Electrical Measurements

1. UNIT III
ELECTRICAL MEASUREMENTS
BASIC ELECTRICAL,
ELECTRONICS AND
MEASUREMENT ENGINEERING

2. CLASSIFICATION OF
INSTRUMENTS
 Electrical measuring instruments are mainly
classified as:
 1. Indicating Instruments
 2. Recording Instruments
 3. Integrating Instruments
 1. Indicating Instruments
 These instruments make use of a dial and pointer
for showing or indicating magnitude of unknown
quantity.
 Examples of this instruments are ammeter,
voltmeter, wattmeter etc.

3.  2. Recording Instruments
 These instruments give a continuous record of the given
electrical quantity
 The examples are various types of recorders. In such
recording instruments, the readings are recorded by
drawing the graph. The pointer of such instruments is
provided with a marker
 3. Integrating Instruments
 Integrating Instruments are those instruments which
totalize the events over a specified period of time. The
output of such instruments is the product of time and an
electrical quantity.
 For example, a house energy meter , Unit of energy is
kwhr.

4. BASIC PRINCIPLE OF INDICATING
INSTRUMENTS
 Three types of operating forces
 i) Deflecting force
 ii) Controlling force and
 iii) Damping force
 i) Deflecting Torque/Force
 The deflecting torque’s value is dependent upon the
electrical signal to be measured; this torque/force helps in
rotating the instrument movement from its zero position.
 The system producing the deflecting torque is called the
deflecting system.

5.  ii) Controlling Torque/Force
 The act of this torque/force is opposite to the deflecting
torque/force.
 When the deflecting and controlling torques are equal in
magnitude then the movement will be in definite position or
in equilibrium.
 Spiral springs or gravity is usually given to produce the
controlling torque.
 The system which produces the controlling torque is called
the controlling system.
 iii) Damping Torque/Force
 When a deflection force is applied to the moving system,
its deflects and it should come to rest at a position where
the deflecting force is balanced by the controlling force.
 The moving system cannot immediately settle at its final
position but overshoot or swings ahead of it. So in order to

6. Controlling System
 It is the system that provides a force equal and
opposite to the deflecting force. Controlling forces are
applied in two ways.
 i) Spring Control (used in modern instruments)
 ii) Gravity Control ( not properly used)
 i) Spring Control
 A spring attached to the moving system produces a
controlling torque. The requirements for spring are
 1. They should be non- magnetic.
 2. They should be free from mechanical fatigue.
 3. They should have a small resistance, where springs are
used to lead the current into moving system.

7.  Flat spiral spring A and B as shown in figure.

8.  ii) Gravity Control
 Figure shows the gravity control, in which two weights,
balance weight and control weight are attached to the
spindle of the moving system.
 The balance weight is used to balance the weight of the
pointer. The controlling torque is produced by control
weight.
 The controlling torque is proportional to the sine of the
angle deflection 'θ'

9. Damping System
 Damping system is provided in order to bring the pointer
to rest within short time.
 The quickness with which the moving system settles to
the final steady position
 When the moving system oscillates about final steady
position with a decreasing amplitude and takes some
time to come to rest, then the instrument is said to be
under damping.
 When the moving system moves rapidly but smoothly to
its final steady position, then the instrument is said to be
critically damped or deadbeat.
 When the instrument is over damped, the moving

10.  Generally, underdamped system is preferred for any
instrument.
 Various methods used for producing damping torque
are,
 i) Air friction damping
 ii) Fluid friction damping and
 iii) Eddy current damping

11. i) Air Friction damping
 The air friction damping system which consists of a
light aluminium piston attached to the moving system
(i.e., pointer).
 The piston moves in a fixed air chamber which is
closed at one end.
 The clearance between the piston and chamber wall
is uniform throughout the chamber and it is very
small.
 When there is oscillations in the pointer, the piston will
move inside the air chamber.

12. ii) Fluid Friction Damping
 As the viscosity of oil is greater than air, the damping
force of this type of damping the greater than air friction
damping.
 The disc is dipped into the oil pot and it is completely
submerged in oil. When the moving system moves, the
disc moves in oil and which always opposes the motion.

