UNIT-II

MOTION, PROXIMITY AND RANGING SENSORS

Motion sensors -Potentiometers –Resolvers-Encoders- Optical, Magnetic and Inductive
Encoders-Linear Variable Differential Transformer (LVDT)-Rotary Variable Differential
Transformer (RVDT) -Syncro, Microsyn, Accelerometer-GPS, Bluetooth-Range Sensors-RF
beacons, Ultrasonic Ranging, Reflective beacons, Laser Range Sensor (LIDAR)

MOTION SENSORS
INTRODUCTION
The study of specific measuring devices with motion measurements. Based on two fundamental
quantities in nature (length and time) and so many other quantities (such as Force, Pressure and
Temperature, etc) are often measured by transducing them to motion and then measuring this
resulting motion. It is mainly concerned with electromechanical transducers which convert motion
quantities into electrical quantities.
POTENTIOMETER:
The potentiometer, commonly referred to as a ‘pot’, is a three-terminal mechanically operated
device is used in a large variety of electrical and electronic circuits. They are passive devices,
meaning they do not require a power supply or additional circuitry in order to perform their basic
linear or rotary position function. Basically, they convert a Mechanical signal to output voltage

Construction:
✓ 3 terminal device
✓ It consists of a Sliding contact / wiper and a resistive element
✓ Resistive element is made of 0.01mm diameter platinum or nickel alloy
✓ A long conductor whose effective length is variable
✓ Based on motion of wiper it is classified into
✓ Linear / Translatory
o Resistive element is straight and the stroke is of 2mm to 0.5m
o Used to measure linear displacement
o Stroke: 2mm to 0.5mm
✓ Rotary / Rotational
o Resistive element is circular in shape
o Used to measure angular displacement
✓ Helipot
o Resistive element is shaped as helix
Application:
One of the most common application is measuring of displacement
✓ To measure the displacement of the any medium, it is connected to the sliding element / wiper
located on the potentiometer
✓ Resistance wire is wound on the conductor whose one end is fixed, while the position of the other
end is decided by the slider position that move along the whole length of the conductor
✓ When there is displacement in that medium, the slider also moves along the conductor so its
effective length changes, due to which it resistance also changes
✓ The effective resistance is measured as the resistance between the fixed position of the conductor
and the position of the sliding contact
✓ Due to this the voltage across these points also changes
✓ The change in resistance or the voltage is proportional to the change in the displacement of the body
✓ Thus, the voltage change indicates the displacement of the body
✓ Since these potentiometers work on the principle of resistance, they are also called as the resistive
potentiometer
Materials used:
1. Wire wound
✓ Materials includes platinum, nickel chromium, nickel copper
✓ Carry large current at high temperature
✓ Resistance temperature coefficient is small
✓ Resolution is 0.025-0.05mm
✓ Resolution is limited by number of turns
✓ Response is limited to 5Hz

2. Non wire wound / continuous potentiometer

✓ Has improved resolution and life
✓ As resolution is not limited by no of turns
✓ Sensitive to temperature change
✓ Materials used:
✓ Cermet:
 o It uses Precious metal particles fused into ceramic base
o Advantage is large power rating at high temperature
o Low cost
o Moderate temperature coefficient of order 100*10-6 to 200*10-6 / °C
o Useful for ac applications
o Resolution is infinite
✓ Hot Moulded Carbon
o Element is fabricated by moulding together a mixture of carbon and thermosetting plastic
binder
o Useful for ac applications
✓ Carbon Film:
o Thin film of Carbon is deposited on non-conductive base
o Advantage is low cost
o Temperature coefficient is upto 1000*10-6/° C
✓ Thin metal film:
o Very thin vapour deposited layer of metal on glass or ceramic base
o Advantage is excellent resistance to changes in environment
o Useful for ac applications
o cost is moderate
o Resolution is infinite
Advantages:
✓ Inexpensive
✓ Simple to operate
✓ Easily available
✓ Used to measure large displacements

