TRANSDUCERS: VARIABLE RESISTIVE/CAPACITIVE/ INDUCTIVE

1. TRANSDUCERS:
VARIABLE
RESISTIVE/CAPACITIVE/
INDUCTIVE
10/28/17 EE & I 1

2. Introduction Of Transducers
10/28/17 EE & I 2
““ENERGY CANENERGY CAN
NEITHER BE CREATEDNEITHER BE CREATED
NOR BE DESTROYED,NOR BE DESTROYED,
IT CAN ONLY BE TRANSFORMEDIT CAN ONLY BE TRANSFORMED
FROMFROM
ONE STATE TO ANOTHER”ONE STATE TO ANOTHER”

3. Introduction Of Transducers
10/28/17 EE & I 3
Transducer is a device, usually electrical,
electronic, or electromechanical, that
converts one type of energy to another for
various purposes including measurement
or information transfer.
It must produce the output which depends
on some way on the input
quantity/measurand.

4. Introduction Of Transducers
10/28/17 EE & I 4
Its output will depends on its input if it
processes upon the actual input.
So transducer extracts some energy
from the measured medium.
Some energy is required to convert the
in input to output.
This energy can be supplied from
outside or by the measurand only.

5. Introduction Of Transducers
10/28/17 EE & I 5
Transducers are divided into two types:
• Active Transducer
• Passive Transducer

6. Active Transducers
10/28/17 EE & I 6
• An active transducer does not require an
external power supply to provide an output
signal.
• These transducers usually rely upon
magnetic inductance or piezoelectric
effects to produce their output signals.
• Ex. Piezoelectric crystal used for
acceleration measurement.

7. Active Transducers
10/28/17 EE & I 7
• Measurand is converted into output
without any other form of energy
requirements.
• They are also called self generating type
transducers.

8. Passive Transducers
10/28/17 EE & I 8
• Passive transducers need an external
power supply in order to amplifies the
input and generate an output signal.
• All passive transducers change one of,
or a combination of, the three general
circuit parameters; resistance,
inductance, or capacitance.
• Ex. Potentiometer is used for
measurement of pressure,
displacement and position.

9. Basic Requirements Of
Transducer
10/28/17 EE & I 9
• Linearity: Linearity of any transducer is the
prime requirement. A transducer having
linear input output characteristics is a big
plus.
• Repeatability: A transducer having this
quality produces the same result again and
again when the same input signal is applied
repeatedly under same environmental
conditions ex. Temperature, pressure,
humidity etc.

10. Basic Requirements Of
Transducer
10/28/17 EE & I 10
• Ruggedness: A transducer should be
robust in construction. It should be
mechanically rugged, so that is capable of
withstanding overload.
• High Signal To Noise Ratio: The quality
of output signal of transducer should be
good; it should be free from the internal
and external noise.

11. Basic Requirements Of
Transducer
10/28/17 EE & I 11
• Highly Reliable: Output of the transducer
should be highly reliable and stable it should
be precise. It should give minimum error in
measurement for temperature variations,
humidity vibrations.
• Good Dynamic Response: A transducer
may be called upon to respond to either
slowly varying or dynamic signals. Its output
should be faithful to input when taken as a
function of time.

12. Basic Requirements Of
Transducer
10/28/17 EE & I 12
• No Hysteresis: A good transducer is
free from hysteresis. It should not
introduce any hysteresis during
measurement while input signal is varied
from its low value to high value and vice
versa.
• Residual Deformation: There should be
no deformation of testing material after
the removal of any pressure after long
period of application.

13. Variable Resistance Type
Transducer
10/28/17 EE & I 13
• The variable resistance transducers are
one of the most commonly used types
of transducers. The variable resistance
transducers are also called as resistive
transducers or resistive sensors.
• They can be used for measuring
various physical quantities like
temperature, pressure, displacement,
force, vibrations etc.

14. Variable Resistance Type
Transducer
10/28/17 EE & I 14
• These transducers are usually used as the
secondary transducers, where the output
from the primary mechanical transducer
acts as the input for the variable
resistance transducer.
• The output obtained from it is calibrated
against the input quantity and it directly
gives the value of the input.