13. iii) Eddy Current Damping
 The eddy current damping which is the most effective
way to provide damping. It is based on Faraday's law
and Lenz's Law.
 When a conductor moves in a magnetic field, it cuts the
magnetic field and hence emf is induced. This induced
emf opposes the causes producing it, thus opposing

14.  In this method, an aluminium disc is connected to the
spindle which is inturn connected to the pointer.
 A part of the aluminium disc is inserted into the damping
magnet which is a permanent magnet.
 When the pointer oscillates, the aluminium disc rotates
which inturn cuts the magnetic field of the damping
magnet.
 So, an emf is induced in the disc. As the disc is a closed
path, eddy current flows through the disc which
opposes the cause producing it i.e., the pointer
oscillation, thus hunting the pointer oscillation.

15. MOVING IRON INSTRUMENTS
 There are two types
 Moving iron attraction type instruments
 Moving iron repulsion type instruments
 Moving iron attraction type instruments
 It consist of fixed coil C and moving iron piece D.
 The coil is flat and has a narrow slot like opening.
 The moving iron is a flat disc or a sector eccentrically
mounted on the spindle.
 The spindle is supported between the jewel bearings.
The spindle carries a pointer which moves over a
graduated scale.

16.  Following consequences happens
 Current in coil→ produce magnetic field → attract disc →
pointer moves → so measure current.
 Tc provide by spring
 Td provide as air friction damping

17. Moving iron repulsion type
instruments
 Two vanes inside the coil, one is fixed and other is
movable
 When the current flows in the coil, both the vanes are
magnetised with like polarities induced on the same
side.
 Both vanes gets magnetized and get repulsive each
other. So pointer moves
 It has two types
 Radial vane type
 Coaxial vane type

18.  1. Radial Vane Repulsion Type Instrument
 The two vanes are radial strips of iron. The fixed vane is
attached to the coil.
 The movable vane is attached to the spindle and
suspended in the induction field of the coil.
 Even though the current through the coil is alternating,
there is always repulsion between the like poles of the
fixed and the movable vane. Hence the deflection of the

19.  2. Co-axial Vane Repulsion Type Instrument
 In this type of instrument, the fixed and moving vanes are
sections of co axial cylinders

20.  The controlling torque is provided by springs.
 The damping torque is provided by air friction
damping.
 Eddy current damping cannot be used in moving iron
type instruments, because introduction of permanent
magnet required for eddy current damping would
distort the operating magnetic field of MI instruments
which is very weak.
 Moving Iron type of instruments can be used for both
AC and DC measurements

21. Torque Equation of Moving Iron
Instruments
 The energy stored in the coil in the form of magnetic
field = (1/2)LI2.
 As soon as the current changes to (I+dI), deflection in
the pointer becomes dƟ resulting into change in
inductance of coil from L to (L+dL).
 Let this deflection in pointer is due to deflection torque
Td.
 Mechanical work done = Td . dƟ ………………..(1)
 Energy stored in Coil = (1/2)(L+dL)(I+dI)2
 Change in stored energy of coil = Final Stored Energy –
Initial Stored Energy

22. = (1/2)(L+dL)(I+dI)2 – (1/2)LI2
= (1/2)[ (L+dL)(I+dI)2 – I2L]
= (1/2)[ (L+dL)(I2+2IdI+(dI)2 – I2L]
= (1/2)[ LI2+2LIdI+L(dI)2 + dL.I2+2IdI.dL+dL.(dI)2 – I2L]
 Neglecting second order and higher terms of differential
quantities
i.e. L(dI)2, 2IdI.dL and dL . (dI)2 = (1/2)[ 2LIdI+dL.I2]
= LIdI +(1/2)dL.I2 ……………………(2)
 We can write as, e = d(LI) / dt
= IdL/dt + LdI/dt
 But electrical energy supplied by the source = eIdt
= (IdL + LdI)
. I

