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TRANSDUCERS

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Transducers

The term transducer is applied to any

device which converts a mechanical orother measurable phenomenon into an

electrical one or vice versa.

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Resistive transducers Temperature sensors (metals)

Rt= Ro(1+t+t2+t3+.)where Rt is resistance of a length of wire at t

oC

Ro is the resistance at 0oC

, , are the temp coefficient of resistance with >>. For most metals resistance increases reasonably

linearly with temp and , etc. can be neglected.

Then Rt= Ro(1+t)

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Temperature sensors (Semiconductors)

Resistance changes with temp.

A group of transducers based on this are thermistors Made from mixture of metal oxides such as Cr, Co, Fe, Mn, Ni

Formed into various forms such as beads, discs and rods.

The resistance-temp graph is highly non-linear and described by

the exponential relationshipt

t eKR

=

where Kand are constant.

The resistance decreases with an

increase in tempchange in

resistance is larger than change in

temp

Decrease in resistance causes

increase in current which in turn

increases temp further.

This effect continues until heatdissipates and power supplied

equalizes.

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Sliding-Contact Devices

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Variable-Inductance Transducer Elements

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S i

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Strain gauges When a length of wire, or metal foil or

semiconductor, is stretched its resistancechanges. The fractional change in resistance,R/R is directly proportional to the strain

R/R = G (where G is a constant called gaugefactor)

For most material G is positivemeans resistanceincreases when strain is increased, i.e. tensionincreases resistance, compression decreases it.

The resistance of a strain gauge is changed notonly by a change in strain but also a change intemp.

Wires of Cu or Fe

Semiconductorstrips of Si doped with n-type or p-

type material.

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Capacitive Transducers

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Photoconductive cells It is light-dependent resistorits resistance decrease as

the intensity of light falling on it increases. Cadmium sulphidecommonly used, however, othersare also in use

Resistance variesmega ohms in the dark to few

hundred ohms in bright light Response timetypically of the order of 50 m.

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Photoemissive detectors:

A cathode-anode combination in an evacuated glass

In a proper circuit (several hundred volts required) light

impingement on the cathodecurrent produced and

amplified.

Photovoltaic cells

Sandwich of unlike

materials such as ironbase covered with thin

layer of iron selecide.

When exposed to light,a voltage is developed

across the sandwich.

Requires no external

power other than light.

Pi l t i

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Piezoelectric sensors

Piezoelectric effect

Certain materials can generate an electric charge when

subjected to mech. strain or, conversely can change

dimensions when subjected to voltage.

E.g. quartz, Rochelle salt (potassium sodium tartarate),

barium titanate, ammonium dihydrogen phosphate,

certain organic polymers, and even ordinary sugar.

None of the above materials possess all the desirableproperties, such as stability, high output, insensitivity to

temp extremes and humidity, and the ability to be formed

into desired shape Rochelle salt provides a high output, but requires protection from

moisture, and cannot be used above 45oC

Quartzmore stable but has low output15

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What is a crystal?

A class of materials arranged in a definite,geometric pattern in three dimensions(table salt and sugar are commonexamples)

Quartz Crystal is silicon and oxygenarranged in a crystalline structure (SiO2).

SiO2 is also found abundantly in nature in

a non-crystal structure (amorphous) assand.

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A method for predicting the

behavior of a crystal: The unit cell

+ Represents silicon atom

- Represents oxygen atom

Not actually correct, but this method allows

a good understanding of quartz crystals

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The unit cell of crystal silicon dioxide

-

+

-

+

-

+

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-

+

+

-

+

-

Unit Cell at Rest

-

+

+

-

+

-

Neutral Charge

Unit Cell Under MechanicalCompression (pushing force):

Electrical polarity as shown

-

+

+

-

+

-

Unit Cell Under mechanical

Tension (pulling force):

Electrical polarity reverses.

+ + +

- - -

- - -

+

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The Piezoelectric Effect

Crystal material at rest: No forces applied,

so net current flow is 0

Crystal

+ - + - + -

+ - + - + -

Current Meter= 0

Charges cancel

each other, sono current flow 21

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The Piezoelectric Effect

Crystal material with forces appliedin direction of arrows..

Crystal

- - - - -

+ + + + +

Force

Current Meter

deflects in +

direction

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The Piezoelectric Effect

Changing the direction of the

applied force..

Crystal

+ + + +

- - - - -

Force Current Meter

deflects in -

direction

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The electromechanical effect

Now, replace the current meter with a power source capable

of supplying the same current indicated by the meter.

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+ - + - + -

+ - + - + -

switch

Crystal

charges cancel power source

. With the switch open, the crystal material is now at rest again:the positive charges cancel the negative charges.

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The electromechanical effect

When the switch is closed, and you apply the exact amountof power to get the same current that resulted when you squeezed

the crystal, the crystal should deform by the same amount!!

Crystal

power source

(battery)

- side

+ + + +

- - - - -

+ side

. and, the crystal should get shorter and fatter.25

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The electromechanical effect

What will happen if you switched the battery around??

