement, plastics, packaging machines and they are abundantly
also placed into machines like cranes, elevators, conveyers etc. Sensors form a very essential
segment as input devices in any automation system and the following is a simple treatise to the
exciting world of “Sensors for Automation”.
We humans interact with the physical world around us through the senses endowed to our being.
Our mind and body act or react according to the feedback given by our senses. Now, let us ponder
how a machine or a control mechanism would act or react to the physical environment around it and
function as per its design and expectations. It needs to be imparted with some sensing devices. So it
does through sensors which are devices that interact with the physical environment around them
and covert these physical parameters or changes in parameters to the equivalent signals that can be
relayed to the control system it has been designed to operate with. Sensors are widely used in
industrial applications as well as automated functions in our everyday life as shown in fig (2.1). The
simplest example is controlling water level in a closed tank fitted with a filling pump. It is easily done
with the help of level sensors which are connected to a controlling device which based on the signals
from these sensors controls the pump.
Figure 2.1
2.1 Introduction to sensors

Going back to the pre industrialization era, exploiting power of the wind or using weights or
gravitation was the only key to actuation for the simplest mechanisation in the society. Probably a
water wheel or a windmill is the closet and simplest visualization of how things used to get done in
simple yet ingenious ways during those times.
With the advent of steam and electricity, engineers did pioneering work in the field of sensing by
developing sensors that used to measure a host of physical parameters and convey them accurately
for the best control and actuation. A good example being that of measurement and control of pres-
sure through a spring release valve for control of pressure in a engine shown in fig (2.2A).
The real boon for sensing dawned by exploiting the physics behind principles of electromagnetism
shown in fig (2.2B), behaviour of light , changes in physical properties of materials due change in
temperature and pressure and likewise. However, the biggest leaps in the development of technology
and products for industrial sensing were with the inception of semiconductors and digital technology
as shown in fig (2.2C).
It shall be prudent to understand some of the physical processes and changes in physical properties of
material and environment that enable proper sensing of the processes and manufacturing that takes
place in the Industry. Various types of sensors, their working principles, uses and applications are
hence discussed in subsequent sections of this chapter.
(A) (B) (C)
2.2 History of sensors
Figure 2.2

Automation is the key to efficient manufacturing and it enables production of goods having
consistent quality, better economy with lesser demand on the resources. As we tend to automate, the
first necessity is to efficiently and accurately measure or detect the desired process or physical
parameter that needs to be controlled or automated. The solution lies in correctly identifying and
then applying the best possible fit of a sensor.
A plethora of sensors exist to detect, analyze, measure physical parameters like changes in length,
height, appearance, position, displacement etc. Many of these sensors play an important role in
predicting changes and prevention of hazards due to typical manufacturing environments like those
which deal with oils and chemicals.
Speaking holistically, in the Industrial world, the sensors are generally used for -
 Detection and measurement of physical and chemical attributes.
From the simplest need of sensing presence or absence of the desired object/material, today
sensors perform very complicated tasks of accurate measurement and communication of measured
parameters to different control systems. Most of the manufacturing or machinery functions along the
output from the controllers which are primed by the inputs from these sensors.
 Better yield and improved quality through manufacturing.
Sensors enable the automation system to closely monitor, speed up and optimize any process or
assembly thereby raising the throughput yield of a manufacturing loop. Not only do they aid in
speeding up the various processes and measurements , even the end quality of the product depends
heavily on sensors placed for these specific purposes. For example lack of accurate control of
temperature may lead to a malformed molds of plastic . Also, consider for example, lack of monitoring
the viscosity of oil may yield an unacceptable product or by products.
 Checking the quality and quantity of the end produce (Yield).
There are mandatory and compliance issues on commercially produced and sold goods. Sensors help
concurrence along the design of production process without fatigue or human error. Imagine if you
receive lesser number of candies in a box as many as claimed by its manufacturer. A sensor aptly and
consistently measures the number of candies that go into the box prior to its final packaging at the
end of the production line that produces these candies.
2.3 Needs of sensing

 Hazard control and risk management
Lot of detection and measurement centers around keeping the plant, personnel and machinery safe
during a manufacturing exercise, assembly or a process as shown in fig(2.3).
Visualize what may happen because of uncontrolled and inaccurately measured pressure—It shall
surely lead to catastrophic failures and accidents. Another example being monitoring of jamming of a
conveyer line through sensors which otherwise would lead to terrible accidents and loss of material.
Sensors play a vital role in ensuring safety of the life and limb of the operators of the machinery and
protect the machines and other assets at an industrial locale. All these are impossible without sensors
designed, tested and certified for the specific purpose of operation and safety.
Difference between sensors and transducers
People generally get confused between these two entities as they both achieve sensing and
conversion of physical parameters to measureable signals, but there is a significant difference
between these two. A sensor can be defined as the complete assembly required to detect and
communicate a particular readable signal on a standardized platform, while a transducer is the
element within the sensor assembly which accomplishes the task of conversion between the two
forms of energy as shown in fig (2.4). A simple distinction is to use the term 'transducer' for the
sensing element itself and the term ‘sensor’ for the sensing element plus any associated circuitry. All
transducers would thus contain a sensor and most (though not all) sensors would also be transducers.
Figure 2.3
Figure 2.4

Why sensing is so essential in industrial application?
The best analogy would be to equate a factory or a machine to a human being. Just as the
sensors endowed to us help our minds to make decisions in favour of the task to be achieved by
initiating the needed movement of our limbs, similarly, sensors in the industrial domain help the
needed information to be collected and relayed to the control system of the machine or the plant for it
to act and meet the desired objective.
Generic and high-precision sensors are used to detect and measure critical attributes in plants
and machinery so that a host of parameters can be validated and error-proofed automatically
in-process rather than post-process. This aids smooth manufacturing with high accuracy, reliability,
saves multiple and repeated steps in the manufacturing process, eliminates significant costs and
wastages and so on.
Some of the very common applications which are easily and importantly met by the usage of sensors
are
 Part Presence/Positioning
Verifying that a critical part is in place and in the correct position
before the next step in the process can be initiated is a common
requirement as shown in fig(2.5). Modern sensors can easily
achieve this objective and are able to detect even the smallest
and narrowest edges of the target objects. The sensors selected
must however be capable of reliably detecting the desired target
given the parameters associated with it.
 Metal detection
Many parts or metal assemblies have to be inspected with
Inductive sensors. Oftentimes the user needs to determine if any
one or more of several parts are if missing or in the proper
position in an assembly. An inductive sensors system can check
an entire part for missing or misplaced components once they
are positioned properly as shown in fig(2.6).
2.4 Industrial applications of sensors
Figure 2.5
Figure 2.6

 Image recognition
Oftentimes an entire product area must be
inspected because the flaw could be anywhere in the
defined area. Vision sensors shown in fig(2.7) are
designed to easily solve these applications with a camera
that counts pixels and then compares the count to a
pre-determined reference count.
 Manual pick to light aid and verification
Error proofing in sequential manual assembly shown in fig
(2.8) is also a huge challenge and is prone to mistakes. A
PLC-controlled "picking" system uses lights which glow
sequentially onto the item to be picked next, and a light
screen in front of each bin also verifies if the correct part
has been actually picked up. These systems help to mitigate
quality issues by reducing missing parts, and / or parts
assembled in the wrong order. Along with error-proofing
the assembly process, they also increase worker efficiency
by verifying at all times where an assembler last left off
during the assembly process, even after a break or work-
stoppage. Such a visual aid is a very effective solution which also circumvents training obstacles such
as language barriers, and technical ability. The illustration shows a warehouse application where a
worker has to sequentially pick the correct items in a particular order only.
 Measurement
 Short range measurement
With the advancement of sensing technology in the last several
years, users are now able to integrate cost effective measuring
capabilities into their processes with resolution as high as 0.0001
inch. Parts can be automatically inspected for critical attributes
prior to the next manufacturing step. In the illustration, laser
sensors shown in fig(2.9) are checking a freshly casted wheel
rotating in a fixture so as to assure that there are no voids or
excessive run-outs prior to its machining.
Figure 2.7
Figure 2.8
Figure 2.9

 Long range measurement
New, long range sensors can look inside a machine or process from a distance where a shorter range
sensor will not fit or survive (like high temperature or soiling conditions) , or is intrusive to the process.
Long range sensors can now measure very accurately distances up to several hundred meters with a
couple of millimeters accuracy as shown in fig(2.10 &11). The illustration shows a single sensor
measuring the range of motion of an automotive seat back to verify it is able to adjust in three angles
of recline, without the measurement getting affected due to material of the seat or its colour
and texture.
 Counting while filling
Packaging machines must ensure that the correct count of items are in a package to be built or
sealed before moving it down the line for further storing or transportation as shown in fig(2.12). Many
a times it should not touch or contaminate the item being counted and should count very reliably
irrespective or the shape , size and color of the target material being counted. The same gets done
aptly though a slot shaped photoelectric sensor with outstanding speed and accuracy.
Figure 2.10 Figure 2.11
Figure 2.12