15. Principle Of Working Of Variable
Resistance Type Transducer
10/28/17 EE & I 15
The variable resistance transducer
elements work on the principle that the
resistance of the conductor is directly
proportional to the length of the conductor
and inversely proportional to the area of
the conductor. Thus if L is the length of the
conductor (in m) and A is its area (in m
square), its resistance (in ohms) is given
by:
R = ρ L/A.

16. Principle Of Working Of Variable
Resistance Type Transducer
10/28/17 EE & I 16
L is the length ,A is cross sectional area and
p is resistivity of the resistance material.
So resistance can be changed if any of
these value p ,L or A is changed.
Measurand is connected to the resistance in
such a way that it varies any one of its
parameters.
A change in the value or R is proportional to
the measurand.
Thus the measurand can be measured by
measuring the change in resistance.

17. Principle Of Working Of Variable
Resistance Type Transducer
10/28/17 EE & I 17
• Where ρ is called as resistivity of the
material and it is constant for he materials
and is measured in ohm-m.
• The resistance of some materials also
changes with the change in their
temperature. This principle is primarily used
for the measurement of temperature.

18. Variable Resistance Type
Transducer
10/28/17 EE & I 18
 Some of the variable resistive
transducers are:
Sliding wire(potentiometer)
Strain gauge
Load cell
RTD
Thermistor
Hot wire anemometer
Carbon microphone
Humidity sensor

19. Potentiometer
10/28/17 EE & I 19
• A potentiometer informally, a pot, is a
three-terminal resistor with a sliding
contact that forms an adjustable voltage
divider. If only two terminals are used (one
side and the wiper), it acts as a variable
resistor or rheostat.
• Potentiometers are commonly used to
control electrical devices such as volume
controls on audio equipment.

20. Potentiometer
10/28/17 EE & I 20
The potentiometer is a displacement
transducer.
This is a passive transducer.
It consists of resistive material whose
resistance is proportional to its length.
Slider displacement x is proportional to
the output voltage.
The most common problem is dirt under
the slider.

21. Potentiometer
10/28/17 EE & I 21
One end of conductor is fixed.
The position of the other end is decided
by the slider/brush.
This contact can move along the whole
length of the conductor.
When the body moves the slider also
moves along the conductor so its effective
length changes.
Due to which its resistance also changes.

22. Potentiometer
10/28/17 EE & I 22
• The effective resistance value is
measured between the fixed position of
the conductor and the position of the
slider.
• The resistance is calibrated against the
input quantity, whose value can be
measured directly.

23. Potentiometer
10/28/17 EE & I 23

24. Potentiometer
10/28/17 EE & I 24

25. Potentiometer
10/28/17 EE & I 25

26. Thermistor
10/28/17 EE & I 26
• Thermistors are thermally sensitive
resistors, generally composed of
semiconductor materials. All resistors vary
with temperature, but thermistors are
constructed of semiconductor material
with a resistivity that is especially sensitive
to temperature. This resistance changes
predictably with temperature.

27. Thermistor
10/28/17 EE & I 27
• Although Positive Temperature
Coefficient(PTC) units are available, most
thermistors have a Negative Temperature
Coefficient(NTC); that is, their resistance
decreases with increasing temperature.
• The thermistor is an extremely non-linear
device.

28. Thermistor
10/28/17 EE & I 28
• The price we pay for this increased
sensitivity is loss of linearity.
• These are usually connected to an
electronic circuit that reads out temprature
digitally.

29. Thermistor
10/28/17 EE & I 29
• The resistance temperature relation is
generally of the form:
R = R0 exp[β(1/T – 1/T0)]
• R = Resistance at temp. T, Ω
• R0= Resistance at temp. T0, Ω
• β = Constant, Characterstics of
material
• T, T0 Absolute tempratures, K

30. Thermistor
10/28/17 EE & I 30
• NTC thermistors operates over a range of
-200ºC to +1000ºC.NTCs should be
chosen when a continuous change of
resistance is required over a wide
temperature range. They offer
mechanical, thermal and electrical
stability, together with a high degree of
sensitivity.
• PTC thermistors are temp dependent
resistors manufactured from Barium
Titanate and should be chosen when a
drastic change in resistance is required at
a specific temperature or current level.

31. Thermistor
10/28/17 EE & I 31

32. Advantages Of Thermistor
10/28/17 EE & I 32
• Inexpensive
• Rugged
• Reliable
• Respond Quickly
• Highly Sensitive
• Manufactured in a wide range of shapes,
sizes and values.

