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EXPERT SYSTEMS AND SOLUTIONS

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EI1361-MEASUREMENT EI1361-MEASUREMENT AND INSTRUMENTATION AND INSTRUMENTATION

-C.KAMALA KANNAN

Lecturer/EEE

UNIT 5

TRANSDUCERS AND DATA

ACQUISITION SYSTEMS

TRANSDUCERS AND DATA

ACQUISITION SYSTEMS Classification of transducers Selection of transducers Resistive transducers Capacitive transducers Inductive transducers Piezoelectric transducers Optical transducers Digital transducers Elements of data acquisition system - A/D, D/A

converters.

Reference TEXT BOOKS 1. E.O. Doebelin, ‘Measurement Systems

– Application and Design’, Tata McGraw Hill publishing company, 2003.

2. A.K. Sawhney, ‘A Course in Electrical & Electronic Measurements & Instrumentation’, Dhanpat Rai and Co, 2004.

REFERENCE BOOKS

1. A.J. Bouwens, ‘Digital Instrumentation’, Tata McGraw Hill, 1997.

2. D.V.S. Moorthy, ‘Transducers and Instrumentation’, Prentice Hall of India Pvt Ltd, 2003.

3. H.S. Kalsi, ‘Electronic Instrumentation’, Tata McGraw Hill, 1995.

4. Martin Reissland, ‘Electrical Measurements’, New Age International (P) Ltd., Delhi, 2001.

5. J. B. Gupta, ‘A Course in Electronic and Electrical Measurements’, S. K. Kataria & Sons, Delhi, 2003.

TRANSDUCERS

It’s a device which convert one form of energy to another form

Non electrical quantity is converted into an electrical form by a transducer.

Another name is pick up

Advantage of Electrical Transducers

Electrical amplification and attenuation can be done easily.

Mass inertia effects are minimized. The effect of friction is minimized. The electrical or electronic systems can be

controlled with a very small power level.

Conti..

The electrical output can be easily used , transmitted and processed for the purpose of measurement.

Telemetry (aerospace – remote indication / recording)

Miniaturization on account of use of IC’s.

Two parts/element of transducer Sensing or detector element.

A detector or a sensing element is that part of a transducer which responds to a physical phenomenon or a change in a physical phenomenon.

Transduction element.

A transduction element transforms the output of a sensing element to an electrical output. The transduction element in a way acts as a secondary transducer.

Classification of Transducers

On the basis of transduction form used. As primary and secondary transducers As passive and active transducer. As analog and digital transducer. As transducers and inverse transducers.

Classification based upon principle of transduction

Resistive Inductive Capacitive etc

Depending upon how they convert the input quantity into resistance, inductance or capacitance respectively.

Eg piezoelectric, thermoelectric, magneto restrictive, electro kinetic and optical

Primary and Secondary Transducers

LVDT (Linear Variable Differential Transformer)

Primary- Pressure to displacement (bourdon tube)

Secondary-Displacement into analogous voltage (LVDT).

Passive and Active Transducer

Active Transducer:

Also known as self generating type, develop their own voltage or current from the physical phenomenon being measured.

Velocity , temperature , light intensity and force can be transduced with the help of active transducer.

Conti.. Passive Transducer:

Also known as externally powered transducers, i.e., derive the power required for energy conversion from an external power source.

e.g. POT (Potentiometer)-used for the measurement of displacement .

Analog and Digital Transducer.

Analog Transducers : It converts the input quantity into an analog output which is a continuous function of time.

E.g. LVDT, Thermocouple or a thermistor

(gives output which is continuous function of time)

Conti..

Digital Transducer: Converts input quantity into an electrical output which is in the form of pulse.

Transducers and Inverse Transducers

Transducer: Non electrical to electrical quantity

Inverse transducer: Electrical quantity into non electrical quantity.

Characteristics and Choice of Transducer

Input Characteristics Transfer Characteristics Output Characteristics.

Input Characteristics

Type of Input and Operating Range Loading effect.Type of Input :The type of input, which can

be any physical quantity, is generally determined in advance .