There are many a perspective with which one may go about categorizing sensors. It would primarily
depend on the functionality to be derived out of the sensor and the physical environment in which it
has to operate. However, we may appropriately select sensor according to a combination of the
following factors depending upon the application -
 Physical and chemical properties of the material to be sensed
Many a times the very characteristics of the material to be sensed lends an easy way out to choose the
appropriate sensor. As an example, one may look at the color of the material being the differentiating
criteria. A color detection sensor thereby may be suitably used to detect or differentiate the material it
senses.
 The environment and operating distance
The sensors used should not get affected and also not cause a change or contamination in the
environment or the product it is expected to operate in or detect. It should be designed and classified
according its withstand capability in the typical environmental conditions like temperature, pressure,
humidity which it will get subjected to.
 Operating distance, range and contact ability
The operating principle and construction of the sensor plays a defining role on it being chosen given
the distance, range or contact ability of the media it has to measure or detect. For example one would
need a contactless measurement of level of an acid in a tank which holds concentrated acid. Any
contact with a sensor otherwise would corrode the sensor immediately.
 Material used and construction
One of the core governing factors in the choice and classification of sensors is their shape, dimension
and material of construction. Like we would have to use an inert material of construction of a sensor
to be used in a corrosive environment. On the other hand a miniature machine would ask for smaller
sized sensors without any compromise on their functionality.
 Type of the output and connection
The output of the sensor can be in binary/digitized or continuous/analog form. Generally a simple
signal of yes or no would be through the Boolean expression 0 and 1 in its appropriate electrical form.
On the other hand a continuous measurement leads to a continuous or analog output to be derived
from the sensor. Importantly—the interconnection between the sensor and the controller would be
the very determining factor on the electrical specifications and forms of output from the sensor. In
Industrial norms a step signal depending on the voltage of the power supply defines the electrical ‘1’
or ‘0’ from the sensor and DC current levels of 0/4-20mA and DC voltage levels of 0-10v or 1-5v are the
most popular for analog inputs.
2.5 Evaluation of sensors

Further, there are a lot many sensors these days which are on wireless connections or networked
through industrial bus communications directly to the controllers.
 Power supply and operating conditions like voltage variation and EM noise
Leveraging on the suitability and control system architecture, the power supply to sensors and
automation systems should be of normal and standard industrial voltage levels like
110VAC/220VAC/60Vdc/24VDC
Further the sensors are classified according to the direct and indirect electrical parameters like circuit
voltage adaptability during operation or their electromagnetic noise withstand capability, short circuit
and /or overvoltage and/or reverse polarity protection.
 Approval and certification
Approvals and certifications are the additional important classification criteria that categorizes the
sensors in context of their tested reliability, safety, ingress protection. Some very common approvals
and certifications are for example CE / UL / CSA / BIS approvals.
2.5.1 Specifications for evaluating sensors
Not only are the choice of external factors important in proper selection of sensors, certain other key
performance specifications which play a decisive role in their design, selection and operation are :
 Resolution
The resolution of the sensor is the least physical value it is capable of measuring per the least change
in the media it has been designed to detect or measure. This must also transact into a corresponding
or the desired change in the output of the sensor.
 Accuracy
The accuracy of the sensors can be defined as the difference between the measured value (value
detected by the sensor) and the expected ideal value.
 Response time
The sensors do not produce output immediately with the change in their input parameters, therefore
the time taken up by the sensors to change from its present state to its final acceptable/perceptible
value is known as response time.
 Sensitivity
This is can be defined as the minimum change in the input that can cause a significant or measureable
change in the output is called as sensitivity of the sensor.

 Repeatability
Repeatability is defined as the range of actual positions the system takes while being repeatedly
commanded to the same location under identical conditions.
 Range
The range of a sensor defines the least and the maximum value like that of length, weight, pressure
etc it can accurately measure or detect. sensors
 Sensing distance (Sn)
The maximum space for which it is possible to sense an object or it is the maxi-mum distance between
the sensing face and the object.
 Hysteresis
An artificial difference between the two switching point of the sensors. Switching hysteresis of
an sensor describes the distance between the turn-on point while approaching an object and the
turn-off point during the separation of it from the sensor. Therefore, sensor should be capable of
following the changes of the input parameter regardless of which direction the change is made.
2.5.2 Different output types associated with sensors
The output from a sensor is characteristic to the functionality desired in the application and as needed
by the architecture of the control system or the actual interfaced device.
Simple confirmation of presence or absence warrants a corresponding binary output in a relevant
connectable or readable form to the interfaced system (Like a controller) . Whereas the need to know
the actual process value (measured value) would appropriate that a respective analogue electrical
signal be sent out by the sensor. Most of the modern sensors use solid state output which uses
transistor as a switch in many a configuration ( 2 wire, 3 wire) and analogue current / voltage output
defining the corresponding process values.
Figure 2.13

Broadly speaking, some key types and explanations for the outputs associated with sensors are
 Binary / Digital output
Digital representation of ON/Off (1 or 0) is used widely for triggering alarms , actuating relays or sim-
ple yet important confirmation to the controllers or supervisory systems. It is generally a step voltage
output which defines a change from the sensors original state. Like for TTL , open collector output one
may have a voltage sourcing or sinking output typical to the power supply voltage the open collector
has been tied down to.
 NPN output
A transistor output that switches the common or negative voltage to the load. The load is connected
between the positive supply and the output of the sensor . When the output switch is ON, the current
flows from the load to ground through the output transistor. NPN output is also known as current
sinking or negative switching.
 PNP output
As shown in fig (2.14) transistor output that switches the positive voltage to the load. The load is
connected between common and output . When the switch output is ON, the current flows from
output transistor to the load and then onwards to the ground. PNP output is also known as current
sourcing or positive switching.
 Analog output
As shown in fig (2.15) contiguous voltage or current output
within the defined standards that would represent the
process value as measured. General convention is to use
0/4 - 20mA for the current type and 0/1-5/10VDC as the
voltage type. The full span ( like pressure of 0-2 bar) may
be calibrated to give an output of 0/4 mA for 0 bar of pressure as measured and 20mA as output when
the sensor measures 2 bar. (What would be the output of such a sensor when it measures 1 bar ?)
Figure 2.15
Figure 2.14

 Intrinsically safe output
Intrinsically safe output sensors shown in fig (2.16) have characteristic voltage and current values that
are kept below certain threshold levels so these sensors can be used in potentially explosive
environments. The power-limiting function is implemented in the respective field and control
apparatus which when mutually interfaced renders the plant/machinery or equipment safe by their
inability to spark an ignition or an explosion.
 Relay output /potential free contact
Many a times controllers and field conditions demand that no voltage or current be input from the
field device to other systems yet controllers be able to interrogate the field condition though state of a
relay (ON/OFF) in the field though its potential free contacts.
2.5.3 Electrical specifications for output
 NO (normally open)
For the sake of simplicity, let us take an example of an operation of a relay in which its two mechanical
contacts are not shorted thereby not permitting the flow of current through them. We can say that
this is an open circuit under normal conditions ( Normally Open) .
Once the relay operates the two contacts develop an electrical short thereby permitting the flow of
current through them. Similarly, in transistorized output of the sensors, the current is unable to flow
through the two output wires (Normally Open contact) and when the sensor senses the change it has
been designed to, analogically, these two wires get shorted internally thereby permitting flow of
current through them.
Figure 2.16

 NC (normally closed)
Now, on the other hand, consider the relay contacts to be shorted under normal condition thus
permitting the flow of current through them ( Normally Closed ). So when the relay switches , the
mechanical contacts move away from each other thereby opening the circuit and disrupting the flow of
current through them. Similarly, in case of sensors the current flows through the circuitry under normal
condition and on detection of the change by the sensor, the sensor output breaks away the
connection between the output wires thus prohibiting the flow of current.
 Maximum switching current
The amount of continuous current allowed to flow through the sensor without causing damage to its
circuitry. It is generally protected by a short circuit override which activates should the current flowing
in the output circuitry exceed its safe limits .
 Minimum switching current
It is the minimum current value, which should flow through the sensor in order to guarantee a faithful
operation.
 Maximum peak current
The maximum peak current indicates the maximum current value that the sensor can bear for a limited
period of time. This takes care of the sudden load condition or transition stages in the output circuitry
of the sensor.
 Light ON / Dark ON types of Output
The terminology is specifically used in the case of photoelectric sensors and is slightly different than
other conventional sensors because of the duality in their operation. A change in the output stage is
defined when the light is received by the receiver LED (Light ON) or when it is curtailed from the
receiver (Dark ON).

