unit-2 transducers and strain gauges
TRANSCRIPT
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Unit-2 TRANSDUCERS AND STRAIN GAUGES
2.1 Definition
A transducer is a device used to convert position displacement, thermal and
optical signal into electrical signal that may be a
mplified, recorded and processed in the instrumentation system. Transducers are
also known as prime sensors or pickups or signal generators
Examples of common transducers:
i. Microphone. (Converts sound into electrical impulses. Sound energy into
electrical energy)
ii. Loud speaker. (Converts electrical impulses into sound. Electrical energy into
sound energy.)
iii. Electric motor.(Converts electrical energy into mechanical energy or motion)
iv. Thermocouple. (Converts thermal energy into electrical energy) etc.
2.2 Characteristics of Transducers
The characteristics of transducers are:
1. It should be small in size and less weight
2. It should have exceptional reliability
3. Should possess low cost.
4. It should be accurate for fast transient pressures.
5. It should high sensitivity
6. It should maintain stability with environmental changes
7. It should develop linear relationship between input and output
2.3 Requirements of Transducers or Factors to be considered for the
selection of transducer
Factors are listed bellow
1. It should have good frequency response i.e either study state , transit or
dynamic.
2. It should have high sensitivity.
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3. High accuracy and precision.
4. It should have linear relationship between input and
output.
5. Less affected by mechanical hysteresis i.e., friction,
backlash, loose screws.
6. Magnitude of creep should be less.
7. Transducer should possess less loading effect ( due to its
large size and weight, it consumes the power)
8. It should be compact and precise.
9. It should have good repeatability.
10. Less affected by environmental conditions.
11. It should be capable of withstanding heat, over range and
power dissipation ratings.
12. It should be calibrated easily.
13. Good compatibility.
14. It should be less expensive.
15. Less maintenance.
2.4 Classification of Transducers
Transducers are broadly classified into
1.Analogue transducers.
2. Digital transducers.
1. Analogue transducers
i) Electromechanical types
a) Potentiometric resistance type
b) Inductive type transducers
i) self generating type
ii) Non self generating types(LVDT)
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c) Capacitive type transducers
d) Piezo-electric transducers
e) Resistance strain gauges
f) Ionisation transducer.
g) Michano-electronic transducer.
ii) Opto-electrical transducers.
a) Photo emissive transducer.
b) Photo conductive transducer.
c) Photo voltaic transducer.
2. Digital transducers.
i) Frequency domain transducers.
a) Electromechanical frequency domain transducer
b) Opto-electrical frequency domain transducer
c) Vibrating string transducer
ii) Digital encoders
2.4.1 Another way of classification of transducer is As follows
Type Operation
I Mechanical
A. Contacting spindle, pin or finger
B. Electric member
1. Proving ring
2. Bourdon tube
3. Bellows
4. Diaphragm
5. Spring
C. Mass
1.Seismic mass
2.Pendulum scale
3.Manometer
D. Thermal
1. Thermocouple
Displacement to displacement
Force to displacement
Pressure to displacement
Pressure to displacement
Pressure to displacement
Force to displacement
Forcing function to displacement
Force to displacement
Pressure to displacement
Temperature to electric current
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2. Bimaterial (includes mercury in
glass)
3. Temp stick
E. Hydro pneumatic
1. Static
a) Float
b) Hydrometer
3. Dynamic
a) Orifice
b) Venturi
c) Pitot tube
d) Vanes
e) Turbines
II . Electrical
A. Resistance
1.Contacting
2.Variable-length conductor
3.Variable area of conductor
4.Variable dimensions of conductor
5.Variable resistivity of conductor
B. Inductive
1. Variable coil dimensions
2. Variable air gap
3. Changing core material
4. Changing coil positions
5. Changing core positions
6. Moving coil
7. Moving permanent magnet
8. Moving core
Temperature to displacement
Temperature to phase
Fluid level to displacement
Specific gravity
Velocity to pressure
Velocity to pressure
Velocity to pressure
Velocity to force
Linear to velocity
Displacement of resistance change
Displacement of resistance change
Displacement of resistance change
Strain to resistance change
Ai velocity to resistance change.
