Unit III - Solved Question Bank- Robotics Engineering -

VMKV ENGINEERING COLLEGE, SALEM
AERONAUTICAL ENGINEERING COURSE
UNIT III - SENSORS
Sensor devices, Types of sensors - contact, position and displacement sensors, Force and torque sensors -
Proximity and range sensors - acoustic sensors - Robot vision systems - Sensing and digitizing - Image processing
and analysis.
PART A
1. What do you mean by a sensor?
Ans. Sensors are used to collect information about the internal state of the robot or to communicate with
the outside environment. Sensor is a device to make a measurement of a physical variable of interest and
convert it into electrical form.
2. What is the common imaging device used for robot vision systems?
Ans. Black and white videocon camera, charge coupled devices, solid-state camera, charge injection
devices.
3. What is segmentation in higher level vision?
Ans. Segmentation is the method to group areas of an image having similar characteristics or features into
distinct entities representing part of the image.
4. What is image thresholding?
Ans. Thresholding is a binary conversion technique in which each pixel is converted into a binary value
either black or white.
5. Differentiate between internal and external purpose sensors.
Ans. Internal sensors are used to monitor and control the various joints of the robot; they form a
feedback control loop with the robot controller. Examples of internal sensors include potentiometers and
optical encoders, while tachometers of various types can be deployed to control the speed of the robot arm.
External sensors are external to the robot itself, and are used when we wish to control the operations
of the robot with other pieces of equipment in the robotic work cell. External sensors can be relatively
simple devices, such as limit switches that determine whether a part has been positioned properly, or
whether a part is ready to be picked up from an unloading bay.
6. Name the advanced sensor technologies used in robotics.
Ans. Advanced sensor technologies used in robotics
7. What are the functions of machine vision system?

Ans. i. Sensing and digitizing image data
ii. Image Processing and analysis
iii. Application
8. Differentiate between contact and noncontact sensors.
Ans. Contacting sensors: Respond to a physical contact.
Tactile/touch sensors – switches, Photo-diode/LED combination
Slip sensors
Tactile sensors – resistive/capacitive arrays
Non-contacting sensors: Detect variations in optical, acoustic or electromagnetic radiations or
change in position/orientation.
Proximity sensors – Inductive, Capacitive, Optical and Ultrasonic
Range sensors – Capacitive and Magnetic, Camera, Sonar, Laser range finder, Structured
light
Colour sensors
Speed/Motion sensors – Doppler radar, Doppler sound, Camera,
Accelerometer, Gyroscope
Identification – Camera, RFID, Laser ranging, Ultrasound
Localisation – Compass, Odometer, GPS
9. What do you mean by range sensing?
Ans. Range sensors measure distance of objects at larger distances. Uses electromagnetic or electrostatic or
acoustic radiation. Looks for changes in the field or return signal. Highly reliable with long functional life and no
mechanical parts.
Four main kinds of range sensing techniques in robots
i. Triangulation
ii. Structured lighting approach
iii. Time of flight range finders
iv. Vision
Applications: i. Navigation in mobile robots, ii. Obstacle avoidance, iii. Locating parts.
10. Define sensors and transducer.
Ans. Sensor is a transducer that is used to make a measurement of a physical variable of interest.
Transducer is a device that converts one form of information into another form without changing
the information content.
11. What do you mean by Region growing?
Ans. Region growing is a collection of segmentation techniques in which pixels are grouped in regions
called grid elements based on attribute similarities.
12. What do you mean by Feature Extraction?
Ans. In vision applications distinguishing one object from another is accomplished by means of features
that uniquely characterize the object. A feature (area, diameter, perimeter) is a single parameter that
permits ease of comparison and identification.
13. What are the various techniques of image processing and analysis?
Ans. i. Image data reduction

