The document discusses various types of transducers used to convert non-electrical quantities to electrical signals. It describes transducers such as strain gauges, temperature transducers including thermocouples and thermistors, LVDT, capacitive, piezoelectric, and opto-electronic transducers. Strain gauges use resistance variation to measure strain or force. Temperature transducers include RTDs with resistance variation with temperature, thermocouples producing voltage related to temperature difference, and thermistors with negative resistance-temperature coefficient. LVDT and capacitive transducers convert displacement to an electrical signal. Piezoelectric transducers generate charge when subjected to pressure.

1. Mr. Vikas Karade
Asst. Professor
P.V.P.I.T. Budhgaon

2.  Transducer
 Classification
 Strain Gauge
 Thermistors
 Thermocouple
 LVDT
 Capacitive Transducer
 Piezo-electric Transducer
 Opto-electronic Transducer

3.  Sensors
◦ Elements which generate variation of electrical quantities (EQ) in
response to variation of non-electrical quantities (NEQ)
 Examples of NEQ
◦ Temperature, displacement, humidity, fluid flow, speed, pressure,…
 Sensor are sometimes called transducers
Sensing
element
Signal
conditioning
element
Signal
conversion/
processing
element
Output
presentation
Non-
electrical
quantity
Electrical
signal

4. The transducer is defined as the device which convert the non
electrical form of energy into electrical form of the energy
Example:
 Temperature transducers
 Thermocouples
 Resistance-Temperature Detectors (RTD)
 Thermistors
 Resistive position transducers
 Displacement transducers
 Strain gauge

5.  On the basis of transduction form used.
 As primary and secondary transducers.
 As passive and active transducers.
 As analog and digital transducers.
 As transducers and inverse transducers

6.  Resistive Transducers.
 Capacitive Transducers.
 Inductive Transducers.
 Voltage and current Generating Transducers.

7. Example
LVDT and bourdon tube

8.  If transducers derive the power require for transduction
from an power source, then this kind of transducer are
known as passive transducer
Example:
(1) LVDT
(2) RVDT
 When there is no need for any source then these type of
transducers are Active transducers
Example :
(1)Thermocouple
(2)Piezoelectric crystal

9. These type of transducers convert a electrical quantity into
non-electrical quantity
Example
 Piezoelectric crystal
 Analog ammeter
 voltmeter

10.  Analog sensor
 Gives an output that varies continuously as the input changes
 Output can have infinite number of values within the sensor’s range
 Digital sensor
 Has an output that varies in discrete steps or pulses or sampled form
and so can have a finite number of values
 The switching operations are counted by an electronic counter
10

11.  Operating principle
 Sensitivity
 Operating range
 Accuracy
 Errors
 Environmental capability
 Insensitive to unwanted Signal
 Stability

12. 12
Strain Gauge
The strain gauge is an example of a passive transducer that uses
electric resistance variation in wires to sense the strain produced by a
force on wires. It is a very versatile detector and transducer for
measuring weight, pressure, mechanical force, or displacement.
The construction of a bonded strain
gauge (see figure) shows a fine wire
element looped back and forth on a
mounting plate, which is usually
cemented to the member undergoing
stress. A tensile stress tends to
elongate the wire and thereby
increase its length and decrease its
cross-sectional area.

13. A
L
R


13
The combined effect is an increase in resistance:
Where,
ρ: the specific resistance of the conductor material in ohm meters
L : length of conductor (meters)
A : area of conductor (m2)
As consequence of strain, 2 physical qualities are particular interest:
1) The change in gauge resistance
2) The change in length
The relationship between these two variables called gauge factor,
K, is expressed mathematically as

14. 14
LL
RR
K
/
/



Where
K= the gauge factor
R=the initial resistance in ohms (without strain)
∆R= the change in initial resistance in ohms
L= the initial length in meters (without strain)
∆L=the change in initial length in meters
∆L/L same unit with G, therefore
G
RR
K
/


15. 15
From Hooke theory, stress, S, is defined as internal force/area.
Where
S= the stress in kilograms per square meter
F= the force in kilograms
A= area in square meters
Then the modulus of elasticity of material E or called Young’s
modulus (Hooke’s Law) is written as:
A
F
S 
G
S
E 
Where,
E= Young modules in kg per square meter
S= the stress in kilograms per square meter
G= the strain (no units)

16. 16
Metallic strain gauge – formed from thin resistance
wire or etched from thin sheets of metal foil.
Wire gauge (small) – to minimum leakage – for high T
applications
Semiconductor strain gauge – high output transducers
as load cells
Strain gauge is generally used as one arm of bridge

17. 17
Temperature transducers can be divided into
four main categories:
1. Resistance Temperature Detectors (RTD)
2. Thermocouples
3. Thermistor
4. Ultrasonic transducers

18. 18
Detectors of wire resistance temperature common employ platinum, nickel
or resistance wire elements, whose resistance variation with temperature has
high intrinsic accuracy.
where
R = the resistance of the conductor at temperature t (0C)
R0= the resistance at the reference temperature, usually 200C
α = the temperature coefficient of resistance
ΔT = the difference between the operating and the
reference temperature
1) Resistance Temperature Detector (RTD)
)1(0 TRR  

