Chapter06 Transducer

TRANSDUCERCHAPTER 6

1.0 What is transducer?Non-electrical physical

quantity: temperature, sound or light

Electrical signal

Example1. Thermoelectric:

Thermocouples (Gandingan terma)Resistance Temperature Detector (RTD)Thermistors (termistor)

2. Electromechanical (or actuators):Strain Gauge (tolok terikan)

3. Electromagnetic (position transducer):Capacitive transducers, inductive transducers, resistive transducers …

1.1 Difference between Transducer & Sensor

SensorSensor

A physical device or A physical device or biological organ that biological organ that detects, detects, or sensesor senses, a signal or physical , a signal or physical condition and chemical condition and chemical compoundscompounds

TransducerTransducer

An electrical or An electrical or electronic, that electronic, that converts one converts one type of energy to anothertype of energy to another. . Most transducers are either Most transducers are either sensors or actuators. Often, sensors or actuators. Often, transducers are transducers are equipped with equipped with signal conditioning circuitssignal conditioning circuits to to convert physical quatities to convert physical quatities to electrical signals electrical signals

1.2 Classification of transducerSelf generating type (active transducer) – do not require an external power, and produce an analog voltage or current when stimulated by some physical form of energy

Thermocouple, Photovoltaic cell, Moving coil generator

Modulator (Passive transducers)- require an external power, and their output is a

measure of some variation, such as resistance or capacitance. E.g.:▫ Slide-wire resistor- Slider or contact varies the resistance in a

potentiometer, rheostat or bridge circuit. ▫ Resistance strain gauge- Resistance wire, foil or semiconductor

stress.▫ Differential transformer - Transformer with differential secondary

coils and movable magnetic core

1.3 signal conditioning

1.4 Choices of transducer criteria

2.0 TRANSDUCER•Temperature transducers

▫Thermocouples▫Resistance-Temperature

Detectors (RTD)▫Thermistors

•Resistive position transducers•Displacement transducers•Strain gauge

Temperature

Electrical signal

2.1 Thermocouple• In 1821, T.J. Seeback discovered that an electric

potential occurs when 2 different metals are joined into a loop and the two junctions are held at different temperatures.

• Seeback emf – a voltage difference between the two ends of the conductor that depends on the temperature difference of the ends and a material property.

• If the ends of the wire have the same temperature, no emf occurs, even if the middle of the wire is hotter or colder.

2.1.1 Thermocouple - Principle

Twisting or welding of 2 wires

Measuring junction - "hot" junctions

which is exposed to measured temperature.

Reference junction - "cold" junction

The other junction which is kept at a known temperature.

http://en.wikipedia.org/wiki/Image:Tc-dia.jpg

In normal operation, cold junction is placed in an ice bath

2.1.1 Thermocouple - Principle

2.1.2 Different Thermocouple Types

T/C Type

Conductors – Positive

Conductors – Negative

Temp Range

Sensitivity

E Nickel-chromium alloy

Copper-nickel alloy

-200C up to +90C 68 μV/C

J Iron Copper-nickel alloy

-40C to +75C 55 μV/C

K Nickel-chromium alloy

Nickel-aluminum alloy

-200C up to 1100C

41 μV/C

N Nickel-chromium-silicon alloy

Nickel-silicon-magnesium alloy

can withstand temperatures in excess of 1200C

39 μV/C at 900C

T Copper Copper-nickel alloy

-200C up to +350C

42 μV/C

Table: Compositions and Letter Designations of the Standardized Thermocouples

2.1.3 Magnitude of thermal EMF

wherec and k = constants of the thermocouple materialsT1 = the temperature of the ‘hot’ junctionT2 = the temperature of the ‘cold’ or ‘reference’ junction

)()( 22

2121 TTkTTcE

2.1.4 ProblemA thermocouple was found to have linear calibration

between 0⁰C and 400⁰C with emf at maximum temperature (reference junction temperature 0⁰C) equal to 20.68 mV.

a) Determine the correction which must be made to the indicated emf if the cold junction temperature is 25⁰C.

b) If the indicated emf is 8.82 mV in the thermocouple circuit, determine the temperature of the hot junction.

2.1.4 Solution(a) Sensitivity of the thermocouple

= 20.68/(400-0)= 0.0517 mV/⁰C

Since the thermocouple is calibrated at the reference junction of 0⁰C and is being used at 25⁰C, then the correction which must be made, Ecorr between 0⁰C and 25⁰C

Ecorr = 0.0517 x 25Ecorr = 1.293 mV

2.1.4 Solution(b) Indicated emf between the hot junction and

reference junction at 25⁰C => 8.92 mV

Difference of temperature between hot and cold junctions = 8.92/0.0517 = 172.53⁰C

Since the reference junction temperature is 25⁰C,hot junction temperature = 172.53 + 25 = 197.53⁰C.