23.  According to law of conservation of energy, this
electrical energy supplied by the source is converted
into stored energy in the coil and mechanical work done
for deflection of needle of Moving Iron Instruments.
 I2dL + LIdI = Change in stored energy + Work done
⇒ I2dL + LIdI = LIdI +(1/2)dL . I2 + Td . dƟ
⇒ Td . dƟ = (1/2)dL.I2
⇒ Td = (1/2)I2(dL/dƟ)
 Thus deflecting torque in Moving iron Instruments is
given as
Td = (1/2)I2(dL/dƟ)
 In moving iron instruments, the controlling torque is
provided by spring. Controlling torque due to spring is
given as
T = KƟ in N-m

24.  In equilibrium state,
 Deflecting Torque = Controlling Torque
 ⇒ Td = Tc
 ⇒ (1/2)I2(dL/dƟ) = KƟ
 ⇒ Ɵ = (1/2)(I2/K)(dL/dƟ)
 Ɵ α I2
 The deflection torque is unidirectional whatever may
be the polarity of the current.
 Hence, the MI instruments can be used for both AC
and DC.

25. Errors in Moving Iron
Instruments
 1. Errors with both A.C and D.C work:
 (a) Hysteresis error.
 (b) Stray magnetic field error.
 (c) Temperature error.
 (d) Friction error.
 2. Errors with A.C work only:
 (e) Frequency error.
 (f) Error due to reactance of the instrument coil.
 (g) Error due to eddy current.
 (h) Error due to waveform.

26. Advantages of Moving iron
Instruments
 Used for the measurement of AC and DC quantities.
 These types of instruments have high value of torque to
weight ratio. Due to this error because of friction is quite
low.
 It is very cheap due to simple construction.
 There is no moving part in the instrument which carries
current.
 These instruments can be designed to provide precision
and industrial grade accuracy. A well designed moving
iron instruments have a error of less than 2 % or less for
DC. For AC, the accuracy of the instrument may be of
the order of 0.2 to 0.3 % at 50 Hz.
 Not damaged even under sever overload conditions.

27. Disadvantages of Moving Iron
Instruments
 These instruments suffer from error due to hysteresis,
frequency change and stray losses.
 The scale of moving iron instrument is not uniform.
Accurate readings are not possible at lower range.
 If it is used at 50 Hz, calibration must also be done at
the same frequency i.e. 50 Hz.
 Moving Iron Instruments are suitable for low frequency
application. Moving iron instruments are not suitable for
frequency above 125 Hz.
 The reading of the instrument is affected by
temperature variation.

28. PERMANENT MAGNET MOVING
COIL (PMMC) INSTRUMENTS
 The permanent magnet moving coil instrument is the
most accurate type for d.c. measurements.
 Basic Principle
 The action of these instruments is based on the motoring
principle.
 When a current carrying coil is placed in the magnetic field
produced by permanent magnet, the coil experiences a
forced and moves.
 As the coil is moving and the magnet is permanent, the
instrument is called permanent magnet moving coil
instrument.
 The basic principle is called D' Arsonval principle.

29. Construction of PMMC
Instruments

30.  The moving coil is either rectangular or circular in
shape.
 The controlling torque is provided by the method of
spring control with the help of two phosphor bronze hair
springs.
 The damping torque is provided by the movement of the
aluminium former in the magnetic field produced by the
permanent magnet.
 The scale markings of the basic d.c. PMMC instruments
are usually linearly spaced

31. Torque Equation for PMMC
 The deflecting torque is given by,
Td = NBAI
 Td = GI
Where, G = NBA = constant
 The controlling torque is provided by the springs
 Tc = KØ
For the final steady state position,
Td = Tc
Therefore GI = KØ
Ø = (G/K)I or I = (K/G) Ø
Ø α I

32. Errors in PMMC Instrument
 Errors due to permanent magnets
 Error may appear in PMMC Instrument due to the
aging of the spring.
 Change in the resistance of the moving coil with
the temperature

33. Advantages of Permanent Magnet
Moving Coil Instruments
 The scale is uniformly divided
 Power consumption is also very low
 A high torque to weight ratio. So operating current
is small.
 The sensitivity is high
 It has high accuracy
 Instrument is free from hysteresis error
 Extension of instrument range is possible
 Not affected by external magnetic field called
stray magnetic fields.

