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.

Education

Unit III - Solved Question Bank- Robotics Engineering -