Operating Range : Choice of transducer depends upon the useful range of input quantity.

Conti..

Loading Effect : The transducer, that is selected for a particular application should ideally exact NO force, power or energy from the quantity under measurement in order that is measured accurately.

Transfer Characteristics

1. Transfer function.

2. Error.

3. Response of transducer to environmental influences.

Transfer function.

The transfer function of a transducer defines a relationship between the input quantity and the output. The transfer function is

Where are respectively output and input of the transducer.

)(0 iqfq

iandqq0

Conti..

Sensitivity,

Scale Factor, Inverse of sensitivity.i

o

dq

dqS

i

o

dq

dq

S

1

Error

The error in transducer occur because they do not follow, the input output relationship.

Example.. Instead of qo, we might get a output as qo

’, then the error of the instrument is

0, qqo

Three components of error

1. Scale error.

2. Dynamic error

3. Error on account of noise and drift.

Scale error.

Zero error. Sensitivity error Non conformity. Hysteresis.

Zero error

Output deviates from the correct value by a constant factor over the entire range of the transducer.

Theorectical Curve.

Practical Curve.

Input

Output

Sensitivity Error

Observed output deviates from the correct value by a constant value.

Theorectical Curve.

Practical Curve.

Input

Output

Non conformity

Transfer function deviates from the theoretical transfer function for almost every input.

Theorectical Curve.

Practical Curve.

Input

Output

Hysteresis

Increasing input

Decreasing input

Input

Output

Response of transducer to environmental influences.

It should not be subjected to any disturbances like stray electromagnetic and electrostatic fields, mechanical shocks and vibrations temperature changes, pressure and humidity changes, changes in supply voltage and improper mechanical mountings.

Output Characteristics

Type of Electrical Output. Output Impedance Useful Range.

Type of Electrical Output.

The type of output which may be available from the transducers may be available from the transducers may be a voltage, current , impedance or a time function of these amplitudes.

Output Impedance Ideally the value of output impedance should

be zero if no loading effects are there on the subsequent stage.

Since zero output impedance is not possible , it should be kept as low as possible, since it determines the amount of power that can be transferred to the succeeding stages of the instrumentation system.

Useful Output Range

The output range of a transducer is limited at the lower end by noise signal.

The upper limit is set by the maximum useful input level.

Factors Influencing the choice of Transducer.

1. Operating Principle2. Sensitivity3. Operating Range4. Accuracy5. Cross sensitivity6. Errors7. Transient and frequency response

Conti..

8. Loading effects.

9. Environmental compatibility

10. Insensitivity to unwanted signals

11. Usage and Ruggedness

12. Electrical aspects

13. Stability and Reliability

14. Static characteristics.

1. Operating Principle: The transducer are many times selected on the basis of operating principle used by them. The operating principle used may be resistive, inductive, capacitive , optoelectronic, piezo electric etc.

2. Sensitivity: The transducer must be sensitive enough to produce detectable output.

3. Operating Range: The transducer should maintain the range requirement and have a good resolution over the entire range.

4. Accuracy: High accuracy is assured.

5. Cross sensitivity: It has to be taken into account when measuring mechanical quantities. There are situation where the actual quantity is being measured is in one plane and the transducer is subjected to variation in another plan.

6. Errors: The transducer should maintain the expected input-output relationship as described by the transfer function so as to avoid errors.

7. Transient and frequency response : The transducer should meet the desired time domain specification like peak overshoot, rise time, setting time and small dynamic error.

8. Loading Effects: The transducer should have a high input impedance and low output impedance to avoid loading effects.

9. Environmental Compatibility: It should be assured that the transducer selected to work under specified environmental conditions maintains its input- output relationship and does not break down.

10. Insensitivity to unwanted signals: The transducer should be minimally sensitive to unwanted signals and highly sensitive to desired signals.

11. Usage and Ruggedness: The ruggedness both of mechanical and electrical intensities of transducer versus its size and weight must be considered while selecting a suitable transducer.