34. Disadvantages of Permanent
Magnet Moving Coil Instruments
 These instruments cannot measure AC quantities.
 The cost of these instruments is high
 Ageing of permanent magnet and the control springs
introduces the errors.
 The friction due to jewel-pivot suspension.

35. ELECTRODYNAMOMETER
WATTMETER
 Fixed coil
 Current Coil (C.C), which is connected in series with the
load and it carries the current through the load.
 Moving coil
 Across the load and it carries the current proportional to
the voltage across the load.
 Pressure Coil (or) P.C.

36.  Fixed Coil
 Carry the load current of the circuit.
 Generally they are divided into two halves but connected
in series.
 The fixed coils are wound with heavy wire with less
number of turns
 The maximum current range of wattmeter is 20A
 Moving Coil
 The moving coil is generally attached to the spindle which
is connected to the pointer.
 It is made of thin wire but has more number of turns
 A series resistor is used in the voltage circuit in order to
limit the current to a small value in the order of 100mA.
 The voltage rating of the wattmeter is limited to 600 V.
 Control Torque- Control torque is provided by springs

37. Errors in electrodynamometer type
wattmeter
 Error due to pressure coil inductance
 Error due to pressure coil capacitance.
 Error due to the effect of manual inductance.
 Error due to wrong connection of current coil and
pressure coil.
 Eddy current error.
 Stray magnetic field error.
 Error caused by vibration of moving system.
 Temperature error.

38. INDUCTION TYPE ENERGY
METER
 Energy meters is an integrating instrument which
measures quantity of electricity.
 These meters record the energy in kilo-watt-hours
(kWh).
 Energy meter is an instrument used to measure energy
which is the total power consumed over a specific
interval of time.
 Unit of energy is kWh or Joules.
 Energy = Power x Time

39.  Basic Principle
 The operation of the induction type energy meter is based
on the passage of alternating current through two coils
 Magnetic field which interacts with a aluminium disc
supported near the coils and make the disc rotates.
 The current coil carries the line current and develops
magnetic field. This magnetic field is in phase with the
line current.
 The pressure coil is highly inductive, hence the current
through it lags behind the supply voltage by 90̊.
 Due to this, a rotating field develops which interacts with
the disc to rotate.

40.  Construction Details
 i) Driving System ii) Moving System
 iii) Braking System iv) Registering System

41.  i) Driving System
 The coil of one of the electromagnets, called current
coil, is excited by load current which produces flux. This
is called as a series magnet.
 The coil of another electromagnet is connected across
the supply and it carries current proportional to supply
voltage. The coil is called pressure coil. This is called
shunt magnet.
 The flux produced by the shunt magnet is bought in exact
quadrature with supply voltage
 ii) Moving System
 Moving System consists of an aluminium disc
 The moving system is connected to a hardened steel
pivot which is screwed to the foot of the shaft.
 The Pivot is supported by a jewel bearing. In this type of
energy meter, as there is no controlling torque

42.  iii) Braking System
 The braking system consists of a permanent magnet
positioned near the edge of the aluminium disc.
 The aluminium disc moves in the field of this magnet and
this provides a braking torque.
 iv) Registering System / Counting System
 The function of a registering or counting mechanism is to
record continuously a number which is proportional to the
revolutions made by the moving system.

43.  Operation
 The C.C carries the load current. It produces the magnetic
fields in phase with the line current.
 The P.C carries current proportional to the supply voltage.
 The magnetic field due to pressure coil lags approximately
90̊ behind the supply voltage
 The magnetic field due to current coil develops eddy
current in the aluminium disc which react with magnetic
field due to the pressure coil.
 Thus a torque is developed in the disc then it rotates.
 The braking magnet produces mechanism so that the
electrical energy consumed in the circuit is directly given in
KWh (Kilo Watt hour)

44.  Advantages of induction type energy meters
 The construction is simple and strong.
 It is cheap in cost.
 It has high torque to weight ratio, so frictional errors are
less and we can get accurate reading.
 It has more accuracy.
 It requires less maintenance.
 Disadvantages of induction type energy meters
 The main disadvantage is that it can be used only for a.c.
circuits.
 The creeping can cause error.
 Lack of symmetry in magnetic circuit may cause errors.

