unit iv transducers c.lokanath reddy assistant professor · 2019. 10. 23. · 23-oct-19 1...

23-Oct-19

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C.Lokanath ReddyAssistant Professor

Department of Electronics and Communication Engineering

Unit IVTRANSDUCERS

Electronic Measurements & Instrumentation

Contents

Classification

Strain Gauges, Bounded, unbounded;

Force and Displacement Transducers

Resistance Thermometers

Hotwire Anemometers

LVDT

Thermocouples

Synchros

Special Resistance Thermometers

Digital Temperature sensing system

Piezoelectric Transducers

Variable Capacitance Transducers

Magneto Strictive Transducers.

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Introduction

Instrumentation system generally consists of three major

elements.

a. Primary Sensing Element

b. Data Conditioning Element

c. Data Presentation Element

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Transducer

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Transducer is a device that convert one form of energy to other

form. It converts the measurand to a usable electrical signal.

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Block Diagram of Transducer

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A transducer is also called as PICK UP


Electrical Transducer

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It is a device that is capable of converting the physical quantity

into a proportional electrical quantity such as voltage or current.

Pressure Voltage

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Advantages of Electrical Transducer

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Electrical amplification and attenuation can be done easily

Mass – Inertia effects are minimized

Effects of friction are minimized

The output can be modified to meet the requirements of the

indicating or controlling units

The signal can be conditioned or mixed to obtain any

combination with outputs of similar transducers


Selecting a Transducer

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Operating Principle

Sensitivity

Operating Range

Accuracy

Transient and frequency response

Environmental Compatibility

Usage and ruggedness

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Classification of Electrical Transducer

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Active Transducer

Passive transducers



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Active Transducer

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Passive Transducers


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Passive Transducers

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Passive Transducers


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Strain Gauge

When a gauge is subjected to a positive stress, its length increases

while its area of cross section decreases. Since the resistance of a

conductor is directly proportional to its length and inversely

proportional to its area of cross section the resistance value of a

conductor increases with positive strain.

The change in resistance value of a conductor under strain is more

than for an increase in resistance due to its dimensional changes. This

property is the piezo-resitive effect.

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Strain Gauge


Strain Gauge

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A stress tends to elongate the wire & there by increases its

length and decreases its cross sectional area. The combined

effect is an increase in resistance,

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Strain Gauge

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As a result of strain, two physical parameters are

1. The change in gauge resistance

2. The change in length

The measurement of the sensitivity of a material to strain is

called gauge factor.


Strain Gauge

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Strain is defined as the change in length divided by the original

length

Now

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Strain Gauge

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Strain Gauge

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Strain Gauge

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Strain Gauge

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Strain Gauge

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Strain Gauge

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Introduction

Wire Strain Gauges

Foil Strain Gauges

Semiconductor Strain Gauges

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Types of Strain Gauges


Wire Strain Gauges

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Resistance wire gauges are used in two basic forms

a. Unbonded type

b. Bonded type

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Wire Strain Gauges

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An unbonded type strain gauge consists of a wire stretched between two points in an insulating medium.


Wire Strain Gauges

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A bonded type strain gauge consists of a fine wire element is

looped back and forth on a carrier(base) or mounting plate,

which is usually cemented to the member undergoing stress.

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Wire Strain Gauges

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Types of wire strain gauges

a. Grid type

b. Rossette type

c. Torque type

d. Helical type


Wire Strain Gauges

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Wire Strain Gauges

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Half Bridge Configuration


Wire Strain Gauges

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Full Bridge Configuration

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Foil Strain Gauges

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Foil types of strain gauges is an extension of the resistance wire

strain gauge. The strain is sensed with the help of a metal foil.


Semiconductor Strain Gauges

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Semiconductor gauge are used in application where a high gauge

factor is desired. A high gauge factor means relatively higher

change in resistance that can be measured with good accuracy

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Potentiometer

The resistance of a wire is given by the formula

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lR

A


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Introduction

The resistance of conductor changes when its temperature is

changed.

The resistance thermometer is an instrument used to measure

electrical resistance in terms of temperature.

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Resistance Thermometer

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The main part of a resistance thermometer is its sensing

element. The characteristics of sensing element determines the

sensitivity and temperature range of instrument.

The sensing element may be any material that exhibits a

relatively large resistance change with change in temperature.

The material used should be stable in its characteristics.

Platinum, nickel and copper are the metals most commonly used

to measure temperatur.



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The change in resistance caused by changes in temperature are

detected by a wheatstone bridge.

The temperature sensing element, which may be copper,

platinum, nickel contained in a bulb or well along with balancing

bridge, form the essential components of a temperature

measuring system.

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The sensing element is made of a material having high

temperature coefficient and R1, R2 and R5 are made of

resistances that are practically constant under normal

temperature changes.

When no current flows through the galvanometer, the normal

principle of wheatstone’s bridge states the ratio of resistance is



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The sensing element is away from the indicator and its leads

have resistance R3, R4

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Advantages

The measurement is very accurate

It has a lot of flexibility with regards to choice of measuring

equipment.

