This presentation gives detailed description about transducers and it's importance

1. TRANSDUCERS
• Device that converts one form of energy to
another form for various purposes including
measurement or information transfer.
• Energy may be of any kind electrical,
mechanical, chemical, optical.

2. Basic Requirements of transducers
• Linearity : i/p – o/p characteristics should
be linear.
• Stability : o/p should be stable to change in temp
& other environmental factors.
• Ruggedness : capable of withstanding overloads,
with measures of overload protection.
• Repeatability : should produce identical output signals.
• Dynamic Response: should respond to changes in i/p as
quickly as possible.
• Reliability: should withstand mechanical strains without
affecting the performance of the transducer.

3. Classification of Transducers
• Active Transducers
self generating, does
not require external
source.
 Thermocouple
 Photovoltaic Cell
 Piezoelectric Transducer
• Passive Transducer
Requires an external
power source.
 Resistive Transducer
Strain Gauge, Thermister,
Thermometer.
 Inductive Transducer
Linear Variable Differential
Transducer
 Capacitive Transducer
Photoemissive Cell
Photomultiplier Tube

4. Strain Gauge
• A device whose electrical resistance varies in
proportion to the amount of strain in the device.
• The sensitivity of stain gauge is described in terms of
Gauge factor.
• Gauge factor-Change in resistance per unit change in
length.
G = ΔR/R = ΔR/R
Δl/l S

5. Bonded strain gauge
• Consist of a grid of fine resistance wire cemented to
the base.
• Base may be thin sheet of paper or bakelite.
• Covered with thin sheet of paper or thin bakelite sheet
to avoid mechanical damage.
• Bonded to the structure under study with
an adhesive.

6. Bonded Strain gauge

7. Unbonded strain gauge
• Unbonded strain gages consist of a wire stretched
between two points
• Force acting on the wire (area = A, length = L,
resistivity = r) will cause the wire to elongate or
shorten.
• This will cause the resistance to increase or
• decrease proportionally according to:
R = ρL/A
and ΔR/R = GF· ΔL/L,
where GF = Gauge factor (2.0 to 4.5 for metals, and
more than 150 for semiconductors).

8. Unbonded Strain Gauge

9. Thermistor
• Two terminal resistor whose resistance changes
significantly when its temperature changes.
• The resistance of a thermistor decreases with increase
of temperature.
• Resistance at any temp is given by
RT=R0 exp β(1/T-1/T0)
Where RT = thermister resistance at temp T(K)
R0 = thermister resistance at temp T0(K)
β = A constant determined by calibration.

10. Symbol
Miniature Epoxy Coated Thermistors

11. • Three parameters characterizing the thermistor are
 Time Constant - time for thermistor to change its
resistance by 63%.(1 to 50ms)
 Dissipation Constant - power necessary to increase the
temp of thermistor by 1ºC.(1 to 10 mW/ºC)
 Resistance Ratio - ratio of resistance at 25ºC to that at
125ºC.(3 TO 60).
• Uses:
To measure temp, flow, pressure, composition
of gases, liquid level etc.

12. Thermocouple
• Junction between to dissimilar metals or
semiconductors that generates a small voltage.
• The two junctions reference and sensing are
maintained at different temp.
• Each junction is made by welding the two dissimilar
metals together.
• Reference junction has a fixed temp usually 0ºC.
• And the output voltage depends on the temp of
sensing junction.

13. Milli voltmeter
Sensing
junction
Reference
junction
iron
constantan
iron
constantan
Thermocouple circuit

14. Inductive Transducers
• In the first diagram the variable inductor is part of an
oscillator circuit.
• If the position of the core is moved then the oscillator
frequency changes.
• The change in frequency can be displayed as a change
in millimetres.

15. variable inductance type

16. Variable reluctance type
• As the air gap changes the reluctance of the circuit
changes.
• This causes a change of inductance.

17. • This can be used as shown by the next illustration.
• As the inductance changes so the frequency of the
oscillator changes.
• The output of the oscillator can be converted to DC
for display on a digital meter calibrated in inches etc.

18. Linear Variable Differential
Transformer
• There is one primary and two secondary windings.
• If AC is applied to the primary then voltages are
induced in the secondaries.
• The secondaries are connected so their outputs are
opposite.
• When the core is central the two voltages are equal in
amplitude and cancel out.

19. LVDT
• If the core is moved then there will be more voltage
in one secondary than the other.
• The voltages will not cancel out and there will be an
AC signal at the output proportional to the distance
the core has moved.
• Using a phase detector circuit it is also possible to
indicate the direction the core has moved.
• The graph representation shows the output
voltage/position characteristics.

