MEASUREMENT AND
INSTRUMENTATION
K.ANISH
Lecturer/EEE
UNIT 5
TRANSDUCERS AND DATA
ACQUISITION SYSTEMS
TRANSDUCERS AND DATA
ACQUISITION SYSTEMS
 Classification of transducers
 Selection of transducers
 Resistive transducers
 Capacitive transducers
 Inductive transducers
 Piezoelectric transducers
 Optical transducers
 Digital transducers
 Elements of data acquisition system - A/D, D/A
converters.
Reference
TEXT BOOKS
 1. E.O. Doebelin, ‘Measurement Systems –
Application and Design’, Tata McGraw Hill
publishing company, 2003.
 2. A.K. Sawhney, ‘A Course in Electrical
& Electronic Measurements &
Instrumentation’, Dhanpat Rai and Co,
2004.
REFERENCE BOOKS
1. A.J. Bouwens, ‘Digital Instrumentation’, Tata
McGraw Hill, 1997.
2. D.V.S. Moorthy, ‘Transducers and
Instrumentation’, Prentice Hall of India Pvt Ltd,
2003.
3. H.S. Kalsi, ‘Electronic Instrumentation’, Tata
McGraw Hill, 1995.
4. Martin Reissland, ‘Electrical Measurements’, New
Age International (P) Ltd., Delhi, 2001.
5. J. B. Gupta, ‘A Course in Electronic and Electrical
Measurements’, S. K. Kataria & Sons, Delhi, 2003.
TRANSDUCERS
 It’s a device which convert one form of
energy to another form
 Non electrical quantity is converted into an
electrical form by a transducer.
 Another name is pick up
Advantage of Electrical
Transducers
 Electrical amplification and attenuation can
be done easily.
 Mass inertia effects are minimized.
 The effect of friction is minimized.
 The electrical or electronic systems can be
controlled with a very small power level.
Conti..
 The electrical output can be easily used ,
transmitted and processed for the purpose of
measurement.
 Telemetry (aerospace – remote indication /
recording)
 Miniaturization on account of use of IC’s.
Two parts/element of transducer
 Sensing or detector element.
A detector or a sensing element is that part
of a transducer which responds to a physical
phenomenon or a change in a physical
phenomenon.
 Transduction element.
A transduction element transforms the
output of a sensing element to an electrical
output. The transduction element in a way
acts as a secondary transducer.
Classification of Transducers
 On the basis of transduction form used.
 As primary and secondary transducers
 As passive and active transducer.
 As analog and digital transducer.
 As transducers and inverse transducers.
Classification based upon principle
of transduction
 Resistive
 Inductive
 Capacitive etc
Depending upon how they convert the
input quantity into resistance, inductance or
capacitance respectively.
Eg piezoelectric, thermoelectric, magneto
restrictive, electro kinetic and optical
Primary and Secondary Transducers
LVDT (Linear
Variable
Differential
Transformer)
Primary- Pressure to displacement
(bourdon tube)
Secondary-Displacement into analogous
voltage (LVDT).
Passive and Active Transducer
 Active Transducer:
Also known as self generating type,
develop their own voltage or current from
the physical phenomenon being measured.
Velocity , temperature , light intensity and
force can be transduced with the help of
active transducer.
Conti..
 Passive Transducer:
Also known as externally powered
transducers, i.e., derive the power required
for energy conversion from an external
power source.
e.g. POT (Potentiometer)-used for the
measurement of displacement .
Analog and Digital Transducer.
 Analog Transducers : It converts the input
quantity into an analog output which is a
continuous function of time.
 E.g. LVDT, Thermocouple or a thermistor
(gives output which is continuous function of
time)
Conti..
 Digital Transducer: Converts input quantity
into an electrical output which is in the form
of pulse.
Transducers and Inverse Transducers
 Transducer: Non electrical to electrical
quantity
 Inverse transducer: Electrical quantity into
non electrical quantity.
Characteristics and Choice of
Transducer
 Input Characteristics
 Transfer Characteristics
 Output Characteristics.
Input Characteristics
 Type of Input and Operating Range
 Loading effect.
Type of Input :The type of input, which can
be any physical quantity, is generally
determined in advance .
Operating Range : Choice of transducer
depends upon the useful range of input
quantity.
Conti..
 Loading Effect : The transducer, that is
selected for a particular application should
ideally exact NO force, power or energy
from the quantity under measurement in
order that is measured accurately.
Transfer Characteristics
1. Transfer function.
2. Error.
3. Response of transducer to environmental
influences.
Transfer function.
 The transfer function of a transducer defines a
relationship between the input quantity and the
output. The transfer function is
q0  f (qi )
Where q0 andqi are respectively output and input
of the transducer.
Conti..
 Sensitivity,
dqo
S
dqi
 Scale Factor, Inverse of sensitivity.
1 dqo