Detection of a defined target in a particular area or any movement reaching to its end or desired
position, measurement of the distance or positioning of the target, measurement of temperature,
pressure, level, current etc. are the simplest and the most common detection and measurements to
be done in the Industry. To achieve these tasks we can leverage on a variety of physical phenomena
and exploit them to deploy a wide range of sensors on the shop floor. We encounter numerous
applications of sensors in our day to day life without sometimes noticing them to be at play in the
background. Just, for an example, take note that an automated elevator uses inductive sensors to
count the number of floors it is crosses or photo sensors in action when they arrest or trigger
retraction in the movement of the doors of a lift after sensing a human being or other material in
the path of the closure of the doors.
2.6.1 Limit switches
All along the yesteryears, the simplest implementation of physical sensing or detection was done
through limit switches. Limit Switches are electro-mechanical devices fig (2.17) which consist of an
actuator arm which is mechanically linked to a set of contacts. When an object comes into contact
with the actuator, the device operates the contacts to make or break an electrical connection the
output types of which are shown in fig (2.18).
However, the usage of limit switches in the industry has been on the wane as
 The mechanical wear and tear within the switch which creeps in with time.
 It is prone to sparking and carbonization within its electrical contacts
 It needs regular maintenance of mechanical parts and a clean up of electrical contacts.
To overcome these challenges, modern automation heavily depends upon non contact sensors in-
stead of switches.
2.6 Sensors for automation
Figure 2.18
Figure 2.17

Selection Criteria Points for selection appropriate Limit Switch Reference:
a) Number of Poles & Number & nature of contact operation.
b) Preferred Material casing
c) Insulation Voltage & thermal current.
d) Type of Head- Plunger or Rotary Head.
e) Type of Plunger
f) Type of Operator
g) Type of cable entry connection.
h) Degree of Protection
i) Operating Temperature
j) Application usage.
6.2 Magnetic sensors
Magnetism is a naturally occurring phenomenon which can be well utilized for detection of
presence and absence of a magnet or in its advanced versions be used for accurate positioning .
 Operating principle
Magnetic sensors detect the change in the linkage of the magnetic lines of forces as from a defined
magnet shown in fig (2.19) and derive information on parameters such as direction of movement,
presence/absence and angle of rotation with relation to a defined magnet . Magnetic field sensors
are used to detect the presence of magnets (electromagnetic and permanent magnets) or
ferromagnetic objects. Permanent magnets are predominantly used in automation technology as
they can be used without a power supply.
S N
Figure 2.19

Magnetic sensors have a longer operating range and are easily mounted, without the need for
mounting slots or holes. In addition, they can be fully encapsulated inside metal housings. This opens
up new areas of application, especially in automation technology and automotive engineering.
Some typical Magnetic Field Sensors considerably simplify the detection of position of pistons inside
the thick-walled hydraulic cylinders. In such applications, the magnetic field sensors render
ferromagnetic materials ‘‘transparent’’ so that actuating components can be detected through the
thick cylinder walls.To achieve this task, a permanent magnet is entrenched at the side of the head
of the piston and a magnetic sensor is placed outside the cylinder as shown in fig no. (2.20). Simply
speaking, as soon as the piston reaches to the end of a stroke the sensors detects the presence of the
permanent magnet through the cylinder walls and it switches the output accordingly.
Another application of these sensors includes testing for advanced magnetic data storage media in
hard disk drives as well as forensic analysis of conventional media from cassettes and other
ubiquitous magnetic data storage media.
2.6.3 Inductive sensors
Inductive sensors are one of the most effective, technically viable and commercially popular solution
for reliable and non-contact detection of ferrous targets for ranges up to 100 mm. Owing to their
reliable and rugged construction, inductive sensors are extensively used across the board throughout
the spectrum of Industrial automation and including very demanding and tough operating
conditions.
Figure 2.20

As the name suggests, these sensors operate under the simple principles of electromagnetic
induction. The first law of electromagnetic induction states that an EMF in induced in a conductor
when it passes through a varying magnetic field. And so is put to a very effective use for sensing .
Basic construction of an Inductive sensor constitutes four basic elements shown in fig (2.21)
a) Coil
b) Oscillator
c) Detection circuit
d) Output circuit
To create a varying magnetic field, these sensors have a tight coil wound around a concentric ferrite
core. An oscillator pushes a high frequency alternating current into this coil which generates a
fluctuating doughnut shaped magnetic field around the winding of the coil that emanates from the
sensing face as shown in fig(2.22) .
Figure no.-
Figure 2.21
Figure 2.22

When a ferrous metal object moves into this alternating field, eddy currents get induced in the
metallic object. As per the Lenz’s law, these eddy currents so induced flow in such a direction so that
they oppose the very alternating field which produces them. So the magnetic field of the eddy
currents push back the alternating field of the sensor by increasingly damping it and forcing the
oscillator to stall. A trigger circuit designed to monitor the oscillator’s strength gets activated under
such conditions and triggers the output circuitry. The output circuit then switches its state to target
present state.
 Types of inductive sensor
Inductive sensors may be primarily distinguished based on the criteria of coil winding which in turn
effects the shape of the magnetic field produced by them thereby influencing the sensing around
the sensor and its sensing range.
a) Shielded b) Unshielded
Shielded inductive sensor uses a ferrite core to direct the coil’s magnetic field so that it emanates
only from the front of the detection face of the sensor as shown in fig(2.24). On the other hand, in
an Unshielded inductive sensor, a peeled back ferrite core shielding allows for a longer sensing
distance yet also allows targets to be sensed along the sides of the face of the sensor as shown in
fig(2.25).
Figure 2.23
Figure 2.25
Figure 2.24

Industrial applications using inductive sensors.
The simplicity and reliability in working of an inductive sensor renders them very welcome to
Industrial environments so they are casted into a host of shapes, sizes and variants like
 Cylindrical
The sensing field is in the front of the sensing device as shown in fig(2.29). The sensor is activated
when a target enters the sensing field in an axial or lateral direction.
Jam detection
Figure 2.26
Monitoring speed of a conveyor belt
Figure 2.27
Figure no.-2.29
Transfer station
Figure 2.28
Sealing machines
Figure 2.25

Limit Switch Style
This sensor shown in fig(2.30) contains a sensing face that is field changeable to any one of five
positions, from front to top to both sides to bottom. This sensor has the same mounting dimensions
as a standard style limit switch, therefore, this sensor is a replacement for electro-mechanical limit
switch.
 Surface mounted / rectangular
Depending on the surface mount sensor that is used, some sense from the top and some from
the side shown in fig(2.31).
 Slot
The sensing field is concentrated between two coils on a common axis shown in fig(2.32). The
sensor is activated when a metallic object (target) enters the area between the coils.
 Ring
The sensing field is concentrated inside the ring shown in fig(2.33).
The sensor is activated when a metallic object (target) enters the ring.
Figure 2.31
Figure 2.32
Figure 2.33
Figure 2.30

 Operational criteria
 Sensing Range
One of the major factor for selecting an application specific inductive sensor is sensing range. The
sensing range of an Inductive sensor is affected by the :
Target composition : In the world of inductive proximity sensors, not all metals are treated
equally. Different metals exhibit different quantum of induced currents and behaviour and so the
sensing distance of an inductive sensor varies with different metals and alloys. Inductive proximity
sensors will detect non-ferrous metals such as aluminium better than they sense iron. “All metal
sensing” inductive proximity sensors will detect all metallic materials at the same sensing distance.
Examine the sensing distance reductions for typical inductive proximity sensors below.
Stainless Steel=Standard Sensing Distance * 0.8
Brass = Standard Sensing Distance * 0 .5
Aluminium = Standard Sensing Distance * 0 .4
Copper = Standard Sensing Distance * 0 .3
Target size : For Inductive sensors an ideal target size is equal to diameter of the proximity switch
or 3 times its nominal sensing range whichever is greater. This can be easily understood by follow-
ing the example-.
If Sensor diameter is 18 mm and its sensing range is 5 mm then 3 x sensing range is 15mm which is
less than its diameter. Therefore, target should be 18 x 18 x 1 mm. (The same are defined with a mild
steel target sheet approximately 1mm thick as an industrial standard).
Switch mounting : The switch mounting depends
on the type of the inductive proximity sensor
discussed in earlier section. The example
diameter shown in fig(2.34) shows a shielded
sensor flush mounted in a metal plate and an
unshielded sensor mounted in a metal plate
with dimensions for safe installation. Figure 2.34

 Important variants of Inductive sensors
 Reduction factor 1 sensor
Reduction factor 1 sensors are capable of detecting all kinds of metal targets within the same
sensing distance and over significantly greater operating distances than what is possible with the stan-
dard inductive sensors. As a result the users benefit from greater flexibility, increased pro-
ductivity and reduced operating costs to optimize machine designs in virtually any application and are
free to use combination of different metal types and alloys.
 Inductive analog output sensor
Inductive analog sensors give current (0/4-20mA) or voltage (0/1-5/10vdc) output corresponding to
how far or near or wherever the defined target is within their sensing range. As the target further
changes its position within the sensing range, the sensor output changes in the same proportion.
 Linear and rotary displacement Inductive sensors
If instead of single coil, multiple coils as a system get organized and controlled through electronics,
one can measure liner and rotary displacement of defined target along the range of the sensor as
shown in fig (2.35). And so the linear and angular displacement as output as analog voltage or current
find high utility in the industry.
Figure 2.35

2.6.4 Capacitive sensors
Capacitive sensors shown in fig (2.36) are very similar to inductive sensors. The main difference
between these two types is that the capacitive sensors produce an electrostatic field instead of an
electromagnetic field. Therefore, capacitive sensors are capable of sensing metallic as well as
nonmetallic materials such as papers, glass, liquids, food items, cloth etc.
Capacitive sensors work on the basis of change in capacitance which happens when free air in front of
the sensor is replaced by any other substance. The sensing front of a capacitive sensor is formed by
two concentrically shaped metal electrodes of an unwound capacitor. When an object increasingly
nears the sensor, it enters the electrostatic field of the sensor and changes the capacitance as seen by
the circuitry of the sensor oscillator circuit. The prime reason of it is the change in the dielectric of the
insulator of the capacitor getting formed in front of the sensor. The capacitance changes when
compared as between free air and any other material occupying that free space subsequently. As a
result, the oscillator begins oscillating as shown in fig(2.37) .The trigger circuit reads the amplitude of
produced oscillation and when it reaches a specific yet adjustable threshold level and it then switches
the output state of the sensor.
Figure 2.36
Figure 2.37