45. INTRODUCTION TO
TRANSDUCERS
 A transducer is a sensing device, it converts physical
phenomenon into electrical, pneumatic or hydraulic
output signal.
 Mostly use definition in electrical instrumentation field
is, transducer is a device, which converts physical
quantity into electrical quantity.
 Basic Requirements of Transducer
 1. Ruggedness
 2. Linearity
 3. Repeatability
 4. High output signal quality
 5. High reliability and stability
 6. Good dynamic response
 7. No hysteresis
 8. Residual reformation

46. CLASSIFICATION OF
TRANSDUCERS
 Classification based on transduction principle
used
 Classified as resistive, inductive, capacitive depending
upon how they convert input quantity resistance,
inductance, and capacitance respectively.
 Primary and Secondary transducers
 Primary
 Transducers senses the input physical quantity directly and
convert directly into electrical quantity output.
 Secondary
 Input signal is sensed by other some detector or sensor and then
its output is given to transducer in other form then the transducer
converts the secondary signal into electrical.

47.  Active and Passive transducers
 Active
 Converts physical quantity into electrical quantity directly
 So it is called self generating type transducers.
 Passive
 In this transducer the electrical parameters resistance, inductance,
and capacitance changes with change in input signal are called
passive transducers.
 Analog and Digital transducers
 Analog transducers:
 Output of analog transducer is continuous function of time.
 Digital transducers:
 Output of this type transducer is pulses or discrete form

48.  Transducers and Inverse transducers
 Transducers:
 Transducer is a device, which converts input
physical quantity into output electrical quantity.
 Inverse transducers:
 Inverse transducer is a device, which converts
Input electrical quantity and output physical
quantity.

49. Capacitive transducer
 The principle based on capacitance of a parallel
plate capacitor
 C= εA/d= εoεrA/d
 The change capacitance caused by
 Change in overlapping area
 Change in distance “d” between the plates
 Change in dielectric constant
 These are changes due to changing the force,
displacement and pressure
 The change in capacitance causes change in
dielectric constant. Also measure the liquid level

50. variation of overlapping area of
plates
 C α A, capacitance changes linearly with change
in area of plates

51.  The area changes linearly with the displacement
and also the capacitance
 Char are linear. But initially non linearity due to
edge effects
 Parallel plate capacitor, the capacitance is
 C= εA/d= (εXW/d)* F
 X= length of overlapping portion of plates in m
 W= width of overlapping portion of plates in m
 Sensitivity as
 Cylindrical capacitor whose over lapping area is
varied by varying length of over lapping portion of
cylinder
 Cylindrical transducer as shown

52.  Capacitance as
 S=const. Then relationship between capacitance
and displacement is linear
 Fig shows two plate capacitor

53.  Angular displacement to measured is applied
movable plate
 The angular displacement changes the effective
area between area of plate and thus changes the
capacitance

54. Capacitive transducers- By variation
of distance between the plates
 C α (1/d), used to measure linear displacement

55.  Here one plate fixed and other plate moving
 Moving plates moving away from or towards the
fixed plate as per displacement under
measurement, so capacitance decreases or
increases
 Capacitance measured by AC bridges circuit, so
displacement of moving plate is determined
 Curve is non linear. Sensitivity is high for initial
portion of curve

56. Capacitive transducer-
Differential arrangement
 To achieve linear char, differential arrangement as
shown

57.  It have three plates,
 P1, P2→ Fixed plate
 M → Movable plate
 So two capacitor with differential o/p
 M-midway between P1 & P2
 AC voltage E applied between P1 & P2
 C1=C2, E1=E2=E/2 (i.e) exactly midway between
2 plates

60. Advantages of capacitive
transducers
 Have very high i/p impedance, so min loading
effect
 Have good freq response. This response as high
as 50KHz and very useful for dynamic studies
 Not affect by stray mag fields
 High sensitivity, higher resolution
 Force requirement of capacitive transducer is
very small and require small power to operate
them

