This document discusses different types of displacement, position, and proximity sensors. It describes various contact and non-contact sensors such as potentiometers, strain gauges, LVDTs, capacitive sensors, Hall effect sensors, optical encoders, eddy current sensors, inductive sensors and microswitches. It provides details on their working principles, construction, applications in automation, metrology and other fields. Selection of appropriate sensors depends on factors like required accuracy, resolution, displacement size and cost.

1. ME 407
MECHATRONICS
DHANESH S
ASSISTANT PROFESSOR-MECHANICAL ENGINEERING

2. DISPLACEMENT,PROXIMITYAND,
POSITION SENSORS
Displacement Sensor-used to measure amount by which some object has been moved
Position Sensor-Used to determine position of some object with reference to some
reference point
Proximity Sensor-Uses to trace when an object has moved in particular critical distance
of a transducer
Displacement Sensors can be: Potentiometer, Resistant Strain Gauge, LVDT, Push Pull
Displacement Sensor
Positions Sensors can be: photo electric sensors, Hall effect sensors, Optical Encoders
Proximity Sensors can be: Pneumatic Proximity Sensor, Eddy current proximity sensor,
Inductive proximity sensor, Micro switch and Reed Swtich

3. DISPLACEMENT,PROXIMITYAND,
POSITION SENSORS
Displacement and Positions sensors: classified into
Contact Sensors
Non-Contact Sensors
Contact Senors:
Measured object is in mechanical contact with the sensor
A sensing shaft which is in direct contact with object being monitored
Movement of shaft used to make change in electrical voltage, capacitance or
resistance
Non-Contact Sensors:
Measured object causing change in air pressure in the sensor or a change in
inductance or capacitance. No physical contact between measured object and sensor

4. DISPLACEMENT,PROXIMITYAND,
POSITION SENSORS
Factors considered while selecting sensors for this applications :
Accuracy required
Resolution required
Size of the displacement
Displacement type
Cost and material made

5. DISPLACEMENT SENSORS
Potentiometer Sensors:
Potentiometer-used to convert rotary or linear displacement to a voltage
Two types: Rotary potentiometer and Linear Potentiometer
Potentiometer consist of resistance element with a sliding contact that can be mover
from one end to other.
This sliding contact is called Wiper
Motion of sliding contact may be linear or angular
Linear wire wound potentiometer: consist of resistance element with number of turns
of wire wound around a non conducting former together with a movable contact which
travels over bare wires

6. DISPLACEMENT SENSORS
Linear or angular displacement of wiper contact is directly proportional to input
voltage
Rotary potentiometer: consist of a circular wire-wound track over which a
rotatable sliding contact can be rotated
Wire wound track-can be single turn of helical turn
Object of whose displacement is to be measure is connect to slider by using:
A rotating shaft for angular displacement
A moving rod for linear displacement
A cable that is kept stretched during operation

7. DISPLACEMENT SENSORS

8. DISPLACEMENT SENSORS

9. DISPLACEMENT SENSORS

10. DISPLACEMENT SENSORS
Applications:
Used in control systems with a feedback loop to ensure moving member or
component reaches its commanded position
Used on machine tool controls, elevators, automobile throttle controls, injection
moulding machines, printing, spraying, robotics etc
Advantages of Resistance Potentiometer:
Simple and inexpensive
Useful for measurement of large amplitudes of displacement
Electrical efficiency is high
Simple in operation

11. DISPLACEMENT SENSORS
Strain Gauge
Strain: ratio of change in length to original length
Resistance strain guage: exhibits change is electrical resistance when it is stretched or
strained
When strain gauges subject to strain-resistance R changes and change is resistance is
proportional to strain
G is known as Gauge Factor

12. DISPLACEMENT SENSORS
G value is between 2 to 4 and resistance are take in order of 100 Ohm
Strain gauges: two forms, wire and foil
Design of strain gauge is very long in order to give enough nominal resistance
Strain gauges are bonded or cemented directly on to the surface of body or structure
which is examined
Metal wire strain gauges: a wire stretched between two points in insulting material
such as air
Wires may be made of Copper nickel, chrome nickel or nickel iron alloys
About 0.003mm in diameter and gauge factor of 2. Length of wire is 25mm or less

13. DISPLACEMENT SENSORS

14. DISPLACEMENT SENSORS

15. DISPLACEMENT SENSORS

16. DISPLACEMENT SENSORS
Applications :
Experimental stress analysis and diagnosis of machines and failure analysis
Multi-axial stress fatigue testing, proof testing, residual stress and vibration
measurement, torque measurement, bending and deflection measurement,
compression and tension measurement etc
Used as sensors for machine tools and safety in automotives
Force measurement in tools, hydraulic or pneumatic press and as impact sensor in
aerospace vehicles

