This document discusses different types of transducers. It begins by defining a sensor and transducer. A sensor detects and responds to a signal from some type of energy and converts it to an output representation. A transducer converts one type of energy to another. Transducers can be classified based on their operating principle, type of output, and application. Examples of different transducer types are provided, including resistive, inductive, and capacitive transducers. Specific transducers like LVDT, strain gauge, and capacitive level sensors are described in more detail.

1. SENSORS
• A sensor is a device that receives and responds to a signal.
• This signal must be produced by some type of energy, such as heat,
light, motion, or chemical reaction.
• Once a sensor detects one or more of these signals (an input), it
converts it into an analog or digital representation of the input
signal

2. Transducer
• A transducer is defined as a substance or a device that converts (or
transfers) an input energy into a different output energy.

3. Classification of Transducers

4. The Transducer can be classified
I. On the basis of principle of transduction form used
II. As primary and secondary transducer
III. As passive and active transducer
IV. As analog and digital transducer
V. As transducers and inverse transducer
On the basis of principle of transduction form used
• The transducers can be classified based upon how they convert the input
quantity into resistance, capacitance ,inductance.
Resistive Transducer:
Resistance of an electrical conductor is given by,
R = ρl/A
Where
R = Resistance in Ω
ρ = Resistivity of the conductor (Ω - cm)
l = Length of the conductor in cm.
A = Cross-sectional area of the metal conductor in cm2

5.  It is clear from the equation that, the electrical resistance can be varied by
varying
(i) Length (ii) Cross-sectional area (iii) Resistivity or combination of these.
 The change in electrical resistance is caused by means of pressure,
temperature, force, displacement.
 Eg : Potentiometer, Strain gauge, RTD, Thermistor.
S NO Transducer Basic Principle (Variation of) Measurement
1 Potentiometer R due to movement of slider caused by
external force
Pressure Displacement
2 Resistance
Strain Gauge
R of a wire/semiconductor due to
elongation /compression caused by
externally applied stress
Force
Displacement
3 Resistance
thermometer
R of pure metal (having positive
temperature co-efficient) with change of
temperature
Temperature
4 Thermistor R of pure metal oxide (having negative
temperature co-efficient) with change of
temperature
Temperature

6. Inductive Transducer
 The inductive transducer works upon one of the following three principles:
a) Change of self inductance b) Change of mutual inductance
c) Production of eddy current
 Inductive transducer are mainly used for measurement of displacement.
 Eg : LVDT ,Variable Reluctance, Magnetostriction gauge
S.No Transducer Basic Principle(Variation of) Measurement
1 Differential
Transformer
Differential Voltage of two secondary windings
due to change in position of the core
Displacement,
Position,
Pressure ,Force
2 Variable
Reluctance
Reluctance of magnetic circuit due to change in
position of core or coil
Position
Force
Pressure
3 Magnetostriction
gauge
Magnetic properties changes due to application
of force or stress
Force
Sound
Pressure

7. Capacitive Transducer
• As Capacitance C is a function of ϵr , A, d (i.e) C = f (ϵr , A, d),
when anyone of these quantities changes, the capacitance varies.
This leads to the design of a variable capacitance transducer.
• Eg : Capacitor microphone, dielectric gauge, Variable capacitance
pressure gauge

8. Active and Passive Transducer
Passive Transducer :
 These transducer derive the power required for transduction from an
auxillary power source.
 They are also known as “externally powered transducer”
 Eg : Resistive ,Inductive and Capacitive transducer
S .NO Transducer Basic Principle(Variation of) Measurement
1 Variable
Capacitance
Gauge
C changes due to change of distance between two
parallel plates caused by externally applied force
Displacement
Pressure
2 Capacitor
Microphone
C changes due to sound pressure on a movable
diaphragm
Sound
Music
Noise
3 Dielectric
Gauge
C due to change in dielectric Liquid Level
Thickness

9. Active Transducer
 These transducer do not require an auxillary power source to
produce their output.
 They are also known as self generating type since they develop
their own voltage as output.
 The energy required for production of output signal is obtained
from the physical quantity being measured.
 Eg : Piezoelectric crystal, thermocouple

