Unit-2 TRANSDUCERS AND STRAIN GAUGES

Mechanical Measurement – 15ME33T 2020-21

DEPARTMENT OF MECHANICAL ENGINEERING, GOVT POLYTECHNIC KALGI -585312 Page 1

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.



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)



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




Forcing function to displacement



Temperature to electric current



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 force

Linear to velocity

Displacement of resistance change



Strain to resistance change

Ai velocity to resistance change.

Temperature to resistance

Displacement to inductance change





Velocity to inductance change





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 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.



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.



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



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



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.



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



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



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.



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.



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.



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



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.



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

https://en.wikipedia.org/wiki/Deformation_%28mechanics%29#Strain



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.



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.



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.



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.



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,



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



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

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Unit-2 TRANSDUCERS AND STRAIN GAUGES