Robotics unit3 sensors

UNIT-3
Sensors and machine vision
Dr. B. Janarthanan
Professor
Dept. of Mechanical Engineering
Mohamed Sathak A.J. College of Engineering
ROBOTICS – OIE 751

Sensors
• Sensor is a device that acquires a physical parameter
and changes it into a signal that can be processed by
the system
• Eg., temperature, pH, velocity, rotational rate, flow
rate, pressure and many others.
• Today, most sensors do not indicate a reading on an
analog scale, but, rather, they produce a voltage or a
digital signal that is indicative of the physical variable
they measure.
• Those signals are often imported into computer
programs, stored in files, plotted on computers and
analyzed to depth.

Need of sensors
• The significance of sensor technology is
constantly growing.
• Sensors allow us to monitor our surroundings in
ways we could barely imagine a few years ago.
• New sensor applications are being identified
everyday which broadens the scope of the
technology and expands its impact on everyday
life.

Need of sensors
In Industry
• On the factory floor, networked vibration sensors
warn that a bearing is beginning to fail.
• Mechanics schedule overnight maintenance,
preventing an expensive unplanned shutdown.
• Inside a refrigerated grocery truck, temperature and
humidity sensors monitor individual containers,
reducing spoilage in fragile fish or produce.

Need of sensors
In the Environment
• Networks of wireless humidity sensors monitor fire
danger in remote forests.
• Nitrate sensors detect industrial and agricultural
runoff in rivers, streams and wells, while distributed
seismic monitors provide an early warning system for
earthquakes.
• Built-in stress sensors report on the structural
integrity of bridges, buildings and roadways, and
other man-made structures.

Need of sensors
For Safety and Security
• Fire fighters scatter wireless sensors throughout a
burning building to map hot spots and flare-ups.
Simultaneously, the sensors provide an emergency
communications network.
• Miniature chemical and biological sensors in
hospitals, post offices, and transportation centres
raise an alarm at the first sign of anthrax, smallpox or
other terror agents.

Characteristics of a sensor
1. Range: It is the minimum and maximum value of physical
variable that the sensor can sense or measure. For example,
a Resistance Temperature Detector (RTD) for the
measurement of temperature has a range of -200 to 800oC.
2. Span: It is the difference between the maximum and
minimum values of input. In above example, the span of
RTD is 800 – (-200) = 1000oC.
3. Accuracy: The error in measurement is specified in terms of
accuracy. It is defined as the difference between measured
value and true value. It is defined in terms of % of full scale
or % of reading.
Xt is calculated by taking mean of infinite number of
measurements.

4. Precision: It is defined as the closeness among a
set of values. It is different from accuracy. Let
Xt be the true value of the variable X and a
random experiment measures X1, X2, …. Xi as the
value of X. We will say our measurements X1,
X2,… Xi are precise when they are very near to
each other but not necessarily close to true value
Xt. However, if we say X1, X2,… Xi are accurate, it
means that they are close to true value Xt and
hence they are also close to each other. Hence
accurate measurements are always precise.

5. Sensitivity: It is the ratio of change in output to change
in input. It should be as high as possible
6. Linearity: Linearity is the maximum deviation between
the measured values of a sensor from ideal curve. The
sensory device should exhibit the same sensitivity over
its entire operating range
7. Hysteresis: It is the difference in output when input is
varied in two ways- increasing and decreasing.
8. Resolution: It is the minimum change in input that can be
sensed by the sensor.

9. Reproducibility: It is defined as the ability of sensor to
produce the same output when same input is applied.
10. Repeatability: It is defined as the ability of sensor to
produce the same output every time when the same
input is applied and all the physical and measurement
conditions kept the same including the operator,
instrument, ambient conditions etc.
11.Response Time: It is generally expressed as the time at
which the output reaches a certain percentage (for instance,
95%) of its final value, in response to a step change of the
input.