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

Linear Variable Differential Transformer or simply LVDT is a well-established inductive transducer used
for measuring the linear displacement. Generally, it is an electromechanical transducer which converts the
movement of the core, which is coupled mechanically, into a corresponding electrical output. LVDT can
measure the displacement up to 1.5 m.
Construction
Identical to a transformer, LVDT consists of primary and secondary windings; in particular, there is one
primary winding and two identical secondary windings. The constructional arrangement of LVDT is shown
in Figure. From the figure, the primary winding is centred between a pair of identically wound secondary
windings which are symmetrically spaced about the primary winding.
These coils are uniformly wound on a thermally stable glass reinforced polymer which is of hollow shape.
It is then encapsulated against moisture and wrapped in a high permeability magnetic shield. Finally, the
coil assembly is protected in cylindrical stainless steel housing and this is usually considered as the
stationary element of the LVDT sensor.

Figure: Constructional arrangement of LVDT

The moving element of the LVDT is called as a core which is simply a separate tubular armature made up
of magnetically permeable material. The core is able to move axially within the hollow shaped coil
assembly. It is mechanically coupled to the object whose position is to be measured. It is shown in Figure
4.7. Since the hollow bore assembly is large sized enough, there will be considerable space between the
stationary and moving elements, and so there is no physical contact between them.

Figure: Core element of LVDT

Principle of Working
Initially, an alternating current of appropriate amplitude and frequency called as primary excitation is
given to the primary winding of LVDT. The electrical output signal from LVDT is obtained as the
differential ac voltage between its two secondary windings. This output voltage is varied based on the
variation in the axial movement of the movable core within the stationary coil assemble of LVDT.
It is mathematically expressed as;
 𝐸0 = 𝐸𝑠1 − 𝐸𝑠2

Where
𝐸0 = 𝐸𝑠1 − 𝐸𝑠2
𝐸𝑠1 is the secondary- 1 voltage
𝐸𝑠2 is the seconday- 2 voltage

Case 1 Core is at Maximum LEFT Position

The LVDT core is moved to the left of the primary winding, i.e., closer to the secondary windin(𝑠1)
than secondary winding-2 as shown in Figure. Due to this movement, more amount of flux is linked with
𝑠1than 𝑠2 Hence the emf induced in 𝑠1 is greater than in 𝑠2

Since 𝐸𝑠1 > 𝐸𝑠2

𝐸0 = 𝐸𝑠1 − 𝐸𝑠2= Positive

Case II Core is at NULL Position The LVDT core is moved to the midway between
the secondary winding-1 and secondary winding-2 as shown in Figure.
 Figure.LVDT core at NULL Position

The midway position is also termed as NULL position. Due to this movement, equal amount of fluxis
linked to both the secondary windings, and hence the voltages induced in both the secondary
windings are equal. Since 𝐸𝑠1 = 𝐸𝑠2

𝐸0 = 𝐸𝑠1 − 𝐸𝑠2= Zero

Microsyn:
 RESOLVERS:
A resolver is very precise electromagnetic device comprising of two stator and two rotor
windings. The output of the transducer is in the form of two signals, one proportional to the sine
of the angle and the other proportional to cosine of the angle. Uses: For conversion of angular
position of a shaft into cartesia co-ordinates.
Construction:
The construction of a resolver is similar to that of a two phase, two pole wound rotor induction
motor. The stator windings are identical and are housed in a magnetic structure, with the axis of
two windings 90⁰ to each other. Similarly , the two rotor windings are placed in a magnetic
structure and are mutually perpendicular to each other.
WINDING CONFIGURATION OF RESOLVER
Stator windings are supplied with an alternating voltage that produces an alternating magnetic
flux which induces voltages in the two rotor windings. The output voltage of the rotor windings
is proportional to the stator voltage and the coupling between stator and rotor windings. The way
in which the windings are placed, the rotor output voltages are proportional to the sine and cosine
of the rotor angle.
When one of the stator windings S1S3 is excited by an A.C source, with the other stator
windings
S2S4 short circuited, the following output voltage are obtained from the
rotor. ER 1-3 = ES 1-3 Cosθ -------------------------(1)
ER 2-4 = -ES 1-3 Sin θ ---------------------- (2)
When the two stator windings are excited, the outputs are us under