Capacitive sensors can be very effective in detecting presence, measuring density, thickness, and
location of non-conductors as well. The capacitance changes in relationship to the thickness or density
of the material. Additionally the dielectric constant determines how different non-conductive
materials also affect capacitance between two conductors. And so the sensing distance of different
material with the same capacitive sensor will be different .
Standard targets are specified for each capacitive sensor. The standard target is usually defined as
metal and/or water. Capacitive sensors depend on the dielectric constant of the target. The larger the
dielectric number of a material the easier it is to detect. The graph in fig (2.38) shows the relationship
of the dielectric constant of a target and the sensor’s ability to detect the material based on the rated
sensing distance (Sr).
The property of capacitive sensor to detect both conductors and non conductors opens a wide scope of
applications in the industrial environment .
 Level detection and empty pouch detection
Capacitive sensor reliably detect level even through a barrier like water in a plastic bottle or filled
material inside a plastic/thin film pouch. For example, water has a much higher dielectric than plastic.
This gives the sensor the ability to see it through the plastic as shown in fig (2.39 & 2.40) .
Figure 2.38

 Mounting considerations
Capacitive sensors provide ranges from 1mm to 50mm and some models are available with
potentiometers too.
Standard Capacitive Sensor Product features includes:
a) Cylindrical & surface mount housing styles
b) 12, 18 and 30 mm cylindrical styles of stainless steel or plastic housings
c) 5mm thin rectangular and long range 80 mm x 80 mm x 40 mm styles
d) Sensor outputs of 3-wire DC and NAMUR output types
e) Models with hazardous area approvals
f) 10-30VDC & 115/ 230VAC rated models
 Shielding
Shielded sensors will detect conductive material such as copper, aluminium, or conductive fluids, and
nonconductive material such as glass, plastic, cloth, and paper. Shielded sensors can be flush mounted
as shown in fig(2.41) without adversely affecting their sensing characteristics. Care must be taken to
ensure that this type of sensor is used in a dry environment.
 Other operating criteria
 Target Size
The target size is a primary consideration when selecting a probe for a specific application. When the
sensing electric field is focused , it creates a slightly conical field that is a projection of the sensing
area. The minimum target diameter for standard calibration is 30% of the diameter of the sensing
area.
Figure 2.41

 Range of Measurement
The range in which a probe is useful is a function of the size of the sensing area. Greater will be the
area, the larger the range. The driver electronics are designed for a certain amount of capacitance at
the probe. Therefore, a smaller probe must be considerably closer to the target to achieve the desired
amount of capacitance.
 Target material
The sensing electric field is seeking a conductive surface. Provided that the target is a conductor,
capacitive sensors are not affected by the specific target material.
Because the sensing electric field stops at the surface of the conductor, target thickness does not affect
the measurement.
The electronics are adjustable during calibration but there is a limit to the range of adjustment. In
general, the maximum gap at which a probe is useful is approximately 40% of the sensing area
diameter. Standard calibrations usually keep the gap considerably less than that.
 Maximizing accuracy
Now that we’ve discussed the basics of how capacitive sensing works, we can now form strategies for
maximizing effectiveness and minimizing error when capacitive sensors are used. Accuracy requires
that the measurements be made under the same conditions in which the sensor was calibrated.
Whether it’s a sensor calibrated at the factory, or one that is calibrated during use, repeatable results
come from repeatable conditions. If we only want distance to affect the measurement, then all the
other variables must be constant. The following sections discuss common error sources and how to
minimize them.
a) Target Size
b) Target Shape
c) Surface Finish
d) Parallelism
e) Environment

2.6.5 Photoelectric sensors
A photoelectric sensor is a device that detects a change in the intensity of light focussed onto its
receiver. The LEDs which are the light sources used by these sensors emit light in the range of visible
green to invisible infrared as shown in fig(2.42) of the light spectrum. A lot many photo sensors are in
use in the Industrial and commercial domain. If we look around we shall see them helping to safely
control the opening and closing of doors of the lifts, garage doors, turn on the sink faucets with the
wave of a hand and even to detect the high speed winning car which reaches the finish line first at the
racing events.
The easiest way to describe the operating principle of a photoelectric sensor would be the detection of
the typical emitted beam of light which is sent out by its emitter, which in some fashion is directed to
and detected by the receiver transistor . So, to simplify, any photoelectric sensor is made of a light
source (Emitter LED), a receiver (phototransistor), a signal converter, and an amplifier shown in fig
(2.43).
An LED is the prime component used to radiates visible red/green or infra-red light. The emitter
circuitry of the photoelectric sensor uses this LED to emit light signal in the form of pulses along a
fixed frequency. This light gets reflected back through a dedicated reflector or the target to be sensed
itself. The receiver circuitry uses a photo transistor within the sensor to then receive back this light and
evaluates it for the designed output.The signal received is amplified and is synchronized with the
generator signal. This mode of signal evaluation results in a distortion reduction. The signals will be
integrated in the comparator and afterwards amplified.
Figure 2.42

Figure 2.43
Understanding the differences among the available photoelectric sensing modes is the first step
toward determining which sensor will work best for an application . The reflectivity and transparency
of the targets towards the incident light beam are different. Some targets are plainly opaque while
others including the ones that may be black in colour may be highly reflective. In part, the best
technique to use depends on the optical nature of the target. The optical system of any conventional
photoelectric sensor is designed for either one of three basic sensing modes:
a) Thru-Beam
b) Retro-reflective
c) Diffused
 Thru–Beam photoelectric sensors
Thru-beam mode is also called as the opposed mode. It is the most reliable method of detection of
presence of a target using the photoelectric sensors. This mode uses two separate entities, one for the
emitter and the other one for the receiver shown in fig(2.44). The light from the emitter is aimed at the
receiver. Under the normal circumstances, the light continuously falls on the receiver and whenever
the target to be detected falls into the path of this light beam, the beam as received by the receiver
gets interrupted. The receiver circuitry therefore activates the binary output signaling presence or
detection of the target which was otherwise absent.
Figure 2.44

This mode is one of the most reliable modes, and allows the longest possible sensing ranges for the
photoelectric sensors. However, the challenge arises in the installation and alignment of an infra red
sensor comprising separate emitter and receiver in two opposing locations, which may be quite a
distance apart. One of the very readily seen application is detection of personnel or lets us say a car
moving into a parking slot/washing area/service bay and also checking presence of paper or cloth in a
set location as shown in the fig (2.45) and fig (2.46)
Thru-beam mode sensors are available in a variety of styles. Other than the most commonly used style,
which has separate housings, different types of styles are also available, like “slot” or “fork” photoelec-
tric sensors and Light grids.
An added bonus to through-beam photoelectric sensors is their ability to effectively sense an object in
the presence of a reasonable amount of airborne contaminants such as dirt. Yet if contaminants start
to build up directly on the emitter or receiver, the sensor does exhibit a higher probability of false
triggering. To prevent false triggering from build up on the sensor face, some manufacturers
incorporate an alarm output into the sensor’s circuitry. This feature monitors the amount of light
arriving on the receiver. If the amount light decreases below a certain level without a target in place,
the sensor sends a warning out by means of a built in LED and/or an output wire.
 Slot Type Sensor
Slot type photoelectric sensor shown in fig(2.47 & 2.48) is a special design of a through-beam sensor
called a slot sensor. It incorporates both transmitter and receiver into one housing and is used where
only a short sensing distance is needed.

A very important application of a thru beam photoelectric sensor is safety light curtains. In this, instead
of a single sensor we use array of sensors mounted on an aluminium profile which can be used for
multiple operations some of which are discussed below -
Selection criteria for Photoelectric sensor :
1) sensor diameter
2) system (diffuse/reflex polarized/reflex/thru beam/ multimode)
3) output type (PNP/NPN/ Relay)
4) output function (NO/NC)
5)Connection (cable, connector, terminal)
6) Type of housing required- Plastic / metal
7) sensing distance
8)Supply Voltage , wiring technique - Two wire / Three wire
 Safety light Curtains
Automated processes require increased operator protection and accident prevention. Safety light
curtains are an advanced method of safeguarding personnel around many hazardous machines. Also
called light screens, optical guards, and presence sensing devices, safety light curtains offer freedom,
flexibility and reduced operator fatigue when compared with traditional guarding methods such as
mechanical barriers, sliding gates and pull-back restraints. A photoelectric transmitter projects an array
of synchronized, parallel infrared light beams to a receiver unit shown in fig (2.49). When an opaque
object interrupts one or more beams the control logic of the light curtain sends a stop signal to the
guard machine and a takes a necessary action according to controller.

Figure no.-2.51 Figure no.-2.52
The Retro-reflective range is the distance from the sensor to the Retro-reflective target. The effective
beam is usually cone shaped and connects the periphery of the retro sensor lens (or lens pair) to that
of the Retro-reflective target.
A reason one would suggest a retro-reflective sensor over a through beam is because only one location
needs to be wired for installation. The opposing side simply requires installation of a dedicated
reflector. This could result in easier installations and be a good help where one encounter space
constraints.
Retro reflective sensors are very commonly used in a host of industries owing to certain inherent
advantages. They reliably detect targets irrespective of their colour or reflectivity. Even shiny targets
can be reliably detected by using polarised filter reflector so that sensor triggers through absence/
presence of light reflected only through such a specific reflector. The sensitivity of a retro reflective
sensor can be so tuned so as to measure even very minor changes in light when passing through a very
finely transparent material. This renders them to be of good use in detection of very transparent
material like clear glass or thin plastic films. Some of the applications we generally encounter in our day
to day life are control of toll gate at highways, fig (2.51) and visitor count at the gates of mall, airports
fig (2.52).