33. Humidity Measurement
10/28/17 EE & I 33
• Humidity is the amount of water vapour
in the air and Humidity Measurement is
a measure of relative amount of water
vapour present in the air or a gas.
The humidity can be expressed in
different ways:
• Absolute Humidity
• Relative Humidity
• Dew Point

34. Humidity Measurement
10/28/17 EE & I 34
• Devices that indirectly measure
humidity by sensing changes in
physical or electrical properties in
materials due to their moisture content
are called hygrometers.
• The three major instruments used for
measuring humidity in industry are:
• The Electrical Hygrometer
• The Psychrometer
• The Dew Point Meter

35. Humidity Measurement
10/28/17 EE & I 35

36. Resistance Hygrometer
10/28/17 EE & I 36
• This is an electrical hygrometer.
• It is an active transducer.
• These instruments are suitable for
measuring moisture levels between 15%
and 95%.
• It has typical measurement uncertainty of
3%.
• Atmospheric contaminates and operation
in saturation conditions both cause
characteristics drift.

37. Principle Of Resistance
Hygrometer
10/28/17 EE & I 37
• Some Hygroscopic Salts exhibit a
change in resistivity with humidity.
Resistive hygrometer humidity sensors
use the change in resistance of a
hygroscopic material between two
electrodes on an insulating substrate.
• The hygroscopic salt is deposited
between two electrodes. The resistance
of the element changes when it is
exposed to variations in humidity.

38. Resistance Hygrometer
10/28/17 EE & I 38
• The Resistance Hygrometer should
not be exposed to conditions of 100%
humidity as the resulting condensation
may damage the device.
• These are accurate to within ± 2.5 %
or ± 1.5 % in some cases.
• Response times are typically of the
order of a few seconds.

39. APPLICATIONSAPPLICATIONS
• Humidity sensors can be used not only to
measure the humidity in an atmosphere
but also to automatically control:
-> Humidifiers
-> Dehumidifiers
-> Air conditioners for adjustment.
10/28/17 39EE & I

40. Strain Gauge Pressure Transducer
10/28/17 EE & I 40
• When the tension is applied to the
electrical conductor, its length increases
while the cross section area decreases.
• So its resistance changes.
• This change can be measured to
measured.
• Used for the measurement of force,
stress and strain.

41. Strain Gauge Pressure Transducer
10/28/17 EE & I 41
A strain gauge is a passive type
resistance pressure transducer whose
electrical resistance changes when it is
stretched or compressed
A pressure transducer contains a
diaphragm which is deformed by the
pressure which can cause a strain
gauge to stretch or compress. This
deformation of the strain gauge causes
the variation in length and cross
sectional area due to which its
resistance changes.

42. Strain Gauge Pressure Transducer
10/28/17 EE & I 42
The wire filament is attached to a structure
under strain and the resistance in the
strained wire is measured.

43. Strain Gauge Pressure Transducer
10/28/17 EE & I 43
• Construction and Working

44. Strain Gauge Pressure Transducer
10/28/17 EE & I 44
• Construction and Working

45. Strain Gauge Pressure Transducer
10/28/17 EE & I 45
Strain gauge pressure transducers are
used for narrow pressure span and for
differential pressure measurments
 Available for pressure ranges as low
as 3 inches of water to as high as
200,000 psig
Inaccuracy ranges from 0.1 % of span
to 0.25 % of full scale

46. RESISTANCE TEMPERATURE
DETECTOR (RTD)
10/28/17 EE & I 46
Resistance temperature detector (RTD)
devices are conductors used for
temperature sensing.
They can be used in bridge method as
well as ohmmeter method to take the
output.
The change in resistance of material per
unit change in temperature should be as
large as possible.

47. RESISTANCE TEMPERATURE
DETECTOR (RTD)
10/28/17 EE & I 47
• The material should have high value of
resistivity to get required value in less
space.
• Resistance and temperature relation
should be continuous and stable.
• Platinum, nickel and copper are the most
commonly used.
• Tungsten and nickel alloy are also used.

48. APPLICATIONS OF RTDAPPLICATIONS OF RTD
• They can be used in average and
differential temp. measurement.
• Differential temp. sensing to an accuracy
of 0.05º have been accomplished in a
nuclear reactor coolant heat rise
application.
10/28/17 48EE & I