Transducer

Device when activated from one form energy converted to another quantity

or

Device which converts one physical quantity or condition to another

Physical quantity – heat , intensity of light, flow rate, liquid level, humidity etc

Sensor : sense physical quantity

Classification of transducers

1. Based on principle of transduction

2. Active & passive

3. Analog & digital

4. Inverse transducer

Based on principle used

• Thermo electric

• Magneto resistive

• Electro kinetic

• Optical

Passive transducer

Device which derive power reqd. for transduction from auxiliary power source

- externally powered

Eg : resistive, inductive, capacitive

Without power they will not work

Active transducer

• No extra power reqd. to produce I/p

• Self generating

• Draw power from input applied

• Eg. Piezo electric x’tal used for accelartion measurement

Analog transducer

• convert I/p quantity into an analog o/p

• Analog o/p- a continuous fn. Of time

• Eg. Strain gauge, L VDT, thermocouple

Digital transducer

• Converts I/p into an electrical o/p in the form of pulses

Inverse transducer

• Which converts electrical signal to physical

• quantity

Transducer for pressure measurement

• What is pressure?

force/ unit area

Unit – (N/sq.m) Pa

Pressure measured can be absolute , gauge or differential depending on type of reference

Types of pressure transducers

• Gravitational

eg : manometer

• Elastic – force converted to strain

eg. Diaphragm, capsule, bellows, bourdon tubes

Diaphragm

Common pressure sensing elastic element

Thin circular plate stretched & fastened at its periphery

Made of elastic alloys of bronze,phosphor bronze, stainless steel or alloys like Monel, Nickel span -C

Structure – flat or corrugated

Top view of flat type

Top view of corrugated type

• Flat type – high natural frequency

• Corrugated – for large deflections

Capsule

Capsule

• Two corrugated metal diaphragms sealed together at periphery

• Forms a shell like structure

• One diaphragm has a port at centre to admit pressure to be measured

• Other diaphragm linked to a moveable mechanical part.

Capsule

• Displacement proportional to difference b/w outer & inner pressure.

Bourdon tubes

• curved or twisted metallic tubes with elliptical cross section

• Sealed at one end

• Tends to straighten when pressure applied.

• Angular sensitivity proportional to pressure applied

Spirial bourdon tubes

Bourdon tubes –C type

Helical type

• Tube in the form of helix

Bourdon tube

• More sensitive to shock & vibrations

• Good repeatability

Bellows

Bellows

• Thin walled cylindrical sheets with deep convolutions sealed at one end

• Sealed end moves axially when pressure is applied

• No. of convolution s – vary from- 2 to 50 – depends on range, operating temp

• Used for low pressure measurement

Bellows

displacement y = 2.n. A q P Rx 2 /( Et 2 .) where n – no. of convolutions

A q- effective area

Et - young’s modulus of elasticity

Rx – radius of diaphragmP – pressure ie, Y P

Resistive type

Eg. Strain gauge

Capacitive

Inductive type

• Eg. LVDT

Measurement of velocity

Velocity – linear or angular

Linear velocity

• Electromagnetic transducers• O/p voltage E = /t

• = N I/ R2 . dR/dt

where N I/ R2 is a constant

R- reluctance

so E proportional to reluctance

R proportional to air gap & air gap proportional to velocity

• Types

• Moving coil

• Moving magnetic

Angular velocity

• Tachometer - types

Mechanical

Electrical

MHD sensor – magneto hydrodynamic sensor

highly sensitive

High precision

Transducer for vibration

• Vibrations give early warning of impending conditions which may develop &vlead to complete failure & destruction of equipment

• Used in power plants, turbines

• Most vibrations are sinusodial in nature

• Got amp & freq.

• Amp gives displacement

• By measuring displacement, velocity or acceleration - vibrations measured

Vibration transducer

Accelometer – measures shock or vibration

Pot type or LVDT type

Pot type accelrometer

LVDT accelorometer

THANK UTHANK U

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