12. It should be suitable for the environment in
which it is employed
13. It should have a suitable physical size, cost and
ease of operation
14. It should not disturb or have any effect upon the
quantity it senses or measures
15. It should include isolation from receiving excess
signals or electrical noise that could give rise to
the possibility of misconception or damage of
the sensor, circuit or system

Types of sensors
• Internal state sensors
• External state sensors
• Tactile sensors
• Non-tactile sensors

Internal state sensors
• These sensors deals with the detection of
variable such as arm joint position, which are
used in robot control.
• Internal sensors as the name explains it is
used to measure the internal state of a robot.
• It measures position, velocity, acceleration
etc. of robot joints and/or end effector
i. Potentiometers
ii. LVDT or RVDT
iii. Optical encoders
iv. Tachometer
v. Accelerometer

External state sensors
• These type of sensors are used to monitor the
robot’s geometric and /or dynamic relation to its
task, environment of the objects that is handling.
i. Strain gauges
ii. Pressure transducer
iii. Proximity devices
iv. Ultrasonic sensors
v. Electromagnetic sensors
• External state sensors may be further classified as
1. Tactile (Contact) sensors.
2. Non-tactile (Non-contact) sensors.

Tactile sensors
• These are contact sensors that must be
brought in contact with the object to obtain
signals to measure the necessary quantities
i. Force sensors
ii. Torque sensors
iii. Touch sensors
iv. Position sensors

Non-tactile sensors
• These are contactless sensors which sense
signals remotely but only within the specified
range of distance from the object
• Non-contact sensors rely on the response of
variations in acoustic or electromagnetic
radiation.
i. Proximity sensors
ii. Range imaging sensors
iii. Ultrasonic sensors
iv. Electro-optical vision sensors
v. Magnetic sensors

Position sensors
• As the name implies, Position Sensors detect the position
of something which means that they are referenced either
to or from some fixed point or position.
• These types of sensors provide a “positional” feedback.
• One method of determining a position, is to use either
“distance between two points such as the distance
travelled or moved away from some fixed point, or by
“rotation” (angular movement). determine its distance
travelled along the ground.
• Either way, Position Sensors can detect the movement of
an object in a straight line using Linear Sensors or by its
angular movement using Rotational Sensors.

Proximity Sensors
• This type of sensor is capable of pointing out the
availability of a component.
• The proximity sensor will be placed in the robot
moving part such as end effector.
• This sensor will be turned ON at a specified
distance, which will be measured by means of feet
or millimeters.
• It is also used to find the presence of a human being
in the work volume so that the accidents can be
reduced.

Range Sensors
• The distance between the object and the robot hand is
measured using the range sensors, within it is range of
operation.
• The calculation of the distance is by visual processing.
Range sensors find use in robot navigation and
avoidance of the obstacles in the path.
• The location and the general shape characteristics of
the part in the work envelope of the robot is done by
special applications for the range sensors.

Sensors
Tactile Sensors:
A sensing device that requires the contact between an
object, and sensor is considered as the Tactile Sensor.
This sensor can be sorted into two key types namely:
• Touch Sensor, and
• Force Sensor.

Touch Sensors
• The touch sensor has got the ability to sense
and detect the touching of a sensor and object.
• Some of the commonly used simple devices as
touch sensors are micro – switches, limit
switches, etc.
• If the end effector gets some contact with any
solid part, then this sensor will be handy one to
stop the movement of the robot.
• In addition, it can be used as an inspection
device, which has a probe to measure the size
of a component.

Force Sensors
• The force sensor is included for calculating the
forces of several functions like the machine loading
& unloading, material handling, and so on that are
performed by a robot.
• This sensor will also be a better one in the assembly
process for checking the problems.
• There are several techniques used in this sensor like
Joint Sensing, Robot –Wrist Force Sensing, and
Tactile Array Sensing.

Position Sensors (LVDT)
• The LVDT full form is “Linear Variable Differential
Transformer” is LVDT.
• Generally, LVDT is a normal type of transducer.
• The main function of this is to convert the
rectilinear movement of object to the equivalent
electrical signal.
• LVDT is used to calculate displacement and
works on the transformer principle.
• The working principle of the linear variable
differential transformer or LVDT working theory is
mutual induction.

• Linear Variable Differential Transformer (LVDT) is an
Electromechanical type Inductive Transducer that
converts rectilinear displacement into the Electrical
Signal.
• Since LVDT is a secondary transducer, hence physical
quantities such as Force, Weight, Tension, Pressure, etc
are first converted into displacement by a primary
transducer and then LVDT is used to measure it in terms
of corresponding Electrical signal.
• As LVDT is an AC controlled device, so there is no
electronics component inside it. It is the most widely used
Inductive Sensor due to its high accuracy level.
• Its electrical output is obtained because of the difference
of secondary voltages, hence it is called Differential
Transformer.