61. Disadvantages of capacitive
transducers
 Very high o/p impedance. So complicated
measuring circuit
 Stray capacitance including that cables etc in
parallel with o/p impedance of transducer also
causes error and introduces non linearity
 The cable connecting the transducer to the
measuring point is also a source of error. The
cable may br source of loading resulting in loss of
sensitivity. Also loading makes the low freq
response error
 The instrumentation circuitry used with these
transducer is very complex

62. Application of capacitive
transducers
 Use to measure both linear and angular
displacement
 Use to measure force and pressure. Here first
convert displacement causes change of
capacitance
 Able to measure pressure directly in all those
cases in which permittivity of a medium changes
with pressure, such as in case of benzene
permittivity vary by 0.5%, in pressure range of 1 to
1000 times the atm pressure
 Use to measure humidity. Since the permittivity of
gases varies with variation in humidity. Though the
variation in capacitance due to variation in
humidity is quite small but is detectable

63. Capacitor microphone
 Most commonly used as studio
 Thin electrically conductive diaphragm is
suspended over back plate forming a flexible
capacitor
 One plate is diaphragm, it mounted not touching.
Other plate is back plate
 Battery connected to both plates, which produces
electrical potential or change between them
 Sound wave exit the diaphragm. Distant between
plate change the capacitance, due to change the
voltage
 This is excellent choice for mixing vocals, acoustic
guitar, piane, sound effect.

65. Inductive transducer
 Either self generating or passive type
 Self generating type utilize basic generator
principle
 An inductive transducer is a device that convert
physical motion into a change in inductance
 The principle used as
 No of turns
 Geometric configuration
 Permeability of magnetic material or magnetic
circuit

66. Transducer based on principle of change
in self inductance with no of turns
 o/p changes w.r.to no of turns
 Measure displacement of linear and angular
movement as shown
 Here no of turns changes, inductance changes,
then o/p changes

67. Transducer working on principle of change in
self inductance with change in permeability
 Inductive transducer on principle of variation of
permeability causing change in self inductance as
shown
 Iron core surrounded by winding. Here
permeability changes, then L-changes
 Iron move out of winding, permeability↓, then L↓
in coil. Source to measure displacement

68. Variable reluctance inductance
transducer
 Use to measure linear displacement
 Hence length of magnetic path varies with the
displacement and reluctance of magnetic circuit
changes causing in self inductance of the coil

69. Linear Variable Differential
Transformer (LVDT)
 Construction
 It is widely used inductive transducer to translate
linear motion into electrical signal
 LVDT is differential transducer consist of one
primary (P) and two secondary (S1 & S2). Both
wound on non magnetic material
 S1 & S2 have equal no of turns and identical placed
on either side of pri winding
 Displacement to be measure is applied to arm
attached to the soft iron core
 In order to overcome the problem of eddy current
losses in the core, nickel-iron alloy is used as core
material and is slotted longitudinally

70. Construction as shown

71. Working
 Primary winding voltage range as 5-25V and freq
as 50Hz-20kHz
 Primary winding exited AC current source, so
produces AC mag field which induces AC voltages
 Es1- o/p voltage of S1
 Es2-o/p voltage of S2
 Both S1 & S2 are in series opposing.
 Differential o/p voltage = Eo=Es1-Es2

73.  Case (i): When the core is at its normal (NULL)
position
 Core is normal null position
 Both sec. have equal flux linkages (i.e) Es1=Es2
 So Eo=Es1-Es2=0
 Case (ii): The core is moved to the left of the NULL
position
 Core moved left of NULL position (i.e) at A
 Flux linkages more in S1 and less in S2
 So Es1>Es2, Eo= Es1-Es2
 Eo=+ve which is in phase wih o/p
 Case (iii): The core is moved to the right of the null
position
 Core move right of NULL position (i.e) at B
 Flux linkages less in S1 and more in S2

74.  Eo α (movement of core) (i.e) linear motion
 Eo↓ or Eo↑ depends on direction of motion
 o/p of one secondary increases and other
secondary decreases. So which use to measure
displacement
 Variation of Eo w.r.to displacement of core as
shown. For small displacement only linear char.
Small changes
 At ‘O’ position of core, Eo not equal to zero due to
have some residual magnetism (i.e) 1% of Emax
 Residual voltage due to mag unbalance or
electrical unbalance
 Due to harmonics & saturation of iron core
contribute residual voltage. Also due to stray mag
field.