 Diffuse photoelectric sensor
Another very popular and useful method by which object detection can be carried out using photo
sensors is diffuse mode. The sensors using this mode are very popular and perform very well in a
wide range of industrial processing applications. Here too the emitter and receiver are in the same
housing .The advantage is that a secondary device, such as a reflector or a separate receiver, is not
required.
In such a sensor an object directly in front of a sensor is detected by the reflected light back by the
object onto the receiver of the sensor. In this mode, the emitted light strikes the surface of an object
and is then diffused at many angles as shown in fig(2.53). Even when the receiver is placed at an
arbitrary angle, at least a small portion of the diffused light gets detected.
The diffuse mode is simple in operation however, suffers from variation in performance should the
colour of the target change. They also get dramatically influenced by the reflectivity of the surface
that is being sensed. The extremes being, a bright white surface will be sensed at a greater range than
a dull black surface. Most diffuse-mode sensors use lenses to collimate (i.e., make parallel) the emitted
light rays and to gather more received light. Although lenses extend the range of diffuse sensors, they
also increase the criticality of the sensing angle to a shiny or glossy surface.
To tide over the challenges of the reflectivity and target type, the sensor is designed to work with
adjustable sensitivity. Exploiting the properties of diffusion sensors industries use the for achieving
their task such as detecting the polarity of capacitor, a capacitor can be placed in a circuit board in one
of two orientations. Since they are polarity-sensitive, an inspection process is needed to ensure proper
insertion. The diffuse sensor shown in fig (2.54) look for the polarity marks on the side of the capacitor.
The sensor is programmed to output if the mark is not detected another application of diffusion
sensors can be Ink- jet printing registration, bottles are channeled through guide rails shown in fig
(2.55) ,the diffuse mode sensor consistently triggers the printer to provide accurate printing
registration at the same point on the circumference of each bottle.
Figure 2.53

Detecting polarity of capacitor Ink- jet printing registration
 Diffused Mode with Background Suppression
Interestingly, there are quite a few variants to the diffused photo sensor such as fixed-focus and
sharp-cut off modes. They are among what we may say “Background suppression functionality”. A true
background-suppression photo sensor is designed specifically for applications which require the
sensor to see a target very close to a reflective background. This background suppression is particularly
effective when the target and background have similar reflectivity shown in fig(2.56) (e.g., light
reflected back to the sensor from the target is roughly equal to the light reflecting from the
background) or when dark targets are to be sensed against a lighter, more reflective background. This
functionality which when used enables the sensors to detect targets which under normal
circumstances or by use of normal diffused sensors may not get sensed reliably. Background
suppression technology, in its true form, uses light triangulation to create a distinct focal plane that is
the effective sensing area. Targets beyond the focal plane will not be detected. Unlike fixed-focus and
sharp-cut off sensors that achieve background suppression through their inability to see the
background, true background-suppression sensors actively sense both target and background.
Figure 2.56
Figure 2.55
Figure 2.54

 Diffused Mode with Background Evaluation
Another interesting mode of operation is the “Background evaluation photoelectric sensor“ which
works by establishing a light path to a reference background, object and back to the sensor. Targets are
detected when they pass in front of the sensor and disrupt the light path. Even rounded or curved
targets that otherwise may reflect light away from the sensor, and targets with poor reflectivity, are
reliability detected. This is the opposite principle to that of Background Suppression (BGS). In contrast
to BGS they are tested and can be designed to be self-monitoring. Such sensors have no blind area in
their sensing range and are more suitable for detecting difficult objects, especially the ones that are
highly reflective. They are engineered to detect all targets regardless of color or shape, making them
well suited for use in material handling and packaging applications.
 Special types of photoelectric sensors
 Contrast / Print mark sensors
Contrast sensors as shown in fig(2.57), also called as color mark or print mark
sensors, detect the difference between two colors often corresponding to a
target color and a background color or change of contrast between two shades
of the same color. At the heart of this sensor is a microprocessor which controls
three different color LED’s namely Red, Green and Blue (which are also the
naturally occurring colors and all other colors being the different derivatives of
the mix of these) . The sensor is taught its reference value of the contrast which is combination of RGB
light as stated earlier. When the sensor faces the target, it then compares the actual RGB light as
reflected by the target to the reference RGB light it has been taught to switch its output state to the
desired level. Such a sensor operates through number of lenses and detects the target only form a
fixed pre determined distance.
 Color sensors
Color is an obvious and an important product characteristic. Recognition and
reliable detection of colors play an increasingly important role in industrial
automation. Just like the contrast sensor, a color sensor shown in fig (2.58) also
works on the principle of RGB light detection. However, instead of a change in
the RGB mix in the reflected light from the target as in the earlier case, the
color sensor looks for the specific mix of the RGB light it has been programmed
to and enables the switch in its output. Color sensors can be applied for sensing color marks at a fixed
distance, detecting colored objects at variable distances.
Figure 2.57
Figure 2.58

 Fibre optics photoelectric sensors
Fibre optic photoelectric sensors consist of similar circuitry as that of other sensors with transmitter
and receiver (sensing head) ,but with a difference that the light from transmitter to receiver and back
to the transmitter is guided through an optical fibre cable as shown in fig(2.59) .
 Principle of operation
The light source (a LED) transmits the light beam down the fibre optic cable
by repeatedly reflecting the light off the boundary between the fibre core
and its sheath. When it reaches the end of the fibre the light it gets
dispersed. When the light gets dispersed it spreads out and forms a beam
much like as that of other sensors, but on a smaller scale, with smaller light
source and lens area the sensing range is on the whole much shorter.
Fibre Optics systems have small optically active area, which makes them
suitable for detecting small details of near applications. Due to large
opening angle of the light aperture of the optical fibre, fibre optics is
generally used for shorter distances.
Fibre Optics systems have small optically active area, which makes them suitable for detecting small
details of near applications. Due to large opening angle of the light aperture of the optical fibre, fibre
optics is generally used for shorter distances.
 Sensing head with fibre optic cable
 Plastic fibre optic
 Glass fibre optic
 Amplifier
Each fibre optic requires a powerful analyser unit. Amplifiers are available in various designs depending
on the requirement and applications.
Figure 2.59

 Distance Measurement
Determining distances is one of the most common metrological applications in automation technology.
With a long measuring range, the sensor reliably detects distance in diffuse mode using Pulse Ranging
Technology (PRT) as a measuring principle. A laser diode is employed to emit brief light pulses, which
are reflected back by the target and gets detected by a light-sensitive receiver shown in fig(2.60).
Figure 2.60
Pulse Ranging Technology (PRT) can be used in industrial sensors of small sizes for a wide range of
commercial applications. Unlike indirect processes such as phase correlation and analog chip-based
processes, the time of flight is measured directly. The first distance measurement sensors with pulse
ranging technology available to the market boast outstanding performance data that clearly
demonstrate the superior nature of the process.
Success at the speed of light! The superiority of this technology lies in the power density of the light
pulses, which is 1000 times greater than in sensors with constant light sources. The benefits that this
technology delivers include large measurement ranges, high detection ranges, and absolute levels of
precision. Negative influences such as extraneous light or different reflection characteristics cannot
impact the function of PRT sensors. PRT—the technology for high quality results.

 Accuracy
a. Direct measurement method delivering precise, reliable, and clear measurements
b. Greater accuracy over longer distances
c. No offset of the measured value during prolonged operation
d. Clear measurement result, even when several targets are present in the detection range.
 Immunity
a. High degree of immunity to extraneous light
b. No mutual interference
c. Minimal susceptibility to changes in the measurement path caused by environmental influences
d. Reliable suppression of interfering influences such as dust or fog
e. Reliable blanking of objects in the distant background
 Insensitivity
a. Little influence on the properties of the object, practically no black-white difference
b. Reliable operation, even in applications in the frozen storage sector at temperatures to -30 °C
c. Fully capable of detecting multiple targets, can be used in safety-critical applications
 Future trends
These reflection light beam switches are available with various types
 Polarization filter
A polarizing filter is utilized to eliminate false signals that may occur if a shiny target passes in front of
the retro-reflective sensor.
 Clear object detector
The clear object detector is a special retro-reflective mode photoelectric sensor that detects clear
objects by the use of a low hysteresis circuit, the sensor detects small changes in light typical when
sensing clear objects.
 Foreground suppression
The foreground suppression sensor is a retro-reflective mode photoelectric that can identify glossy
targets as the reflector when they are within a certain distance.
 Area sensor
With several transmitters and receivers in single housing, area sensor forms a continuous wide or high
detection area over the relevant sensing range.

