Chapter 2 TRANSDUCERS AND STRAIN GAUGES

Department of Collegiate and Technical

Education

Chapter 2 TRANSDUCERS AND STRAIN GAUGES

PREPARED BY

THAURYA NAIK

LECTURER/ME

GPT KAMPLI

TRANSDUCERS

2.1 Introduction:

Sensing of input parameter is very essential in all measurement.

This is effectively done using sensors and transducers.

Definition:

Transducers are generally defined as devices that transform

values of physical quantities in the form of input signals

into corresponding electrical output signals.

The physical quantity may be heat, intensity of light, flow rate,

etc.

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TRANSDUCERS (Cont...)

Transducer :

➢ A transducer is a device used to convert position

displacement, thermal and optical signal into electrical

quantities that may be amplified, recorded and otherwise

processed in the instrumentation system.

➢ Transducers are also known as prime sensors or pickups or

signal generators.

➢ The function of the transducer is to present the input

information into analogous form.

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Introduction (Cont…)

Simple Block diagram of Transducer

Examples of common transducers:

▪ Microphone (Converts sound into electrical impulses. Sound energy into electrical energy)

▪ Loud speaker. (Converts electrical impulses into sound. Electrical energy into sound energy.)

▪ Electric motor.(Converts electrical energy into mechanical energy or motion)

▪ Thermocouple. (Converts thermal energy into electrical energy) etc.

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Characteristics of Transducers

The characteristics of transducers are as follows:

▪ It should be compact, small in size and less weight

▪ It should have exceptional reliability

▪ It should high sensitivity

▪ It should maintain stability with environmental changes

▪ It should develop linear relationship between input and output

▪ It should be available at lowest possible cost and ease of

producing, fabricating it.

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Requirements of Transducers1) Nature of measurement to be made

2) Mechanical input characteristics

a. Linearity

b. Mechanical hysteresis

c. Viscous flow or Creep

3) Loading effect of the transducers

4) Environmental factors (Ability to withstand environmental

conditions).

5) Capability of the transducer

6) Compatibility of the transducer and measuring system.

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Requirements of Transducers (Cont…)

7) Economical factors or considerations

8) Smaller in size and weight.

9) High sensitivity.

10) Low cost.

Nature of measurement to be made:

The measurement parameters are either steady state, transient or

dynamic

Ex: A strain gauge located on the structured member of an

aircraft would indicate steady loading on smooth flight,

dynamic loading and transient loading during landing.

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Mechanical input characteristics:

For static measurements, it is very important that transducer must

have linear relationship between inputs and output.

Example : Mechanical hysteresis in transducers like springs,

pressure capsule etc

Factors like friction, backlash, loose screws etc., are

responsible for hysteresis in the transducer.

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Requirements of Transducers (Cont…)Loading Effect of transducer:

If the attachments or installation of transducer in anyway affect or changes the value of parameters being measured, errors may be introduced.

Ex: If the mass of the thermocouple junction is too large, it physically affects the process by absorbing the heat in the system.

Environmental considerations :

The following environmental factors affect the performance of transducer

a) Temperature

b) Shocks

c) Vibration

d) Electromagnetic interference. 9


Transducer Capability:

▪ Transducer capabilities should be investigated to determine the

effect of overheating, over range and power dissipation rating.

▪ When sensing the input signal, the output power of the

transducer must not exceed the specified maximum power.

▪ Some factors involved in the selection of transducers are cost,

Basic simplicity, reliability and low maintenance.

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Factors To Be Considered While Selecting

Transducer

• It should have high input impedance and low output

impedance, to avoid loading effect.

• It should have good resolution over is entire selected range.

• It must be highly sensitive to desired signal and insensitive to

unwanted signal.

• Preferably small in size.

• It should be able to work in corrosive environment.

• It should be able to withstand pressure, shocks, vibrations etc..

• It must have high degree of accuracy and repeatability.

