An Instrumentation System

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Lucy Lawson
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1 Transducer As Input Elements to Instrumentation System An Instrumentation System Input signal (measurand) electrical or non-electrical Input Device Signal Conditioning Circuit Output Device? -amplifier -filter -meter -oscilloscope -recorder Input device receives the quantity under measurement and delivers a proportional electrical signal to the signal-conditioning circuit.

2 Definition Transducer a 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 nonelectrical signal.

3 Classification Signal Type -Physical, Chemical or Biosensors Input Signal -Thermal, Radiation, Mechanical, Magnetic, Chemical, Biological Function -Temperature, Pressure, Humidity Principle - Thermoelectric, elastoresistive

4 Basic Sensor Characteristics Output (electrical quantity) Full scale output (FSO) Sensitivity = dy dx offset Measurand range Measurand Example of a Calibration Curve

5 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 R T2 = R ref [1 + α(t 2 - T 1 )] α -Temperature coefficient of resistance Nickel Copper Silver Iron Platinum (Pt100) Mercury Carbon Material α ( C -1 ) at 0 C (Rather Linear) (Linear) Pt 100 (-190 to +600 C, 100 Ω at 0 C)

6 Resistance Temperature detector (RTD)

7 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 Reference junction Reference junction 0ºC Sensing Junction V V = s T Metal#2 T s: Thermoelectric coefficient

8 Thermocouple

9 Thermocouple Copper Cu Cu + DVM Constantan J 1 Meter Cu J 3 J 2 - Constantan J 1 Cu V = V J1 V J2 = s (T J1 T J2 ) It is necessary that the temperature of one of the junctions be known and constant. DVM Ice Bath Cu Constantan J 2 J 1 V Thermopile Reference junction Metal#1 Metal#2 If T J2 = 0 C; V = st J1

10 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 o C) Operating temperature: -75 to +150 C

11 Force transducer: Strain Gage A resistive input transducer whose change in resistance is related to changes in length. The sensitivity of a strain gage: gage factor (K) K R/ R R/ R = = L/ L σ For metallic strain gage K ~ Hooke s law S = Eσ where R = resistance L = length σ = mechanical strain where E = Young s modulus S = mechanical stress

12 Strain Gage Example A resistance strain gage with a gage factor of 2 is fastened to a steel member subjected to a stress of 1,050 kg/cm 2. Calculate the change in resistance, R, of strain-gate element due to the applied stress. SOLUTION Hooke s law gives: σ L S 1, 050 = = = = L E The sensitivity of the strain gage K = 2. Therefore, R = K σ = = R or 0.1% This example illustrates that the relatively high stress of 1,050 kg/cm 2 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.

13 Measurement Circuit for resistive transducer 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 o C R+ R R Vout = Vs 2R+ R 2R R=100 Ω 100 Ω 1 + R/ R 1 R/ R = Vs = V 2 + R/ R 2 4(1 + R/2 R) R If R/R <<1 then Vout Vs 4R At normal condition; R A = 100 Ω Constant Temp. s V s = 1.0 V R A =R+ R V 100 Ω (Temp. compensate) Vout (mv) Strain gauge R D R A (Ω)

14 Displacement Transducer Force-Summing Device A mechanical element that are used to convert the applied force into a displacement. The displacement created by the action of the force-summing device is converted into a change of some electrical parameter Capacitive Transducer Deflected Diaphragm Static Plate C = Aεε d 0 r Pressure Dielectric

15 Piezoelectric Transducer Certain crystalline materials (Rochelle salt, quartz) and ceramics (barium titanate) generate a voltage when deformed Compression Tension BT V BT V V Sign and Magnitude Pressure Port Forcesumming plunger Output Piezoelectric crystal

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.

16 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. V out = emf 1 emf 2 Output Voltage A emf 1 V out 0 B Core Position V in emf 2 Core at A Core at 0 (Null Position) Core at B The position of the movable core determines the flux linkage between the acexcited primary winding and each of the two second windings.

17 Light Transducers Photoconductive cell A device which exhibits a change in resistance when exposed to different intensities of light. Metal case Photoconductive material Glass window Ceramic substrate Base pin Spectral response of cadmium selenide (CuSe)

18 Photodiode p-type n-type - Flow - - hν hν + + E I + Flow Spectral response of Si photodiode Anode V R Symbol Cathode qv I = I (exp[ ] 1) kt o I p I p : photocurrent Light intensity Photodiode Solar cell

19 Phototransistor Emitter Base - Collector hν + - hν E + V Collector R Collector Symbol Equivalent circuit Emitter Emitter

20 Photomultiplier Amplify photocurrent to detect extremely low light situations by impact ionization Dynode 2 Dynode 4 Dynode 6 Photocathode Dynode 1 Dynode 3 Dynode 5 Anode

Sensors and Transducers

Sensors and Transducers Transducers-Transducer is a device which converts one form of energy into another form of energy. Electrical transducers are those which convert one form of energy into electrical