ER 1-3 = ES 1-3 Cosθ + ES 2-4 Sin θ -------------------

(3) ER 2- 4 = ES 2-4 Cosθ - ES 1-3 sin θ ----------------
---(4)
When the two rotor windings are excited , the output from the stator windings

ES 1-3 = ER1-3 Cosθ - ER 2-4 Sin θ --------------------

(5) ES 2- 4 = ER 2-4 Cosθ + ER1-3 sin θ --------------
---(6)
where θ= angular displacement of rotor

Classification of Resolvers
I)Computing Resolvers: Uses:
For generating sine, cosine and tangent functions as well as for solving geometric relationships.
II)Synchro resolvers:
Uses: For data transmission It perform the same functions as synchro transmitters, receivers
and control transformers but with a better accuracy.
 APPLICATIONS:
Vector Resolution Vector Composition
Vector angle and component resolution
Pulse amplitude control and pulse resolution
Phase shifting

ROTARY VARIABLE DIFFERENTIAL TRANSFORMER (RVDT)

RVDT is used to sense the angular displacement and it is similar to the LVDT except that its core
is cam shaped and may be rotated between the windings by means of shaft are shown in figure.

Operations:
The operation of a RVDT is similar to that of LVDT. At the null position of the core, the output
voltages of secondary winding S1 and S2 are equal and in opposition. Therefore, the net output is
zero. Any angular displacement from the null position will result in a differential voltage output.
The greater this angular displacement, the greater will be the differential output. Hence the
response of the transducer is linear. Clockwise rotation produces an increasing voltage of a
secondary winding of one phase while counter clock - wise rotation produces an increasing voltage
of opposite phase. Hence the amount of angular displacement and its direction may be ascertained
from the magnitude and phase of the output voltage of the transducer.

GPS (GLOBAL POSITIONING SYSTEM):

The Global Positioning System (GPS) is a space-based navigations system that provides location
and time information in all weather conditions, anywhere on or near the earth where there is an
unobstructed line of sight to four or more GPS satellites. GPS is "space-based satellite navigation
system" which can show the exact position on or near the Earth surface, anytime, anywhere, in
any weather condition. The GPS system provides critical capabilities to military, civil and
commercial users around the world.

Advantages and Disadvantages:

The principle advantage of using GPS over land based beacons is that the GPS signal is readily
available which reduces the deployment cost and time of the system. Further, a GPS system is less
susceptible to damage since the satellites, the beacons of the GPS, are maintained by international
reputed organizations. GPS is extremely effective for outdoor ground-based and flying robots. A
disadvantage of this technique is that the stationary receiver must be installed, its location must be
measured very carefully, and of course the moving robot must be within kilometers of this static
unit in order to benefit from the DGPS technique.

System Description

1. The Space Segment: Consists of satellites and transmitted signals.

Special Features of the Space Segment:

The Operational GPS Constellation consists of minimum 24 satellites, each in its own orbit,
approximately about 20,200 km. above the Earth, in 12 hours (nearly 11hrs 58 min). There are
often more than 24 operational satellites as new ones are launched to replace older satellites.
The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each
day. The orbit altitude is such that the satellites repeat the same track and configuration over any
point approximately each 24 hours (4 minutes earlier each day).

2. The Control Segment:

Consists of ground stations (located around the world) that make sure the satellites are working
properly. Control Segments formerly consists of 5 tracking stations situated at Hawaii, Ascension
Island, Diego Garcia, Kwajalein and the Master Control facility is located at Schriever Air force
Base(Formerly Falcon AFB) in Colorado Springs. Newly added control stations after 2005 are
Washington DC England, Ecuador, Argentina, Bahrain and Australia. These Monitor stations
measure signals from the SVs, which are incorporated into orbital models for each satellite. Master
stations collect the data about the satellites of this system continuously from the other tracking
stations. MCS process the tracking data for computation of satellite ephemerides (or co-
ordinate) and satellite clock parameters. The Master control station uploads ephemeris and clock
data to SVs.