Transducer
Decoupling layer
Integral skin foam
Piezo-ceramic
2.6.6 Ultrasonic principle
Sound waves with a frequency over 20kHz are inaudible to the human ear. Sound waves with such a
frequency profile are called as ultrasonic waves and the sensors that exploit these waves as the basis
of their operation are called ultrasonic sensors. Ultrasonic waves travel at a velocity of 330 m/s
through the air at 25 Deg C which incidentally is the same for audible sound. By determining the run
time (the time a signal moves from the transmitter to the target and reaches back after reflection)
the distance of an object can be precisely measured (because velocity of sound at the measured
temperature is standard and known).
Ultrasonic sensors shown in fig(2.61), measures the time taken up by the sound to travel from
sensor to target and back to sensor (direct detection) or check whether the transmitted signal has
been received by a separate receiver (thru beam detection). A decoupling layer of special material
(ceramic) is used to decouple the ultrasonic from air. Ultrasonic sensors uses piezoelectric transducer
to send and receive high frequency sound signals and convert the echo pulse into a voltage. The
integrated controller computes the distance from the echo time and the velocity of sound. When a
target enters the sound cone the sound is reflected back to the sensor, causing it to energize or
de-energize the output circuit. The ultrasonic sensor detects objects within its sensing range,
regardless of whether these objects approach the sensor axially or move laterally through the sound
cone. The ultrasonic transducer is encapsulated in a polyurethane foam and forms part of an indus-
trial housing with proper connection and mounting elements to shape a well performing ultrasonic
sensor.
The transmitted pulse duration ‘t’ with the delay time of the ultrasonic transducer results in the
Figure 2.61

creation of an blind area in which the ultrasonic sensor can not detect an object within its sensing
range.
Types of ultrasonic sensors
Ultrasonic Sensors can be classified into three types based on their principle of operation. These are:
1. Through beam
2. Retro reflective
3. Direct detection
 Thru-beam ultra sonic sensors
Thru-beam ultrasonic sensors consist of a emitter and a receiver located under different housings as
shown in fig(2.63), thus, it is also called as a double head type. If the beam between the transmitter
and the receiver is interrupted by the target the output of the receiver switches state. Therefore,
object acts as the sound reflector.
Thru beam ultrasonic sensors are widely used in industries due to their ease of use and large sensing
range. A very interesting variant of such a sensor in the thru beam mode is the one that even measures
the intensity of the ultrasonic waves which may pass through the targets which may come between the
Figure 2.63
Figure 2.62

Popularly called as Double sheet detector as shown in fig(2.64). The waves at the receiver end are
evaluated by a microprocessor which sets the appropriate switching output depending upon its
programming for the target being single or double sheet.
 Retro – Reflective ultrasonic sensor
In retro reflective mode of ultrasonic sensors emitter and receiver are mounted in the same housing,
therefore, it is known as single head type. The ultrasonic beam is reflected back to the receiver by
the target as shown in fig(2.65). Objects entering the sensing range are detected by the changes in
the measured distance or by reflector signal loss due to absorption. It has high detection reliability of
objects even with sound absorbing targets or objects with angled surfaces.
The applications of retro reflective ultrasonic sensors are finding large acceptability in the industrial
environment as they tend to reduce the errors in manufacturing and packaging processes. Level
detection of solids and liquids within closed or open tanks remains to be the most popular solution
using ultrasonic sensors within the industry. Figure(2.66) depicts an example of detection of height
of the containers on a running conveyor belt enabling their routing to the appropriate packaging
area.
Figure 2.64
Figure 2.65

 Direct detection ultrasonic sensors
This type of sensor is also called as a single head type as the emitter and receiver are mounted in a
single housing shown in fig(2.67). Objects, traveling in any direction into the operating range of the
sound cone, will cause the sensor output to switch its states. The sensing range depends on the
reflection attributes of the object like surface and angle deviation. These influences can be
compensated by sensitivity adjustment.
The direct detection method has found its versatile
applications in automated industries one of which is
lubrication of plants, an ultrasonic sensor shown in fig
(2.68) is applied to cover of the grease reservoir in a way
that the actual filling level can be monitored continuously .
Due to the TEACH-IN (the output switching points are
taught by the microprocessor) of the adequate switching
points, both overspill and run-dry prevention are ensured
reliably. Due to the sensors modular mechanical
construction, connector types (threads and flanges) can be
realized according to the need of the applications.
Figure 2.68
Figure 2.67
Figure 2.66

 Mounting considerations
 Target characteristic
Various operations performed by ultrasonic sensors depends upon the characteristics of the target
to be sensed which are as follows-
A) Absorption
Acoustically hard materials shown in fig(2.69), such as plastic, metal, stone,
wood, liquids, have good reflection and poor sound attenuating properties.
Acoustically soft materials, such as textiles, foam, fur or foam rubber have poor
reflection and good sound attenuating properties. The detection range of an
ultrasonic sensor depends on the material of the object .
to be detected. The effect of these influences and whether poorly reflecting objects can be reliably
detected must often be determined by trial and error.
B) Angle
Maximum permissible angular deviation from the sensor axis is + 3° shown in fig(2.70) if the object is
smooth (directional reflection) and a larger angular deviation is possible if the object surface is
rough.
The definition of smooth/rough surface depends on the wave length ‘L’ of the ultrasonic signal as
well as on the operating principles and technology used.
If surface roughness >> l, it produces diffuse reflection from the target.
If surface roughness << l, then the regular deviation takes the form of directional reflection which is
also called as ”mirror effect “.
Figure 2.69
Figure 2.70

C) Wave Bending
Echoes from the target edge are possible. Objects that are smaller than the wave length of the
ultrasonic signal cause bending. If customer complaints the output of sensor is fluctuating, the sharp
edge of interference may be a reason.
D) Reflection
Reflection is the function of roughness / smoothness of the reflecting material. Material having
intermediate reflective surface produce a mix of reflection and scattering. More rough material lead to
a reduced sensing range but higher tolerance to angular deviation
 Temperature Influence
Sound wave needs a medium to move, air is the medium used by ultrasonic to move. For a correct
measurement of the velocity of air must be known.
Because of this reason most of the sensors have got temperature compensation implemented. For
temperature compensation a temperature sensor is used, with such a sensor an absolute accuracy of
± 2% can be reached. But for successful implementation of temperature compensation, one factor
should be carefully monitored, which is self heating of the sensor.
 Environmental Influence
Sound travel time can be affected by physical properties of the air. This in turn can affect the preset
operating distance of the sensor .
 Other operational considerations
 Operating modes of ultrasonic sensors
Following are 4 operating modes of operation in ultrasonic sensors-
A. Switching point operation
In this operation mode, Sensors with independent switching points change their output state when the
object passes the switching point A1 or A2 shown in fig(2.71). Within the sensing range these
switching points are adjustable and teachable.

A1 Switch O/P 1
Figure 2.71
B. Window operation mode
In this operation mode the output of the sensor changes, if
the first incoming echo (normally caused by the object) is
coming from any point inside the switching window.
The window borders are adjustable. If there is more than one
echo reaching the sensor at different moments and one of them is
closer to the sensor than A1 shown in fig(2.72), the sensor doesn’t switch even when other signals
are coming from inside the switching window. The sensor always evaluates only the first incoming
echo signal.
C. Hysteresis operation (Latch) mode
In and outside of the hysteresis-window the
sensors output condition doesn’t change. An
approaching object causes an output change at
point A1. This output returns when the object
breaks away farer than point A2 or it enters the blind
zone shown in fig(2.73). Both switching points A1 and A2 form a wide switching hysteresis. This
mode suits for many applications like level monitoring. In this mode you need only one output
whereas two outputs would be necessary in switching point operation mode.
D. Field monitoring mode (Area monitoring)
The ultrasonic sensor monitors the evaluation window. The output switches only if an object is
detected in the window. Echoes other than those from the evaluation window are ignored by the
sensor software. This is due to the active masking of the foreground in the area of monitoring mode;
echoes from the areas outside of the switching window (foreground) do not cause interference.
A2 Switch O/P 2
A1
A2
Figure 2.72
Figure 2.73

For the selection of Ultrasonic sensor , please find the selection criteria below :
1.) Sensing distance ??
2.) Diameter of the sensor ??
3.) Type of output ( PNP / NPN ) ??
4.) Degree of protection ??
5.) Function ( NO / NC ) ??
6.) Supply voltage ?? ( 12- 24 V DC )
Some of the most common and important sensing happens around measurement of voltage or current
in a circuit or for establishing of position of a target in an area. Current/Voltage transducers, LVDT and
Hall effect devices come to the aid in meeting challenges along these lines of applications.
2.7.1 Current transformer
A current transformer (CT) is used for measurement of alternating electric currents flowing in a wire.
Current transformers, are also known as instrument transformers. When current in a circuit is too high
to apply directly to measuring instruments, a current transformer reduces current accurately propor-
tional to the current in the circuit, which can be conveniently connected to measuring and recording
instruments. Current transformers are commonly used in metering and protective relays in
the electrical power industry.
Like any other transformer, a current transformer has a primary winding, a magnetic core and a sec-
ondary winding as shown in fig(2.74). The alternating current flowing in the primary produces an alter-
nating magnetic field in the core, which then induces an alternating current in the secondary winding
circuit. An essential objective to be met in case of current transformer is to ensure the primary and sec-
ondary circuits are efficiently coupled, so that the secondary current is linearly proportional to the pri-
mary current.
Figure 2.74
2.7 Other sensors