ii. Segmentation
iii. Feature extraction
iv. Object recognition
14. What is an inductive type proximity sensor?
Ans. Inductive proximity sensors operate under the electrical principle of inductance. Inductance is the
phenomenon where fluctuating current, which has a magnetic component induces an electromotive force
(emf) in a target object. To amplify a devices inductance effect, a sensor manufacturer twists wire into a
tight coil and runs a current through it.
15. Classify the position sensors.
Ans. i. Incremental encoders
ii. Absolute encoders
iii. Resistive position sensors
iv. Linear variable differential transformer.
v. Encoders
vi. Potentiometer
vii. Resolver.
16. What are the feedback devices used in robotics?
Ans. i. Position Sensors
ii. Velocity Sensors
17. What do you mean by triangulation?
Ans. Triangulation with active beacons is widely used in the absolute localization of mobile robots. In
trigonometry and geometry, triangulation is the process of determining the location of a point by forming
triangles to it from known points. Triangulation today is used for many purposes, including surveying,
navigation, metrology, astrometry, binocular vision, model rocketry and gun direction of weapons.
18. What is frame grabber?
Ans. It is a hardware device used to capture and store the digital image.
19. List the types of encoders.
Ans. i. Incremental encoders
ii. Absolute encoders
20. What do you mean by proximity sensing?
Ans. i. Detect presence of an object near a robot or manipulator.
ii. Works at very short ranges (<15-20 mm).
iii. Frequently used in stationary and mobile robots to avoid obstacles and for safety during
operation.
Four main types of proximity sensors
i. Inductive proximity sensors
ii. Capacitive proximity sensor
iii. Ultrasonic proximity sensor
iv. Optical proximity sensors

PART B
1. Explain the characteristics of Sensors.
Ans. i. Resolution:
It is the minimum step size within the range of measurement of a sensor. In a wire-wound
potentiometer, it will be equal to resistance of one turn of wire. In digital devices with ‘n’ bits,
Resolution = Full range/2n
ii. Sensitivity:
It is defined as the change in output response divided by the change in input response.
Highly sensitive sensors show larger fluctuations in output as a result of fluctuations in
input.
iii. Linearity:
It represents the relationship between input variations and output variations.
In a sensor with linear output, any change in input at any level within the range will produce
the same change in output.
iv. Range:
It is the difference between the smallest and the largest outputs that a sensor can provide, or
the difference between the smallest and largest inputs with which it can operate properly.
v. Response time:
It is the time that a sensor’s output requires to reach a certain percentage of total change.
It is also defined as the time required to observe the change in output as a result of change
input for example, ordinary mercury thermometer response time and digital thermometer response
time.
vi. Frequency response:
The frequency response is the range in which the system‘s ability to resonate to the input
remains relatively high.
The larger the range of frequency response, the better the ability of the system to respond to
varying input.
vii. Reliability:
It is the ratio between the number of times a system operates properly and the number of
times it is tried.
For continuous satisfactory operation, it is necessary to choose reliable sensors that last
long while considering the cost as well as other requirements.
viii. Accuracy:
It shows how close the output of the sensor is to the expected value.
For a given input, certain expected output value is related to how close the sensor‘s output
value is to this value.
ix. Repeatability:
For the same input if the output response is different each time, then repeatability is poor.
Also, a specific range is desirable for operational performance as the performance of robots
depends on sensors.
Repeatability is a random phenomenon and hence there is no compensation.
x. Interfacing:

Direct interfacing of the sensor to the microcontroller/microprocessor is desirable while
some add-on circuit may be necessary in certain special sensors.
The type of the sensor output is equally important. An ADC is required for analogue output
Sensors. For example, potentiometer output to microcontroller.
xi. Size, weight and volume:
Size is a critical consideration for joint displacement sensors.
When robots are used as dynamic machines, weight of the sensor is important.
Volume or spaces also critical to micro robots and mobile robots used for surveillance.
Cost is important especially when quantity involved is large in the end application.
2. Explain the various techniques of Image Processing and Analysis.
Ans. In the industrial applications the algorithms and programs are developed to process the images
captured, digitized and stored in the computer memory.
The size of data to be processed is huge, of the order of 106 which is to be substantially executed in
seconds.
The difficult and time consuming task of processing is handled effectively by the following
techniques.
(1) Image data reduction
(2) Segmentation
(3) Feature extraction
(4) Object recognition.
1. Image Data Reduction:
The purpose of image data reduction is to reduce the volume of data either by ellimination
of some or part processing, leading to the following sub-techniques.
(a) Digital conversion
Digital conversion is characterized by reduction in number of gray levels. For a 8-bit
register each pixel would have 28-256 gray levels. When fewer bits are used to represent pixel
intensity the digital conversion is reduced, to suit the requirements.
2. Segmentation:

