transducers electronic measurement and instruentation

1. BETL 305 ELECTRONIC
MEASUREMENT &
INSTRUMENTATION
BY PRASHANT KUMAR
ASST. PROFESSOR
MITS GWALIOR

2. UNIT II TRANSDUCERS
Transducer Block Diagram
A transducer will have basically two main components.
They are
1. Sensing Element
The physical quantity or its rate of change is sensed
and responded to by this part of the transistor.
2. Transduction Element
The output of the sensing element is passed on to the
transduction element. This element is responsible for
converting the non-electrical signal into its proportional
electrical signal.
• There may be cases when the transduction element
performs the action of both transduction and
sensing. The best example of such a transducer is a
thermocouple.
• There are transducers which don’t require external
Excitation
• measurand=quantity under measurement

3. TRANSDUCER
Defnition of transducer: A transducer is a device, usually electrical,
electronic, electro-mechanical, electromagnetic, photonic, or
photovoltaic that converts one type of energy or physical attribute (eg
displacement, change in velocity) to another (generally electrical or
mechanical) for various measurement purposes including
measurement or information transfer (for example, pressure sensors).

4. STATIC CHARECTERISTICS OF
TRANSDUCERS1. Linearity
Its input vs output characteristics should be linear and it should produce these characteristics in
balanced way.
(Linearity–can be defined as the variation in the constant of proportionality between the input
physical quantity and the output electrical signal. A sensor or a transducer is therefore said to be
linear when the constant of proportionality has the same value within the whole measurand range
(i.e. when the graph relating input to output is a straight line))
2. Ruggedness
A transducer should be capable of withstanding overload and some safety arrangements
must be provided with it for overload protection.
3. Repeatability
The device should reproduce the same output signal when the same input signal is applied again
and again under unchanged environmental conditions, e.g., temperature, pressure,
humidity, etc.
4. High Reliability and Stability
The transducer should give minimum error in measurement for temperature variations,
vibrations and other various changes in surroundings.

5. CONTINUED.......
5. High Output Signal Quality (High SNR)
The quality of output signal should be good, i.e., the ratio of the
signal to the noise should be high and the amplitude of the output
signal should be enough.
6. No Hysteresis
It should not give any hysteresis during measurement while input
signal is varied from its low value to high value and vice versa.
7. Residual Reformation
There should not be any deformation on removal of input signal after
long period of use.

6. Resolution is the minimal change of the input necessary to produce a detectable
change at the output
Precision: The capacity of a measuring instrument to give the same reading when
repetitively measuring the same quantity under the same prescribed conditions
Precision implies agreement between successive readings, NOT closeness to the
true value
Two terms closely related to precision
Repeatability : The precision of a set of measurements taken over a short time
interval
Reproducibility : The precision of a set of measurements BUT
i) taken over a long time interval or
ii) Performed by different operators or
iii) with different instruments or

7. Hystheresis: The difference between two output values that
correspond to the same input depending on the trajectory followed
by the sensor (i.e., magnetization in ferromagnetic materials)
The basic equation is y = mX + B, where X is the scaling or multiplier,
and B is the offset. Using a little basic algebra we can apply this same
set up to calculate scale and offset of any linear sensor or device with
a linear output

8. CLASSIFICATION OF
TRANSDUCERS

9. ACTIVE TRANSDUCERS
Active transducers are those which do not require any power source
for their operation. They work on the energy conversion principle.
ACTIVE TRANSDUCER PARAMETER
1.PEIZOELCTRIC TRANSDUCER Pressure/strain/displacement to
Electric potential
2.THERMOCOUPLE Temperature to electrical potential
(emf)
3. Electromagnetic Change in area/velocity
/displacement to Induced Emf