Temperature to resistance
Displacement to inductance change
Displacement to inductance change
Displacement to inductance change
Displacement to inductance change
Displacement to inductance change
Velocity to inductance change
Velocity to inductance change
Velocity to inductance change
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C. Capacitive
1. Changing air gap
2. Changing plate areas
3. Changing dielectric
D. Electronic
E. Piezoelectric
F. Photoelectric
G. Streaming potential
Displacement to capacitance change
Displacement to capacitance change
Displacement to capacitance change
Displacement to current
Displacement to voltage
Light intensity to voltage
Flow to voltage
2.5 Transducer Actuating· Mechanisms
The transducer actuating mechanisms are the elastic members
which when subjected to a pressure, they get deformed. The deformation
may be measured by mechanical or electrical means. These mechanisms
are convenient to use and can cover a wide range of pressure, depending
on the design of the elastic elements.
2.5.1 Types of Actuating Mechanisms :
Following are the different types of transducer actuating m echanisms.
1. Diaphragm
2. Currugated diagphragm
3. Capsule
4. Bellows
5. Circular bourdon
6. Twisted bourdon tube
7. Helical tube
8. Straight tube
9. Magnetic diaphragm
10. Spiral bourdon tube, etc.
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2.6 Voltage and current generating analog transducers: (Inductive
type transducers)
In these type of transducers, the magnetic characteristics of an electric
circuit changes due to the motion of the object.
These are two types:
i ) Self generating types
ii) Non self generating types.
i) Self generating types: In self-generating type transducers a voltage signal
is generated in the transducer, because of relative motion of a conductor and a
magnetic field.
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Types:
1. Single coil transduce or electromagnetic transducer
2. Two coil self- inductive transducer
3. Electrodynamics transducer
4. Electrodynamics transducer for rotary motion
5. Eddy current transducer.
1. Single coil transduce or electromagnetic transducer.
Figure shows the single coil electromagnetic transducer, in which a voltage
is induced in the coil when the magnetic flux about it is varied due to the motion of
the object of ferromagnetic material. In this case, the flux intensity changes due to
chant in the air gap by the to and fro motion of the object. The change in the
inductance can be measured by suitable circuit.
2. Two Coil Self Inductive Transducers.
Two coil self-inductive transducer in which an output voltage signal is
generated because of relative motion of a conductor and a magnetic field.
two coil self-inductance transducer is as shown in figure. It may also be
called as single coil with center tap transducer. Two coils are wound over the non-
magnetic material and tapped at the center. The movement of armature or core
alters the annular space between magnetic and non magnetic material which
changes the relative inductance of the two coils, this provides an output. these
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devices are generally incorporated in some form of inductive bridge circuit, in
which variation in the inductance ratio between the two coils provides the
output. It is used as a secondary transducer for pressure measurement.
3. Electrodynamics Transducer
Figure shows the electrodynamic type of transducer. A coil is wound on a
hollow cylinder of non magnetic material which moves in the annular space
of a fixed magnet. The moment of coil generates a voltage in the coil wich is
Proportional to the rate of the change of flux and hence the velocity of a moving
Object changes. The coil cylinder is connected to the moving object and thus
this Is the contact type transducer.
4. Electrodynamics Transducer for Rotary Motion
Figure Shows the electrodynamic transducer for measuring the rotary
motion. The coil moves in the annular space between a magnet and a
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soft iron core, which generates a voltage in coil and which can be measured
by a suitable circuit.
5. Eddy Current Transducer
Figure shows the eddy current type transducer. A non-ferrous plate
moves in a direction perpendicular to the lines of flux of a magnet. The
two magnets are placed as shown in figure and coil is wound on one
magnet to measure the output voltage when the plate moves in direction
perpendicular to the lines of flux of a magnet, the eddy currents are generated
in the plate. These are proportional to the velocity of the plate. These eddy
currents set up a magnetic field in a direction opposing the magnetic field that
creates them. The output voltage 'e' is produced which is proportional to
the rate of change of eddy current or the acceleration of the plate. Since
the air gap remains constant the transducer has linear characteristics.