2.7.2 Potential transformer
Potential transformers shown in fig(2.75) are too
called as instrument transformers. They allow the control
devices to take readings from the higher voltage side
and reciprocate proportionately to the safer and instrumenta-
tion compatible levels. Primary of this transformer is con-
nected across the phases or to the ground depending upon the
requirement. Just like the transformer, it used for stepping down
purpose.
 Operating design
The potential transformer works along the same principle as of other transformers. When the system
high voltage is applied across the terminals of primary winding then proportionate secondary voltage
appears across the secondary terminals of the PT. It converts voltages from high to low. It will take the
thousands of volts behind power transmission systems and step the voltage down to something that
control circuitry or devices can handle.
These transformers work for single and three phase systems, and are attached at a point where it is
convenient to measure the voltage. They lend a good hand in motor control and electrical system
balancing.
2.7.3 Hall effect sensors
A Hall effect sensor is a transducer that alters its output voltage in response to a magnetic field. Hall
effect sensors are used for switching, positioning, speed detection, and current sensing applications.
When a current carrying conductor is placed in a magnetic field, at right angle to the path of the elec-
trons, the electrons are deflected from its straight line path, this is the operating principle of Hall effect
sensors. Therefore, one side of the conductor becomes negative portion and the other side becomes
positive one. The charge separation continues until the force on the charged particles from the electric
field balances the force produced by magnetic field. The transverse voltage measured is known as Hall
Voltage as shown in figure no.(2.76). If the current is kept constant, then the Hall voltage is a measure
of change in magnetic flux density.
 Types of Hall effect sensor
There are two forms of Hall effect sensors
 One is linear where the output voltage linearly varies with the magnetic flux density.
Figure 2.75

2.7.4 Linear Variable Differential Transducer (LVDT)
Linear Variable Differential Transducer or LVDT is a well-established device which have been used
since long for the accurate measurement of displacement and for the control of positioning within the
closed loops .
Operating principle
In its simplest form, the design consists of a cylindrical array of primary and secondary windings with a
cylindrical core which passes through the centre as shown in Fig no. (2.77a and c). The primary wind-
ings are energised with a constant amplitude A.C. supply at a frequency of 1 to 10 kHz. Thus,
producing an alternating magnetic field in the centre of the transducer which induces a signal into the
secondary windings depending on the position of the core. Movement of the core within this area
causes the secondary signal to change as shown in Fig no. (2.77 b). As the two secondary windings are
positioned and connected in a push-pull arrangement, when the core is positioned at the centre, a
zero/no signal is derived. Movement of the core from this point in either direction causes the signal to
increase as shown in Fig no. (2.77 b). As the windings are wound in a particular precise manner, the
signal output has a linear relationship with the actual mechanical movement of the core.
The secondary output signal is then processed by a phase-sensitive demodulator which is switched at
the same frequency as the primary energising supply. This results in a final output which, after rectifi-
cation and filtering, gives D.C. or 4-20mA output, proportional to the core movement and also indi-
cates its direction, positive or negative from the central zero point as shown in Fig no. (2.77 d).
The distinct advantage of using a LVDT displacement transducer is that the moving core does not make
contact with other electrical components of the assembly, unlike resistive types, as so offers high
reliability and long life. The LVDT design lends itself for easy modification to fulfil a whole range of dif-
ferent applications in both research and industry.
Figure 2.77

2.7.5 Position sensors
Position sensors are basically used for measuring the distance travelled by the body starting from its
reference position that is, how far the body has moved from its reference or initial position is sensed
by the position sensors and often the output is given as a fed back to the control system which takes
the appropriate action. Motion of the body can be either rectilinear or curvilinear. Accordingly position
sensors are called linear position sensors or angular position sensors.
Position sensors use different sensing principles to sense the displacement of a body.
Depending upon the different sensing principles used for position sensors, they can be classified as
follows also shown in fig (2.78):
 Resistance-based or Potentiometric position sensors
 Capacitive position sensors
 Linear voltage differential transducers
 Magnetoresistive linear position sensor
 Eddy current based position sensor
 Hall effect based magnetic position sensors
 Fiber-optic position sensor
 Optical position sensors
Figure 2.78

Rotary encoders shown in fig (2.79) are among the most useful and versatile pieces of equipment
available to the automation industry, providing accurate position measurement and speed feedback.
A basic encoder is a sensor that generates digital signals in response to movement. It is an
electro-mechanical device that converts the mechanical motion of shaft or linear motion into an
electric impulse. This analogy or digital code can be processed by counters, tachometers, logical
controllers, and industrial PCs. A rotary encoder is also known as shaft encoder which can also be
used to measure linear movement, speed, and position. In automation technology rotary encoders
are used as sensors for angle, position, speed, and acceleration. By using spindles, gear racks,
measuring wheels, or cable pulls, linear moments can also be monitored by a rotary encoder. Rotary
encoders are used in many applications that require precise shaft rotation including industrial
controls, robotics, and expensive photographic lenses.
 Construction & working
Rotary encoders use a glass or plastic disc with alternating transparent and opaque fields, with a light
source on one side and a light-sensitive sensor on the other shown in fig (2.80). As the disc rotates,
the light source is alternately blocked and revealed to the sensor. Whenever the light source hits the
sensor, the encoder transmits an electric pulse that can be interpreted by a controller. The pulse
ends when an opaque field on the disc blocks the light source. Rotation of the disc results in a square
-wave pulse output. Most rotary encoders use an infra-red light emitting diode as a light source and
photodiodes or phototransistors as receivers. If no other functions are added to the encoder, the
only output is a square wave that indicates that the disc is rotating. A rotary encoder typically has 2
outputs. These outputs emit signals that are 90 degrees out of phase with respect to each other. The
output signals may be square wave or sine wave. Sine wave outputs are typically used in higher
resolution encoder applications. For simplicity, we will talk about square wave output encoders in
this application note.
2.8 Rotary encoders
Figure 2.79

Classification under choice of output: Incremental Rotary Encoders and Absolute Encoders.
Classification under sensing technology: Optical and magnetic Encoders
Classification under design and the mounting systems: Solid Shaft, Hollow Shaft and Recessed Shaft
Encoders.
2.10.1 Linear encoder
A linear encoder is a sensor, transducer or read head paired with a scale that encodes position. The
sensor reads the scale in order to convert the encoded position into an analogue or digital signal,
which can then be decoded into position by a digital readout (DRO) or motion controller. A typical
linear encoder consists of a scanning unit and a scale. The scale is generally glass and is cemented to
a support, usually an aluminum extrusion. The scanning unit contains a light source, photocells, and a
second graduated piece of glass called the scanning reticule. This scanning reticule sits a short
distance from the scale. In operation, a parallel beam of light produced by the light source and lens
passes through four windows on the scanning reticule, through the glass scale, and onto a set of
photo sensors. The four windows in the scanning reticule are each phase shifted 90° apart.
Figure 2.80
Figure 2.81

The system combines the phase-shifted signals to produce two symmetrical sinusoidal outputs phase
shifted by 90°. When the scanning unit moves, the scale modulates the light beam, creating a
sinusoidal outputs from the photo sensor.
To obtain high resolution, a fine-scale pitch is used. Because of the diffraction effects of the scale grat-
ing, spacing between the fixed scale and scanning reticule must be extremely narrow and constant.
Consequently, the entire scanning unit mounts on a carriage that runs on ball bearings along the glass
scale. The scanning unit connects to the machine slide via a coupling that compensates for alignment
errors between the scale and the machine guide ways.
 Incremental rotary encoders
Incremental rotary encoders supply a certain number of pulses for each shaft revolution shown in fig
(2.82). Measuring the cycle duration or counting the number of pulses during a pre-determined unit of
time determines rotational speed. Incremental rotary encoders emit pulses as the shaft is rotated, and
the number of pulses is used to calculate angular position. If the pulses are measured, after a reference
point is added, the calculated value represents a parameter for a scanned angle or the distance cov-
ered.
 Absolute rotary encoders
Absolute encoders provide a uniquely coded numerical value for each shaft position. Absolute rotary
encoders eliminate the need for expensive input components in a positioning application because they
have built-in reference data. The sampling unit in an absolute encoder reads the code disk to
determine the shaft position and the data is transmitted by parallel or serial interface. In addition,
reference runs after a power failure or when the machine is switched off are not required because the
encoder provides the (incomplete).
Figure 2.82

2.9.1 Temperature
Temperature is perhaps the most important and commonly used parameter in all walks of life and so is
the necessity to measure it accurately. Whether the purpose is operation or maintenance of a process
or safety and protection, proper measurement of temperature is very important. Successful and
accurate measurement of temperature holds the key to the most vital aspects of control of processes,
quality of products, yield of many a manufacturing plant.
Temperature measurement is based on either one of the following physical phenomenon with change
of temperature:
 Material expansion
 Change in Electrical resistance
 Change in contact voltage between two dissimilar metals
 Change in radiated energy.
Fig 2.83 shows a temperature sensor
Substances and compounds that exhibit either of the foregoing changes are placed into well defined
thermowells and placed into or onto the walls of the vessels of which the temperature has to be
measured. These thermowells are adapted to the mounting considerations versus chemical and
physical nature of the environment that they have to stand up to.
Figure 2.83
2.9 Sensing process conditions