LVDT Construction:
• LVDT consists of one primary winding P and two
secondary windings S1 & S2 mounted on a
cylindrical former.
• Both the secondary windings (S1 & S2) has an
equal number of turns and placed identically on
either side of the primary winding in such a way
that the net output will be the difference of the
voltage of both secondary windings.
• There is a movable soft iron core placed inside
the former.

LVDT working principle:
• The working principle of LVDT is based on the mutual
induction principle.
• When AC excitation of 5-15 V at a frequency of 50-400Hz
is applied to the primary winding, then a magnetic field is
produced.
• This magnetic field induces a mutual current in secondary
windings. Due to this, the induced voltages in secondary
windings (S1 & S2) are E1 & E2 respectively.
• Since both the secondary windings are connected in series
opposition, So the net output voltage will be the difference
of both induced voltages (E1 & E2) in secondary windings.
• Hence Differential Output of LVDT will be E0 = E1 – E2

Case 1: When the core moves towards S1 (Max
Left).
• When the core of LVDT moves toward Secondary
winding S1.
• Then, in this case, the flux linkage with S1 will be
more as compared to S2.
• This means the emf induced in S1 will be more than
induced emf in S2.
• Hence E1>E2 and Net differential output voltage E0
= E1 – E2 will be positive.
• This means the output voltage E0 will be in phase
with the primary voltage.

Case 2: When the core is at Null position.
• When the core is at the null position then the flux
linkage with both the secondary windings will be
the same.
• So the induced emf (E1 & E2) in both the windings
will be the same.
• Hence the Net differential output voltage E0 = E1
– E2 will be zero (E0 = E1 – E2 = 0).
• It shows that no displacement of the core.

Case 3: When the core moves towards S2 (Max
Right).
• When the core of LVDT moves toward Secondary
winding S2.
• This means the emf induced in S2 will be more
than induced emf in S1.
• Hence E2>E1 and Net differential output
voltage E0 = E1 – E2 will be negative.
opposition (180 degrees out of phase) with the
primary voltage.

1. The direction of the movement of an object can be identified
with the help of the differential output voltage of LVDT. If the
output voltage E0 is positive then this means an object is
moving towards Left from the Null position.
2. Similarly, If the output voltage E0 is negative then this means
the object is moving towards the Right of the Null position.
3. The amount or magnitude of displacement is proportional to
the differential output of LVDT. The more the output voltage,
the more will be the displacement of the object.
4. If we take the core out of the former then the net differential
the output of LVDT will be zero.
5. In fact corresponding to both the cases, whether the core is
moving either Left or Right to the Null position. Then the
output voltage will be increased linearly up to 5mm from the
Null position and after 5 mm output E0 will be non-linear.

Advantages of LVDT
1.Smooth and Wide Range of Operation :- 1.25mm to 250 mm.
2. High Sensitivity:- 40V/mm.
3.Low Hysteresis Losses:- LVDT gives low hysteresis losses hence
repeatability is excellent under all the conditions.
4.Low Friction Losses:- As the core moves in a hollow Former, So
there is no concept of friction losses. Hence it gives accurate output
value.
5.Rugged Operation:- It can tolerate a high degree of shock and
variation, especially when the core is loaded with spring.
6.Low Power consumption:- LVDT consume very low power of
approx 1W during its operation.
7.Direct conversion to Electrical Signal:- They convert linear
displacement directly to the corresponding electrical voltage signal
which are easy to process further.
8.Fast dynamic Response:- Due to the absence of Friction, Its
dynamic response becomes very fast to change in a core position.

Disadvantages of LVDT
1. Since LVDT is Inductive Transducer, so it is
sensitive to Stray Magnetic Field. Hence an
extra setup is required to protect it from Stray
Magnetic Field.
2. Since it is an electromagnetic device, so it also
gets affected by the vibrations and
temperature variation.

Application of LVDT
1. LVDT is used to measure the physical quantities
such as Force, Tension, Pressure, Weight, etc. These
quantities are first converted into displacement by
the use of primary transducers and then it is used to
convert the displacement to the corresponding
Electrical voltage signal.
2. It is mostly used in industries as well as a
servomechanism.
3. It is also used in Industrial Automation, Aircraft.
Turbine, Satellite, hydraulics, etc.