2.1.5 Thermocouple - applications• Thermocouples are most suitable for measuring over a large

temperature range, up to 1800 K.

• They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications, thermistors and RTDs are more suitable.

• disadvantages▫ Low output voltage▫ Amplifiers are needed▫ Difficult to measure low temperature change

2.2 Resistance temperature detector (RTD)

Resistance temperature detectors (RTDs), also called resistance thermometers, are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature.

Temperature Metal Resistance Temperature Metal Resistance

The resistance ideally varies linearly with temperature.

2.2.1 Resistance vs Temperature Approximations

•A straight line has been drawn between the points of the curve that represent temperature, T1 and T2, and T0 represent the midpoint temperature.

2.2.1 Resistance vs Temperature Approximations

Straight line equationStraight line equation

R(T) = approximation of resistance at temperature T R(T0) = resistance at temperature T0

αo = fractional change in resistance per degree of temperature at T0 ΔT = T - T0

21]1)[()( TTTTTRTR oo

2.2.1 Resistance vs Temperature Linear Approximations

Straight line Straight line equationequation

R2 = resistance at T2

R1 = resistance at T1

)()(

1

12

12

0 TTRR

TRo

Example

2.2.2 RTD – quadratic approximationMore accurate representation of R-T curve over some spanof temperatures.

R(T) = quadratic approximation of resistance at temperature T R(T0) = resistance at temperature T0

α1 = linear fractional change in resistance with temperatureα2 = quadratic fractional change in resistance with temperature ΔT = T - T0

212

21 ])(1)[()( TTTTTTRTR o

Example

Solution

Platinum: Platinum: very repeatable, sensitive, expensivevery repeatable, sensitive, expensive

Nickel: Nickel: not quite repeatable, more sensitive, less expensivenot quite repeatable, more sensitive, less expensive

Performance of RTD is based on:

1. Sensitivity 2. Response time 3. Temperature range4. Material considerations

Platinum

CopperTungsten

Nickel

2.2.3 Performance Specifications

•Sensitivity is shown by the value αo

▫Platinum – 0.004/ °C▫Nickel – 0.005/ °C

•Thus, for a 100Ω platinum RTD, a change of only 0.4 Ω would be expected if the temperature is changed by 1°C

1 00ºC

0.4 Ω

100 ºC

0.5 Ω

2.2.3 RTD - sensitivity

2.2.3 RTD – response time Type of Sensor Time Response

Direct immersion 0.3 to 3 seconds Aerospace/nuclear style sensor

1/8 inch diameter 2 to 3 seconds 300 series SST Sheath

3/16 inch diameter 4 to 5 seconds 300 series SST Sheath

Generally 0.5 to 5 seconds or more The slowness of response is due principally to the slowness of thermal conductivity in bringing the device into thermal equilibrium with its environment.

Temperature range 2.2.3 The operation range of RTD depends on the type of metal.

Example:• Platinum RTD have a operation

range of -100C to 650C. • Nickel RTD have a operation

range of -180C to 300C.

2.2.4 Construction of a platinum resistance thermometer

Wire is in a coil to achieve small size and improve thermal conductivity to decrease response time.

2.2.4 Construction of a platinum resistance thermometer

Protect from the environment

2.2.5 Advantages and Disadvantages of RTD

Advantages

Stable, reliable and repeatable than thermocouples.

Linear relationship

between temperature, T and resistance, R.

Disadvantages

More expensive than thermocouple.

Requires external current source to operate.

Slower response time than thermocouple.

Self-heating occurs.Smaller temperature range

(operation range) than thermocouple.

2.3 Thermistor•Semiconductor resistance sensors•Unlike metals, thermistors respond negatively to

temperature and their coefficient of resistance is of the order of 10 times higher than that of platinum or copper.

• Temperature semiconductor resistanceTemperature semiconductor resistance

•Symbol

http://en.wikipedia.org/wiki/Image:Thermistor_symbol.png

2.3.1 Thermistor: resistance vs temperature

• Advantages:▫ Stable, reliable and repeatable than RTD and

thermocouples. ▫ More accurate than RTD and thermocouple.▫ More sensitive than RTD and thermocouple.

• Disadvantages:▫ Smaller temperature range (operation range) than RTD

and thermocouples. ▫ Nonlinear relationship between temperature, T and

resistance, R.