An image can be broken into regions that can then be used for later calculations. In effect
this method looks for different self-contained regions, and uses region numbers instead of pixel
intensities.
A simple segmentation algorithm might be,
1. Threshold image to have values of 1 and 0.
2. Create a segmented image and fill it with zeros (set segment number variable to one).
3. Scanning the old image left to right, top to bottom.
4. If a pixel value of 1 is found, and the pixel is 0 in the segmented image, do a flood fill for
the pixel onto the new image using segment number variable.
5. Increment segment # and go back to step 3.
6. Scan the segmented image left to right, top to bottom.
7. If a pixel is found to be fully contained in any segment, flood fill it with a new segment as in
steps 4 and 5.
3. Feature Extraction

4. Object Recognition
Form Fitting : It can sometimes help to relate a shape to some other geometric primitive using
compactness, perimeter, area, etc.
- Ellipse
- Square
- Circle
- Rectangle
3. Describe in detail the advanced sensor technologies used for robotics.
Ans.
4. Describe briefly various approaches of range sensing.
Ans.

5. Discuss in brief proximity sensors and its types.
Ans.

An ultrasonic proximity sensor uses a piezoelectric transducer to send and detect sound waves. Transducer
generate high frequency sound waves and evaluate the echo by the detector which is received back after
reflecting off the target. Sensors calculate the time interval between sending the signal and receiving the
echo to determine the distance to the target. When the target enters the operating range the output switches.
The ultrasonic proximity switches are equipped with temperature sensors and a compensation circuit, in
order to be able to compensate for changes in operating distance caused by temperature fluctuations. The
ultrasonic sensor can work in diffuse, reflex or thru-beam mode.
Thru-Beam: In this case the emitter and detector are 2 separate units. The emitter emits the light which is
detected by the detector. A target is detected when it passes in-between the emitter and detector.
Diffuse Reflective: In this case the emitter and detector are put in the single package in such a way that
their field of view cross. Here the emitter continuously emits the light. When the target comes within the
operating range of the sensor the light from the emitter is reflected off the target and detected by the
detector.
Retro-Reflective: the main components of this sensor are the emitter, detector and the Retro-reflector. The
emitter and the detector are in the same package. The Retro-reflector is placed little far from the sensor.
The light from the emitter is reflected off the Retro-reflector and detected by the detector. When the target
passes between the sensor and the Retro-reflector the beam is not reflected back to the detector. Here the
problem can be that the beam could reflect from the target itself. For this the polarising filter is used in the
sensor. Hence only the light reflected by the retro-reflector is detected by detector.
The advantages of an Ultrasonic proximity sensor are
No physical contact with the object to be detected, therefore, no friction and wear.
Unlimited operating cycles since there is no mechanical contact with the target.
Ultrasonic proximity sensors are not affected by target colour or atmospheric dust, snow, rain etc.
Can work in adverse conditions.
Sensing distance is more compared to inductive or capacitive proximity sensors

The targets to be detected can be in the solid, liquid, granular or powder state.
The disadvantages of the ultrasonic proximity sensor are:
The sensor has a blind zone of several millimetres in front of it.
The application of ultrasonic proximity sensor is
Proximity detection
Optical proximity sensors generally cost more than inductive proximity sensors, and about the
same as capacitive sensors. They are widely used in automated systems because they have been available
longer and because some can fit into small locations. These sensors are more commonly known as light
beam sensors of the thru-beam type or of the retro reflective type. Both sensor types are shown below.