The temperature sensitive resistive element can be easily installed

and replaced

Resistive elements can be used to measure differential temperatures

Resistive thermometers have a wide working range without loss of

accuracy and can be used for temperature ranges -200C to +650 C



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Advantages

The resistive element response time is of order of 2 to 10s

The limit of error of a resistive element are +0.25% of the scale

reading

Stability of performance over long periods of time.

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Limitations

High cost

Need for bridge circuit and power source

Possibility of self heating



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Hotwire Anemometer

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Hotwire Anemometer

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Hotwire Anemometer

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Hotwire Anemometer

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Hotwire Anemometer

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Introduction

The differential transformer is a passive inductive transformer. It

is also known as a Linear Variable Differential Transformer.

The transformer consists of a single primary winding P1 and two

secondary windings S1 and S2 wound on a hallow cylindrical

former.

An movable soft iron core slides within the hollow former and

therefore affects the magnetic coupling between the primary

and the two secondary

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Linear Variable Differential Transducer

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The displacement to be measured is applied to an arm attached to

the soft iron core.



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The primary coil is excited with an AC current, the secondary coils are

wound such that when a ferrite core is in the central linear position,

an equal voltage is induced in to each coil.

The secondary are connected in opposite so that in the central

position the outputs of the secondary cancels each other out.

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The excitation is applied to the primary winding and the

armature assists the induction of current in to secondary coils.

When the core is exactly at the center of the coil then the flux

linked to both the secondary winding will be equal. Due to equal

flux linkage the secondary induced voltages (Eo1 & Eo2) are

equal but they have opposite polarities. Output voltage E0 is

therefore zero. This position is called “null position”



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Now if the core is displaced from its null position toward sec1 then

flux linked to sec1 increases and flux linked to sec2 decreases.

Therefore Eo1 > Eo2 and the output voltage of LVDT E0 will be

positive

Similarly if the core is displaced toward sec2 then the Eo2 > Eo1

and the output voltage of LVDT E0 will be negative.



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By comparing the magnitude and polarity or phase of the

difference output voltage with that of source, the amount and

direction of the movement of the core and hence of the

displacement may be determined.

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Advantages

Linearity

Infinite resolution

High output

High sensitivity

Ruggedness

Less friction

Low hysteresis

Low power consumption

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Disadvantages

Larger displacements are required for appreciable differential

output.

They are sensitive to stray magnetic fields.

The dynamic response is limited mechanically by the mass of the

core and electrically by the applied voltage.

Temperature also affects the transducer.


Introduction

Thermocouple is used as a primary transducer for temperature

measurement in which changes in temperature are directly

converted into an electrical signal.

The thermocouple behavior can be explained on the basis of

thermoelectric phenomena namely Seeback effect .

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Thermocouple

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Thermocouple

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If the two wires of different metals are joined together forming

closed circuit and if the two junctions are at different

temperatures, an electric current flows around a closed circuit.

This is called SEEBACK effect.


Thermocouple

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Construction

Thermocouple is made up of two wires of dissimilar metals joined

together to form two junctions

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Thermocouple

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Construction

Out of two junctions T1 & T2, T2 is kept at constant reference

temperature. Hence it is referred as cold junction.

While the temperature changes to be measured are subjected to

the junction T1 which is referred as hot junction.

When the hot junction temperature is greater as compared to

cold junction, emf is generated due to temperature gradient. The

magnitude of the emf generated depends on the material used

for wires and temperature difference between the two junctions.


Thermocouple

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Materials used

The thermocouples are made from a number of different metals including copper-

constantan, platinum-platinum-rhodium etc. They cover wide range of temperature from-

200c to 2800c

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Thermocouple

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Thermoelectric Laws

The application of heat to single homogeneous metal is in itself not

capable of producing an electric current.

A thermoelectric e.m.f is produced when the junctions of two dissimilar

homogeneous metals are kept at different temperatures. The e.m.f is

not affected by temperature gradients along the conductors.


Thermocouple

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Thermoelectric Laws

In a thermocouple having the junctions at different temperatures, the

emf developed will not be affected when a third homogeneous metal is

made part of the circuit, provided that the temperatures of its two

junctions are same.

This is called Law of Intermediate metals

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Thermocouple

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Thermoelectric Laws

If a simple thermocouple circuit develops an emf E1 when its junctions

are at temperature T1 &T2 and an emf E2 when its junctions are at

tenperatures T1 & T2

This is called Law of Intermediate temperatures


Thermocouple

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Thermoelectric Laws

The algebraic sum of the emfs produced in a circuit containing two or

more thermocouples are at same temperature is zero.

The total emf of a circuit containing two thermocouples is unaffected

by the addition of more thermocouples at the same temperatures

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Thermocouple

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Advantages

The thermocouple is rugged in construction.

It covers a wide range temperature from -270c to 2700c

Using extension leads and compensating cables, long transmission

distances for temperature measurement are possible.

The thermocouple is comparatively cheaper in cost.