17. DISPLACEMENT SENSORS
Capacitive Element Bases Sensor(Push Pull Displacement Sensor):
Non-contact type sensor and primarily used to measure linear displacements
from few millimetres to hundreds of millimetres
Three plates, with upper pair forming one capacitor and lower pair another
Linear displacement has two forms:
One of the plates is moved by displacement so that plate separation changes
Area of overlap changes due to displacement

18. DISPLACEMENT SENSORS

19. DISPLACEMENT SENSORS
Capacitance C of parallel plate is given by
A is are of overlap between two plates and d is the plate separation
When central plate movers near to top plate or bottom one due to movement of
element/workpiece of which displacement is measured, separation in between
plate changes

20. DISPLACEMENT SENSORS
When C1 and C2 are connected to Wheatstone’s bridge , resulting out of
balance voltage would be proportional to displacement x
Applications:
Small vessel pump control
Grease level monitoring
Level control of liquids
Metrology applications
Assembly line testing

21. DISPLACEMENT SENSORS
Linear Variable Differential Transformer(LVDT):
Its inductive displacement sensor
Construction of LVDT Sensor

22. DISPLACEMENT SENSORS
Working of LVDT Sensor

23. DISPLACEMENT SENSORS
Consist of three coils spaces along an insulated tube
Central coil is primary coil and other two are secondary coils
Secondary coils are connected in series in such a way that their outputs oppose
each other
Magnetic core attached to element of which displacement is to be monitored is
placed inside insulated tube
Alternating voltage input to primary coil result in alternating electromagnetic
forces generated in secondary coils

24. DISPLACEMENT SENSORS
When magnetic core is centrally places with half portion in each of secondary
coil regions-resultant voltage is zero
If core is displaced more in secondary coil 1 than in coil2-more emf is
generated in one coil i.e.coil 1 than other and there will be resultant voltage form
coils
If magnetic core is displaced, then value of resultant voltage increases in
proportion with displacement
Signal process devices like low pass filters, demodulators, can be measured by
LVDT sensors

25. DISPLACEMENT SENSORS
Have good repeatability and reproducibility
Highly reliable
Applications:
Provide displacement feedback for hydraulic cylinders
Control weight and thickness of medicinal products
Automatic inspection of final dimensions of products being packed for dispatch
Measure distance between approaching metals during friction welding process
Monitor fluid level as part of leak detection system
Detect number of currency bills dispensed by an ATM

26. POSITION SENSORS
Photo-Electric Sensors:
Mostly light emitting diodes for light source and phototransistor is used to
detect light radiation
Two types of photoelectric sensors for sensing objects:
A reflective type photoelectric sensor detects the objects based on reflection of
light onto detector from target
Trasmissive type of sensors is used to measure change in light quantity caused
by targets crossing optical axis

27. POSITION SENSORS
Hall Effect Sensors:
 When a beam of charged particles passes
through a magnetic field , the beam is
deflected from its straight line path due to
forces acting on the particles. A current
flowing in a conductor, like a beam , is
deflected by a magnetic field.
 This effect is called Hall Effect

28. POSITIONS SENSORS
Works on the principle that when a beam of charge particles passes through a
magnetic field, forces act on particles and current beam is deflected from its
straight line path
One side of disc will become negatively charged and other side will be positive
charge
This charge separation generates a potential difference which is a measure of
distance of magnetic field from disc carrying current
Have the advantages of being able to operate as switches and it can operate upto
100KHz

29. POSITION SENSORS
Applications:
Uses a magnetic to electric transducer
Used for measurement of position, displacement and proximity
Used for measurement of current
Used for measurement of power
Measurement of fluid level in a container
Optical Encoder;
Encoder is a device that provides a digital output in response to a linear or
angular displacement
An optical encoder is a suitable transducer for measurement of angular
position and has added advantage of having digital output

30. POSITION SENSORS
Optical Encoder has four main parts: a light source, a code disk, a light detector
and a signal conditioner
Rotary encoder compose of a glass or plastic code disk with a photographically
deposited radial pattern organized in tracks
Digital encoder is a device that converts motion into a sequence of digital
pulses
Counting or decoding these bits, pulses can be converted into relative or
absolute position measurements
Optical encoders are in rotary or linear configurations
Rotary encoders are two forms: absolute encoder and incremental encoder

31. POSITION SENSORS
 Widely used in servo motors to
measure rotation of shafts
 Comprises of a disc with three
concentric tracks of equally spaced
holes
 Three light sensors are employed to
detect light passing through holes
 These sensors produce electric pulses
which give angular displacement of
mechanical element e.g.shaft on
which optical encoder is optical
encoder is mounted