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II. Non-Self Generating Type Transducer
These are the external power source types of transducers in which an external
source needed to energize a coil/coils the inductance of which would change
due to the motion of the object, The following types of transducers belongs to
this category
1 Variable inductance transducer 2. Air gap type
3. LVDT type 4. Magnetostrictive transducer
1. Variable Inductance Transducer
An inductance transducer shown in figure.. The core, made up
of high permeability steel is attached to the moving object. The
motion of the object changes the length of the core inserted in
the coil and thus the inductance of the coil gets changed that to
change of reluctance of the magnetic flux path.
Variable inductance transducer
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When the core moves up a n d down, the inductance of one half increases
while that of the other half decreases. The two inductance L1 and L2 form
adjacent arms of a wheat-stone bridge a shown in fig (c) The output is
supplied to the phase sensitive demodulator which eliminates the carrier
frequency and gives, an output corresponding to the motion frequency.
Variable Inductance Transducer for Rotary Motion
A variable inductance transducer for rotary motion as shown in
figure for measuring the angular displacement or tortional motion. One
half of the core is made up of a magnetic material while the other half
of non magnetic material. The inductances of two halves of the coil
depend upon the amount of magnetic material in their flux path. The
two inductances L1 and L2 form the adjacent arms of the wheat-stone
bridge as shown in figure.
2. The Air gap type Variable Inductance 'Transducer
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A small air gap type or proximity type of variable inductance
transudecer as shown in figure.
A small air gap in the magnetic flux path of an electromagnet is
varied the inductance of one coil increases, while that of other decreases
as shown in figure.
The two coils are wound over the two magnetic material will
form the arms of the wheat-stone bridge network 'The object is made
to move in the air gap between two magnetic materials When the
object moves, the inductances of the coil gets changed which is
proportional to the displacement of the object.
3. LVDT (Linear variable differential transformer) type of
transducer
Fig 2.18 shows the LVDT type transducer, it consists of a soft iron core which
moves up and down between a primary coil and two secondary coils connected in
series opposition as shown in figure. The core provides the magnetic coupling between
a primary coil and two secondary coils.
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When core is at center and both secondaries are identical, the voltag them are equal in
magnitude. However the output is zero as both the secondaries are in series opposition.
As the core moves up and .down, the induced voltage Or one secondary coil increases
while that of the other decreases. The output voltage which is modulated, isthe
difference wheatstone bridge network where ne output is measured which is
proportional tothe displacement of the iron core figure shows the LVDT rotary type of
transducer for measuring the angular displacement.
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4. Magneostrictive type of Transducer
Figure shows the simple magnetostrictive type of transducer is based on the principle
that the magnetic permeability of a ferromagnetic material like Ni changes when the
material is subjected to mechanical stress.
The magnetic permeability of Ni increases when the material is subjected to
compressive force and decreases when it is subjected to tensile force. This change
causes the change in inductance of the coil and produces the exciting current the
terminals. The magnitude and frequency of the exciting current can be measured
which is the measure of change in the inductance of the coil
These transducers can be used to measure the force, motion etc. These have
high mechanical impedance and thus resonant frequency is high with a good dynamic
response.
2.7 Piezo-electric Transducer
This operates on the principle that when a crystalline material like quartz or
barium titanate is distorted, an electrical charge is produced, this is known as piezo-
electric effect. As shown in fig 2.21, a piezo electric crystal is connected to an
amplifier whose output voltage is ie.
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Disadvantages
1. It is limited to dynamic measurements.
2. Output is quite low.
3. Output is affected by change in temperature.
Piezoelectric transducers are mainly used in the roughness, in acceirometers
and vibration pickups
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2.7.1 Various Piezo-slectric Materials
The various pieoz0-electric material are
1. Quartz
2. Barium titanate
3. Berlinite
4. Sucrose
5. Rochelle Salt
6. Topaz
7. Lead titanate
8. Tourmaline-group minerals
9. Silk
10. Tendon
11. Langasite
12. Gallium orthophosphate
13. Potassium niobate
14. Lithium niobate
15. Sodium tungstate
16. Lithium tantalate
17. Lead Zirconate titanate etc.
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2.8 STRAIN GAUGES:
2.8.1 Introduction:
It is not possible (currently) to measure stress directly in a structure. However,
it is possible to measure strain since it is based on displacement. There are a number
of techniques to measure strain but the two more common are extensometers
(monitors the distance between two points) and strain gauges.