• Selected transducer must be free from errors.

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Classification Of Transducers

1. Based on the physical phenomenon,

• Primary transducer

• Secondary transducer

2. Based on the power type

• Active transducer

• Passive transducer

3. Based on the type of output the classification of

transducers are made,

• Analog transducer

• Digital transducer

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Classification Of Transducers (Cont…)

4. Based on the electrical phenomenon

• Resistive transducer

• Capacitive transducer

• Inductive transducer

• Photoelectric transducer

• z Thermoelectric transducer

• Piezoelectric transducer

• Photovoltaic transducer

5. Based on the non-electrical phenomenon

• Linear displacement

• Rotary displacement

6. Based on the transduction phenomenon

• Transducer

• Inverse transducer

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Primary and Secondary Transducers

Primary : The function of this element is to respond to a

physical phenomenon or a change in the physical

phenomenon.

Secondary: Function is to transform the output obtained by the

sensing element to an analogous electrical output.

Example: The bourdon tube acts as the primary sensing element

and the LVDT as the secondary transducer.

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Active transducer and Passive transducer

Active transducer : These transducers do not need any external

source of power for their operation. Therefore they are also

called as self generating type transducers.

➢ Example: Thermocouples, Piezoelectric transducer,

Photovoltaic cell etc.

Passive Transducers: These transducers need external source of

power for their operation.

➢ Example: Resistance thermometer, thermistors, differential

transformer, Potentiometric device etc.

Active transducer Passive transducer 15

Analog and Digital transducer

Analog Transducer: These transducers convert the input

phenomenon into an analogous output which is a continuous

function of time.

➢ Example: Strain gauge, Thermocouple, Thermistors, LVDT

etc.

Digital transducer: These transducers convert the input

phenomenon into an electrical output which may be in the

form of pulse.

➢ Examples of Digital Transducer Applications as Turbine meter

used in for flow measurement.

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Direct and Inverse Transducers

Direct Transducers : When a measuring device measures

and transforms a non-electrical variable into an electrical

variable, it is called a direct transducer

➢ A thermocouple that is used to measure temperature,

radiation, and heat flow is an example of a transducer.

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Direct and Inverse Transducers (Cont…)

Inverse Transducers: If an electrical quantity is transformed

into a non-electrical quantity, it is termed an inverse transducer

➢ A very common example of an inverse transducer is a

piezoelectric crystal wherein a voltage is given as the input.

➢ This is because when a voltage is applied across its surfaces,

its dimensions are changed causing a mechanical

displacement.

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Actuating Mechanisms

Actuator:An actuator is a device that converts energy into motion.

Therefore, it is a specific type of a transducer.

Actuating mechanisms used in some devices are

1. Bellows

2. Diaphragms- Flat, Corrugated and Capsule.

3. Bourdon tubes- Circular and Twisted.

4. Vanes

5. Rectilinear motions.

6. Shaft rotations – Pivot Torque and Unrestrained.

7. Proving rings.

8. Cantilever beams.

9. Beam elongation.

10. Seismic mass and springs and others.

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Actuating Mechanisms(Cont…)

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Voltage and current generating analog

transducersPiezoelectric Transducers:

Certain materials can produce an electrical potential when

subjected to mechanical strain or can change the dimension

when subjected to voltage this effect is known as

piezoelectric effect.

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Piezoelectric Transducers (Cont…)

• When force is applied to the plate, a stress will be produced in

the crystal and a corresponding deformation with certain

crystals. This deformation produces a potential difference at

the surface of the crystal, and this effect is known as

piezoelectric effect.

• The induced charge on the crystal is proportional to the

impressed force.

• E=gtp

Where g=Voltage sensitivity

t=Crystal thickness

p=Pressure

The induced charge will be given by the relation.

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Piezoelectric Transducers (Cont…)

The induced charge will be given by the relation.