Position Sensors - Resolver
(RVDT)

Position Sensors (RVDT)
• RVDT full form stands for a Rotary variable differential
transformer.
• It is an electro-mechanical type of inductive transducer
that converts angular displacement into the
corresponding electrical signal.
• As RVDT is an AC controlled device, so there is no
electronics component inside it. It is the most widely
used inductive sensor due to its high accuracy level.
• Since the coil of RVDT is designed to measure an
angular position, so it is also known as an angular
position sensor.
• The electrical output of RVDT is obtained by the
difference in secondary voltages of the transformer, so it
is called a Differential Transformer.

RVDT Construction:
• The design and construction of RVDT is similar to
LVDT.
• The only difference is the shape of the core in
transformer windings.
• LVDT uses the soft iron core to measure the linear
displacement whereas RVDT uses the Cam-shaped
core (Rotating core) for measuring the angular
displacement.

RVDT working principle:
• The working principle of RVDT and LVDT both are the
same and based on the mutual induction principle.
• When AC excitation of 5-15V at a frequency of 50-400
Hz is applied to the primary windings of RVDT then a
magnetic field is produced inside the core.
• This magnetic field induces a mutual current in
secondary windings.
• Then due to transformer action, the induced voltages
in secondary windings (S1 and S2) are Es1 and
Es2 respectively.
• Hence the net output voltage will be the difference
between both the induced secondary voltages.
Hence Output will be E0 = Es1 – Es2

Case 1: When the core is at Null position.
• When the core is at the null position then the
flux linkage with both the secondary windings
will be the same.
• So the induced emf (Es1 & Es2) in both the
windings will be the same.
• Hence the Net differential output voltage E0 =
Es1 – Es2 will be zero (E0 = Es1 – Es2 = 0).
• It shows that no displacement of the core.

Case 2: When the core rotates in the clockwise
direction.
• When the core of RVDT rotates in the clockwise
direction.
induced emf in S2.
• Hence Es1>Es2 and Net differential output
voltage E0 = Es1 – Es2 will be positive.
with the primary voltage.

Case 3: When the core rotates in the anti-
clockwise direction.
• When the core of RVDT rotates in the anti-clockwise
direction.
induced emf in S1.
• Hence Es2>Es1 and Net differential output
voltage E0 = Es1 – Es2 will be negative.
opposition (180 degrees out of phase) with the
primary voltage.

Advantages of RVDT
• High Accuracy.
• Compact and strong construction.
• The consistency of RVDT is high.
• Long life span.
• Very high Resolution.
• Low cost.
• High durability
• Linearity is excellent.
• The performance is repeatable.
• Easy to handle

Applications of RVDT
• Actuators for controlling flight as well as engine.
• Fuel valve as well as hydraulics.
• Brake with a cable system.
• Modern machine tools.
• Nose wheel steering systems.
• Weapon and Torpedo system.
• Engine fuel control system
• Aircraft and avionics.
• Engines bleed air systems.
• Robotics.

Potentiometer
• The instrument is designed for measuring
the unknown voltage by comparing it with
the known voltage, such type of instrument
is known as the potentiometer.
• In other words, the potentiometer is the
three terminal device used for measuring
the potential differences by manually
varying the resistances.
• The known voltage is drawn by the cell or
any other supply sources.

Potentiometer
• The potentiometer uses the comparative
method which is more accurate than the
deflection method.
• So, it is mostly used in the places where
higher accuracy is required or where no
current flows from the source under test.
• The potentiometer is used in the
electronic circuit, especially for controlling
the volume.

Potentiometer
The following are the important characteristics
of the potentiometer.
1. The potentiometer is very accurate because
it works on the comparing method rather
than the deflection pointer method for
determining the unknown voltages.
2. It measures the null or balance point which
does not require power for the
measurement.
3. The working of the potentiometer is free
from the source resistance because no
current flows through the potentiometer
when it is balanced.

Potentiometer
• The construction of the potentiometer is
categorised into two parts.
➢ They are the sliding and non-sliding
parts. The sliding contact is a called
wiper.
• The motion of the sliding contacts is either
translatory or rotational.
• Some potentiometer uses both the
translatory and rotational motions.
• Such type of potentiometer uses the
resistor in the form of a helix, and hence
they are called heliports.

Potentiometer
• The potentiometer has three terminals, the
two terminals are connected to the resistor,
and the third terminal is connected to the
wiper which is movable with the wire.
• Because of this moving wire, the variable
potential is tapped off.
• The third terminal is used for controlling the
variable resistor.
• The potential of the third terminal is
controlled by changing the applying potential
at the end of the resistor.
• The body of the potentiometer is made up of
resistive material, and the wire is wound on
it.