10. EXAMPLE OF PRECISION &
ACCURACY (SHOOTING DART)

11. STRAIN GAUGE
MECHANICAL
STRAIN =
ELECTRICAL
STRAIN
ELECTRICAL
OUTPUT
=
STRAIN
GAUGE
l
l

R
R

Wheatston
e bridge
R
R

GENERAL
WORKING
OF STRAIN
GAUGE
STRAIN: It is defined as
extension or reduction per
unit length
l
S
l



12. STRESS STRAIN CURVE

13. The change in the value of resistance by the application of force can
be explained by the normal dimensional changes of elastic material.
If a positive strain occurs, its longitudinal dimension (x-direction)
will increase while there will be a reduction in the lateral dimension
(y-direction).
The reverse happens for a negative strain.
Since the resistance of a conductor is directly proportional to its
length and inversely proportional to its cross sectional area, the
resistance changes.
The resistivity of a conductor is also changed when strained. This
property is known as piezoresistive effect.

14. GAUGE FACTOR
2
resistance of unstrained gauge=R = .....( )
to find R we need to differentiate R w.r.t stress S
...............( )
both side by R = we get
1 1 1
L
i
A
dR L L A L
ii
dS A S A S A x
L
dividing
A
dR L A
R dS L S A S

    
  

 
 

  
 
1
...............( )iii
x

 

As we observe it from eq
(iii)
The effective change in
resistance w.r.t strain is due
to three factors
 Per unit change in length
 Per unit change in area
 Per unit change in
resistivity
(Peizoresitivity)
The piezoresistive
effect is a change in
the electrical
resistivity of a
semiconductor or
metal when
mechanical strain is
applied
L
L

A
A





15. GAUGE FACTOR…..
Continued..
2
2
2. .
4 4
1 1 2
.2. . ......( )
4
4
this value of eq (iv) in (iii) we get
1 1 2 1
poisson's ratio
strain
=
/
D A D
Area D
S S
A D D
D iv
DA S S D S
substituting
dR L D
R dS L S D S S
now from
Lateral
Longitudnal axia
   
 
   
  
  
   

  
  
  
starin
.
D
D
Ll
L
D L
D L


 

 
 

16. 1 1 2 1
when 0 it can be approximated as
1 1 2 1
2
........( )
factor = G
eq (v) both side by
get
1 2
1
f
f
f
R L L
R S L S L S S
S
R L L
R S L S L S S
R L L
v
R L L
R
RGauge
L
L
L
dividing
L
we
G
L
L
G








   
  
   
 
   
  
   
   
  





  

  2 ( )
L
strain
S L




 

17. Continued…..
1 1 2 1
factor = G
eq (v) both side by
get
1
1 2 ( )
2
f
f
f
dR L L
R dS L S L S S
R
RGauge
L
L
L
dividing
L
we
G
L
L
L
G strain
S L


  
   


  





 
  






18. Desirable characteristics
in a material for strain
Gauge
1.High Gauge factor
2.High resistivity
3. Low temp. sensitivity
(Low temp coefficient of
resistance)
4. Low Hysteresis
5. Linearity (Strain Vs
Resistance should be
linear)
6. High yield point

19. STRAIN GAUGE

20. A bonded strain gauge will be either a wire
type or a foil type as shown in the figure
below. It is connected to a paper or a thick
plastic film support. The measuring leads are
soldered or welded to the gauge wire. The
bonded strain gauge with the paper backing
is connected to the elastic member whose
strain is to be measured.
Unbonded Strain
Gauge is used at
places where the
Gauge is to be
detached and used
again and again

21. BASIC STRAIN GAUGE WORKING USING WHEATSTONE
BRIDGE

22. A quarter bridge output corresponding to the application of a
force is shown below. Initially, the circuit will be balanced
without the application of any force. When a downward force is
applied, the length of the strain gauge increases and thus a
change in resistance occurs. Thus an output is produced in the
bridge corresponding to the strain.