75. Advantages of LVDT
 Upto 5mm, it have linear displacement
 High sensitivity, range as 10mV/mm – 40mV/mm
 Give high o/p. No need of amplification
 Use freq upto 20kHz, more reliable
 Have low hysterisis, hence repeatability is
excellent under all condition
 Rugged construction, vibration without any
adverse effect
 Power consume < 1W, small weight
 Stable and easy maintanance

76. Disadvantages of LVDT
 Require large displacement of o/p
 Sensitive with stray mag field
 Performance affected by vibration
 Receiving instrument select to operate AC signal
or a demodulator network must be used if a DC
o/p is required
 The dynamic response is limited mechanically by
mass of the core and electrically by freq of applied
voltage. The freq of the carrier should be at least
10 times the highest freq component to be
measured
 Performance is affected with temperature

77. Application of LVDT
 LVDT use to measure
 Displacement
 Force
 Weight
 Pressure
 Position

78. STRAIN GAUGES
 Piezo resistive Effect
 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 it is strained and this property is called
piezo resistive effect.
 Uses of strain gauges
 Used for measurement of strain and associated stress in
experimental stress analysis.
 Many detectors and transducers notably the load cells,
torque meters, pressure gauges, temperature sensors,
accelerometers and flow meters, employ strain gauges as
secondary transducers.

79. Classification of Strain gauges
 Wire strain gauge
 Foil strain gauge
 Thin film strain gauge
 Semiconductor strain gauge

80. 1. Wire strain gauges
 It is small size, min leakage, employ high temp
 It has two types
 Unbounded resistance wire strain gauge
 Bonded resistance wire strain gauge
 Unbounded resistance wire strain gauge
 It consist of wire stretched between 2-point of insulating
medium (i.e) air
 Dia=25µm
 Wire have high tension. So that no sag & no vibration
 Load applied, resistance changes, unbalances the bridges.
 So V0 changes, V0 α strain, displacement ≈ 50µm

82.  Bonded resistance wire strain gauge
 The schematic as shown
 Dia of wire≈25µm
 Loop as back and forth
 The grid of fine wire is cemented on a carriers which
may be a thin sheet of paper, backelite or teflon
 Wire converted on the top with thin material, so not
damaged mechanically
 Spreading of wire permits uniform distribution of
stress

83. 2. Foil strain gauge
 It is extension of resistance wire strain gauge
 Metal & alloys use for foil. Nichrome, constantant
use for wire
 It have high dissipation capacity. So use high temp
gauge. It have better bonding due to larger area
 Advantage as fabricate to larger scale, any shape.
 Etched foil gauge construction consist of first
bonding layer of strain sensitive material to a thin
sheet of paper of paper or bakelite
 Etched foil strain gauge made thinner than
comparable wire units. More flexible. So it placed
remote & restricted places and curved placed.

85. 3. Thin film strain gauges
 This can be produced by depositing a thin layer of
metal alloy an elastic metal specimen by means of
vacuum deposition
 This technique, relatively new and extensively
used to produces a strain gauge that is
molecularly bondes to the specimen under test
and so the drawback of epoxy adhesive bond are
eliminated
 Thin technique is most widely used for transducer
application such as in disphragm type pressure
gauges.

86. 4. Semiconductor strain gauge
 It have high sensitivity have gauge factor
 It required high value of gauge factor. It is 50 time
higher then wire strain
 Resistance change w.r.to applied strain
 Semiconductor used as germanium & silicon
 The schematic as shown

87.  Consist of strain material and leads placed in
protective box. Thickness of wafer 0.05mm used
 Bonded on suitable insulating subsrate, such as
teflon
 For making contact use gold leads
 For soldering leads use cadmium material
 It have both +ve and –ve gauge factor for p and n-
type silicon respectevely