12. Electrical aspects: The electrical aspects that need consideration while selecting a transducer include the length and type of cable required.

13. Stability and Reliability : The transducer should exhibit a high degree of stability to be operative during its operation and storage life.

14. Static Characteristics :Apart from low static error, the transducer should have a low non- linearity, low hysteresis, high resolution and a high degree of repeatability.

Resistive Transducers

m- material;conductor ofy resistivit

mconductor; of area sectional-cross A

m ;conductor oflength L

; resistance

,

2

RA

LR

Any method of varying one of the quantities involved in the above relationship can be the design basis of an electrical resistive transducer.

The translational and rotational potentiometers which work on the basis of change in the value of resistance with change in length of the conductor can be used for measurement of translational or rotary displacement.

Strain gauge work on the principle that the resistance of the conductor or a semiconductor changes when strained. This property can be used for measurement of displacement, force and pressure.

The resistivity of the material changes with change of temperature thus causing a change of resistance. This property may be used for measurement of temperature.

Potentiometers POT Resistive potentiometer used for the

purposes of voltage division is called POT. Resistive potentiometer consist of a

resistive element provided with a sliding contact.

Sliding Contact-Wiper

POT It’s a Passive Transducer. Linear Pot –Translational Motion Rotary Pot-Rotational Motion Helipots- Combination of the two motions

(translational as well as rotational). In Electrical Measurement , Standard

potentiometer are used to measure the unknown voltage by comparing it with a standard known voltage.

Resistive potentiometer

Translational, rotational and helipots

Consider a translational potentiometer

If the distribution of the resistance with respect to translational movement is linear, the resistance per length is

ter;potentiome theof resistance totalR

m, position; zero its from wiper ofnt displaceme

m, pot; onal translatioflength total

V, ly;respective tagesoutput vol andinput e and e

i

0i

p

t

x

x

t

p

xR

The output voltage under ideal conditions is:

For Rotational Motion

ii

0

ex e

ageinput voltx inalsinput term at the resistance

minalsoutput ter at the resistance

t

i

p

tip

x

x

R

xxR

e

i0 ex t

ie

Strain Gauges If a metal conductor is stretched or

compressed , its resistance changes on account of the fact that both length and diameter of conductor change.

Also there is a change in the value of resistivity of the conductor when strained and this property is called piezoresistive effect.

Resistive strain gauges are also known as piezoresistive gauges.

(2)- 2 sA

L

s

A

A

L

s

L

Ads

dR

)1(A

LR

(3)- 1111

ss

A

As

L

Lds

dR

R

Let a tensile stress s be applied to the wire.

Divide equation (2) by A

LR

A

A

L

L

From (3) , per unit change in resistance is due to

Per unit change in length=

Per unit change in Area =

Per unit change in resistivity =

Area = )4(.4

.2,4

2

s

DD

s

AD

)5(2

)4(

)42(12

s

D

Ds

D

D

D

s

A

A

Equation (3) can be written as

(6)- 1211

ss

D

Ds

L

Lds

dR

R

)7(strain allongitudin

strain lateral

LL

DDv

Poisson’s ratio ,

Or )8( LLvDD

(9)- 1211

ss

L

Lv

s

L

Lds

dR

R

For small variation , the above relationship , can be written as

(10)- 2

L

Lv

L

L

R

R

The gauge factor is defined as the ratio of per unit change in resistance to per unit change in length.

)13(L

)12(

LRor

(11)- L

R

Lstrainwhere

G

LGR

L

RG

f

f

f

(12)- 21L

R

LLv

L

RG f

(13)- 21L

R

vL

RG f

Resistance change due to change in length

Resistance change due to change in area

Resistance change due to change in piezoresistive effect.

Types of strain gauges

Unbonded metal strain gauge Bonded metal wire strain gauge Bonded metal foil strain gauge Vacuum deposited thin metal film strain gauges. Sputter deposited thin metal film strain gauge. Bonded semiconductor strain gauges. Diffused metal strain gauge.