A complete optical proximity sensor includes a light source, and a sensor that detects the light. The
light source is supplied because it is usually critical that the light be "tailored" for the light sensor system.
The light source generates light of a frequency that the light sensor is best able to detect, and that is not
likely to be generated by other nearby sources. Infra-red light is used in most optical sensors. To make the
light sensing system more foolproof, most optical proximity sensor light sources pulse the infra-red light on
and off at a fixed frequency. The light sensor circuit is designed so that light that is not pulsing at this
frequency is rejected.
The light sensor in the optical proximity sensor is typically a semiconductor device such as a
photodiode, which generates a small current when light energy strikes it, or more commonly a
phototransistor or a photo-darlington that allows current to flow if light strikes it. Early light sensors used
photoconductive materials that became better conductors, and thus allowed current to pass, when light
energy struck them. Sensor control circuitry is also required. The control circuitry may have to match the
pulsing frequency of the transmitter with the light sensor. Control circuitry is also often used to switch the
output circuit at a certain light level. Light beam sensors that output voltage or current proportional to the
received light level are also available.
Through beam type sensors are usually used to signal the presence of an object that blocks light. If
they have adjustable switching levels, they can be used, for example, to detect whether or not bottles are
filled by the amount of light that passes through the bottle. Retroflective type light sensors have the
transmitter and receiver in the same package. They detect targets that reflect light back to the sensor.
Retroreflective sensors that are focused to recognize targets within only a limited distance range are also
available.
6. Explain Optical Proximity sensor with the help of a neat and labelled diagram.
Ans. Already explained in Q. No.5.
7. Explain Ultrasonic sensor with the help of a neat and labelled diagram.
Ans. Already explained in Q. No.5.
8. Discuss in brief touch sensors and its types.

Ans.

Touch sensors are finding their way into many applications, from mobile phones to remote controls
and appliance control panels. Mechanical button and switch replacement continues to be implemented in a
wide variety of applications. Touch sensors with simple linear or rotational sliders, rotary wheels and touch
pads offer significant advantages for more intuitive user interfaces. They are more convenient to use
without moving parts and provide increased reliability. Using touch sensors allows the designer greater
freedom, while reducing overall system cost. The consumer can now enjoy a more appealing, intuitive
interface often with a more contemporary look.
Touch sensors are designed to detect touch and even the presence of objects without relying on
physical contact. Touch sensors can support multiple electrodes, where several different applications can be
controlled by one sensor. By multiplexing the electrodes, the single sensor becomes an extension for
detection at multiple points. For example, capacitive touch sensors are user interface controllers that
manage multiple configurations of touch pads, sliders, rotary positions and mechanical keys. Freescale
offers a broad portfolio of touch sensors as both standard products and software solutions for applications
ranging from gaming controllers to occupant detection. Target markets include consumer, appliance,
automotive, industrial, medical and networking.
Applications
• Gaming controllers
• Home entertainment
• Home appliances
• Cellular handsets
• Portable media devices