20. LVDT
Model Graph

21. CLASSIFICATION OF
MEASURING INSTRUMENTS

22. Depending on working principle
• Moving iron type
instruments
a). Attraction type
b). Repulsion type
• Moving coil type
instruments
a). Permanent magnet
type
b). Dynamometer type

23. MOVING IRON INSTRUMENTS – ATTRACTION TYPE
Principle
• A soft iron piece gets magnetized when it is brought into a magnetic
field produced by a permanent magnet.
• The same phenomenon happens when the soft iron piece is brought
near either of the ends of a coil carrying current.
• The iron piece is attracted towards that portion where the magnetic flux
density is more.
• This movement of soft iron piece is used to measure the current or
voltage which produces the magnetic field.

24. Construction
• A soft iron disc is attached
to the spindle
• To the spindle, a pointer is
also attached, which is made
to move over calibrated
scale
• The moving iron is pivoted
such that it is attracted
towards the center of the
coil where the magnetic
field is maximum

25. Principle
• When the current to be measured is passed
through the coil or solenoid, field is produced
which attracts the eccentrically mounted disc
inwards, thereby deflection the pointer which
moves over a calibrated scale

26. Deflecting Torque
• Produced by the current or the voltage to be measured.
• It is proportional to the square of the voltage or current.
• Hence, the instrument can be used to measure d.c. or a.c.
• Scale is non- uniform
Control torque : Spring or gravity
Damping : Air friction damping

27. MOVING IRON INSTRUMENT - REPULSION
TYPE
Principle
• Two iron piece kept with close proximity in a
magnetic field get magnetized to the same polarity.
Hence, a repulsive force is produced.
• If one of the two piece is made movable, the repulsive
force will act on it and move it on to one side.
• This movement is used to measure the current or
voltage which produces the magnetic field.

28. Construction
• There are two iron pieces-fixed and moving.
• The moving iron is connected to the spindle to
which is attached a pointer. It is made to move over a
calibrated scale.

29. Working
• When the current to be measured is passed through the fixed
coil it sets up its own magnetic field which magnetizes the two
rods similarly the adjacent points on the lengths of the rods
will have the same magnetic polarity.
• Hence, they repel each other with the result that the pointer is
deflected against the controlling torque of a spring or gravity.
• The force of repulsion is approximately proportional to the
square of the current passing through the coil
• Whatever be the direction of current in the coil, the two irons
are always similarly magnetised.

30. Deflecting torque
• Produced by the current or the voltage to be measured.
• It is proportional to the square of the voltage or current.
• Hence, the instrument can be used to measure d.c. or a.c.
Control torque : Spring or gravity
Damping : Air friction damping

31. Advantages and disadvantages:
• The instruments are cheap ,reliable and robust
• The instruments can be used on both A.C and
D.C
• They cannot be calibrated with high degree of
precision with D.C on account of the effect of
hysteresis in the iron rods or vanes .

32. MOVING COIL INSTRUMENT – PERMANENT
MAGNET TYPE
Principle :
when a current carrying conductor is placed in
magnetic field it is acted upon by a force which tends
to move it to one side and out of the field. This
movement of coil is used to measure current or
voltage.

33. Construction
• This instrument consists of a
permanent magnet and a
rectangular coil of many turns
wound on a light aluminium or
copper former inside which is an
iron core
• The sides of the coil are free to
move in the two air gaps between
the poles and core
• To the moving coil spindle is
attached, a pointer is attached to the
spindle to move over a calibrated
scale.

34. Working
• A magnetic field of sufficient density is produced by
the permanent magnet.
• The moving coil carries the current or a current
proportional to the voltage to be measured.
• Hence, an electromagnetic force is produced which
tends to act on the moving coil and moves it away
from the field.
• This movement makes the spindle move and so the
pointer gives a proportionate deflection

35. • Deflecting torque : It is directly proportional to the
current or the voltage to be measured. So, the instrument can
be used to measure direct current and dc voltage.
• Control torque : Spring control.
• Damping torque : Eddy current damping.Damping is
electromagnetic by eddy currents induced in the metal frame
over which the coil is wound. Since the frame moves in an
intense magnetic field, the induced eddy currents are large and
damping is very effective.

36. The permanent-magnet moving coil (PMMC) type instruments
have the following advantage and disadvantages:
ADVANTAGES
1. They have low power consumption
2. Their scales are uniform and can be designed to extend over and arc of 1700 degree or
so
3. They possess high (torque/weight) ratio.
4. They can be modified what the help o f shunts and resistances to cover a wide range of
currents and voltages.
5. They have no hysteresis loss.
DISADVANTAGES
1. Due to delicate construction and the necessary accurate machining and assembly of
various parts, such instruments are somewhat costlier as compared to moving iron
instruments.
2. Some errors are set in due to the ageing of control springs and the permanent magnets.