Crystal

power source

(battery)

+ side

- - - - -

+ + + + +

- side

. the crystal should get longer and skinnier.26

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Summary of the Piezoelectric &

Electromechanical Effect

A deformation of the crystal structure (eg: squeezing

it) will result in an electrical current.

Changing the direction of deformation (eg: pulling it)

will reverse the direction of the current.

If the crystal structure is placed into an electrical field,

it will deform by an amount proportional to the

strength of the field.

If the same structure is placed into an electrical fieldwith the direction of the field reversed, the

deformation will be opposite.

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Most of the piezoelectric materials are single

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Most of the piezoelectric materials are single

crystal; barium titanate is polycrystalline, hence

shaping and sizing is easy. Polarizing treatment is necessary to induce the effect:

Element heated above Curie point of 120oC

A high DC voltage (of the order of 10,000 V/cm)

Element cooled with the voltage applied.

Piezoelectric polymers, such as polyvinylidenefluoridelow cost piezoelectric transducer with

relatively high voltage outputs.

Formed into thin films (~30 micron thick)with silveredelectrodes on either side

Light, flexible and easily manipulated

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Hall Effect Sensors

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Hall Effect Sensors

The Hall effect is

the appearance ofa transversevoltage difference

on a conductorcarrying a currentperpendicular to a

magnetic field.Present in any

conductor carrying

a magnetic fieldbut morepronounced in

semiconductors.

Thermoelectric transducers

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Thermoelectric transducers

Thermocouples

There is a potential difference between

two different metals across the junction.

The potential difference depends on the

metals used and the temperature.

A thermocouple is just wires of two

different metals forming a complete circuit.

When same temp at both junctionssame potential difference across

each junction.

A temp difference between two junctionproduces a net e.m.f.the

value depends on the two metals and the temp. Usually one metal is held at 0oC

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Ass mption one of the j nction is at 0oC

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Assumptionone of the junction is at 0oC

If other temp.then law of intermediatetemperatures has to be used to determine the

e.m.f.

Statementemf of a thermocouple withjunctions at 1 and 3 is the algebraic sum of

the emfs of the two thermocouples of thesame materials with junctions at 1 and 2,

and 2 and 3.

Compensation circuits can be used when the

reference junction is not at a constant temp. but

allowed to vary with the ambient temp.

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Electrochemical transducers

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Ion selector electrodes It gives response whichdepends mainly on the

conc. of single type of ionin the solution.

A reference electrode is

also used. E.g. hydrogen ion

measurement for

measuring pH.

Elastic transducers

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Proving ring

A steel ring which deforms

from its circular shape under

the action of forces.

The amount of deformation

is a measure of forces and

can be measured by dial

indicator.

Are capable of high

accuracy

Are used for forces in therange of 2 kN to 2000 kN.

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Load CellsTh d f ti f li d b d th ti f f

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The deformation of a cylinder on a box under the action of forces canbe used as a measure of forces. Such a system is known as load

cell. The deformation is measured by means of strain gauges.

Four identical gauges

If the load cell is under compression then R1 and R3 are in

compressionthe strain = -(F/AE), where F is the force applied, A itscross-sectional area and E the tensile modulus of the cell material.

Strain gauges R2 and R4 are in tension, the strain being +(F/AE)where is Poissons ratio of the cell material.

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Diaphragms

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p g

The movement of the center

of a circular diaphragm whenthere is a pressure difference

between its two sides is the

basis of a pressure gauge.

Capsules and bellows Can be considered to be just

two diaphragms back-to-

back. Bellows being a stack

of capsules

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Bourdon tubes

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A bourdon tube is an almost

rectangular or ellipticalshaped tube made frommaterials such as stainlesssteel or phosphor bronze.In

one form the tube is C-shaped.

When pressure inside tube

increases the C opens outhence displacementmeasures pressure.

A helical form gives greater

deflections. Also exists in twisted form

the pressure changes the

tube to untwisted.37

Pneumatic transducers

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Flapper nozzle

Air at constant pressure Ps flows throughthe orifice and escapes through the

nozzle.

At flapper closed, i.e., x=0; no air

escapesWhen x increases pressure

drops so becomes measure of

displacement of the flapper

The transducer has high sensitivity but asmall range of measurement, typically

0.05 mm.

)/(161422

on

s

dxd

P

P +=

where dn is the dia of the nozzle

do is the dia of the orifice

Differential pressure transducers

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Flowmeters When a fluid flows from a wider dia. to a narrower dia. pipe its

velocity increases and the pressure drops.

If the fluid is incompressible, Bernoullis equation gives for ahorizontal pipe

g

P

g

v

g

P

g

v

2

2

21

2

1

22+=+

Where v1 is the fluid velocity, P1 the pressure at the pipe, v2 the

velocity and P2 the pressure at the constriction, and the fluid

density.

Hence, since the density does not change, the volume of the fluid Q

passing through the wide section per second must equal the volume

passing through the constriction. Hence Q=A1v1=A2v2 where A1 is the

area of the tube and A2 that at the constriction. In practice, the flow is not frictionless and the cross-sectional area of

the moving fluid may not be the same as the pipe. Hence, a

correction factor is usually applied.

There are number of forms of flowmeters based on the measurementsystem of the pressure difference between the flows.

Mechanical transducers

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