Touch Sensor types:
There are various touch sensor types such as 5-wire(or 4-wire) resistive, surface capacitive, projected
capacitive, surface acoustic wave and Infrared sensors.
Wire resistive Sensor : In this type of sensor, when user touches screen, two metallic layers make contact.
This results into flow of current. The point of contact is determined based on change in the voltage. This type of
touch sensors are more affordable but they are damaged with the use of sharp objects.
Surface capacitive Sensor: This sensors are activated with the touch of human skin or a stylus holding an
electrical charge. In this type of monitor, a transparent electrode film is placed on top of the glass panel. When
exposed finger touches the monitor screen, it reacts to the static electrical capacity of the human body. Some of the
charge will get transfer from the screen to the user. The change in capacitance(decreased) is detected by sensors
located at the four corners of the screen. This allows the controller to determine the touch point.
Projected capacitive Sensor: This type of touch sensor is similar to surface capacitive type. It offers two
merits compare to surface capacitive. It can also be activated with the application of surgical gloves as well as thin
cotton gloves. It also detects multiple touch points.
This type of sensor has sheet of glass with embedded transparent electrode films and an IC chip. This
create 3 dimensional electrostatic field. When a finger comes in direct contact with the screen, ratios of electrical
current will change and hence system will detect touch points.
Surface Acoustic Wave Sensor: SAW touchscreen monitors utilize a series of piezoelectric transducers
and receivers. This creates grid of ultrasonic waves on the surface. The other element is placed on the glass
referred as reflector. When a panel is touched, portion of the wave is absorbed. This will help receiving transducer
to locate the touch point and send this data to the system.
Infrared Sensor: This type of touch screen sensor is based on interruption of light path in an invisible light
grid in front of the screen. If an obstable appears inside the grid matrix. it will interrupt the light beams and will
cause reduction in measured photo current in the detectors. Based on these informations, X-Y co-ordinates can be
determined.
In this type of infrared sensor, array of emitters are placed behind two adjacent bezels of the screen frame.
This creates the optical grid as mentioned above.
9. Explain Analog sensor with the help of a neat and labelled diagram.
Ans. Sensors help translate physical world attributes into values that the computer on a robot can use. The
translation produces some sort of output value that the Microcontroller can use.
In general, most sensors fall into one of two categories: Analog Sensors and Digital Sensors.
An analog sensor, such as a CdS cell (Cadmium Sulfide cells measure light intensity), might be
wired into a circuit in a way that it will have an output that ranges from 0 volts to 5 volts. The value can
assume any possible value between 0 and 5 volts.
An 'Analog Signal' is one that can assume any value in a range. An interesting way to think about
this is an Analog Signal works like a tuner on an older radio. We can turn it up or down in a continuous
motion. We can fine tune it by turning the knob ever so slightly.

Digital sensors generate what is called a 'Discrete Signal'. This means that there is a range of
values that the sensor can output, but the value must increase in steps. There is a known relationship
between any value and the values preceding and following it. 'Discrete Signals' typically have a stair step
appearance when they are graphed on chart. If we consider a television sets tuner, it allows us to change
channels in steps.
For example, consider a push button switch. This is one of the simplest forms of sensors. It has two
discrete values. It is on, or it is off. Other 'discrete' sensors might provide you with a binary value. A digital
compass, for example, may provide you with your current heading by sending a 9 bit value with a range
from 0 to 359. In this case, the Discrete Signal has 360 possibilities. The most common discrete sensors
used in robotics provide us with a binary output which has two discrete states. The distinction between
Analog and Digital is important when we are deciding which type of sensor we wish to use. Part of this
decision depends on the type of resources available on our Microcontroller.
Analog to Digital Conversions
Microcontrollers almost always deal with discrete values. Controllers such as the 68HC11 deal with
8 bit values. An important part of using an Analog Signal is being able to convert it to a Discrete Signal
such as a 8-bit digital value. This allows the Microcontroller to do things like compute values and perform
comparisons. Fortunately, most modern controllers have a resource called an Analog to Digital converter
(A/D converter).

The function of the A/D converter is to convert an Analog signal into a digital value. It does this
with a mapping function that assigns discrete values to the entire range of voltages. It is typical for the
range of an A/D converter to be 0 to +5 volts.
The A/D converter will divide the range of values by the number of discrete combinations. For
example, the table shows 5 samples of an Analog Signal that have been converted into digital values.
>= Volts < Volts
Conversion
0.0000 0.0195 0
0.0195 0.0391 1
0.0391 0.0586 2
0.0586 0.0781 3
0.0781 0.0977 4
The range of the Analog Signal is 0 to +5 volts. It is a 8-bit A/D converter, which has 256 discrete
values. Therefore, the A/D converted divides 5 volts by 256 to yield approximately .0195 volts per unit.
The table shows how voltages map to specific conversion values. We have only included the first five, but
the table would continue up to conversion value 255.
The Chart below shows the results of the A/D conversions for 14 samples. The sample numbers are
shown along the X axis at the bottom. The left hand Y axis indicates the voltage of the Analog sample that
was fed into the A/D converter. On the right hand side, the 8-bit value assigned to the conversion is show.
As you can see from the blue line, this was an analog function just like the original Analog Signal
graph shown above. The A/D converter has mapped a set of discrete values onto this graph.
There are many types of A/D converters on the market. An important feature is the resolution of the
converter. An 8-bit converter is fairly common on Microcontrollers. There are others. A 10-bit converter,
for example, will divide by 1024 samples. A 16-bit A/D converter can do 65356 discrete values. The
resolution required for your application depends on the accuracy your sensor requires. The higher the
resolution, the greater the accuracy.