10. VARIABLE AREA-BASED CAPACITIVE TRANSDUCER
• The capacitance is directly proportional to the area of cross-section
of the parallel plates.
• The capacitance changes linearly with change in area of the plates.
• Hence, the capacitive transducers based on the variable area are
suited for the measurement of both linear and angular
displacements.
MEASUREMENT OF LINEAR DISPLACEMENT

11. • The linear displacement to be measured by the capacitive transducer
is applied to the plate which is movable.
• Now, there will be overlapping of the plates.
• Due to this, there will be change in capacitance.
• The amount of capacitance change is proportional to the displacement
applied to the movable plate.
• The above parallel plate capacitive transducer is suitable for linear
displacement measurements in the range of 1 mm to 10 mm with an
accuracy of 0.005%
• The capacitance is directly proportional to the area of the plates and
varies linearly with changes in the displacement between the plates.
• Instead of a two parallel plate capacitive transducer, a cylindrical
capacitive transducer can also be used for the measurement of linear
displacement.
• It consists of two cylindrical electrodes.
• One cylindrical electrode is a fixed one, whereas the other cylindrical
electrode is movable one

12. • Similar to the parallel plate capacitive transducer, the linear
displacement to be measured is applied to the movable
cylinder which results in overlapping of two cylinders which
in turn further makes a change in capacitance between the
two cylindrical electrodes.
• The amount of capacitance is proportional to the
displacement applied to the movable cylinder.

13. MEASUREMENT OF ANGULAR DISPLACEMENT
• It consists of a set of semi-circular plates forming the two plates of the
capacitor, out of which one plate is fixed and the other plate is
movable.
• The angular displacement to be measured is applied to the movable
plate of the transducer assembly.
• Hence there is a change in effective area between the semicircular
plates which in turn changes the capacitance.
• The amount of capacitance change is depending on the angular
displacement applied to the movable plate, and it is observed that it is
maximum when the plates overlap each other which means the
angular displacement θ = 180 °.

14. • At various angular displacements of θ, the capacitance is
determined by
C =Ɛ θ r2 / d
Where,
r is the radius of semi-circular plates.
𝜃 is the angular displacement.
D is the gap between two plates.
VARIABLE DISTANCE-BASED CAPACITIVE
TRANSDUCER
• The capacitance is inversely proportional to the distance
between the two plates of the capacitor .
• With this principle, variable distance based capacitive
transducer used for the measurement of linear
displacements.

15. • Variable distance based capacitive transducer consists of
two plates among which one plate is fixed, and the other
one is movable.
• The linear displacement to be measured is applied to the
movable plate.
• When the displacement is towards left, the distance
between the plates decreases and hence the capacitance
increases. Whereas when the displacement is towards right,
the distance between the plates increases and hence the
capacitance decreases.

16. • Since the capacitance is inversely proportional to the
variation in distance between the plates, the response of
the transducer is found to be non-linear.
• Due to this nonlinearity in response, it is useful for the
measurement of only smaller displacements.

17. VARIABLE DIELECTRIC-BASED CAPACITIVE TRANSDUCER
• This kind of capacitive transducer works based on variation
in the dielectric material and its corresponding dielectric
constant.
• This arrangement is suitable for large linear displacements.
• Here, the two capacitor plates are kept in fixed position,
whereas the solid dielectric material available between the
fixed plates is movable thereby varying the capacitance of
the transducer assembly.
• The figure shows that how in a capacitor the position of the
dielectric is varied to vary the capacitance

18. • If the area (A) of and the distance (d) between the plates of
a capacitor remain constant, capacitance will vary only as a
function of the dielectric constant (e) of the substance
filling the gap between the plates.
• If the space between the plates of a capacitor is filled with
an insulator, the capacitance of the capacitor will change
compared to the situation in which there is vacuum
between the plates.
• The change in the capacitance is caused by a change in the
electric field between the plates.
• Many factors will cause the dielectric constant (e) to
change, and this change in dielectric constant (e) will vary
for different materials.
• The major factors that will cause a change in dielectric
constant (e) are moisture, voltage, frequency, and
temperature, moisture, humidity, material bulk density, and
particle size etc.