88. Advantages of semiconductor
strain gauges
 Measure very small strain as well as 0.01 micron.
Also high gauge factor between -100 and +150
 Manufacturing very small size range of 0.7- 7mm
use to measure high localized strain
 Chemically inert and low sensitivity
 Have excellent hysteresis char.
 Disadvantages
 Sensitive to change w.r.to temp, more expensive
 Poor linearity char

89. Hall effect

90.  Ic flows downwards in semiconductor pellet which
placed in magnetic field perpendicular to pellet
surface, an VH created in pellet in direction
perpendicular in both Ic and magnetic field. This
process called as hall effect.
 Electromagnetic force act on charged particle
according to F.L.H.R, the charged particle are
biasing to left side of semiconductor pellet.
 The magnitude of emf VH, which is called the hall
voltage
 VH=1/d(BIcRH)
 RH= hall constant
 B=flux density
 D=thickness of semiconductor
 Semiconductor device which are made use in

91. Conceptual Diagram of Hall
Effect Transducer
(15)
(15)

92.  A constant current runs through a conductive Hall strip
inside the sensor.
 The diagram shows a rotating magnet placed near the
Hall sensor.
 The alternating field from this rotating magnet will
cause an alternating Hall voltage to be generated
across the Hall strip.
 This alternating voltage waveform is fed into the digital
circuitry. This digital circuitry converts alternating
voltage waveform into square waveform i.e., digital
signal (ON or OFF) / + 5 V DC or 0 V DC).
 Sensors are available with a verity of output voltages
and polarities. If the sensor is placed in the south
magnetic pole, the sensor is turned ON and remains
ON, after the south pole is removed.

93.  Advantages
 Can operate high speed than mechanical points
 Operating frequency as 100KHz
 Measure wide range of magnetic fields
 Stable, reliable, long lasting
 High resolution and small size
 Disadvantages
 Very low o/p drive capability
 Difficult to operate in strong external magnetic field
 Less accurate
 Application
 BLDC motor, Proximity detector, Speed sensor
(motor control)
 Vending machine, Shaft position sensor, valve
position detector

94. Piezoelectric transducer
 Piezoelectric material is one which an electric
potential appears across certain surfaces of a
crystal surfaces of a crystal if the dimension of the
crystal are changed by the application of
mechanical force. This potential produced by
displacement of changes.
 The effect is reversible also, varying potential
applied to proper axis of crystal, it will changes the
dimension of crystal thereby deform it. This
phenomenon is known as piezoelectric effect
 Piezo is greek word meaning force or pressure.
Element exhibiting piezoelectric quantity are called
electro resistive elements

96. Material for piezoelectric transducer
 Common used material as rochelle salt,
ammonium, dihydrogen phosphate, quartz and
ceramics made with barium titanate, dipotassium ,
lithum sulphate
 The piezoelectric effect can be made to respond to
mechanical deformation of material in many
different modes. These modes are
 Thickness expansion
 Transverse expansion
 Thickness shear
 Face shear
 Mechanical deformation generates a charge and
this charge appears as voltage across electrodes

97.  A tensile force produce a voltage of one polarity while a
compressive force produces a voltage of opposite
polarity
 A crystal between a solid base and the force summing
member. An extremely applied force, entering the
transducer through its pressure, applies pressure to top
of crystal. This produces a voltage across the crystal
proportional to the magnitude of applied pressure.
 Magnitude and polarity of induced surface charges
proportional to mag and direction of force
 Q=F*d
 d= crystal charge sensitivity in coulombs per newton and is
constant for a given crystal cut
 F= force in newton
 The Voltage sensitivity g = E0/tP or g=ε/P ...........

98. Modes of operation of
piezoelectric crystal
 The different modes as
 Thickness shear
 Face shear
 Thickness expansion
 Transverse expansion

100.  Advantages of piezoelectric transducer
 Small size, light weight, rugged construction
 It has self generating type and no need of external
power
 o/p is quite large
 Very good high freq response. Range as 1Hz to
20KHz. Natural frequency as 50KHz
 Disadvantages of piezoelectric transducer
 Eo affect with temp variation of crystal
 Use for dynamic measurement only
 Application of piezoelectric transducer
 Use to measure of force, pressure, temp
 Employ high freq accelerometer