20. 20
The emf of the thermocouple :
E = c(T1 – T2) + k(T1
2 – T2
2)
Where
c and k = constant of the thermocouple
materials
T1 = The temperature of the “hot”
junction
T2 = The temperature of the “cold” or
“reference” junction

21. 21
A thermistor is a semiconductor made by sintering mixtures of
metallic oxide, such as oxides of manganese, nickel, cobalt, copper
and uranium.
Termistors have negative temperature coefficient (NTC). That is,
their resistance decreases as their temperature rises.
Types of thermistor Resistance
Disc 1 to 1MΩ
Washer 1 to 50kΩ
Rod high resistance

22. 22
This figure shows
resistance versus
temperature for a
family thermistor.
The resistance value
marked at the bottom
end of each curve is a
value at 250C
Note!
The resistance
decreases as their
temperature rises-NTC

24. 24
 Small size and low cost
 Fast response over narrow temperature range
 Good sensitivity in Negative Temperature Coefficient
 (NTC) region
 Cold junction compensation not required due to dependence
 of resistance on absolute temperature.
 Contact and lead resistance problems not encountered due
 to large resistance

25. 25
 Non linearity in resistance vs temperature characteristics
 Unsuitable for wide temperature range
 Very low excitation current to avoids self heating
 Need of shielded power lines, filters, etc due to high
resistance

26. 26
Capacitive Transducer
The capacitance of a parallel plate capacitor is given by
where
k = dielectric constant
A = the area of the plate, in m2
εo = 8.854 x 10-12 F/m
d = the plate placing in m
)(0
Farads
d
kA
C



27. 27
Cont’d
Forms of Capacitance Transducers
Rotary plate capacitor
Rectilinear Capacitance
Transducer
Thin diaphragm

28. 28
Rotary plate capacitor:
The capacitance of this unit proportional to the amount of
the fixed plate that is covered, that shaded by moving
plate.

29. 29
Rectilinear capacitance transducer:
It consists of a fixed cylinder and a
moving cylinder. These pieces are
configured so the moving piece fits
inside the fixed piece but insulated
from it.

30. 30
Thin diaphragm:
A transducer that varies the spacing
between surfaces. The dielectric is either
air or vacuum.
Often used as Capacitance microphones.

31. 31
Advantages:
1. Has excellent frequency response
2. Can measure both static and dynamic phenomena.
Disadvantages:
1. Sensitivity to temperature variations
2. the possibility of erratic or distortion signals owing to
long lead length
Applications:
1. As frequency modulator in RF oscillator
2. In capacitance microphone
3. Use the capacitance transducer in an ac bridge circuit

32.  A simple application of
such a transducer is for
liquid measurement
 The dielectric constant
changes between the
electrodes as long as there
is a change in the level of
the liquid
Capacitive transducer for liquid level
measurement.

33. • One Primary and two secondary.
• The LVDT produces a differential voltage at its output which
proportional to the position of the core, from its null position.
Linear Variable Differential Transformer
(LVDT)

34.  As the core moves, these mutual inductances change,
causing the voltages induced in the secondaries to
change.
 The coils are connected in reverse series, so that the
output voltage is the difference between the two
secondary voltages.
 When the core is in its central position, equidistant
between the two secondaries, equal but opposite
voltages are induced in these two coils, so the output
voltage is zero.

35. Output characteristics of an LVDT

36. 36
LVDT Cont..

37.  Measure linear mechanical displacement
◦ Provides resolution about 0.05mm, operating range from 
0.1mm to  300 mm, accuracy of  0.5% of full-scale reading
◦ The input ac excitation of LVDT can range in frequency from
50 Hz to 20kHz
 Used to measure position in control systems and
precision manufacturing
 Can also be used to measure force, pressure,
acceleration, etc..

38.  Appearance of an electric potential across certain
faces of a crystal when it is subjected to mechanical
pressure
 The word originates from the greek word “piezein”,
which means “to press”
 Discovered in 1880 by Pierre Curie in quartz crystals.
 Conversely, when an electric field is applied to one
of the faces of the crystal it undergoes mechanical
distortion.
 Examples --- Quartz, Barium titanate, tourmaline

39.  A piezoelectric crystal is placed between two plate
electrodes
 Application of force on such a plate will develop a
stress and a corresponding deformation
The piezoelectric effect
 With certain crystals, this
deformation will produce
a potential difference at
the surface of the crystal
 This effect is called
piezoelectric effect

40.  Induced charge is proportional to the impressed force
Q = d F
◦ d= charge sensitivity (C/m2)/(N/m2) = proportionality constant
 Output voltage E= g t P
◦ t= crystal thickness
◦ P = impressed pressure
◦ g=voltage sensitivity (V/m)/(N/m2)
 Shear stress can also produce piezoelectric effect
 Widely used as inexpensive pressure transducers for
dynamic measurements

41.  Piezoelelectric sensors have good frequency response
Example: Accelerometer
Piezoelectric accelerometer

42. Photoelectric devices Can be categorized as: photoemissive,
photoconductive, or photovoltaic.
No. Types Characteristics
1. Photoemmisive radiation falling into a cathode causes electrons
to be emitted from cathode surface.
2. Photoconductive the resistance of a material is changed when it’s
illuminated.
3. Photovoltaic Generate an output voltage proportional to
radiation intensity