32. POSITION SENSORS
Inner track has just one hole which is used to locate the home position of disc
Hole on middle track offset form holes of outer track by one-half of width of
hole
This arrangement provides direction of rotation to be determines
When disc rotates in clockwise direction, pulses in outer track lead those in
inner, in counter clockwise direction they lag behind
Resolution can be determine by number of holes on disc

33. POSITION SENSORS
Rotary encoders are two forms:absolute and incremental encoder
Absolute Encoder:
Designed to produce unique digital word corresponding to each rotational position of
shaft that distinguished N distinct positions of the shaft
Eg: if there are 8 tracks, the encoder is capable of producing 256 distinct positions.
Resolution angle of 360/256
Typical absolute encoders may have from 6 to 20 tracks
Each track produces 1 bit of binary numbers according to code that is established by
hole pattern of code disk

34. POSITION SENSORS
 This is the basic form of absolute
encoder
 Rotating disc has four sensors to
detect light pulses
 Slots are arranged such a way
that outputs made in binary code
 Number of bits in binary number
will be equal to number of tracks

35. POSITIONS SENSORS
 Most common types of numerical
encoding uses are gray and natural
binary codes
 Track pattern for simple 4track
encoder is shown

36. POSITION SENSORS

37. POSITIONS SENSORS
Incremental Encoder:
Produces equally spaced pulses from one or more concentric tracks of code disk
Each track has its own light beam
Thus an encoder with 3 tracks will have three light sources and three light sensors

38. POSITION SENSORS
Consist of a pair of opaque discs and has a pattern of windows, one of which is fixed
and other is rotating
Rotating disc is attached to shaft whose angular speed is measured
Fixed disc has only one window and a beam of optical light perhaps a LED which
passes through all the time
Second disk has a track of windows cut into which are spaced equidistantly around
second disc
When disc rotates, then a light beam is alternatively transmitted and stopped and so a
pulsed output is produced and detected by photocells and fed to a counter
Number of pulses is proportional to angle through which disc is rotated, resolution
proportional to number of slots on a disc

39. PROXIMITY SENSORS
Pilot devices used to detect presence of an object without making any physical
contact
Solid state electronic devices, normally of eddy current type or inductive,
capacitive type
Proximity tells central system whether moving part is at a certain place
They come under non contact type sensors

40. PROXIMITY SENSORS
 Eddy Current Proximity Sensors:

41. PROXIMITY SENSORS
Used to detect non-magnetic but conductive materials
Comprise of a coil, an oscillator, a detector and a triggering circuit
When an AC current passed through this coil, an alternative magnetic field is generated
If a metal object comes in close proximity of coil, then eddy currents are induced in
object due to magnetic field
These eddy currents create their own magnetic field which distorts magnetic field
responsible for generation
As a result, impedence of coil changes and so amplitude of AC
Can be used to trigger a swtich at some pre-determined level of change in current
Available in small in size, highly reliable and have high sensitivity for small
displacements

42. PROXIMITY SENSORS
Applications:
Automation requiring precise location
Machine tool monitoring
Final assembly of precision equipment such as disk drives
Measuring dynamics of continuously moving target such as vibrating element
Drive shaft monitoring
Vibration measurements

43. PROXIMITY SENSORS
Inductive Proximity Switch:

44. PROXIMITY SENSORS
Used for detection of metallic objects
Has four components:the coil, oscillator, detection circuit and output circuit
AC current is supplied to coil which generates a magnetic field
When a metal object comes closer to end of coil, inductance of coils changes
This is continuously monitored by a circuit which triggers a switch when a
preset value of inductance change is occurred
Applications:
Industrial automation
Security: detection of metal objects, arms, land mines

45. PROXIMITY SENSORS
Capacitive Sensor:
A device actuated by both conductive and non-conductive materials
Pair of plates of a capacitor is separated by some distance
Depending upon the separation, the capacitance measured will be changed
Proximity of object can be detected if one of plates of capacitor acts as switch
and other being metal object whose proximity is to be detected

46. PROXIMITY SENSORS
Pneumatic Proximity Sensor:

47. PROXIMITY SENSORS
Low-pressure air is allowed to escape through a port which is placed in front
position of sensor
Escaping air reduces pressure in nearby sensor output port which there is not
close by object
If there is a close by object the air will not escape readily, so pressure increases
in sensor output port
This output from sensor depends on proximity of objects

48. PROXIMITY SENSORS
Microswitch and Reed Switch:

49. PROXIMITY SENSORS
Microswitch is used for determine presence of an item on a conveyer belt
This might be actuated by weight of the item on belt depressing the belt by
spring loaded platform neared to sensor the presence of item in conveyer is
determined
Closeness of switch is done by movement of this spring loaded platform
Reedswitch is a non-contact proximity switch used for checking closure of
doors
Consist of two magnetic switch contacts sealed in glass tube
When a magnet is brought close to the switch, magnetic reed are attracted each
other and close the switch contacts