23. SEMICONDUCTOR STRAIN GAUGE
For Semiconductor strain gauge material used is either n-type S.C or P-
type S.C
It is based on Principle of Piezo-resistivity
Advantages :
1. High Gauge factor (>>130)
2. Very Low hystresis (<<0.05%)
Disadvandage
1. Poor linearity
2. prone to error due to high sensitivity to temp.

24. COMPARISON BETWEEN
SEMICONDUCTOR VS METALLIC
GAUGE
PARAMETERS METALLIC STRAIN GAUGE SEMICONDUCTOR STRAIN
GAUGE
GAUGE FACTOR LOW HIGH
HYSTRESIS HIGH LOW
LINEARITY HIGH POOR
COST LOW EXPENSIVE

25. STRAIN GAUGE

26. PRINCIPLE OF WORKING OF
S.GAUGE: Resistance of a
conductor changes when it is
strained. This property can be
used to measure Pressure,
Force and Displacement.
APPLICATION OF STRAIN GAUGE:
1.TO measure pressure
2. To measure Displacement
3. To measure force

27. THERMISTORS

28. MATHEMATICAL EXPRESSSIONS

29. APPLICATION OF THERMISTORS
1. measurement of temp.(thermistors’s large change of resistance
with temp. provides good accuracy and resolution)
2. temperature compensator in in electronic circuits (because of NTC)
3. measurement of power at high frequency
4. measurement of level of liquid/ flow of liquid
5. measurement of time delay

30. MATHEMATICAL ANALYSIS OF
THERMISTOR
 
 
0
0
0 0
1 1
R=R exp ..
eq
....( )
(i)
R is the resistance of thermistor at the temperature T in K
R is the resistance at given temperature T in K
is the material specific constant
differentiat
i
T T
wher
i g
e
n


 

 


2
0
0
w.r.t Temp. T
we get
log
ln ln
1 1
dR R
dT T
taking weget
R R
T T


 



2
1 dR
R dT
T



 


2
1 dR
R dT T

 

31. CAPACITIVE TRANSDUCERS
• PRINCIPLE OF WORKING : A capacitive transducer works on principle of
Capacitance of Parallel plate capacitor.
• Capacitance of a parallel plate capacitor is given by
• Capacitive transducer works on the principle that capacitance is varied by
variation in any one of following parameters
I. Change in effective plate area A /(overlapping area)
II. Change in sepration between parallel plates
III. Variation in dielectric constant
0 r A
C
d
 

r

32. Change in capacitance can be converted into required electrical format using
capacitor equation
by capacitor eqution
Q=CV......(i)
differentiating both side we get
( )C C
dQ dV dC
C V
dt dt dt
dV dC dQ
I C V I
dt dt dt
 
  

33. VARIABLE AREA BASED CAPACITOR
TRANSDUCER
Capacitance of parallel plate capacitor is directly proportional to effective
overlapping area between two plates. 0 r A
C
d
C A
 


C A
C A
 
 
Used to measure Linear
displacement
Used to measure angular
displacement

34. LINEAR DISPLACEMENT MEASUREMENT USNG
VARIABLE AREA BASED CAPACITOR
0 0
0
of overlapping part of plates
of overlapping part of plates
Sensitivity= F/m
r r
r
A wl
C C
d d
l length
w width
wC
l d
   
 
  






35. ANGULAR DISPLACEMENT MEASUREMENT USNG
VARIABLE AREA BASED CAPACITOR
0
2
2
0
2
0
2
2
F/radian
2
r
r
r
A
C
d
r
A
r
C
d
C r
Sensitivity C
d
 

  
 





  


36. VARIABLE DISTANCE BASED CAPACITOR
TRANSDUCER
It is used for linear displacement/pressure measurement.
0
0
2
1
F/meter
r
r
A
C
d
C
d
AC
sensitivity S
d d
 
 



  


37. VARIABLE DIELECTRIC CAPACITIVE
TRANSDUCERLiquid level
measuremen
t using
dielectric
variation
Displacement
measurement