S dqi
Error
 The error in transducer occur because they do
not follow, the input output relationship.
 Example.. Instead of qo, we might get a output
as qo’, then the error of the instrument is
  qo  q0
,
Three components of error
1. Scale error.
2. Dynamic error
3. Error on account of noise and drift.
Scale error.
 Zero error.
 Sensitivity error
 Non conformity.
 Hysteresis.
Zero error
Practical Curve.
Output

Theorectical Curve.
Input
 Output deviates from the correct value by a
constant factor over the entire range of the
transducer.
Sensitivity Error
Practical Curve.
Output

Theorectical Curve.
Input
 Observed output deviates from the correct
value by a constant value.
Non conformity
Practical Curve.
Output

Theorectical Curve.
Input
 Transfer function deviates from the
theoretical transfer function for almost
every input.
Hysteresis
Decreasing input
Output
Increasing input
Input
Response of transducer to
environmental influences.
 It should not be subjected to any
disturbances like stray electromagnetic and
electrostatic fields, mechanical shocks and
vibrations temperature changes, pressure and
humidity changes, changes in supply voltage
and improper mechanical mountings.
Output Characteristics
 Type of Electrical Output.
 Output Impedance
 Useful Range.
Type of Electrical Output.
 The type of output which may be available
from the transducers may be available from
the transducers may be a voltage, current ,
impedance or a time function of these
amplitudes.
Output Impedance
 Ideally the value of output impedance should
be zero if no loading effects are there on the
subsequent stage.
 Since zero output impedance is not possible ,
it should be kept as low as possible, since it
determines the amount of power that can be
transferred to the succeeding stages of the
instrumentation system.
Useful Output Range
 The output range of a transducer is limited at
the lower end by noise signal.
 The upper limit is set by the maximum
useful input level.
Factors Influencing the choice of
Transducer.
1.
2.
3.
4.
5.
6.
7.
Operating Principle
Sensitivity
Operating Range
Accuracy
Cross sensitivity
Errors
Transient and frequency response
Conti..
8. Loading effects.
9. Environmental compatibility
10. Insensitivity to unwanted signals
11. Usage and Ruggedness
12. Electrical aspects
13. Stability and Reliability
14. Static characteristics.
1. Operating Principle: The transducer are many
times selected on the basis of operating principle
used by them. The operating principle used may
be resistive, inductive, capacitive , optoelectronic,
piezo electric etc.
2. Sensitivity: The transducer must be sensitive
enough to produce detectable output.
3. Operating Range: The transducer should maintain
the range requirement and have a good resolution
over the entire range.
4. Accuracy: High accuracy is assured.
5. Cross sensitivity: It has to be taken into account
when measuring mechanical quantities. There are
situation where the actual quantity is being
measured is in one plane and the transducer is
subjected to variation in another plan.
6. Errors: The transducer should maintain the
expected input-output relationship as described
by the transfer function so as to avoid errors.
7. Transient and frequency response : The
transducer should meet the desired time domain
specification like peak overshoot, rise time,
setting time and small dynamic error.
8. Loading Effects: The transducer should have a
high input impedance and low output impedance
to avoid loading effects.
9. Environmental Compatibility: It should be
assured that the transducer selected to work under
specified environmental conditions maintains its
input- output relationship and does not break
down.
10. Insensitivity to unwanted signals: The transducer
should be minimally sensitive to unwanted
signals and highly sensitive to desired signals.
11. Usage and Ruggedness: The ruggedness both of
mechanical and electrical intensities of
transducer versus its size and weight must be
considered while selecting a suitable transducer.
12. Electrical aspects: The electrical aspects that
need consideration while selecting a transducer
include the length and type of cable required.
13. Stability and Reliability : The transducer should
exhibit a high degree of stability to be operative
during its operation and storage life.
14. Static Characteristics :Apart from low static error,
the transducer should have a low non- linearity,
low hysteresis, high resolution and a high degree
of repeatability.
Resistive Transducers
R
L
,
A
R  resistance ; 
L  length of conductor ; m
A  cross - sectional area of conductor; m 2
  resistivit y of conductor material;  - m
 Any method of varying one of the quantities
involved in the above relationship can be the
design basis of an electrical resistive transducer.
 The translational and rotational potentiometers
which work on the basis of change in the value of
resistance with change in length of the conductor
can be used for measurement of translational or
rotary displacement.
 Strain gauge work on the principle that the
resistance of the conductor or a semiconductor
changes when strained. This property can be used
for measurement of displacement, force and
pressure.
 The resistivity of the material changes with change
of temperature thus causing a change of
resistance. This property may be used for
measurement of temperature.
Potentiometers
 POT
 Resistive potentiometer used for the
purposes of voltage division is called POT.
 Resistive potentiometer consist of a
resistive element provided with a sliding
contact.
 Sliding Contact-Wiper
POT
 It’s a Passive Transducer.
 Linear Pot –Translational Motion
 Rotary Pot-Rotational Motion
 Helipots- Combination of the two motions
(translational as well as rotational).
 In Electrical Measurement , Standard
potentiometer are used to measure the
unknown voltage by comparing it with a
standard known voltage.
Resistive potentiometer
Translational, rotational and
helipots
 Consider a translational potentiometer
ei and e 0  input and output vol tages respective ly; V,
xt  total length of translati onal pot; m,
xi  displaceme nt of wiper from its zero position; m,
R p  total resistance of the potentiome ter; 
 If the distribution of the resistance with respect to
translational movement is linear, the resistance per
length is
R
p
xt
 The output voltage under ideal conditions is:
 resistance at the output ter minals
e0  
 resistance at the input term inals
 R p xi xt  
xi