2.3.2 Advantages and Disadvantages of Thermistors


▫Thermocouples▫Resistance-Temperature Detectors

(RTD)▫Thermistors


Distance

Electrical signal

3.0 Resistive position transducersDistance

Electrical signal

3.1 Resistive position transducers

3.1 Resistive position transducers

To VRR

RV21

2

2R

1R

Example 1A displacement transducer with a shaft stroke of 3 cm as shown in Figure 1. The total resistance, RT of the potentiometer is 5 k and the applied voltage VT is 5 V. When the wiper is 0.9 cm from beginning end. What is the value of the output voltage Vo ?

bRLV

TV x

x

aR

LR

Figure 1

Solution

15005000xcm0.3cm9.0

R2

V5.1V0.5x50001500V

RRV T

T

2L

Example 2

•Using the same figure 1, calculate the resistance value when the shaft stroke is located at +0.85 cm and -0.38cm from mid-stroke.

•By assuming that RL >> Ra + Rb, prove that

•Draw the output voltage vs. the displacement

Tba

bL V

RRRV

Solution

!!provenVRR

RV

:equationsimpleraformtoequationthefrominatedlimebecanR,RRRcesin

VRRRRRR

RRV

53

38.05.1Rand53

5.185.0R

Tba

bL

LbaL

TbaLbLa

LbL

21

•Sketch of Output?•VL (V) versus Displacement (cm) – should it be linear

or nonlinear?•Parameters?

▫Displacement range from 0 – 3 cm▫Resistance range from 0 – 5 k▫Output range from 0 – 5.0 V

Example 3


▫Thermocouples▫Resistance-Temperature Detectors (RTD)▫Thermistors


4.0 Displacement transducers•Capacitive transducer• Inductive transducer•Variable inductance transducer

4.1.0 Capacitive transducers•The capacitance of a parallel-plate capacitor is given

by

ε = dielectric constantεo = 8.854 x 1o-12, in farad per meterA = the area of the plate, in square meterd = the plate spacing in meters

dAC o

4.1.2 Capacitive transducers – Value

4.1.2 Capacitive transducers – Value

4.2.0 Inductive transducers

4.2.0 Tachometer with permanent magnet stator

4.2.0 Tachometer with permanent magnet rotor

4.3.1 Variable Inductance Transducers

4.3.2 Variable Inductance Transducers - construction

4.3.3 Variable Inductance Transducers – schematic diagram

4.3.4 Variable Inductance Transducers – operation

When the core is in the center, the voltage induced in the two secondaries is equal.

When the core is moved in one direction from the center, the voltage induced in one winding is increased and that in the others is decreased.

Movement in the opposite direction reverse the effect.


Core at the centerV1 = V2

Vo = 0


Core moves towards S1

V1 > V2

Vo increase


Core moves towards S2

V2 > V1

Vo decrease

4.3.5 Variable Inductance Transducers – with absolute magnitude


▫Thermocouples▫Resistance-Temperature Detectors (RTD)▫Thermistors


5.1 Stress•Stress is a measure of the average amount of force

exerted per unit area. It is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body forces. It was introduced into the theory of elasticity by Cauchy around 1822. Stress is a concept that is based on the concept of continuum.

5.1 StressIn general, stress is expressed as

is the average stress, also called engineering or nominal stress

and is the force acting over the area .

5.2 StrainStrain is the geometrical expression of deformation

caused by the action of stress on a physical body. Strain is calculated by first assuming a change between two body states: the beginning state and the final state. Then the difference in placement of two points in this body in those two states expresses the numerical value of strain. Strain therefore expresses itself as a change in size and/or shape.

5.2 Strain•The strain is defined as the fractional change in length

•Strain is thus a unitless quantity

llstrain

5.2 Strain•The strain is defined as the fractional change in length

•Strain is thus a unitless quantity

llstrain

5.3 Stress-strain curve

5.4 Strain gaugeFrom the equation of resistance,

R = resistanceρ = specific resistance of the conductor materialL = the length of the conductor in metersA = the area of the conductor in square meters

ALR

5.4 Strain gaugeTo measurepressure

When a strain produced by a force is applied on the wires, L increase and A decrease.

5.4 Strain gaugeL – increaseA – decrease

From the equation of resistance,

R – increaseALR

5.5 Strain gauge – the gauge factor

LLRRK//

K = the gauge factorR = the initial resistance in ohms (without strain)ΔR = the change of initial resistance in ohmsL = the initial length in meters (without strain)ΔL = the change of initial length in meters


LLRRK//

K = the gauge factorR = the initial resistance in ohms (without strain)ΔR = the change of initial resistance in ohmsL = the initial length in meters (without strain)ΔL = the change of initial length in meters


GRRK /

e - ppp

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Chapter06 Transducer