 Types of temperature sensors
 Thermocouple
If two dissimilar metal wires are joined at both ends and if one end was at a different temperature
than the other, a current starts to flow between these two junctions. This phenomenon is known as
the Seebeck effect and is the basis of working of all thermocouples. . As per the fig (2.84) one of the
joined end is referred to as the Hot junction. The other end of these dissimilar metals is referred to
as the Cold end or cold junction. The cold junction is actually formed at the last point of
thermocouple material.
Figure 2.84
Thermocouples have been largely standardized over the years and a lot of control and measuring
equipment uses this standardization for implementation and working. (mention standard types)
 RTD
A RTD shown in fig (2.85) is a device which contains an electrical
resistance source (referred to as a “sensing element” or “bulb”)
which changes resistance value depending on its temperature. This
change of resistance with temperature can be measured and used
to determine the temperature of a process or of a material.
 Thermistors
Thermistor shown in fig (2.86) is another type of temperature sensor, whose name is a
combination of the words THERM-ally sensitive res-ISTOR. A thermistor is a special
type of resistor which changes its physical resistance when exposed to changes in
temperature. Thermistors are semiconductor based sensors, manufactured in the
shapes of flat disc, beads, or rods. They are manufactured by combining two or more
metal oxides. When oxides of copper, cobalt, iron, manganese, nickel, magnesium,
vanadium, tin, titanium or zinc are used, the resulting semi-conductor exhibits a
Negative Temperature Coefficient (NTC) of resistance. This means increase in the
temperature, results in decrease in electrical resistance of the NTC thermistor.
Figure 2.85
Figure 2.86

 Applications
2.9.2 Pressure
Pressure remains to be another pivotal parameter that has to be measured and controlled across the
board for effective automation in Industry. Pressure may need to be measured through contact or non
contact means, as that in a closed or an open vessel, as exerted by a gas or liquid or solid to result in a
simple switch output beyond a set value or continuous measurement through a segment of time.
So the before selection of an appropriate pressure sensor, one must truly identify
1) Physical and chemical nature of the media whose pressure needs to be measured.
2) The range over which the pressure has to be measured
3) Measurement if to be done in a closed/open vessel
4) Measurement if to be done through contact/non contact means
5) Kind of standardized output expected over the measurement range
6) If the sensor is to be placed in a hazardous or a safe area
7) Mounting considerations relative to the vessel or media of whose pressure is to be measured
Usually a pressure sensor consist of two main parts, an elastic material which will deform proportion-
ately when exposed to a pressurized medium and electrical device which detects the deformation and
convert it to a standardized and readable signal.
Temperature measurement of
tire and disk brake
Temperature measurement in aircraft
Figure 2.88
Figure 2.87

 Piezoelectric pressure sensors
The word piezoelectricity means electricity resulting from pressure. Piezoelectricity is the electric
charge that accumulates in certain solid materials in response to the applied mechanical stress.
Piezoelectric effect is the linear electromechanical interaction between the mechanical and the
electrical state in crystalline materials with no inversion symmetry. For example ceramic,
quartz ,Rochelle salt etc. A type of sensor shown in fig (2.89) meas-
ures pressure, acceleration, strain or force and converts them into to an electrical charge based on the
piezoelectric principle are known as Piezoelectric pressure sensors.
 Types of pressure transducers
 Absolute pressure transducers – These are used for pressure measurement with reference to
zero pressure.
 Gauge pressure transducers – These are used for the measurement of pressure with reference to
ambient atmospheric pressure.
 Differential pressure transducers – These are used for the measurement of pressure difference
between two pressures.
 Applications
Figure 2.90
Figure 2.89
Integrated pressure sensors in Cars
Figure 2.91
Pressure sensor for measuring pressure
of compressed gas
Figure 2.92

2.9.3 Level
Level sensors detect the level of substances that can flow including liquids, slurries, granular materials,
and powders. Fluids and fluidized solids flow to become essentially level in their containers (or other
physical boundaries) because of gravity whereas most bulk solids pile at an angle of repose to a peak.
The substance to be measured can be inside a container or from its source (e.g., a river or a lake). The
level measurement can be either continuous or point values. Continuous level sensors measure level
within a specified range and determine the exact amount of substance in a certain place, while point-
level sensors only indicate whether the substance is above or below the sensing point. Generally the
latter detect levels that are excessively high or low.
Level measurements can be distinctively done while being in contact with the media being measured
or being off contact.
The simplest of the level measurements can be done by using the principle of conductivity if the media
is electrically conductive. Additionally change in capacitance with the change in level or measurement
of pressure with change in level are put to best use for measurement of level in the industry.
On the other hand usage of Ultrasonic waves or high frequencies is also implemented in non contact
type measurement as shown in fig (2.93). These equipment simply use the distance measurement
techniques by using high frequency waves in the process or the vessel.
 Applications
There are many physical and applications that affect the selection of the optimal level monitoring
method for industrial and commercial processes. The selection criteria include the physical
phase (liquid, solid or slurry), temperature, pressure or vacuum, chemistry, dielectric
constant of medium, density (specific gravity) of medium, agitation (action), acoustical or electrical
noise, vibration, mechanical shock, and tank or bin size and shape.
Important application constraints are: mounting of the instrument and monitoring or control of
continuous or discrete (point) levels, price, accuracy, appearance, response rate, ease of calibration
and programming, physical size.
Figure 2.93

2.9.4 Flow
A flow sensor shown in fig (2.94) is a device for
sensing the rate of fluid flow. Typically a flow sensor is
the sensing element used in a flow meter or flow
logger, to record the rate flow of fluids.
There are various kinds of flow sensors and flow
meters, including some that have a vane that is
pushed by the fluid, and can drive a rotary
potentiometer, or similar devices.
Other flow sensors are based on those sensors which
measure the transfer of heat caused by the moving medium. This principle is common for micro
sensors to measure flow.
 Types of flow meters
 Mechanical type flow meters : Fixed restriction variable head type flow meters using different
sensors like orifice plate, venturi tube, flow nozzle, pilot tube, quantity meters like positive
displacement meters, mass flow meters etc.
 Inferential type flow meters: Variable area flow meters (Rota meters), turbine flow meter,
target flow meters etc.
 Electrical type flow meters: Electromagnetic flow meter, Ultrasonic flow meter, Laser Doppler
Anemometers etc.
 Other flow meters: Purge flow regulators, Flow meters for Solids flow measurement, Cross-
correlation flow meter, Vortex shedding flow meters, flow switches etc.
2.9.5 Humidity
Controlling or monitoring humidity is of paramount importance in many industrial & domestic
applications. In semiconductor industry, humidity or moisture levels
needs to be properly controlled & monitored during wafer processing.
In medical applications, humidity control is required for respiratory
equipment’s, sterilizers, incubators, pharmaceutical processing, and
biological products. Humidity control is also necessary in chemical gas
purification, dryers, ovens, film desiccation, paper and textile
production, and food processing which can be achieved by Humidity
sensors shown in fig (2.95).
Figure 2.94
Figure 2.95

In agriculture, measurement of humidity is important for plantation protection (dew prevention), soil
moisture monitoring, etc. For domestic applications, humidity control is required for living
environment in buildings, cooking control for microwave ovens, etc. In all such applications and many
others, humidity sensors are employed to provide an indication of the moisture levels in the
environment.
2.9.6 Viscosity
For the evaluation of the condition of automotive engine oil, the oil's viscosity is one of the most
important parameters. Using micro acoustic viscosity sensors as shown in in fig (2.96) , an oil-viscosity
measurement can be performed on-board. In this section we will be focusing on the changes in
viscosity of engine oil, its temperature dependence, and the resulting representation in terms of out-
put signals of micro acoustic viscosity sensors. These considerations are illustrated by means of
measurement results obtained for used oil samples, which have been obtained from test cars and fresh
oil samples out of different viscosity classes. Finally, the change in the viscosity occurs due to increase
in soot contamination which has to be determined .
Measuring the viscosity of oil is a rapid method of determining oil condition, and is
considered an important parameter for an oil sample . The viscosity sensor, which can be a com-
plement to IR spectroscopy and other bulk property sensors, can provide instantaneous online
viscosity and temperature data. It has no moving parts with an extremely wide operating range
and offers universal plug-and-play connectivity for integration with and into other handheld products.
2.9.7 pH & Chemical Parameters
pH is the numeric representation of gram-equivalent per litre of hydrogen ion concentration in any
solution. pH value ranges typically from 0(depicting highly acidic) to 14 (depicting highly basic or alka-
line). When a reference electrode is immersed in the solution, potential of the reference electrode
does not change with the changing hydrogen ion concentration. A solution in the reference electrode
also makes contact with the sample solution and the measuring electrode through a junction, complet-
ing the circuit.
Figure 2.96

Output of the measuring electrode changes with temperature (even though the process remains at a
constant pH), so a temperature sensor is necessary to correct for this change in output.
This is done by the analyser or transmitter software as shown in fig (2.97). The pH sensor
components are usually combined into one device called a combination pH electrode. The measuring
electrode is usually made up of glass and is quite fragile.
Figure 2.97
Recent Developments have replaced the glass with more durable solid-state sensors as shown in
fig(2.98).The preamplifier is a signal-conditioning device. It takes the high-impedance pH electrode
signal and changes it into low impedance signal which the analyser or transmitter can accept. The
preamplifier also strengthens and stabilizes the signal, making it less susceptible to electrical noise.
The sensor's electrical signal is then displayed This is commonly done in a 120/240 V ac-powered
analyser or in a 24 V dc loop-powered transmitter.
Figure 2.98

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