Q=k F

Where, Q (charge) is in Coulombs,

F is in Newton and

‘k’ is Piezoelectric constant

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Characteristics of Piezoelectric Materials

Characteristics of Piezoelectric Materials

Rochelle salt: Rochelle salt gives the highest output but needs

protection from moisture in the air and limited to use up to

45OC only.

Quartz: Quartz is the most stable material of all, but has a very

low output. Because of its high stability, quartz is commonly

used for stabilizing the electronic oscillators.

Barium titrate: It is a polycrystalline material and is used to

produce photoelectric effect. It may be formed into variety of

sizes and shapes and used for high temperature range.

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Two Coil Transducer

• It consists of a single coil with centre tap. In this type the

movement of the core (or the armature) varies the relative

inductance of the two coils.

• The variation in inductance ratio between the two coils gives

the output. It is used in secondary transducer for pressure

measurement.

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STRAIN GAUGES

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.

▪ 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.

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STRAIN GAUGES (Cont…)

Definition:

A strain gauge is a device used to measure strain on an object.

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.

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https://en.wikipedia.org/wiki/Deformation_(mechanics)

STRAIN GAUGES (Cont…)

Metals Used In Making Strain Gauges:

The strain gauges are made with the following metals.

1. Constantan

2. Nichrome

3. Dynalloy

4. Platinum alloy

5. Copper Nickel

6. Nickel Chrome

7. Nickel Iron

8. Modified Nickel Chrome

9. Platinum Tungsten28

STRAIN GAUGES (Cont…)Classification:Strain gauges can be classified as follows.

A. Mechanical strain gauges

B. Optical strain gauges

C. Electrical strain gauges

A. Resistance strain gauges

A. Bonded type

B. Un-bonded type

C. Bonded wire type

D. Bonded foil type

E. Semiconductor gauges

B. Capacitive gauges

C. Inductive gauges

D. Piezoelectric gauges29

Mechanical Strain Gauges (Berry-type)

This type of strain gauges involves mechanical means for

magnification.

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Mechanical Strain Gauges (Berry-type)

• 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.

• This is used for structural applications in civil engineering for

long gauge lengths of up to 200 mm.

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Mechanical Strain Gauges (Berry-type)(Cont..)

Advantages

• It has a self contained magnification system.

• No auxiliary equipment is needed as in the case of electrical strain gauges.

Disadvantages

• Limited only to static tests.

• The high inertia of the gauge makes it unsuitable for dynamic measurements and varying strains.

– The response of the system is slow and also there is no method of recording the readings automatically.

• 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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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.

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Optical Strain Gauges(Cont…)

• 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 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.

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• 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.

• 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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Advantage :

• The position of autocollimator need not be fixed relative to the

extensometer, and reading can be taken by holding the

autocollimator in hand.

Disadvantages:

• Limited only for static measurements.

• Large gauge lengths are required.

• Cannot be used where large strain gradients are encountered.

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Mounting Of Strain Gauges

• 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.

• Various traces of grease or oil etc., must be removed by using

solvent like acetone.

• The surface of the strain gauges coming in contact with the test

item should also be free from grease etc.

• Sufficient quantity of cement is applied to the cleaned surface

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Mounting Of Strain Gauges(Cont…)

• The gauges are then allowed to set for at least 8 or 10 hours

before using it.

• 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.

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Problems Associated With Strain Gauge

Installations

The problems associated with strain gauge generally fall in to the

following three categories.

• Temperature effects

• Moisture absorption

• Wiring problems

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Strain Gauge Rosettes

Introduction:

➢ 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.

➢ As 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.

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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.

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.

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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.

• This is also known as rectangular Rosette.

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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,

Where,

ΔR=Change in Resistance.

ΔL=Small change in Length.

R=Initial Resistance.

L=Initial length

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Gauge Factor(Cont…)

• 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

high gauge factor.

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Chapter 2 TRANSDUCERS AND STRAIN GAUGES

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