Potentiometer (working principle )

• The working principle of the potentiometer
is explained through the circuit shown
below.
• Consider S is the switch used for connecting
or disconnecting the galvanometer from the
potentiometer.
• The battery through the rheostat and slide
wire supply the working current.
• The working current may vary by changing
the setting of the rheostat.

• The method of findings the unknown
voltage depends on the sliding position of
the contact at which the galvanometer
shows the zero deflection.
• The zero or null deflection of galvanometer
shows that the potential of the unknown
source E and the voltage drops E1 across
the sliding wires are equal.
• Thus, the potential of the unknown voltage
is evaluated by knowing the voltage drop
across the ac portion of the sliding wire.

• The slide wire has the uniform cross-section
and resistance across the entire length.
• As the resistance of the sliding wire is
known, then it is easily controlled by
adjusting the working current.
• The process of equalising the working
voltage as that of voltage drop is known as
the standardisation.

Optical encoder
• The optical encoder is a transducer commonly used
for measuring rotational motion.
• It consists of a shaft connected to a circular disc,
containing one or more tracks of alternating
transparent and opaque areas.
• A light source and an optical sensor are mounted
on opposite sides of each track.
• As the shaft rotates, the light sensor emits a series
of pulses as the light source is interrupted by the
pattern on the disc.

Optical encoder
• This output signal can be directly compatible with
digital circuitry.
• The number of output pulses per rotation of the
disc is a known quantity, so the number of output
pulses per second can be directly converted to
the rotational speed (or rotations per second) of
the shaft.
• Encoders are commonly used in motor speed
control applications.

Incremental optical encoder
• An incremental optical encoder has two tracks, 90°
out of phase with each other, producing two outputs.
• The relative phase between the two channels
indicates whether the encoder is rotating clockwise or
counterclockwise.
• Often there is a third track that produces a single
index pulse, to indicate an absolute position
reference.
• Otherwise, an incremental encoder produces
only relative position information.

Pneumatic proximity sensor
• Low pressure air is allowed to escape through a
port in front of the sensor

Piezoelectric sensor
• A piezoelectric sensor, also known as a
piezoelectric transducer, is a device that uses the
piezoelectric effect to measure changes in
pressure, acceleration, temperature, strain or force
by converting these into an electrical charge.
• The prefix piezo is Greek for press or squeeze. The
ability of piezoelectric material to convert
mechanical stress into electrical charge is called a
piezoelectric effect.
• Generated piezoelectricity is proportional to the
pressure applied to solid piezoelectric crystal
materials.

Piezoelectric sensor
• In the pressure sensor, a thin membrane is placed on
a massive base to transfer the applied force to
the piezoelectric element. Upon application of pressure
on this thin membrane, the piezoelectric material gets
loaded and starts generating electrical voltages. The
produced voltage is proportional to the amount of
pressure applied.
• In accelerometers, seismic mass is attached to the
crystal element to transfer the applied force to
piezoelectric materials. When motion is applied,
seismic mass load’s the piezoelectric material
according to Newton’s second law of motion. The
piezoelectric material generates charge used for
calibration of motion.

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Position sensors
• Piezo electric sensor
• LVDT
• RVDT or Resolver
• potentiometer
• Optical encoders
• Pneumatic position sensors

Range sensors
• The function of range sensor is to measure the
distance from a reference point (normally on the
sensor itself)to the objects in the field of operation
of the sensor
• Range sensos are employed for robot navigation
and obstacle avoidance
• The following techniques are used for range
sensing
1. triangulation method,
2. structured lighting approach and
3. time-of flight range finders

Triangulation technique
• The object is illuminated by a narrow beam of
light which is swept over the surface
• The sweeping motion is in the plane defined by
the line from the object to the detector and the
line from the detector to the source.
• If the detector is focused on a small portion of the
surface then, when the detector sees the light
spot, its distance D to the illuminated portion of
the surface can be calculated from the geometry
of the Figure,
• since θ is the angle of the source with the base
line and B is the distance between the source and
the detector are known.