Strain gauges are constructed from a single wire that is wound back and
forth. The gauge is attached to the surface of an object with wires in the direction
where the strain is to be measured.
The electrical resistance in the wires change when they are elongated. Thus, the
voltage change in the wires can be collaborated to the change in strain. Most strain
gauge measurement devices automatically collaborate the voltage change to the strain,
so the device output is the actual strain.
2.8.2 Definition
A strain gauge is a device used to measure strain on an object.
2.8.3 Purposes: Strain gauges are used for either of the two purposes.
1) To determine the state of strain existing at a point on a loaded member for the
purpose of stress analysis.
2) To act as a strain sensitive transducer element calibrated in-terms of quantities
such as force, pressure, displacement, acceleration or for the purpose of measuring
the magnitude of the input quantity.
2.8.4 Metals Used In Making Strain Gauges:
The strain gauges are made with the following metals.
1) Constantan
2) Nichrome
3) Dynalloy
4) Platinum alloy
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5) Copper Nickel
6) Nickel Chrome
7) Nickel Iron
8) Modified Nickel Chrome
9) Platinum Tungsten
2.8.5 Classification
Strain gauges can be classified as follows.
1. Mechanical strain gauges
2. Optical strain gauges
3. Electrical strain gauges
A. Resistance strain gauges
i. Bonded type
ii. Un-bonded type
iii. Bonded wire type
iv. Bonded foil type
v. Semiconductor gauges
B. Capacitive gauges
C. Inductive gauges
D. Piezoelectric gauges
2.8.6 Mechanical Strain Gauges (Berry-type)
This type of strain gauges involves mechanical means for magnification.
Extensometer employing compound levers having high magnifications was used. Fig.
shows a simple mechanical strain gauge. It consists of two gauge points which will be
seated on the specimen whose strain is to be measured. One gauge point is fixed while
the second gauge point is connected to a magnifying lever, which in turn gives the
input to a dial indicator. The lever magnifies the displacement and is indicated directly
on the calibrated dial indicator. This displacement is used to calculate the strain value.
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The Berry extensometer as shown in the Fig. is used for structural applications in civil
engineering for long gauge lengths of up to 200 mm.
Advantages
1. It has a self contained magnification system.
2. No auxiliary equipment is needed as in the case of electrical strain gauges.
Disadvantages
1. Limited only to static tests.
2. The high inertia of the gauge makes it unsuitable for dynamic measurements and
varying strains.
3. The response of the system is slow and also there is no method of recording the
readings automatically.
4. There should be sufficient surface area on the test specimen and clearance above it
in order to accommodate the gauge together with its mountings.
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2.8.7. Optical Strain Gauges
The most commonly used optical strain gauge was developed by Tuckerman as
shown in the figure. It combines mechanical and optical system consisting of an
extensometer and an autocollimator. The nominal length of the gauge is the distance
from a knife edge to the point of contact of the lozenge. The lozenge acts like a mirror.
The distance between the fixed knife edge and lozenge changes, due to loading. Then,
the lozenge rotates and if any light beam is falling on it will be deflected. The function
of the autocollimator is to send parallel rays of light and receive back the reflected
light beam from the lozenge on the optical system. The relative movement of the
reflected light as viewed through the eye-piece of the autocollimator is calibrated to
measure the strain directly. This gauge can be used for dynamic measurements of up to
40 Hz using a photographic recorder, and strains as small as 2µm/m can be resolved.
Gauge lengths may vary from 6 mm to 250 mm.
Advantages:
The position of autocollimator need not be fixed relative to the extensometer,
and reading can be taken by holding the autocollimator in hand.
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Disadvantages:
1. Limited only for static measurements.
2. Large gauge lengths are required.
3. Cannot be used where large strain gradients are encountered.
2.8.8. Mounting Of Strain Gauges
1) The surface on which the strain gauge has to be mounted must be properly
cleaned by an emery cloth and bare base material must be exposed.