38. ADVANTAGE OF CAPACITIVE TRANSDUCER
1) High sensitivity
2) Very small power requirement
3) Good frequency response
4) High input impedence so minimum loading effect
DISADVANTAGE OF CAPACITIVE TRANSDUCER
1. Proper insulation of metallic parts required
2. Non linear behaviour due due to edge effect
3. Stray capacitance between metallic wires affect performance
Applications:
Measurement of i) force, ii) angular and linear displacement ,iii)
moisture, iv) pressure v) water level indicator

39. THERMOCOUPLE
It is temperature to electrical (emf) energy conversion based transducer.
WORKING PRINCIPLE:
Themocouple is based on three effects
i) Seeback effect
ii) Peltier effect
iii) Thompson effect
Seeback effect: when the temperature
difference exists between junctions of two
dissimilar metals the the thermo emf is
generated across terminals.
Peltier effect when electric current passes
junction of two dissimilar metals heat is
absorbed at one junction and liberated at
another junction
Thompson effects: when a homogeneous
electrical conductor is subjected to a
temperature gradient then a
corresponding voltage gradient is

40. Materials used in thermocouple
POSITIVE
WIRE
NEGATIVE
WIRE
Temp
(degree
centigrade
)
COPPER CONSTANTAN -253 to
400
Iron CONSTANTAN -200 to
850
platinum Platinum(87%)
+Rhodium(13
%)
0-1400
2
emf generated
( ) ( )
thermal
E a T b T   

41. Adavantage:
High temperature
measurement range
Easy calibration
Fast response
Low cost
Disadvantage
1. Reference junction is
necessary
2. Non linearity

42. LVDT (LINEAR VARIABLE DIFFERENTIAL
AMPLIFIER)
WORKING PRINCIPLE:
LVDT is based on variation in inductance with differential output.
Inductance can be varied by variation in these three quantities
I. Magnetic permeability of material
II. Number of turns
III. Geometric configuration
Secondary winding are connected with opposite polarity to provide
differential output
0 r A
L
l
 


43. CONSTRUCTION
LVDT consists of a cylindrical transformer where it is surrounded by
one primary winding (P1) on one side and two secondary windings
(S1and S2 ) on the other side
The number of turns in both secondary windings are equal, but they
are connected such that both have opposite polarity.
The primary winding is connected to an AC source.
A soft iron core moves within the hollow cylindrical core and changes
the magnetic flux linked with the primary and two secondary
windings.

44. WORKING
Case 1: when iron core is at centre( null position) E0=0, (Es1=Es2)
because the flux linking with both secondary windings is equal,
hence equal emf are induced in them. (Es1 = Es2 ) ( E0=Es1-Es2)
Case 2: When the core is moved to the left of null position (O - A)
more flux links with winding S1 and less with winding S2.
Hence, output voltage is Es1 is greater than Es2. The output voltage is
positive and in phase with input signal.
Case 3: When the core is moved to the right of null position (O- B)
more flux links with winding S2 and less with winding S1
Hence, output voltage Es2 is greater than Es1.The output voltage is
negative and 180o out of phase with input signal.

45. Equivalent
Circuit of LVDT

46. WORKING AND CHARECTERISTICS

47. ..
Adavntage of LVDT
1. high linearity
2. high sensitivity
3. very low hystresis
4. low power consumption
Disadvantages
1.large offset(threshold)
2. easily affected by stray magnetic field
Applications:
1. Linear displacement measurement
2. Measurement of pressure, tension,

48. PEIZOELECTRIC TRANSDUCER
Working principle:
The main principle of a piezoelectric transducer is that a force, when applied
on the quartz crystal, produces electric charges on the crystal surface.
 The charge thus produced can be called as piezoelectricity. Piezo electricity
can be defined as the electrical polarization produced by mechanical strain on
certain class of crystals.
 The rate of charge produced will be proportional to the rate of change of force
applied as input. As the charge produced is very small, a charge amplifier is
needed so as to produce an output voltage big enough to be measured