Triangulation technique
• The above method yields a point measurement.
• However, if the source-detector arrangement is
moved in a fixed plane, then it is possible to
obtain a set of points whose distance from the
detectors are known.
• Then these distances are easily transformed to
three-dimensional co-ordinates by keeping tract
of the location and orientation of the detector as
objects are scanned.

Structured lighting approach
• This approach consists of projecting a light pattern
onto a set of objects and using the distortion of
the pattern to calculate the range.
• Figure shows the structure light approach.
• Specific range values are computed by first
calibrating the system.
• The figure shows the arrangement of top view

Time of flight (lapsed time rangers)

Time of flight (lapsed time rangers)
• These methods involve the use of a lase and
ultrasonics
• Time of flight ranging consists of sending a signal
from a transmitter that bounces back from an
object and is received by a receiver.
• The distance between the object and the sensor is
half the distance traveled by the signal, which can
be calculated by measuring the time of flight of
the signal by knowing its speed of travel.
• Time of flight Range finder uses laser to determine
the range and to measure the time it takes for an
emitted pulse of light to return coaxially
• (along the same path) from the reflecting surface.

Proximity sensors
• Proximity sensor (sometimes referred to as
external state sensors) generally have a binary
output which indicates the presence of an
object within a specified distance interval
• Range sensors in contrast yield an estimate of
the distance between a sensor and a reflecting
object

Types of Proximity sensors
• Contact proximity sensors
• Non-contact proximity sensors
i. Optical proximity sensors
ii. Ultrasonic
iii. Eddy current
iv. Inductive
v. Hall-effect
vi. Capacitive
vii. Pneumatic
viii.Fire optic scanning
ix. Scanning laser

Contact proximity sensors
• Touch sensors are classified into two types
1. Binary
2. Analog

1. Binary

• Binary sensors are basically switches which
respond to the presence or absence of an
object.
• Switches are always microswitches.
• It is constructed in such a way that, a switch is
placed on the inner surface of each finger of a
manipulator hand.
• Multiple binary touch sensors can be used on
the inside surface of each finger to provide
further tactile information.

2. Analog

• Analog sensors will give a signal as output
proportional to the local force.
• The simplest form of these devices consist of a
spring loaded rod, which is mechanically linked
to a rotating shaft in such a way that the
displacement of the rod due to a lateral force
results in a proportional rotation of the shaft.
• The rotation is then measured continuously
using a potentiometer or digitally using a code
wheel.
• The spring constant yields the force
corresponding to a given displacement.

Optical proximity sensors
• Optical proximity sensor consists of a light source called
an emitter and a receiver, which senses the presence or
the absence of the light.
• The receiver is usually a photo transistor, and the
emitter is usually LED.
• The combination of these two creates a light sensor and
is used in many application including optical encoder.
• The sensor is set up in such a way that the light emitted
by the emitter, is not received by the receiver, unless an
object is close by.
• Unless a reflective object is within range of the switch,
the light is not seen by the receiver, therefore there will
be no signal.

Ultrasonic proximity sensors
• An ultrasonic range finder uses an ultrasonic chirp
which is transmitted over a short time period and
since the speed of sound is known for a specified
medium, a simple calculation involving the time
interval between the outgoing pulse and return
echo yields an estimate of the distance to the
reflecting surface.
• The basic element is an electro acoustic
transducer, often of the piezoelectric ceramic
type.
• The resin layer protects the transducer against
humidity, dust and other environmental factors.

Ultrasonic proximity sensors
• It also acts as an acoustical impedance matcher.
• The same-transducer is generally used for both
transmitting and receiving, and fast damping of the
acoustic energy is necessary to detect the object at
closest range.
• This is accomplished by providing acoustic absorbers
and by decoupling the transducer from its housing.
• The housing is designed so that it produces a narrow
acoustic beam for efficient energy transfer and signal
directionality.
• The previously discussed proximity sensors are useful
for detection of ferro-magnetic matter only.
• If the robot has to handle other type of materials
ultrasonic sensors find the application.

Eddy current proximity sensors

Eddy current sensors
• Eddy current sensor operates based on the
inductive eddy-current principle.
• When the sensing coil is supplied with an
alternating current, it causes a magnetic field
to form around the coil.
• If an electrically conducting material is placed
in this field, eddy current field is induced
according to the Faraday’s induction law.
• When the object moves, it causes the change
in the impedance of the coil, which is
proportional to the change in the distance
between the sensor and the target.

