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7/31/2019 EMI Innovative 1

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Innovative -I

In Experimental Methods


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1.Introduction

2.Types

3.Peizo resistive type

4.Peizo electric type

5.Summary

6.Reference


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A semi-conductor may be defined as an electrical

conductor that has a conductivity intermediate

between that of an insulator and that of a

conductor.

The conductivity of semiconductor materials can

be changed radically by small changes in physical

condition.

Semi conductors are prepared by diffusing

impurities to pure silicon or germanium crystals.


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Very high conductor sensitivities are possible

Negative gauge factors are possible in semi-

conductor gauges which permits large

electrical output from Wheatstone bridegecircuit.


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The semi-conductor gauges are of two types

1. PEIZO RESISTIVE GAUGES

2. PEIZO ELECTRIC GAUGES


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Peizo resistive effect is the phenomenon inwhich the change in resistance of a semiconducting material accompaniesdeformation.

It was first demonstrated by Mr.Smith in 1954

on Silicon and Germenium.

This type of strain gauges are useful forstatic strain measurments.


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They are produced from single crystal silicon

or germanium.

Boron is used as trace impurity for producing

p-type conductor. Arsenic is used to produce the N-type

The resistivity od p-type silicon is in the

order of 500 .

This very high sensitivity is reason why theyare used in measuring extremely small

strains .eg.in tranducers


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Semi conductor filaments of even smaller

cross sections can be constructed by

condensation of silicon vapour on the cool

walls of a quartz tube.

The lead material for the semi-conductor

filament is generally doped with a small

amount of antimony. P-type gauges are used in tensile fields and

N-type gauges are used in compressive fields.


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Semi conductor strain gauges can be classified

with respect to following characteristics.

Filament material (p or n type)

Gauge factor Gauge length

Gauge resistance

Temp compensation

Backing

Bonding techniques.

Lead Geometry.


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Sensitivity

The strain sensitivity or gauge factor ,

F=( R/R) /( L/L)

= 1+2v+m

Where m= ( p/p)/( L/L)

v= poissons ratio

The factor m accounts for piezoresistiveeffect and (1+2v ) is dimensional effect.

Usually its value ranges from 50-175.


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Linearity

They depend strongly on strain levels and

are distinctly non-linear.Increase amount of

impurity may improve the linearity.Temperature Effect

Environmental temperature changes affect

the resistance and the sensitivity of

semiconductor which depends on the levelof doping.


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Current carrying Capacity

The maximum permissible current for a

bonded semi conductor strain gauge depends

upon the mechanism of heat dissipation,

mainly conduction to test structure.

Breaking StrengthIt depends mainly on filament cross section

For filament cut from from Gauge crystals

,the breaking strength is 3x10 3 N/m2

Hysteresis and creepSemi conductor filament is not responsible

for any hysteresis or creep errors

experienced in practical gauge installation.


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Fatigue

Fatigue life of semi-conductor gauge ishigher than that of wire and foil gauge.

Humidity

semi-conductor gauges are not that much

affected by moisture.

Temperature compensation

Temperature compensation of semi-

conductor gauges can be achieved in twoways

1. self compensation

2. Circuit compensation


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Introduction

Certain crystals when mechanically strained

in particular directions will becomeelectrically polarised.

The change can be measured by applying

electrodes to the surfaces and measuring the

potential difference between them. Substances which possess this property are

called peizoelectric.


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In single piezoelectric plates,five operation

modes are there1. Thickness expansion mode

2. Length expansion mode

3. volume expansion mode

4. Thickness shear mode

5. Face shear Mode

For strain gauge applications we use

Length-expander mode of operations. Peizoelectric crystals can be stacked and it

can be connected in parallel(higher output)

or in series(lower output).


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The peizoelectric materilas are of two type

A. Naturally occuring type .

B. Synthetic Materialls.

Eg: A type1.Quartz(SiO2)

2.Rochelle salt(NaKC4 O6.4H2o)

B type1.Barium titanate(Ba Ti O3)

2.Lead zirconate titanate


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If a thin slab of peizoelectric material is

cemented to a structure under stress, the

strain in the structure is transmitted to the

slab and a charge is generated across the

crystal.

Out put voltage ,V=ht

h= h-coefficient

=absolute dielectric constantt=thickness of crystal


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The crystals are cheap

Light weight

Highly sensitive to small stains


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The high sensitivity of semi-conductor gauges

make them very useful in steam turbine

blades.

Semi-conductor strain gauges of permits thedesign of wide range of subminiature

transducers which have high natural

frequency and high output signal.


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JAMES.W.DALLY AND WILLIAM F.RILEY

Experimental stress analysis, Mc GRAW-HILL

BOOK Company , second edition 1987,

pp 204-215.

Dr. Sadhu singh , Experimental Stress Analysis ,

Khanna Publishers,4th edition 2004,

pp 245-280.


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Thank You

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