Transducer As Input Elements to Instrumentation System

SignalConditioning

Circuit

Output Device

An Instrumentation System

-amplifier-filter

-meter-oscilloscope-recorder

Input Device

?

Input signal(measurand)electrical or non-electrical

Input device receives the quantity under measurement and delivers a proportional electrical signal to the signal-conditioning circuit.

Definition

Transducera device which, when actuated by energy in one system, supplies energy in the same form or in another form to a second system.

Sensor (input transducer)a device converts the physical or non-physical signal which is to be measured into an electrical signal which can be processed or transmitted electronically.

Actuator (output transducer)a device converts the modified electrical signal into a non-electrical signal.

Classification

• Signal Type -Physical, Chemical or Biosensors

• Input Signal -Thermal, Radiation, Mechanical,Magnetic, Chemical, Biological

• Function -Temperature, Pressure, Humidity

• Principle - Thermoelectric, elastoresistive

Basic Sensor Characteristics

Measurand

Output (electrical quantity)

Measurand range

Full

scal

e ou

tput

(FSO

)

offset

Sensitivity = dydx

Example of a Calibration Curve

Temperature Transducers Resistance Temperature detector (RTD)

employ a pure metal wire such as pure platinum, copper, or nickel etc. to provide a definite resistance value at each temperature

RT2 = Rref[1 + α(T2 - T1)]

(Rather Linear)

α -Temperature coefficient of resistance

-7.0 × 10-4Carbon9.9 × 10-3Mercury3.9 × 10-3Platinum (Pt100)4.0 × 10-3Iron4.1 × 10-3Silver4.3 × 10-3Copper6.7 × 10-3Nickel

α (°C-1) at 0 °CMaterial(Linear)

Pt 100 (-190 to +600 °C, 100 Ω at 0°C)

Resistance Temperature detector (RTD)

Thermocouple

Seeback effect

Whenever two dissimilar metals are in contact, a potential is developed that is proportional to the temperature of the junction.

Metal#1

Metal#2

Reference junction

SensingJunction V

∆T

V = s∆Ts: Thermoelectric coefficient

Reference junction 0ºC

Thermocouple

Thermocouple

DVM

Copper

Constantan

J1

Cu

Constantan

J1

Cu

Cu

Meter J3

J2

V = VJ1 – VJ2 = s (TJ1 – TJ2)

Cu

ConstantanCu

J2

DVM

IceBath

J1

V

Metal#1

Metal#2

Reference junction

Thermopile

+

-

It is necessary that the temperature of one of the junctions be known and constant.

If TJ2 = 0°C; V = s TJ1

Thermistor (Thermal resistor)

A device constructed of a metal oxide or of a semiconducting material with a negative temperature coefficient of resistance which changes dramatically with temperature. (ex R may change up to 6% per oC)

Operating temperature: -75 to +150°C

Force transducer: Strain GageA resistive input transducer whose change in resistance is related to changes in length.

The sensitivity of a strain gage: gage factor (K)

/ //

R R R RKL L σ

∆ ∆= =∆

where R = resistanceL = lengthσ = mechanical strain

For metallic strain gage K ~ 1.5-1.7

where E = Young’s modulusS = mechanical stressHooke’s law S Eσ=

Strain GageExample A resistance strain gage with a gage factor of 2 is fastened to a steel member subjected to a stress of 1,050 kg/cm2. Calculate the change in resistance, ∆R, of strain-gate element due to the applied stress.

SOLUTION Hooke’s law gives:

46

1,050 5 102.1 10

L SL E

σ −∆= = = = ×

×

The sensitivity of the strain gage K = 2. Therefore,

4 32 5 10 10 or 0.1%R KR

σ − −∆= = × × =

This example illustrates that the relatively high stress of 1,050 kg/cm2 results in a very small resistance change of only 0.1%. Therefore, in practical, the bridge circuit is often used to detect the small change in most resistive sensors.

Measurement Circuit for resistive transducer

RA (Ω)90 95 100 105 110

Vout

(mV)

-30

-20

-10

0

10

20

30

V

R=100 Ω 100 Ω

Strain gauge

RA=R+∆R100 Ω

(Temp. compensate)

At normal condition; RA = 100 Ω

RD

2 2

1 / 1 / 2 / 2 4(1 / 2 )

out s

s s

R R RV VR R R

R R R RV VR R R R

+ ∆ = − + ∆ + ∆ ∆ = − = + ∆ + ∆

Constant Temp.

If ∆R/R <<1 then 4out sRV VR

∆ ≈

The major problem with resistive transducers, such as strain gage and RTD, the change of resistance are very small. Example, the resistance of Pt RTD changes only 0.385% per oC

Vs = 1.0 V

Displacement TransducerA mechanical element that are used to convert the applied force into a displacement.

Force-Summing Device

The displacement created by the action of the force-summing device is converted into a change of some electrical parameter

Capacitive Transducer

Pressure

Deflected Diaphragm

Static Plate

Dielectric

0 rACdε ε

=

Piezoelectric Transducer

Certain crystalline materials (Rochelle salt, quartz) and ceramics (barium titanate) generate a voltage when deformed

∆VBT

Compression

∆VBT

Tension

∆V ∝ Sign and Magnitude

Pressure Port

Force-summing

Output

Piezoelectriccrystal

plunger

Linear Variable Differential Transformer (LVDT)

The basis structure consists of a single primary winding and two secondary windings which are placed on either side of the primary winding.

Vout = emf1 – emf2

Vin

Vout

emf1

emf2

CorePosition

OutputVoltage

AB0

Core at A Core at 0(Null Position) Core at B

The position of the movable core determines the flux linkage between the ac-excited primary winding and each of the two second windings.

Light TransducersPhotoconductive cell

Spectral response of cadmium selenide (CuSe)

A device which exhibits a change in resistance when exposed to different intensities of light.

Photoconductivematerial

Glass window

Metal case

Base pinCeramic substrate

Anode

Cathode

Symbol

hν

+

hν

EI

Flow

Flow-n-type

p-type

-

-

++

V R

po IkTqVII −−= )1](exp[

Ip: photocurrent

Spectral response of Si photodiode

Solar cellPhotodiode

Light intensity

Photodiode

Phototransistor

Base

hν

+

-

hν

+

-

E

CollectorEmitter

V RCollector

Emitter

Symbol

Collector

Emitter

Equivalentcircuit

Photomultiplier

Amplify photocurrent to detect extremely low light situations by impact ionization

Photocathode

Dynode 1 Dynode 3 Dynode 5 Anode

Dynode 6Dynode 4Dynode 2