Unbonded metal strain gauge

Used almost exclusively in transducer applications.

At initial preload , the strains and resistances of the four arms are normally equal, with the result the output voltage of the bridge, e0=0.

Application of pressure produces a small displacement , the displacement increases tension in 2 wires and decreasing the resistance of the remaining 2 wires.

This causes an unbalance of the bridge producing an output voltage which is proportional to the input displacement and hence to the applied pressure.

Bonded metal wire strain gauge

It consist of a grid of fine resistance wire of diameter of about 0.025mm.

The wire is cemented to a base. The base – thin sheet of paper or bakelite. Wire is covered with a thin sheet of material

so that it is not damaged mechanically. The spreading of wire permits a uniform

distribution of stress over a grid.

Bonded metal foil strain gauge

Extension of the bonded metal wire strain gauge.

The bonded metal wire strain gauge have been completely superseded by bonded foil strain gauge.

Metal foil strain gauge

Semiconductor strain gauge.

The semiconductor strain gauge depends for their action upon piezo resistive effect. i.e. the change in the value of the resistance due to change in resistivity.

Rosettes

Resistance Thermometers The resistance of the conductor changes when its

temperature is changed. This property is utilized for measurement of temperature.

The variation of resistance R with temperature T(ok) can be represented by the following relationship for most of the metals as

R=R0(1+1T+ 2T2+…+ nTn+………)

Where R0=resistance at temperature T=0 and

1 ,2,n are constants. Platinum – as it can withstand high temperatures while

maintaining excellent stability.

Requirements of a conductor material to be used in RTDs are

The change in resistance of material per unit change in temperature should be as large as possible.

The material should have a high value of resistivity so that minimum volume of material is used for the construction of RTD.

The resistance of material should have a continuous and stable relationship with temperature.

Thermistors

Contraction of a term “thermal resistors” Its composed of semiconductor materials. Used in applications which involve

measurements in the range of -60oC to 15oC The resistance of thermistors ranges from

0.5 to 0.75 M

Thermistors

Composed of sintered mixture of metallic oxides such as manganese, nickel , cobalt, copper, iron and uranium.

Thermocouple

When two metals having different work functions are placed together, a voltage is generated at the junction which is nearly proportional to the temperature. This junction is called a thermocouple.

Variable Inductance Transducer

Change in self Inductance Change in Mutual Inductance. Production of eddy currents.

Transducers working on principle of change of Self Inductance.

Self inductance of a coil

typermeabili

mcoil; oflength

mcoil; ofsection cross of AreaA

factor form geometricA

G where

μGNA

μNμAN

μA

NL Inductance

μAcircuit magnetic theof reluctanceR

turnsofNumber N

where

RNL

2

2222

2

l

l

lll

l

Transducers working on principle of change of Mutual Inductance.

Uses multiple coils. The mutual inductance between two coils is

coupling ofefficient coK

coils twoof sinductance self

LLKM

21

21

LandL

where

Transducers working on principle of production of eddy currents

If a conducting plate is placed near a coil carrying alternating current, eddy currents are produced in the conducting plate.

Linear Variable Differential Transformer (LVDT)

The transformer consists of single primary winding P and two secondary windings S1 and S2 wound on a cylindrical former.

The secondary windings have equal number of turns and are identically placed on either side of the primary winding.

The primary winding is connected to an alternating current source.

LVDT

A movable soft iron is placed inside the former. The displacement to be measured is applied to the

arm attached to the soft iron core. Since the primary winding is excited by an

alternating magnetic field which in turn induces alternating current voltages in the secondary windings.

The output voltage of secondary , S1is Es1 and that of secondary, S2is Es2

Capacitive Transducer The Principle of operation of capacitive

transducer is based upon the familiar equation for capacitance of a parallel plate capacitor.

space. free ofy ermittivit

,ty permittivi relative

F/m ; medium ofty permittivi

m plates, obetween tw distance

mplates, of area goverlappinA

d

A

d

AC

0

2

P

d

where

r

r

r

or

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