3. The User Segment

Consists of receivers, which we can hold in our hand or mount in our car. The GPS user segment
consists of the GPS receivers and the user community. GPS receivers convert SV signals into
position, velocity and time estimates. Four satellites are required to compute the four dimensions
of X, Y, Z (Position) and Time. GPS receivers are used for navigation, positioning, time
dissemination andother research. Navigation in three dimensions is the primary function of GPS.
Navigation receivers are made for aircraft, ships, and ground vehicles and for hand carrying by
individuals. Precise positioning is possible using GPS receivers at reference locations providing
corrections and relative positioning, geodetic control and plate tectonic studies are example.

GPS user segment

Applications of GPS:

1. Road Traffic Congestion ,2. Tectonics 3. GPS and Terrorism 4. GPS of Mining 5. GPS and Tours
6. Navigation 7. Disaster Relief 8. GPS-Equi Radio Sondes and Dropsondes 9. Fleet Tracking 10.
Cellular Telephony 11. Robotics

BLUETOOTH
Bluetooth is a standardized protocol for sending and receiving data via 2.4 GHz wireless link. It's
a secure protocol and it's perfect for short-range, low power, low-cost, wireless transmissions
between electronic devices.

Working of Bluetooth
The Bluetooth protocol operates at 2.4 GHz in the same unlicensed ISM frequency band where
RF protocols like ZigBee and WiFi also exist. There is a standardized set of rules and
specifications that differentiates it from other protocols. BWT-enabled devices operate in the
unrestricted 2.4-gigahertz (GHz) Industrial, Science, Medical (ISM) band. The ISM band ranges
between 2.400 GHz and 2.483 GHz. BWT-enabled devices use seventy-nine 1-megahertz
frequencies (from 2.402 to 2.480 GHz) in the ISM band.
Connection Process:

1. Inquiry
If two Bluetooth devices know absolutely nothing about each other, one must run an inquiry to
try to discover the other. One device sends out the inquiry request, and any device listening for
such a request will respond with its address, and possibly its name and other information.

2.Paging (Connecting)

Paging is the process of forming a connection between two Bluetooth devices. Before
this connection can be initiated, each device needs to know the address of the other
(found in the inquiry process).

3.Connection
After a device has completed the paging process, it enters the connection state. While connected,
a device can either be actively participating or it can be put into a low power sleep mode.

Active Mode: This is the regular connected mode, where the device is actively transmitting or
receiving data.

Sniff Mode: This is a power-saving mode, where the device is less active. It'll sleep and only
listen for transmissions at a set interval (e.g. every 100 ms).

Hold Mode: Hold mode is a temporary, power-saving mode where a device sleeps for a defined
period and then returns back to active mode when that interval has passed. The master
can command a slave device to hold.

Park Mode: Park is the deepest of sleep modes. A master can command a slave to "park", and
that slave will become inactive until the master tells it to wake back up.

4.Bonding and Pairing

When two Bluetooth devices share a special affinity for each other, they can be bonded together.
Bonded devices automatically establish a connection whenever they're close enough.

5. Power Classes

The transmit power, and therefore range, of a Bluetooth module is defined by its power class.
There are three defined classes of power: Some modules are only able to operate in one power
class, while others can vary their transmit power.

6. Bluetooth Profiles

Bluetooth standard to more clearly define what kind of data a Bluetooth module is transmitting.
While Bluetooth specifications define how the technology works, profiles define how it's used.
RANGE SENSORS

Commonly used range sensors in robotics:

1.Tactile and Proximity sensors
2. Ultrasonic Sensors
3. IR Range Sensors
4. Laser Range Finders
5.Vision Systems

Each varies in complexity, size, weight, expense, accuracy, etc.. The detection range is defined
as the maximum distance that the sensor can read reliably from.

