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Instrumentation and Product Testing
Types of Transducers and Their Applications
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1.1 Distinction Between The Sensor And
Transducer
A sensible distinction is to use “sensor” for
sensing element itself and “transducer” for the
sensing element plus any associated circuitry.
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Sensor
+
E
Transducer
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1.2 Active and passive transducers
Passive transducer
A component whose output energy is supplied entirely
or almost entirely by its input signal is commonly called
a passive transducer.
Active transducer
An active transducer requires an auxiliary source of
power which supplies a major part of the output power
while the input signal supplies only an insignificant
portion. Normally, the output magnitude of the active
transducer is higher than that of the passive type.
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We can further classify transducers according to their
 function (displacement, temperature, force)
 physical property (inductive, photo-voltaic,
piezo-electric)
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2.1 Transducer of linear displacement
Force and displacement are closely linked. A
true/ideal displacement transducer is one which
requires a negligible force to make a large
displacement.
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2.2 Analog Methods
Resistive type
The simplest linear displacement transducer is a linear
potentiometer.
Linear displacement
Output vo
Output /Input
function:
+
V
-
L
vo = (V/L) x
noisy
x
Output vo
x
Use a linear potentiometer as a linear displacement transducer
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Features :
1. Simple and cheap
2. Accuracy depends on quality and dimension of
the resistive wire used or the quality of the
resistive film.
3. The force required to operate, although small,
depends on the size of the potentiometer, but
that required to start movement of the slider is
generally about twice that to keep it in motion.
4. The frequency response is limited by the mass of
the system but small transducer can have a flat
response up to 50 or 60 Hz.
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Main demerit :
The output is very noisy when the slider moves.
Cause of the noise are many: dirt and corrosion on the
wire; chatting and vibration of the slider contact;
variation of contact area as the slider moves;
resolution noise when the slider makes and breaks
contact with a turn of wire; etc.
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Inductive type
An important displacement transducer used in industrial
and medical application is the linear variable
differential transformer (LVDT).
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+
+
®
Eo
®
E1
®
E2
- +
-
S2
S1
F
MOTION
C
P
+ ~
-
F. Coil former;
C. Movable core;
P. Primary winding;
S1 and S2. Secondary
windings;
E1. Induced voltage in S1;
E2. Induced voltage in S2.
Eo. Output voltage E1 - E2
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Primary voltage
0
E1
E2
E0=E1-E2
0
E1
E2
E0=E1-E2
E1
E2
displacement
E0=E1-E2
180
(a) Absolute magnitude output voltage; (b) phase-referenced output
voltage as a function of LVDT core position
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Characteristics:
1. Due to no moving contact (non-contact) hence very low
noise level. Resolution is excellent.
2. The frequency response is limited mechanically by the
mass of the core and electrically by the frequency of the
applied primary voltage (carrier), the frequency of this
carrier should be at last ten times that of the highest
frequency component to be measured.
Demerits:
1. Quite expensive.
2. The operation can be severely affected by stray magnetic
A.C. fields or by the presence of large mass of metal near
by.
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Capacitive methods
The capacitance of a capacitor can be changed by varying
its area, gap length or dielectric constant.
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Differential capacitance method
P2
C2E2
d
x
d
M
E
C1E1
P1
x
 E 1 - E 2  E  E
d
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2.3 Digital methods
By means of coding (absolute linear encoder)
Identify the position of a movable test piece by a binary system of
notation.
Relative
motion
5 photo sensors
5-bit digital output for
linear displacement
5-digit scale for digital indication of linear
position of a movable object
 The resolution depends upon the number of bits
the binarydepends
number. upon the number
 comprising
The resolution
The accuracy
obviously depends upon the accuracy
binary number.
with which the scale is drawn.

of bits comprising the
The accuracy obviously depends upon the accuracy with which
the scale is drawn.
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By pulse counting (incremental linear encoder)
opaque strip
Light source
motion
transparent
strip
Photo senser
ck Counter
Reset signal
Digital output for linear displacement
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All the above mentioned methods are impractical
when measuring long displacement (say > 1m) with
good accuracy (say, <0.01mm). Such measurement
may be required in many application areas, e.g. large
machine tools.
What should we do?
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A grating measurement system can be though of as a
development of a well-known mechanical-optical
modulating transducer.
to counter
Photo sensor output:
Photo sensor
Motion
Index grating
Light source
Fixed scale
grating
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Gratings are available fairly cheaply in wide
range of size. The resolution of measurement is
essentially equal to the spacing between the
lines. Gratings up to 1000 lines per mm are
available.
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3.1 Transducers of Angular displacement
All the methods used in linear measurement can be
applied to angular measurement.
Analog Methods:
Resistive, Inductive, and Capacitive
Digital Methods:
Absolute angular encoder, Use of maximal length,
and Incremental angular encoder
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4.1 Transducers of velocity
By electronic differentiator or integrator
As far as we know the linear displacement, finding of
velocity can be done by differentiator. If it is an analog
signal, op-amp differentiator may be used. If it is a digital
signal, then use numerical methods for the differentiation.
However, it is more usual to employ an acceleration
transducer and an integrator, because differentiation
accentuates the high frequency noise while integrating
reduces the high frequency noise.
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Induction method
If a coil is moving inside a magnetic field is moved to
cut directly across the lines of flux, then a voltage is
induced in the coil :
e=Banv
where B is the flux density
a is area of the coil
n is number of turn of the coil
v is relative velocity between the coil and the field
Therefore e is proportional to v, when other parameters
are constant. There are mainly two types --- moving coil
and moving magnet.
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Motion
Pole-pieces
Coil and
former
S
N
S
S
N
S
Motion
Magnet
Soft-iron yoke
Moving coil transducer
Moving magnet transducers
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Doppler effect
It is most common means of measuring remote moving objects.
The police radar trap is a well known example of this technique.
The Doppler effect is a very effective and accurate means of
measuring velocity. If a narrow radio beam or ultrasonic beam
is aimed at an object the beam will be reflected back to the
source. However, if the object is moving the frequency of the
received signal differs from that of the transmitted signal.
The difference between the two frequencies being a measure of
the velocity of the moving object. The received frequency will
be higher than the transmitted frequency if the moving object is
travelling towards the receiver and lower if the object is
travelling away.
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Digital methods
By counting the number of pulse per unit time from the
incremental encoder linear encoder, we can know the
linear velocity. Its principle is the same as the digital
method in angular velocity, hence please refer to that
section for details.
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4.2 Angular velocity
Analog method
Tachometer. There are two main types -
 d.c. tacho and
 a.c. tacho.
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D.C. Tacho
N
Slip rings
S
The device is very convenient
and widely used in control
systems where velocity
feedback is required, but is
unsatisfactory for precision
measurement because of
unavoidable ripple (due to the
finite number of poles) and
because of spikes due to the slip
rings. The a.c. tacho is better in
this respect.
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A.C. Tacho
The below shows two a.c. tachometer in which both the
magnitude of the generated e.m.f. and its frequency are
proportional to the angular velocity.
Coil
S
N
S
N
A.C. tacho with moving magnet
and stationary coils
A.C. tacho with stationary
magnet and coils
Note: The direction of rotation is indicates by the sign of the DC voltage in DC
tachometer, while AC tachometer does not indicate the direction of rotation.
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Digital method
By counting the number of pulses per unit time from a
digital angular displacement encoder.
The resolution depends on number of pulse per
revolution of the encoder and the length of the unit
time base.
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Thank you