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1 BENP 2183 – ELECTRONIC INSTRUMENTATION CHAPTER 4 1 Transducers and Applications Objectives At the end of this chapter, students should be able to: able to: Explain the basic structure of several types of transducers. Describe the characteristics of several types of transducers. Analyze some simple application circuits that 2 Analyze some simple application circuits that uses transducers. Introduction A transducer is defined as a device that receives energy from one system and transmits it to another, often in a different form (electrical, mechanical or acoustical). Basically, there are two types of transducers : 1) Electrical 2) Mechanical The electrical output of a transducer depends on the basic principle involved in the design. The output may be analog digital or frequency 3 The output may be analog, digital, or frequency modulated. Electrical Transducer An electrical transducer is a device that converts a physical, mechanical or optical quantity into a proportional voltage or current quantity. A l ti lt d th th f ll i t An electrical transducer must have the following parameters : 1) Linearity Linear relationship between a physical parameter and the resulting electrical signal. 2) Sensitivity Defined as the electrical output per unit change in the physical parameter (eg V/ 0 C) 4 physical parameter (eg. V/ C) High sensitivity is desirable.

Chap4 Transducers Ver3

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Page 1: Chap4 Transducers Ver3

1

BENP 2183 – ELECTRONIC INSTRUMENTATION

CHAPTER 4

1

Transducers and Applications

Objectives

At the end of this chapter, students should be able to:able to:Explain the basic structure of several types of transducers.Describe the characteristics of several types of transducers.Analyze some simple application circuits that

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Analyze some simple application circuits that uses transducers.

Introduction

A transducer is defined as a device that receives energy from one system and transmits it to another, often in a different form (electrical, mechanical or acoustical).d e e o (e ec ca , ec a ca o acous ca )Basically, there are two types of transducers :1) Electrical2) MechanicalThe electrical output of a transducer depends on the basic principle involved in the design.The output may be analog digital or frequency

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The output may be analog, digital, or frequency modulated.

Electrical TransducerAn electrical transducer is a device that converts a physical, mechanical or optical quantity into a proportional voltage or current quantity.A l t i l t d t h th f ll i tAn electrical transducer must have the following parameters :1) Linearity• Linear relationship between a physical parameter and the

resulting electrical signal.2) Sensitivity• Defined as the electrical output per unit change in the

physical parameter (eg V/0C)

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physical parameter (eg. V/ C)• High sensitivity is desirable.

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Electrical Transducer3) Dynamic Range• Transducer must have a wide operating range.4) Repeatability• The input/output relationship for a transducer should be

predictable over a long period of time.5) Physical Size• The transducer must have minimal weight and volume to

minimize the disturbance to the existing conditions/environment.

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Electrical TransducerElectrical transducers can be classified into two major categories :1) Active transducers• Generates an electrical signal directly in response to the

physical parameter (does not require external power to operate).

• Example : piezo-electric sensor and photo cells.

2) Passive transducers• Requires external power to operate.

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q p p• Example : Strain gauges and thermistors.

Resistive Position TransducerOperates under a principle of resistance change by the physical movement under measurement.One type of resistive position transducer is shown in Figure 13 2 (t tb k)13.2 (textbook).The shaft and wiper can be moved to the left or right causes a change in the circuit resistance in Figure 13.2(b).The output voltage of the circuit is given by :

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2

t

O

RRR

VV

+= (4-1)

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The output voltage is proportional to R2 .• See Example 13.1 and 13.2 (textbook).

Resistive position transducer

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Strain Gauges

The strain gauge is an example of a passive transducer that senses the strain produced by a force on the wires.Wh i bj t d t iti t it l thWhen a gauge is subjected to a positive stress, its length increases while its area of cross-section decreases thus increases its resistance.