RF BEACON (RADIO FREQUENCY BEACON- 1M TO 100 M)

A radio beacon is a transmitter at a known location, which transmits a continuous or periodic

radio signal with limited information content on a specified radio frequency. Occasionally the
beacon function is combined with some other transmission, like telemetry data or meteorological
information.

The Millibot localization system i s based on trilateration, i.e., determination of the position
based on distance measurements to known landmarks or beacons. GPS is an example of a
trilateration system; the position of a GPS unit on earth is calculated from distance measurements
to satellites in space.

Similarly, the Millibot localization system determines the position of each robot based on
distance measurements to stationary robots with known positions. The localization system uses
ultrasound pulses to measure the distances between robots. Periodically, each beacon
simultaneously emits a radio frequency(RF) pulse and an ultrasonic
pulse.

Radio Frequency Beacon

Applications of Radio beacons
1. Air and sea navigation,
2. Propagation research,
3. Robotic mapping,
4. Radio-frequency identification (RFID) / Near Field Communication (NFC) and
5. Indoor guidance, as with real-time locating systems (RTLS) like Syledis or simultaneous
localization and mapping (SLAM).

REFLECTIVE BEACONS
Beacon based Localization
Beacon navigation systems are the most common navigation aids on ships and aircrafts as well
as on commercial mobile robot systems. Active beacons can be detected reliably and provide
accurate positioning information with minimal processing.

As a result, this approach allows high sampling rates and yields high reliability, but it does
also incur high cost in installation and maintenance. Most of the beacon based localization
systems rely on a set of beacons placed at known positions in the environment. The mobile
robot vehicle is equipped with a sensor(s) that can observe the beacons and the navigational
system uses these
observations and knowledge of the beacon positions to locate the robot vehicle.

1.Estimation Process

2. Trilateration
Trilateration is a method to determine the position of an object based on simultaneous
range measurements from three stations located at known sites. In trilateration navigation
systems, there are usually three or more transmitters mounted at known locations in the
environment and one receiver on board the robot.

3.Triangulation
It is the most widespread method used to localize a mobile robot vehicle. In this configuration
there are three or more active transmitters mounted at known locations
ULTRASONIC SENSORS

The sensor sends a sonic pulse signal, which is reflected by the object to be detected. The time,
which the pulse signal requires from the sensor to the object and back, is measured and evaluated.
The distance is calculated from the time and the pulse speed. Ultrasonic sensors are suitable for
use in difficult industrial environments. Disturbances such as dust, soiling or fog do not influence
measurements. Mutually interfering light influences or temperature fluctuations are not a problem
either.

Classification of Ultrasonic Sensors:

1. Short-Range Distance Sensors (Displacement) Precision in the millimetre range

2. Mid-Range Distance Sensors The solution for measuring ranges from 13 mm to 24 m
3. Long-Range Distance Sensors
Developed for maximum ranges from 200 mm to 1,200
m

Advantages & Disadvantages of ultrasonic range sensors

Advantages of ultrasonic range sensors
1. Reliable with good precision
2. Not as prone to outside interference
3. Good maximum range
4. Inexpensive

Disadvantages
1. Sensitive to smoothness & angle to obstacles
2. Poor resolution
3. Prone to self-interference from echos
4. Cannot detect obstacles too close
LIGHT DETECTION AND RANGING (LIDAR)

Laser Range Finders are perhaps the most accurate sensors for measuring distances. Light
distance and ranging (LIDAR) systems use the time taken by the light to fly back and forth to
an object in an effort to measure the distance to this target. Building a LIDAR system can be
made with either a high-speed analog to-digital converter (ADC) or a time-to-digital converter
(TDC).
Lidar systems use one of three techniques:
a) Pulsed Modulation
b) Amplitude Modulation Continuous Wave (AMCW) c) Frequency Modulation Continuous
Wave (FMCW)
Components of a LiDAR system
Laser scanner
High-precision clock
GPS
IMU - Inertial navigation measurement unit
Data storage and management systems
GPS ground station

Electromagnetic Spectrum

Working of Laser
High-voltage electricity causes a quartz flash tube to emit an intense burst of light, exciting
some of the atoms in a cylindrical ruby crystal to higher energy levels. At a specific energy
level, some atoms emit particles of light called photons. At first the photons are emitted in all
directions. Photons from one atom stimulate emission of photons from other atoms and the
light intensity is rapidly amplified.