19. • The dielectric constant (e) in the basic formula is the
effective dielectric constant of the total space between the
electrodes.
• This space may consist of the dielectric material, air, and
even moisture, if present. Physical variables, such as,
displacement, force or pressure can cause the movement of
dielectric material in the capacitor plates, resulting in
changes in the effective dielectric constant, which in turn
will change the capacitance.
CAPACITIVE LEVEL MEASUREMENT
• An insulated metal electrode firmly fixed near and parallel to the
metal wall of the tank.
• If the liquid is non-conductive, the electrode and the tank wall form
the plates of a parallel plate capacitor with the liquid in between
them acting as the dielectric

20. • If the liquid is conductive the rod
and the liquid form the plates of
the capacitor, and the insulation
between them is the dielectric.
• Where the tank is not of metal,
two parallel insulated rods or
electrodes, kept at a fixed distance
apart are used.
• The two rods act as two plates of
a parallel plate capacitor.
• The capacitance of this capacitor
depends, among other factors, upon
the height of the dielectric between
the plates.
• The higher the liquid level, the greater
is the capacitance.
• The lesser the height, the smaller is
the capacitance
• Thus, the capacitance is proportional to
the height of the liquid in the tank.
• The capacitance in the above cases
may be measured and this measured
capacitance is an indication of liquid
levels.

21. Variable Inductance Transducer
The variable inductance transducer works on the following principle:
• Change of self inductance
• Change of mutual inductance
• Production of eddy current
Change of self inductance
Self inductance of a coil is given by,
Where,
N = number of turns.
R = reluctance of the magnetic circuit.
Reluctance R is given by,

23. Thus to measure displacement by varying these parameters leads to
change in self inductance,
• Change in number of turns, N,
• Changing geometric configuration, G,
• Changing permeability
Change of mutual inductance
• Change of mutual inductance principle, use multiple coils.
• Consider two coils having their self-inductance L1 and L2.
• Mutual inductance between these two coils is given by
• Thus mutual inductance can be changed by varying self inductance or
by varying coefficient of coupling, K.
• Now coefficient of coupling depends on the distance and orientation
between two coils.
• Thus for the measurement of displacement we can fix one coil and
make other movable which moves with the source whose
displacement is to be measured.

24. • With the change in distance in displacement coefficient of coupling changes
and it causes the change in mutual inductance.
• This change in mutual inductance can be calibrated with the displacement
and measurement can be done.
Production of eddy current
• When a conducting plate is placed near a coil carrying alternating
current, a circulating current is induced in the plate called “EDDY
CURRENT”. This principle is used in such type of inductive
transducers.
• When a coil is placed near to coil carrying alternating current, a
circulating current is induced in it which in turn produces its own flux
which try to reduce the flux of the coil carrying the current and hence
inductance of the coil changes.
• Nearer the plate is to the coil, higher will be eddy current and higher
is the reduction in inductance and vice versa.
• Thus inductance of coil varied with the variation of distance between
coil and plate. Thus the movement of the plate can be calibrated in
terms of inductance change to measure the quantity like
displacement.

25. LVDT MEASUREMENT OF LINEAR DISPLACEMENT
• An LVDT comprises a primary coil connected to an ac source (typically
a sine wave at a frequency in the 1–10 kHz range) and a pair of
secondary coils, all sharing a common ferromagnetic core.
• The magnetic core serves to couple the magnetic flux generated by
the primary coil into the two secondaries, thereby inducing an output
voltage across each of them.
• The secondary coils are connected in opposition, so that when the
core is positioned at the magnetic center of the LVDT, the individual
output signals of the secondaries cancel each other out, producing a
null output voltage.
• When the rod moves the core away from the magnetic center, the
magnetic fluxes induced in the secondary coils are no longer equal,
resulting in a nonzero output voltage.
• The LVDT is called a “linear” transformer because the amplitude of
the output voltage is a linear function of displacement over a wide
operating range.

27. PRINCIPLE OF WORKING

28. • Based on the position of core three cases exist

30. • Output vs Core Displacement A linear curve shows that output
voltage varies linearly with displacement of core.

31. STRAIN GAUGE
Stress:
In mechanics, stress is defined as a force applied per unit area. It is given by
the formula
σ = F/A
where,
σ is the stress applied
F is the force applied
A is the area of force application
• The unit of stress is N/m^2
Stress applied to a material can be of two types. They are:
 Tensile Stress:
 It is the force applied per unit area which results in the increase in length (or
area) of a body.
 Objects under tensile stress become thinner and longer.
 Compressive Stress:
 It is the force applied per unit area which results in the decrease in length
(or area) of a body.
 The object under compressive stress becomes thicker and shorter.