2) Various traces of grease or oil etc., must be removed by using solvent like
acetone
3) The surface of the strain gauges coming in contact with the test item should also
be free from grease etc.
4) Sufficient quantity of cement is applied to the cleaned surface and the cleaned
gauge is then simply placed on it. Care should be taken to see that there should
not be any air bubble in between the gauge and the surface. The pressure applied
should not be heavy so that the cement may puncture the paper and short the grid.
5) The gauges are then allowed to set for at least 8 or 10 hours before using it. If
possible a slight weight may be placed by keeping a sponge or rubber on the
gauge.
6) After the cement is fully cured the electrical continuity of the grid must be
checked by ohm-meter and the electrical leads may be welded.
2.8.9 Problems Associated With Strain Gauge Installations
The problems associated with strain gauge generally fall in to the following three
categories.
1) Temperature effects: Temperature problems arise due to differential thermal
expansion between the resistance element and the material to which it is bonded.
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Semiconductor gauges offer the advantage of that they have lower expansion co-
efficient than either wire or foil gauges. In addition to the expansion problem, there is
a change in resistance of gauge with temperature which must be adequately
compensated.
2) Moisture absorption: Moisture absorption by the paper and cement can change
the electrical resistance between the gauge and the ground potential and thus
affect the output resistance readings.
3) Wiring problems: This problem arises because of faulty connections between
the gauge resistance element and the external read-out circuit. These problems may
develop from poor soldered connections or from in-flexible wiring, which may pull
the gauge loose from the test specimen or break the gauge altogether.
2.9 STRAIN GAUGE ROSETTES:
2.9.1 Introduction:
A strain gauge rosette is, by definition, an arrangement of two or more closely
positioned gauge grids, separately oriented to measure the normal strains along
different directions in the underlying surface of the test part. Rosettes are designed to
perform a very practical and important function in experimental stress analysis. It can
be shown that for the not-uncommon case of the general biaxial stress state, with the
principal directions unknown, three independent strain measurements (in different
directions) are required to determine the principal strains and stresses. And even when
the principal directions are known in advance, two independent strain measurements
are needed to obtain the principal strains and stresses.
2.9.2 Two Element Rosette Gauges
These are used for the measurement of stresses in bi-axial stresses fields, where
the directions of principal stress is known. Two strain gauges are mounted at 90º to
each other. Whenever the strain gradient along the surface is high and it is important to
approach a ‘point’ as nearly as possible, the grids are staked one on the top of the
other, being insulated between each other. Where there is a high strain gradient,
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perpendicular to the surface, the gauges must be as near to the surface as possible, i.e.
in one plain. It is shown in the following figure.
2.9.3 Three Element Rosette Gauges
These are used in general bi-axial stress fields. In this type also there exist
overlapping type and single plain type of gauges. The choice of either type depends up
on the nature of strain gradient at the point where the gauge is to be mounted. This is
also known as rectangular Rosette. The three strain gauges are oriented as shown in
the following fig.
2.10 Requirements of Ideal Strain Gauges
The following requirements are considered in selection of strain gauges.
a. High gauge factor
b. High resistivity
c. Low temperature sensitivity
d. High yield point
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e. High electrical stability f. High endurance limit
g. Good weldability or soldaribility h. Low hysteresis.
i. Low thermal EMF
j. Corrosion resistant
2.11 Gauge Factor:
Gauge factor is defined as the ratio of electrical strain to the mechanical strain. It is
denoted by ‘F’. It is an important parameter of the strain gauge which measures the
amount of resistance change for a given change. It is given by,
Or
Gauge factor of the conductor can be defined as the change in resistance ‘R’ due to the
strain ϵ i.e
Gauge factor, F =
Where,
ΔR=Change in Resistance.
ΔL=Small change in Length.
R=Initial Resistance.
L=Initial length
Higher the gauge factor of strain gauge, the more sensitivity of the gauge and
greater electrical output for indication and recording purpose. All the efforts is to be
made to develop a strain gauge having