The calibration can be easily checked.

The thermocouple offers good reproducibility


Thermocouple

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Limitations

For accurate temperature measurements cold junction compensation is

necessary.

The emf induced vs temperature characteristics are somewhat non

linear.

Stray voltage pickup is possible

In many applications amplification of signal is required.

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SYNCHROS

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SYNCHROS

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SYNCHROS

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SYNCHROS

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SYNCHROS

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Introduction

A Piezo-electric material is one in which an electrical potential

appears across certain surfaces of a crystal if the dimensions of

the crystal are changed by the application of a mechanical force.

If a varying potential is applied to the proper axis of the crystal, it

will change the dimensions of the crystal therefore deforming it.

This effect is known as Piezo-electric effect.

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Piezo-electric Transducer

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Common piezo-electric materials include Rochelle Salts,

ammonium dihydrogen phospate, lithium sulphate, quartz and

ceramics A&B



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Piezo electric crystal can be made to respond to mechanical

deformations of the material in many different modes

a. Thickness expansion

b. Transverse expansion

c. Thickness shear

d. Face shear

The mode of motion depends on the shape of the body relative to the

crystal axis and location of electrodes

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A piezo electric element used for converting mechanical motion

to electrical signals may be thought as charge generator and

capacitor.

Mechanical deformation generates a charge and this charge

appears as a voltage across the electrodes.

The voltage is E=Q/C



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The piezo electric effect is direction sensitive. A tensile force

produces a voltage of one polarity while a compressive force

produces a voltage of opposite polarity.

The magnitude and polarity of the induced surface charges are

proportional to the magnitude and direction of force applied F.

Charge Q=d×F; coulomb……….1

d=charge sensitivity of the crystal. C/N

F=applied force N

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The force F cause a change in thickness of the crystal

Where A= area of crystal; m2

t= thickness of crystal; m

E= young’s modulus; N/ m2

Young’s modulus E= Stress/Strain= (F/A).(1/∆t)/t

E=Ft/A∆t N/ m2………………3

Area=wl

Where w=width of crystal; m

l= length of crystal; m



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From equ 1&2 we have

Charge Q=dAE(∆t/t) ……..4

The charge at electrodes gives rise to an output

voltage E0 =Q/Cp………5

Where Cp= capacitance between electrodes; F

Cp=ε0εrA/t………6

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From eq 1,5,6

E0 =Q/Cp=dF/(ε0εrA/t)=(dt/ ε0εr ).F/A…..7

F/A=P=pressure or stress in N/ m2

E0 =(d/ ε0εr ).t.P ………8

E0 =g.t.P……9

Where g= d/ ε0εr……..10



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Where g is the voltage sensitivity of the crystal. This is constant for a

given crystal cut.

Its units are Vm/N

g= E0 /tP=(E0 /t)/p……..11

But E0 /t=electric field strength

Let ε=E0 /t=electroic field

g=electric field/stress=ε/P……12

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Crystal voltage sensitivity g can be defined as the ratio of eletric field

intensity to pressure.

From eq 10

Charge sensitivity d= ε0εr g; C/N


Special Resistance Thermometer

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Many special resistance thermometers have utilized the basic wafer

design having the characteristics of small mass combined with good

thermal contact, resulting in an extremely fast time response.

A fine insulated wire of copper, nickel or platinum is usually sandwiched

between two protecting sheets of insulating material and sealed.

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Special Resistance Thermometer

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Thermistor

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Introduction

Thermistor ( THERMally sensitive resISTOR) are non metallic

resistors made by mixture of metal oxides such as manganese,

nickel, cobalt copper and uranium.

Themistors have a negative temperature coefficient

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Thermistor

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Thermistor

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Thermistor

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The smallest thermistor are made in form of beads. They may

come in a glass coating or sealed in tips of glass probes.

The probes are used for measuring temperature of liquids

Where grater power dissipation is required thermistor may be

obtained in disc, washer or rod forms.


Thermistor

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Thermistor

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Thermistor

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Advantages

Small size and low cost

Fast response over narrow temperature range

Good sensitivity in the NTC region

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Thermistor

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Limitations

Non linearity in resistance vs temperature characteristics

Unsuitable for wide range temperature

Very low excitation current to avoid self heating

Needed of shielded power lines, filters etc due to high resistance.


Introduction

Sensistor is a resistor whose resistance changes with temperature.

The resistance increases exponentially with temperature,that is

the temperature coefficient is positive

Sensistors are used in electronic circuits for compensation of temperature

influence or as sensors of temperature for other circuits.

Sensistors are made by using very heavily doped semiconductors so that

their operation is similar to thermistors. However, very heavily doped

semiconductor behaves more like a metal and the resistance change is more

gradual than it is the case for other PTC thermistors.

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Sensistor

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Sensistor


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Variable Capacitance Transducers

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Magnetostrictive Transducers


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Magnetostrictive Transducers

unit iv transducers c.lokanath reddy assistant professor · 2019. 10. 23. · 23-oct-19 1...

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