ei  x ei


Rp
xt


 For Rotational Motion
i
e0  x ei
t

 x input volt age

Strain Gauges
 If a metal conductor is stretched or
compressed , its resistance changes on
account of the fact that both length and
diameter of conductor change.
Also there is a change in the value of
resistivity of the conductor when strained
and this property is called piezoresistive
effect.
Resistive strain gauges are also known as
piezoresistive gauges.
R
L
A
 (1)
Let a tensile stress s be applied to the
wire.
dR  L L A L 

 2

- (2)
ds A s A s A s
L
Divide equation (2) by R 
A
1 dR 1 L 1 A 1 



R ds L s A s  s
- (3)
From (3) , per unit change in resistance is
due to
Per unit change in length=
Per unit change in Area =
Per unit change in resistivity =
Area =
 2 A
 D
D ,
 2. D.
 (4)
4
s
4
s
L
L
A
A


1 A (2 4) D D 2 D


 (5)
2
A s ( 4) D s D s
Equation (3) can be written as
1 dR 1 L 2 D 1 



R ds L s D s  s
- (6)
Poisson’s ratio ,
lateral strain
D D
v

 (7 )
longitudin al strain
L L
Or D D   v L L  (8)
1 dR 1 L
2 L 1 

v

R ds L s
L s  s
- (9)
For small variation , the above
relationship , can be written as
R L
L 

 2v

R
L
L

- (10)
The gauge factor is defined as the
ratio of per unit change in resistance
to per unit change in length.
R R
Gf 
- (11)
L L
or R R  G f  L L
 G f   (12)
where   strain  L L  (13)
R R
 
Gf 
 1  2v 
L L
L L
Gf 
R R
 
 1  2v 
L L

- (12)
- (13)
Resistance change due to
change in length
Resistance change
due to change in
Resistance change due to change
area
in piezoresistive effect.
Types of strain gauges