32. Strain :
• It is defined as the amount of deformation experienced by the body in
the direction of force applied, divided by initial dimensions of the
body.
• The relation for deformation in terms of length of a solid is given
below.
ϵ = δl/L
where,
ϵ is the strain due to stress applied
δl is the change in length
L is the original length of the material.
Depending on stress application, strain experienced in a body
can be of two types. They are:
 Tensile Strain: Ratio of increase in length to the original
length of the body when it is subjected to pull force
 Compressive Strain: Ratio of decrease in length to the
original length of the body when it is subjected to push force

34. Sign convection for strain:
 Tensile strain are considered positive in case of producing
increase in length.
 Compressive strain are considered negative in case of
producing decrease in length.
STRAIN GAUGE
• A Strain gauge is a transducer whose resistance will vary when any
force is applied.
• A strain gauge converts force, pressure, tension, weight, etc., into a
change in electrical resistance which is then measured.
• When any external force is applied to an object, deformation occurs
in the shape of the object.
• The deformation observed in the shape can be both compressive
and tensile. This is called strain. This strain is measured by a strain
gauge.

35. • The small changes in resistance of a gauge are measured using the
concept of Wheatstone bridge.
• If R1/R2 = R4/R3, then the output voltage is zero and the bridge is
said to be a balanced bridge.
• A small change in resistance leads to a nonzero output voltage. If ‘R4’
is replaced with a strain gauge and any changes in the resistance of
strain gauge will unbalance the bridge and produce nonzero voltage.

36. • When an object gets stretched within its limits of elasticity
and does not break or buckle permanently, it becomes
thinner and longer, resulting in high electrical resistance.
• If an object is compressed and does not deform, but,
broadens and shortens, results in decreased electrical
resistance.
• The values obtained after measuring the electrical
resistance of a gauge helps to understand the amount of
stress-induced.
TYPES OF STRAIN GAUGES
• The types of strain gauge are:
1. Unbonded Strain Gauge
2. Bonded Strain Gauge

37. Unbonded Strain Gauge
• The unbonded resistance strain gauge consists of a wire stretched
between two points in an insulating medium, such as air.
• In this type, the strain gauge is not directly bonded to the surface
which is subjected to the stress.
• Instead, the wire is stretched between two frames with the help of
insulation pins. The wires are kept under tension so as to avoid sag
and/or free vibration.
• Unbonded Strain Gauge is usually connected in a bridge circuitry. The
bridge is balanced with no load applied to it.

38. Specifications of unbonded strain gauge
• The typical size is 0.003 mm in diameter and 25mm in length
• The maximum excitation voltage is 5V to 10V
• Construction material – Nichrome, constantan, nickel
Bonded Resistance Strain Gauge
• Bonded strain gauges are directly placed or bonded on the surface of
any device or component which is subjected to stress.
Flat grid wire gauge
In the flat grid wire gauges, the fine wire is wound back and forth (i.e., arranged in the form of
a grid), and then it is pasted on a backing material (ex: epoxy, paper, etc), with the help of an
adhesive.

39. b. Wrap Around Wire Gauge:
• In a wrap-around wire gauge, a fine wire is wound on a thin strip or a
flattened tube of paper.
• They can be made smaller in length for the same value of resistance
as compared to the flat grid type.
• In the wrap-around wire gauge type, the wire grid is in between the
two planes, the gauge has a very high surface thickness.
C. Single Wire Gauge
• Single wire gauge type of wire gauge is mainly designed to avoid
any cross-sensitivity factor.
• These are formed when the single wires are stretched across
and laid as shown below.
• In order to avoid the looping of the same material, thick copper
wires are also attached (welded) at the ends. Due to this, cross-
sensitivity is reduced to a great extent.

40. Foil Type strain gauge
• Foil gauges differ from wire gauges in construction.
• In foil gauges, the required grid pattern is formed with a very thin foil
which is of the same material that is used for wire gauges.
• Due to the larger surface area to the area of cross-section ratio, it
has a higher heat dissipation capability and so has better thermal
stability.
• The strain reproducibility is excellent.
• Construction materials are Nichrome copper, nickel-chromium, or
ferrous nickel alloys.