Applications
1. Factory Automation Optical Proximity Sensor
2. Factory Automation Optical Level Sensor
3. Factory Automation Volume Scanners
4. Drones
INCREMENTAL ENCODERS
The problems caused by reverse motion in the case of tachometer encoder are solved by using
an incremental encoder. The incremental encoder uses atleast two/ three signal generating
elements.
The two tracks the tachometer encoder uses only one track in the case incremental encoder are
mechanically shifted by ¼ cycle relative to each other. This allows detection of motion which
signal rises first thus an up down pulse counter can be used to subtract pulses whenever the
motion reverses. A third output, which produces one pulse per revolution at a distinct point, is
sometimes provided for Zero reference.
Advantage of Incremental encoder:
Able to rotate through as many revolutions as the application requires. Any false pulse resulting
from electric noise will errors that persist even when the noise disappears. The failure of system
power also causes total information about the position data which cannot be retrieved even after
re-application of power.
ABSOLUTE ENCODERS
Generally limited to measurement of a single revolution. They use multiple tracks and outputs,
which are read out in parallel to produce binary representation of the angular shaft input
position. There is a one-to-one correspondence between binary output, position data are recovered
when power is restored after an outage. The transient electric noise causes only transient
measurement errors. Generally limited to measurement of a single revolution. They use multiple
tracks and outputs, which are read out imparallel to produce binary representation of the angular
shaft input position. Since there is a one-to-one correspondence between binary output, position
data are recovered when power is restored after an outage
Classification of Encoders Tachometer Encoders
Has only a single output signal which consists of a pulse for each increment of displacement. If
the motion were always in one direction, a digital counter could accumulate these pulses to
determine the displacement from a known starting point. Any motion in the opposite direction
would also produce identical pulses, which would produce errors. This digital transducer is
usually used for measurement of speed, rather than for displacement and in situations where the
rotation never reverses.
OPTICAL
Incremental Encoder:

Incremental encoders create a series of equally spaced signals corresponding to the mechanical
increment required. For example, if we divide a shaft rotation into 100 parts, an encoder would
be selected to supply 1000 square wave cycles per revolution. By using a counter to count these
cycles, we can find out how much the shaft has rotated

Optical encoders tend to follow one of two principles of operation; They consist of either a system
of coded tracks consisting of transparent and opaque section and associated lamps and photocells
to detect the corresponding switching sequence, or they rely on the use of more fringe techniques,
capable of much higher resolution when used for incremental measurement
SHAFT ENCODERS
The absolute digitiser comprises an assembly consisting of a gray - coded pattern photographically
reproduce on a glass disc mounted on the input shaft. The code consists of ten annular tracks each
with a pattern of opaque and transparent sections. The code reading system employs a filament
lamp and collimating lens from which light passes through the disc and a narrow radial slit, to be
detected by ten photovoltaic cells. Depending on the angular position of the shaft, certain cells
receive light from the transparent portions of the disc and enable the outputs from all ten cells to
reproduce the shaft position directly in parallel - gray - coded form. The output which is noise free,
is suitable for amplification and subsequent processing for use in digital servo systems, computers,
data logging and visual displays.

MAGNETIC ENCODER
In case of magnetic encoders, the conducting portions of the contacting type encoders are
replaced by magnetic tape with magnetized portions and non-conducting portions are
represented by non- magnetized portions as shown in figure below. For magnetizing the portions,
a coating of magnetic material powder is made. The sensing section consists of toroidal cores,
each provided with two coils, namely reading coil (R - coil) and Interrogate coil (I - coil). These
sensing coils are placed closer to the pattern of the magnetic encoder, but there is no contact with
the encoder. The detection of the magnetized portions saturates the toroidal core, and suitable
output signal is generated. When the interrogate coil is energized with a constant voltage signal
of 200 KHz, the reading coil generates the output signals as a transformer action. If the toroidal
core is over the magnetized portion, the output signal from the R - Coil is low and when the core
is over the non - magnetized portion, the output signal from the R - coil is high.