Unbonded metal strain gauge
Bonded metal wire strain gauge
Bonded metal foil strain gauge
Vacuum deposited thin metal film strain gauges.
Sputter deposited thin metal film strain gauge.
Bonded semiconductor strain gauges.
Diffused metal strain gauge.
Unbonded metal strain gauge
 Used almost exclusively in transducer
applications.
 At initial preload , the strains and resistances of
the four arms are normally equal, with the result
the output voltage of the bridge, e0=0.
 Application of pressure produces a small
displacement , the displacement increases tension
in 2 wires and decreasing the resistance of the
remaining 2 wires.
 This causes an unbalance of the bridge producing
an output voltage which is proportional to the
input displacement and hence to the applied
pressure.
Bonded metal wire strain gauge
 It consist of a grid of fine resistance wire of
diameter of about 0.025mm.
 The wire is cemented to a base.
 The base – thin sheet of paper or bakelite.
 Wire is covered with a thin sheet of material
so that it is not damaged mechanically.
 The spreading of wire permits a uniform
distribution of stress over a grid.
Bonded metal foil strain gauge
Extension of the bonded metal wire strain
gauge.
The bonded metal wire strain gauge have
been completely superseded by bonded foil
strain gauge.
Metal foil strain gauge
Semiconductor strain gauge.
The semiconductor strain gauge depends for
their action upon piezo resistive effect. i.e.
the change in the value of the resistance due
to change in resistivity.
Rosettes
Resistance Thermometers
 The resistance of the conductor changes when its
temperature is changed. This property is utilized for
measurement of temperature.
 The variation of resistance R with temperature T(ok) can be
represented by the following relationship for most of the
metals as
R=R0(1+1T+ 2T2+…+ nTn+………)
Where R0=resistance at temperature T=0 and
1 ,2,n are constants.
 Platinum – as it can withstand high temperatures while
maintaining excellent stability.
Requirements of a conductor material
to be used in RTDs are
 The change in resistance of material per unit
change in temperature should be as large as
possible.
 The material should have a high value of
resistivity so that minimum volume of material is
used for the construction of RTD.
 The resistance of material should have a
continuous and stable relationship with
temperature.
Thermistors
 Contraction of a term “thermal resistors”
 Its composed of semiconductor materials.
 Used in applications which involve
measurements in the range of -60oC to 15oC
 The resistance of thermistors ranges from
0.5 to 0.75 M
Thermistors
Composed of sintered mixture of metallic oxides
such as manganese, nickel , cobalt, copper, iron
and uranium.
Thermocouple
 When two metals having different work
functions are placed together, a voltage is
generated at the junction which is nearly
proportional to the temperature. This
junction is called a thermocouple.
Variable Inductance Transducer
 Change in self Inductance
 Change in Mutual Inductance.
 Production of eddy currents.
Transducers working on principle
of change of Self Inductance.
 Self inductance of a coil
L  N2 R
where
N  Number of turns
R  reluctance of the magnetic circuit  l μA
N2
N 2μA
A
 Inductance L 

 N 2μ  N 2μG
l μA
l
l
A
 geometric form factor
l
A  Area of cross section of coil; m 2
where G 
l  length of coil; m
  permeabili ty
Transducers working on principle
of change of Mutual Inductance.
 Uses multiple coils.
 The mutual inductance between two coils is
M  K L1L 2
where
L1 and L2  self inductance s of two coils
K  co  efficient of coupling
Transducers working on principle of
production of eddy currents
 If a conducting plate is placed near a coil
carrying alternating current, eddy currents
are produced in the conducting plate.
Linear Variable Differential
Transformer (LVDT)
 The transformer consists of single primary
winding P and two secondary windings S1 and S2
wound on a cylindrical former.
 The secondary windings have equal number of
turns and are identically placed on either side of
the primary winding.
 The primary winding is connected to an
alternating current source.
LVDT
 A movable soft iron is placed inside the former.
 The displacement to be measured is applied to the
arm attached to the soft iron core.
 Since the primary winding is excited by an
alternating magnetic field which in turn induces
alternating current voltages in the secondary
windings.
 The output voltage of secondary , S1is Es1 and that
of secondary, S2is Es2
Capacitive Transducer
 The Principle of operation of capacitive
transducer is based upon the familiar
equation for capacitance of a parallel plate
capacitor.
C
A
d
where

 r o A
d
A  overlappin g area of plates, m 2
d  distance between tw o plates, m
   r  0  permittivi ty of medium ; F/m
 r  relative permittivi ty ,
 r  Permittivit y of free space.
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