10. Describe briefly the working principle of position sensors with neat sketch.
Ans. Most common way of classifying the wide spectrum of sensors is based on the specific application of
the sensor. Sensor used for measuring humidity is termed as humidity sensor, the one used for
measurement of pressure is called pressure sensor, sensor used for measurement of liquid level is called
level sensor and so on though all of them may be using the same sensing principle.
In a similar fashion, the sensor used for measurement of position is called a position sensor.
Position sensors are basically sensors for measuring the distance travelled by the body starting
from its reference position. 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 rectilinear or curvilinear; accordingly, position sensors are
called linear position sensors or angular position sensors.
Types of Position Sensor
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:
1. Resistance-based or Potentiometric Position sensors
2. Capacitive position sensors
3. Linear Voltage Differential Transformers
4. Magnetostrictive Linear Position Sensor
5. Eddy Current based position Sensor
6. Hall Effect based Magnetic Position Sensors
7. Fiber-Optic Position Sensor
8. Optical Position Sensors
POTENTIOMETRIC POSITION SENSORS
Potentiometric position sensor use resistive effect as the sensing principle. The sensing element is
simply a resistive (or conductive) track. A wiper is attached to the body or part of the body whose
displacement is to be measured. The wiper is in contact with the track. As the wiper (with the body or its
part) moves, the resistance between one end of the track and the wiper changes. Thus, the resistance
becomes a function of the wiper position. The change in resistance per unit change in wiper position is
linear.
Resistance, proportional to wiper position, is measured using voltage divider arrangement. A
constant voltage is applied across the ends of the track and the voltage across the resistance between the
wiper and one end of the track is measured. Thus, voltage output across the wiper and one end of the track
is proportional to the wiper position.
The conductive track can be made linear or angular depending upon the requirements. The tracks
are made from carbon, resistance wire or piezo resistive material.

Three types of potentiometers are used.
a) Wire wound
Wiper slides along coil of Ni-chrome wire.
Wire tends to fail, temperature variations.
b) Cermet
Wiper slides on conductive ceramic track.
Better than wire in most respects.
c) Plastic film
High resolution.
Advantages of these sensors are their ease of use.
CAPACITIVE POSITION SENSORS
Capacitance between any two plates depends upon the permittivity of the dielectric between the plates,
overlapping area between the plates and the distance between the two plates. Any of these three parameters can be
varied in order to design a capacitive sensor.
Capacitive position sensors can use following two configurations:
1. By changing dielectric constant
In this configuration, the body or its part whose displacement is to be measured is connected to the
dielectric material between the plates. As the body moves, the effective dielectric constant between the plates is the
resultant of the dielectric constant due to air and dielectric constant due to the dielectric material. The changing
dielectric constant leads to change in capacitance between the plates. Thus, capacitance becomes a function of the
body position.
This principle is commonly used in level position sensors wherein two concentric tubes are used and fluid
acts as the dielectric. The variation in capacitance with the fluid level is linear.
2. By changing overlapping area
In this configuration, the body or its part whose displacement is to be measured is connected to one of the
plates, the other plate remains fixed. With the movement of the body, overlapping area between the plates changes.
The changing overlapping area between the plates leads to change in capacitance between the plates. Thus,
capacitance becomes a function of the body position.
This principle can be employed for both linear as well as angular motions.