Eddy current sensors
• Eddy current sensors have superior
temperature stability and resistance towards
pressure, temperature, dirt and oil. (sensors
do not recognize non-conductive materials.)
• Capable of operating at pressure up to 4000
bar, they are one of the best wear-free, non-
contact sensors for measuring displacement
and position in harsh industrial environment.
• The sensors are generally miniature in size
and therefore, are suitable for measuring in
an area where access is restricted.
• These sensors are also low-cost.

Applications - Eddy current sensors
• Measure vibrations of actuators in steel
galvanising plants
• Cylinder movements in an internal
combustion engine
• Measure thickness of sheet metals in roller
gap
• Measure movement of hydraulic cylinders
• Used in airplanes to measure movement of
door lock switches and landing gear flaps

Inductive proximity sensors
• This sensor is based on a change of inductance due
to the presence of a metallic object
• The inductive sensor basically consists of a wound
coil located next to a permanent magnet packaged in
a simple, rugged housing.
• The effect of bringing the sensor in close proximity to
a ferromagnetic material causes a change in the
position of the flux lines of the permanent magnet.
• Under static condition, there is no movement of the
flux lines and no current is induced in the coil.
• As ferromagnetic object enters or leaves the field of
magnet, the resulting change in flux line induces a
current pulse whose amplitudes and shape are
proportional to the rate of change of flux.

Inductive proximity sensors
• The voltage waveform observed at the output of
the coil provides an effective means for proximity
sensing
• The polarity of the voltage output of the sensor
depends whether the object is entering or leaving
the field
• It has been observed that the sensitivity falls off
rapidly with increasing distance, and that the
sensor is effective only for fractions of a
millimetre

Hall effect sensors - principle

Hall effect sensors
• If we have thin conductive plate, and if we set
current to flow through it, the charge carriers
would flow in a straight line from one to the other
side of the plate.
• Now if we bring some magnetic field near the plate
we would disturb the straight flow of the charge
carriers due to a force, called Lorentz Force.
• In such a case the electrons would deflect to one
side of the plate and the positive holes to the other
side of the plate.
• This means if we put a meter now between the
other two sides we will get some voltage (Hall
Voltage, VH) which can be measured.

Hall effect sensors
• These sensors can only detect magnetised objects
• However when used in conjunction with a
permanent magnet, they are capable of detecting
all ferromagnetic materials

Capacitive proximity sensors
• Inductive and Hall-effect sensor will detect only
ferromagnetic materials, whereas, capacitive
sensor is capable of detecting all solids and
liquid materials.
• capacitive proximity sensors operate by noting a
change in the capacitance read by the sensor.
• A typical capacitor consists of two conductive
elements (sometimes called plates) separated
by some kind of insulating material that can be
one of many different types including ceramic,
plastic, paper, or other materials.

• The way a capacitive proximity sensor works is
that one of the conductive elements, or
plates, is inside the sensor itself while the
other one is the object to be sensed.
• The internal plate is connected to an oscillator
circuit that generates an electric field.
• The air gap between the internal plate and the
external object serves as the insulator or
dielectric material.
• When an object is present, that changes the
capacitance value and registers as the
presences of the object.

• The other simplest method includes a capacitor
as part of an oscillator circuit designed so that
the oscillator starts only when the capacitance of
sensor exceeds a predefined threshold value.
• The start of oscillation is then translated into an
output voltage which indicates the presence of
an object. This method provides a binary output
whose triggering sensitivity depends on the
threshold value

Fibre-optic scanning sensors
1. Opposed configuration
2. Retro reflective configuration
3. Diffuse configuration

1. Opposed-Mode (Through-Beam) Sensing
• In opposed-mode sensing, also known as through-beam sensing,
the sensor's emitter and receiver are housed in two separate
units. The emitter is placed opposite the receiver so that the light
beam goes directly from the emitter to the receiver.
• The opposed mode should be used whenever possible because it
is the most reliable sensing mode. This is because light passes
directly from the emitter to the receiver. An object is detected
when it breaks the effective beam, which is the column of light
directly between the emitter’s lens and the receiver’s lens.
• It doesn't matter how shiny or dark your object is, or even what
color. The object physically passes between the emitter and
receiver and is detected when it blocks the beam of light.
Therefore, variables such as surface reflectivity, color, and finish
don't affect opposed-mode sensing.