Magnetic encoder
Hence, based on the presence and absence of the magnetized portions, the amplitudes
of the output voltages will vary. If there is low level output voltage, it can be represented
by binary logic - 0, and if there is high level output voltage, it can be represented by
binary logic - 1. This kind of magnetic encoders are very resistant to dust, grease,
moisture and other contaminants common in industrial environments and to shock and
vibration. Hence its applications in industries are high.
INDUCTIVE:

i) Change of self inductance

ii) Mutual Inductance

The mutual inductance between the coils can be varied by variation of self inductances or the
coefficient of coupling.

However, the mutual inductance can be converted into a self inductance by connecting the coils
in series. The self - inductance of such an arrangement varies L1 + L2 – 2 Mto L1 + L2 + 2 M
with one of the coils being stationary while the other is movable. The self inductance of each
coil is constant but the mutual inductanc changes depending upon the displacement of the
movable coil.

iii) Production of Eddy Currents

These inductive transducers work on the principle that if a conducting plate is placed near a coil
carrying alternating current, eddy currents are produced in the conducting plate. The conducting
plate acts as a short - circuited secondary winding of a transformer. The eddy currents flowing
in the plate produce a magnetic field of their own which act against the magnetic field produced
 by the coil. This results in reduction of flux and thus the inductance of the coil is reduced. The
nearer is the plate to the coil, the higher are the eddy currents and thus higher is the reduction in
the inductance of the coil. Thus the inductance of the coil alters with variation of distance
between the plate and the coil.

Types of Inductive Transducer

1.Air Cored Coils
Air cored coil transducers can be operated at a higher carrier frequency because of absence of
eddy current losses in air cores. The inductance of air cored coils is independent of the
current carried by the coil as the permeability of air is constant and does not depend upon
the current carried by the coil. Hence air cored coil tranducer can be used for measurement of
displacement variations occurring at fairly high frequencies.

2. Iron Cored Coils

The greatest disadvantage of iron cored coils transducers is that their inductance is not constant
but depends upon the value of the current carried by the coil. Also, at high frequencies, the
eddy current loss tends to be high and therefore iron cored coil transducers cannot be used
beyond a particular frequency. The frequency of supply voltage should not exceed 20 KHz for
iron core transducer to keep the core losses to acceptable values.

CAPACITIVE TRANSDUCERS

The capacitance transducer works on the principle of change of capacitance which may be
caused by
Change in overlapping area A
Change in the distance d between the plates
Change in dielectric constant
These changes are caused by physical variables like displacement, force and pressure in most
of the cases. The change in capacitance may be caused by change in dielectric constant as is
the case in measurement of liquid or gas levels
. Transducer using Change in Area of Plates

Fig. a Capacitive transducer of cylindrical type

b) Parallel plate capacitive transducer

Transducers Using Change in Distance Between Plates
 Fig: Capacitive Transducer using the principle of change of capacitance with change of
Distance between Plates

Differential Arrangement of Capacitive Transducer

Figure : Capacitive Transducer using Principle of Change in Dielectric Constant for

Measurement of Displacement
Variation of Dielectric Constant for Measurement of Liquid Level
 Case III Core is at Maximum RIGHT Position The LVDT core is moved to the right
of the
primary winding, i.e, closer to the secondary winding-2 than secondary winding-1 as shown in
Figure .

Figure LVDT core at maximum right

Due to this movement, more amount of flux is linked with S than S. Hence the emf induced in

𝑠2 is greater than in 𝑠1

Since 𝐸𝑠1 < 𝐸𝑠2

𝐸0 = 𝐸𝑠1 − 𝐸𝑠2= Negative

MICROSYN