There are three main types of strain gauge :

area sectional-cross length, y,resistivit where ===×

= AlA

lR ρρ

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There are three main types of strain gauge :1) Wire strain gauges2) Foil strain gauge3) Semiconductor strain gauge

Strain Gauges

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Strain Gauges

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Sample of strain gauge

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Strain Gauges

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Strain Gauges

1) Wire Strain GaugesThe figure above shows a metallic bonded strain gauge.A fine wire element is cemented to a thin sheet of paper, bakelite or teflon.The measurement of the sensitivity of a material to strain is

f (G )

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called the gauge factor (GF).The gauge factor (GF) is given by :

l/ ΔlR / ΔRKGF == (4-2)

Strain Gaugeswhere

K = GF = gauge factorΔR = the change in the initial resistance in ΩR = the initial resistance (without strain)Δl = the change in the length in m

l = the initial length in m (without strain)

Strain is defined as Δl/l in equation (4-2).

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Strain GaugesExample :

Given a resistance strain gauge with GF = 2.0.Strain is 1 x 10-6 and R = 120Ω. Calculate ΔR.Strain is 1 x 10 and R 120Ω. Calculate ΔR.

Solution :

=

=

Δl/lKRΔRThereforeΔl/lΔR/RK

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Ω=×××=

=

μ240101120Ω2Δl/lKRΔRTherefore,

6-

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Strain Gauges

The strain gauge is normally used in a bridge arrangement in which the gauge forms one arm of the bridgebridge.A simple DC bridge circuit is shown in Figure above.

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Strain Gauges (bridge arrangement)

Why is compensation needed?

Resistance of fine wire•Resistance of fine wire is very sensitive to temperature and stress variation.•Contributes error.•Both gauges will experience the same effect caused by temperature changes.

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temperature changes.•Unstressed (dummy) strain will balance the bridge.

Strain Gauges

One of the gauge is an active element while the other is a dummy gauge.The dummy gauge is used to compensate the temperature variation.

2) Foil Strain GaugeThi l k lik i t it t l f il

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This gauge looks like a wire gauge except it uses a metal foil as the sensing element.The foil strain gauge is shown in Figure above.Foil strain gauge has the same characteristics as the wire strain gauges.

Strain Gauges3) Semiconductor Strain Gauge

Semiconductor strain gauge has a very high gauge factor (as high as 50 times the wire strain gauge).Figure 13.10 (textbook) shows the construction of a semiconductor strain gauge.The resistive material used to make this gauge is a semiconductor type such as germanium and silicon.Even though semiconductor has a very high gauge factor, it is very sensitive to changes in temperature.Its linearity is poor and it is more expensive

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Its linearity is poor and it is more expensive.

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Sample of semiconductor strain gauge

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Strain Gauges (semicond gauge)

Advantages of s/c strain gaugeS ll iSmaller sizeHigher sensitivity and GFLow hysterisis (more elastic)

Disadvantages of s/c strain gaugeVery sensitive to temperature

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Non-linear resistance-to-strain relationshipVarying 10-20% from a straight line equation

*these limitations can be overcome through software compensation.

CAPACITIVE TRANSDUCER

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Capacitive TransducerCapacitive transducer operates by a linear change in capacitance.The capacitance of a parallel plate capacitor is given by:

wherek = the dielectric constant (or εr)

A = the area of the plate, in m2

ε0 = 8.854 x 10-12 , in Farads / m – electric constant

(Farad)d

kAεC 0= (4-3)

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d = the plate spacing in m.

From equation (4-3), the capacitance increases if (i) the effective area of the plate is increased, and (ii) the material has a high dielectric constant.

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Capacitive TransducerA variable plate area transducer is shown in the figure below.It is made of a fixed plate called Stator and a movable plate called the Rotor.The capacitance of the transducer is changing as the rotor changes its position relative to the stator.This transducer can be used to detect the amount of roll in an aircraft.

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Capacitive Transducer

Another example of capacitive transducer is the capacitive pressure transducer as shown in the figure above.This sensor is designed to measure pressure (in vacuum).A metallic diaphragm will move to the right when pressure is applied to the chamber and to the left when vacuum is

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ppapplied.This diaphragm is used as one plate of a variable capacitor.The capacitive transducer is simple to construct, inexpensive, and effective for HF variations.