2. Retroreflective sensing
• Unlike an opposed-mode sensor, a retroreflective sensor
contains both the emitter and receiver elements in a single unit.
The effective beam is established between the emitter, a
retroreflector, and the receiver.
• As with an opposed-mode sensor, an object is sensed when it
interrupts or "breaks" the effective beam.
• Like opposed-mode sensing, retroreflective sensing is also a
beam-break mode, so objects can often be detected regardless
of their reflectivity.
• For this reason, the retroreflective mode is also a reliable sensing
mode, even if the target’s color or finish is inconsistent.

3. Diffuse-mode sensing
• Diffuse-mode sensing is the most common type of proximity
sensing. In diffuse mode sensing, light emitted from the sensor
strikes the surface of the object to be detected and is diffused,
sending some light back to the receiver element of the sensor.
• With a diffuse-mode sensor, the object is detected when it
"makes" the beam. That is, the object reflects some of the
sensor’s transmitted light energy back to the sensor.

Wrist sensors
• Several different forces exist at the point where a robot
arm joins the end effector. This point is called the wrist. It
has one or more joints that move in various ways.
• A wrist-force sensor can detect and measure these
forces. It consists of specialized pressure sensors known as
strain gauges.
• The strain gauges convert the wrist forces into electric
signals, which go to the robot controller.
• Thus the machine can determine what is happening at the
wrist, and act accordingly.

Compliance sensors
• Constant control of a pressing force of a tip of the
robot hand with force sensor.
• Stable operation on assembling, grinding and
other work by using this function.
• The term compliance refers to flexibility and
suppleness.
• A non-compliant (stiff) robot end effector is a
device which is designed to have predetermined
positions or trajectories.

Slip sensors
• Slip sensor Slip sensors in robotics are used to
provide the robotic manipulator if the object that is
carried by the end effector is slipping.
• Slip may be regarded as the relative movement of
one objects surface over another when in contact.
• The slip of the industrial robot finger means the
relative motion of an object with respect to the
finger in the direction vertical to the grasping finger
force.
• The traveling distance of the grasping point is
called the slip displacement. The slip
sensor detects the slip displacement

Sniff sensors
• Sniff sensors are similar to smoke detectors

Taste sensors
• A taste sensor is a device that determines the
composition of particles in a medium

Image processing versus image
analysis
• Image processing – preparation of an image for
later analysis and use
• Image analysis – process by which a captured and
processed image is analysed to extract information
about the content
• Two measures significantly affect the usefulness of
an image
1. Resolution
2. Quantization

Resolution
• How often a signal is measured and read or
sampled
• Higher number of samples at equally spaced
periodic time result in higher resolution
• The resolution of an analog signal is a function of
sampling rate
• The resolution of a digital signal is a function of pixels

Quantization
• Quantization refers to how accurately the value of
the signal at any given point is converted to digital
form
• This is a function of how many bits are used to
represent the digitized magnitude of the sampled
signal
• Total number of grey level possibilities is 2 𝑛

Machine vision (computer vision
and artificial vision)
• Machine vision is the capability of a computer to
perceive the environment.
• One or more video cameras are used with analog-to-
digital conversion and digital signal processing.
• A machine vision system uses a sensor in
the robot for viewing and recognizing an object with
the help of a computer.
• According to the Automated Imaging Association (AIA),
machine vision encompasses all industrial and non-
industrial applications in which a combination of
hardware and software provide operational guidance to
devices in the execution of their functions based on the
capture and processing of images.

Machine vision
• The operation of the vision system consists of three
functions
1. Sensing and digitizing image data
2. Image processing and analysis
3. Application

Acquisition of images
• There are two types of vision cameras
1. Analog camera.
2. Digital camera.
• An analog camera is not very common any more, but
are still around, they used to be standard at television
stations.
• Digital camera is much more common and mostly
similar to each other.
• A video camera is a digital camera with an added
video-tape recording section

Lighting techniques
• An essential ingredient in the application of machine
vision is proper lighting
1. Diffuse surface devices
2. Condenser projectors
3. Flood spot projectors
4. Collimators
5. Imagers

Image processing and analysis
• There are various techniques to reduce the magnitude
of the image processing problems
• These techniques include
1. Image data reduction
2. Segmentation
3. Feature extraction
4. Object recognition

Robotic applications
• Robotic applications in machine vision fall into the
three broad categories
1. Inspection
2. Identification
3. Visual seroving and navigation

Robotics unit3 sensors

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