LINEAR VOLTAGE DIFFERENTIAL TRANSFORMER
Linear Variable Differential Transformer commonly known by its acronym, LVDT is an electromechanical
transducer which converts rectilinear motion of an object into a corresponding electrical signal. It is used for
measuring movements ranging from microns upto several inches.
LVDT consists of a primary winding and a pair secondary windings. Primary winding is sandwiched
between the secondary windings. Secondary windings are symmetrically spaced about the primary and are
identically wound. The coils are wound on a hollow form of glass reinforced polymer and then secured in a
cylindrical stainless steel housing. The windings form the stationary part of the sensor.
The moving element of an LVDT is called the core made of highly permeable magnetic material; the core
moves freely axially in the coil’s hollow bore. The core is mechanically coupled to the object whose displacement
is to be measured.
When the primary winding of LVDT is energized by alternating current of suitable amplitude and
frequency, AC voltage is induced in the secondary. The output of the LVDT is the differential voltage between the
two secondary windings; the differential voltage varies with the position of the core. Often, differential AC output
voltage is converted into DC voltage for use in measurement systems.
When primary winding is excited, the voltage induced in the secondary depends upon the coupling of the
magnetic flux by the core to the secondary windings. When the core is at the centre, equal flux is coupled to the
two secondary windings and hence, the differential voltage output is zero. However, when the core is at off-centre,

unequal flux is induced in the secondary windings and the amount of flux in the two windings and hence the
differential voltage between the two windings depend upon the position of the core.
LVDTs offer various advantages like Friction-Free Operation, very high resolution, unlimited mechanical
life, high reliability, no cross sensitivity, environmentally rugged, and so on.
For measuring angular motions, a variant of LVDT, i.e, Rotary Voltage Differential Transformer is used.
RVDT is exactly similar to LVDT in terms of operation; difference is in their construction.
MAGNETOSTRICTIVE LINEAR POSITION SENSORS
Magnetostriction refers to the effect wherein a material changes its size or shape in the presence of the
magnetic field the material due to the alignment of the magnetic domains, within the material, with the applied
magnetic field. Materials having such properties are ferromagnetic materials such as iron, nickel and cobalt.
Reverse effect, i.e. property of changing magnetic properties due to applied stress, is called Villari effect.
Primarily comprising of five components, i.e, the position magnet, waveguide, pickup, damp, and
electronics module, a magnetostrictive position sensor measures the distance between a position magnet and the
head end of the sensing rod. The sensing rod is mounted along the motion axis to be measured. The position
magnet is a ring shaped permanent magnet attached to the member that will be moving and it travels along the
sensing rod.

An interrogation (or current) pulse is sent down the waveguide from the electronics module. At the location
of position magnet, magnetic field generated by the current pulse interacts with the magnetic field from the
position magnet. The result is the generation of sonic wave or torsional strain wave in the waveguide. The strain
wave travels towards the head end where the pickup device senses its arrival. Strain wave travelling away from the
head end is removed by the damping module.
Time difference between the generation of the interrogation pulse and the arrival of the return pulse (strain
wave) indicates the location of the position magnet (or the body connected to it).
Eddy Current based position Sensor
Eddy Currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux.
They normally travel parallel to the coil's winding and the flow is limited to the area of the inducing magnetic
field.

Principle of operation of eddy current sensors is as follows:
Applied alternating current fed to the coil induces a primary magnetic field. Primary magnetic field induces
eddy currents in the electrical conducting material (in vicinity of the coil). Eddy currents, in turn, induce secondary
field. This secondary magnetic field has an effect on the coil impedance. Presence or absence of the conducting
material alters the secondary field and in turn, the coil impedance. Change in the coil impedance can be used
measure the distance of the electrical conducting body.
For a defined measuring target the change of coil impedance is a function of the distance. Therefore, the
distance can be derived by measuring impedance change.
Hall Effect based Magnetic Position Sensors
The Hall Effect principle states that when a current carrying conductor is placed in a magnetic field, a
voltage will be generated perpendicular to the direction of the field and the flow of current.
When a constant current is passed through a thin sheet of semiconducting material, there is no potential
difference at the output contacts if the magnetic field is zero. However, when a perpendicular magnetic field is
present, the current flow is distorted. The uneven distribution of electron density creates a potential difference
across the output terminals. This voltage is called the Hall voltage. If the input current is held constant the Hall
voltage will be directly proportional to the strength of the magnetic field.