INDUCTIVE TRANSDUCER

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Inductive TransducerInductive transducers may be either of the self generating or the passive type.The self generating type utilizes the basic electrical

t i i l i ti b t d t dgenerator principle, ie. a motion between a conductor and magnetic field induces a voltage in the conductor.A tachometer is an example of the self generating transducer which directly converts speed or velocity into an electrical signal.An inductive electromechanical transducer converts physical motion into a change in inductance.

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g

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Figure (a) and (b) are transducers used for the measurement of displacement of linear and angular

Inductive Transducer

movement respectively.

In both cases, as the number of turns are changed, the self inductance and the output also changes.

RNL

2

=

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lANLtherefore

AlR

R2

...... μμ

=

Where: N = # turns, R = Reluctance of the materials/path where the flux is going throughµ = permeabilityA = Areal = length

Inductive Transducer

Figure shows an inductive transducer which works on the principle of the variation of permeability causing a change in self inductance.

When the iron core is inside the winding, its permeability is increased, and so is the inductance.

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Inductive Transducer-Figure shows the variable reluctance transducer.- The transducer consists of a

il d f ti

The displacement which is to be measured is applied to a ferromagnetic target.The core and the target are separated by an air gap.Th lf i d t f th il i i l ti l t th l th

coil wound on a ferromagnetic core.

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The self inductance of the coil is inversely proportional to the length of the air gap.When the target is near the core, the length is small, thus increases the self inductance.

Inductive Transducer (variable reluctance)

displacement

t t

targetair gap

core (ferromagnetic)

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output

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Inductive Transducer (variable reluctance)

2

+=

gi RRNL

The inductance of the variable reluctance transducer is given by:

gapair of reluctance partsiron of reluctance

turnsofnumber where

===

g

i

RRN

The reluctance of the iron (target) part is negligible, thus:

lN 2 where l = length of the air gap

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go

gg

g Al

RRNL

×==μ

, where lg = length of the air gap

μo = permeabilityAg = area of the flux path thru air

Rg is proportional to lg, thus L is inversely proportional to lg.

OTHER TYPES OF TRANSDUCERS- LVDT

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LVDT

LVDT is Linear Variable Differential Transformer.An movable soft iron slides within the hollow part of the transformer thus affects the magnetic coupling between the

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transformer thus affects the magnetic coupling between the primary and the secondary windings.The frequency of the ac voltage applied to the primary winding ranges from 50 Hz to 20 kHz.

LVDT

primary

secondary

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Basic construction of LVDT

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LVDT

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LVDTPrevious figure shows the operation and output voltage of an LVDT.

The output voltage of an LVDT is given by :

E0 = ES1 – ES2 (4-4)

Three possible position of the soft iron core:Normal position ES1 = ES2, thus EO = 0VLeft position ES1 > ES2 thus EO = ES1 - ES2

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Left position ES1 > ES2, thus EO ES1 ES2

Right position ES2 > ES1, thus EO = ES2 – ES1

LVDT

The amount of voltage change in either secondary winding is proportional to the amount of movement of the core.

Indication of the amount of linear motion.By noting which output is increasing or decreasing, the direction of the motion can be determined.

Ad t d hi h t t lt f ll h (hi h

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Advantage: produces higher output voltage for small changes (high sensitivity)

E.g: 50mV/mm, 300mV/mmAvailable ranges: ± 0.05 in to ± 25 in.Sensitive enough to measure displacements of well below 0.001 in.

LVDT – examples

Example 13.5 (Kalsi) / example 4.4 (module)

An AC LVDT has the following data.Input = 6.3 V, output = 5.2 V, range ± 0.5 in. Determine:i) the output voltage vs core position for a core movement going from +0.45 in to -0.30 in.

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ii) the output voltage when the core is -0.25 in from the centre.

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OTHER TYPES OF TRANSDUCERSPiezoelectric Transducer

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Piezoelectric Transducer

Piezoelectric transducer consists of a crystal material such as Quartz, Rochelle salt and Barium titanate which produces an emf( l t ti f ) h th l d d t

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(electromotive force) when they are placed under stress.Since the transducer has a very good high frequency (HF) response, its principal use is in HF accelerometers.

