measurement and instrumentation_lecture2

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MEASUREMENT AND INSTRUMENTATION

BMCC 4743

LECTURE 2: PERFORMANCE OF STATICCHARACTERISTICS IN MEASUREMENT

AND INSTRUMENTATION

Mochamad Safarudin

Faculty of Mechanical Engineering, UTeM

2008


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JIM07 - FKM - UTeM 2

Contents

Definition of static

Requirement of static characteristicspreservation

Static calibration

Passive & active sensors


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Definition of Static

The steady staterelationship between input

and output of an instrument

Measurement of quantities that are constantor vary quite slowly with respect to time.

It does not involve differential equations.

All the static performance characteristics areobtained by one form or another via a

process called static calibration.


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Contents


Requirement of staticcharacteristics preservation

Static calibration



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Requirement of static characteristics

preservation Precise & Accurate

Measurements that are close to each other

are precise Measurements that are close to the correct

value are accurate

Measurements can be: Precise but inaccurate Neither precise nor accurate

Precise and accurate


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preservation Repeatability

The ability of an instrument to register readings accuratelyrepeatedly

Linearity The input and output relationship of a linear transducer canbe represented by the following equation:

wherey is theoutput of transducer,xis the input oftransducer, m is theslope of curve (transfer function), c isthe offset.

Often, the straight line approach is used for certain range ofoperation for a non-linear system.

y= mx+ c


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preservation Hysteresis

Hysteresis results in predictable error. May be dueto internal friction, freeplay or looseness in the

mechanism of an instrument.

Also in electrical phenomena (relation between theoutput voltage and the input field current in a d.c.generator) - the effect is due to magnetic

hysteresis of the iron in the field coils. The transfer functions differ with the increase and

decrease of inputs as shown in the followingfigure.


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Generalized

graph

ofo

utput/inputrelationship

wherehysteresisispresent.(FromP.

H.Sydenham,

Han

dboo

k

of

Measuremen

t

Sc

ience,

Vol.

2,

Chichester,

U.K.,

JohnWiley&Sons,198

3)


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Linearity


It is highly desirable that the measurement system has a

linier relationship between input and output means that

the change in output is proportional to the change in the

value of the measurandDeviation from true linearity is called linearity error


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Sensitivity

Sensitivity is the ratio of change in

magnitude of the output to the change

in magnitude of the measurand

Sensitivity=D(output)/D(input)



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Sensitivity and Non Linearity



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Contents



preservation

Static calibration



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Static Calibration

Imagine a situation in which all inputs (desired,interfering or modifying) except one are kept at someconstant values.

The one input under study is varied over some rangeof constant values which causes the output(s) to varyover some range of constant values.

The input/output relationships developed in this waycomprise a static calibration valid under the statedconstant conditions of all other inputs.

The procedure may be repeated for other inputs foroverall instrument static behaviour

Ultimate objective is to define measurement accuracy


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Contents



preservation

Static calibration



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EXERCISE

What is the meaning of the followingwords:

Measurand

Physical quantity

Data

Parameter

Transducer

Actuator


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ANSWER TO EXERCISE

Measurand: Physical quantity being measured

Physical quantity: Variable such as pressure,temperature, mass, length, etc

Data: Information obtained from theinstrumentation/measurement system as a result of themeasurements made of the physical quantities

Parameter: Physical quantity within defined (numeric)limits.

Transducer:A device that converts one form of energy toanother

Actuator: Electronic transducer that converts electrical energyinto mechanical energy


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Next Lecture

Analysis on Experimental Data

End of Lecture 2

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