In position sensors which use hall efffect, the moving part is connected to a magnet. Thus, the sensor
consists of a Hall element and a magnet housed within the sensor shaft. With the movement of the body or its part
the magnet also moves and therefore, the magnetic field across the Hall element and so the Hall voltage. Thus Hall
voltage becomes a function of the position of the moving part.
Commercially available Hall elements are made of Bulk Indium Arsenide (InAs), Thin Film InAs, Gallium
Arsenide (GaAs), Indium Antimonide (InSb).
Fiber-Optic Position Sensor
Optical fibers offer distinct advantages of their immunity to EMI, inability to generate sparks in
potentially explosive environment. Position sensors based on optical fibers can be used for measurement ranging
from few centimeters to few meters where very high resolution is not of paramount importance.
Fluorescence followed by absorption is at the heart of this sensor. Pump source is connected to the body or
its part whose motion is to be sensed. The fiber is fluorescent, and at the ends of the fiber are placed two photo-
detectors.

The logarithm of the ratio of the two signals S1 and S2 is linear in x and independent of the strength of the
pump source.
OPTICAL POSITION SENSOR
Optical sensors are based one of the two mechanisms. In first type, light is transmitted from one end and
received at the other. Change in one of the characteristics- intensity, wavelength, polarization or phase- by the
physical parameter is monitored. In second type, transmitted light is reflected from the object and light returned
towards the source is monitored.
First type of optical sensors are used in optical encoders commonly used to provide feedback to provide
position feedback for actuators. Optical encoders consists of a glass or plastic disc that rotates between a light
source (LED) and light receiver (photodetector). The disc is encoded with alternate light and dark sectors so that
pulses are generated as the disc rotates. Based on the count of the pulses, speed of the disc and hence the angular
position is computed. To identify the direction of movement, two photodetectors are used. Absolute optical
encoders have a unique code that can be detected for every angular position.
An example of second types of sensors is found on machine tools measure the position of the work table is
measured and displayed.
The strip or disc has very fine lines engraved on it which interrupt the beam. The number of interruptions is
counted electronically and this represents the position or angle.

SENSOR SELECTION
When the choices are many, choosing one often becomes an uphill task. As with other types of sensors,
position sensors primarily are selected to suit the application requirement. Parameters which needs to be taken into
account while selecting a position sensors are
· Contact / Non- Contact type
· Motion- Linear/Rotary
· Measurement Range
· Constraints - Dimensions /Weight
· Environment
· Accuracy
· Resolution
· Response Time
· Cost
· Output
Potentiometers are often the cheapest option for position sensing, but needs physical contact with the
moving target. Hall sensors are also cheap but are used in ON/OFF type of applications. It is effective only for
applications where detailed position information is not required. Optical sensors have very fast response as they
are non-contact type, light in weight and don’t need to counter friction. Accuracy is governed by the number of
counts. More the counts, better is the accuracy. However, proper alignment and protection from harshy or dust
environment is necessary. They are relatively costly. Eddy current based position sensors are moderately priced
but are not preferred in applications requiring highly detailed positioning information or where large gaps exist
between the sensor and the target. These can tolerate dirty environments and are good when mounted on stationary
mechanical structure. LVDTs or RVDTs are priced highly but can tolerate dirty or harsh environments. They offer
high accuracy, high precision as well as high sensitivity. They find applications in industrial and aerospace
applications.

Unit III - Solved Question Bank- Robotics Engineering -

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Unit III - Solved Question Bank- Robotics Engineering -