Piezoelectric Transducer

ih

charge generated where −=p

C

QCQE

When pressure is applied, the pressure sensing diaphragm will sense and pressure will be transferred to top of a crystal.

ecapacitancshunt −pC

The crystal will produce emfproportional to the magnitude of the applied pressure.

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Piezoelectric Transducer

Advantage:Needs no external power source self

tigenerating.Disadvantage:

Can not measure static condition since it is a dynamic responding sensor.

Application:

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Application:HF accelerometers

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OTHER TYPES OF TRANSDUCERSTemperature Transducer

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Temperature TransducerTemperature is one of the most widely measured and controlled variable in industry.Most of the temperature transducers are of Resistance Temperature Detectors (RTD), Thermistors and Thermocouples.RTD and Thermistor are passive devices whose resistance changes with temperature, so they need an electrical supply to give a voltage output.Thermocouples are active transducers and are based on the principle of generation of thermoelectricity

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the principle of generation of thermoelectricity.

RTDRTD is Resistance Temperature Detector.RTD commonly use platinum, nickel, or copper wire whose resistance varies with temperature.The RTD resistance at temperature T0C is given by :

whereR(T) = resistance of wire at temperature T0CRref = resistance at the reference temperature

temperat re coefficient of resistance

ΔT)α(1RR(T) ref += (4-5)

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α = temperature coefficient of resistanceΔT = difference between operating and reference

temperature

RTDPlatinum RTDs provide high accuracy and stability. They have the following advantages :1) Linearity over a wide operating range2) Wide operating range (-2000C -> 8500C)3) Higher temperature operation4) Better stability at high temperature

RTD’s are not adaptable to applications requiring fast response or small area temperature sensing.Most RTD instruments use a Wheatstone’s Bridge or its

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Most RTD instruments use a Wheatstone s Bridge or its modified version as shown in Figure 13.38 (textbook).

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ThermistorsThermistor is one of the temperature transducers.Its resistance decreasing as the temperature increasing.Note that the decrease of thermistor’s resistance with temperature is non-linear.Figure 13.12 (textbook) shows a graph of resistance vs temperature for a thermistor.The resistance at room temperature (250C) for typical commercial units ranges from 100Ω to 10 MΩ.Figure 13.13 shows the various type of thermistors.See E ample 13 4 (te tbook)

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See Example 13.4 (textbook).

OTHER TYPES OF TRANSDUCERSPhotodiode / Phototransistor

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PhotodiodeA reverse biased semiconductor diode passes only a very small leakage current (a fraction of 1 uA in typical small silicon diodes) if the junction is not exposed to light.U d ill i ti th t i l t i di tUnder illumination, the current rises almost in direct proportion to the light intensity.Figure 13.35 (textbook) shows the symbol and illumination characteristics of a silicon photodiode.When the device operates with a reverse voltage applied, it functions as a photoconductive device.When operating without the reverse voltage, it functions as a

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When operating without the reverse voltage, it functions as a photovoltaic device.

Photodiode

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PhotodiodeThe photodiode has an advantage over the photoconductive cell.Its response time is much faster, so that it can be used in

li ti i hi h th li ht fl t ti t it hi happlications in which the light fluctuations occur at quite high frequencies.Example 11.13 (Larry textbook) shows a simple circuit using photodiode to sense light illumination.

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PhototransistorPhototransistor is a photodiode with an added junction which makes it an n-p-n device.Phototransistor provides a much larger output current than th h t di d f i f li ht ill i tithe photodiode for a given area of light illumination.This means that phototransistor is more sensitive than photodiode (as much as 100 times).Figure 13.36 (textbook) shows the construction, symbol, and output characteristic of a phototransistor.However, phototransistor has a lower switching time than photodiode.

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photodiode.See Example 11.14 (Larry textbook).

Chapter ReviewDefine transducer and state its function.List four physical quantities that transducer measure.What is the difference between active and passive transducers?What is the difference between active and passive transducers?List 3 types of temperature transducers.Under what condition is a dummy strain gauge used? What is its function?Describe the difference between a variable reluctance type of transducer and LVDT.

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What are the advantages of using foil type strain gauge?Explain the differences between photovoltaic and photoconductive transducers.