SISTEM INTRUMENTASITEL 303

LECTURE 2: PERFORMANCE OF STATIC CHARACTERISTICS IN MEASUREMENT

AND INSTRUMENTATION

Mohammad Iqbal

Teknik Elektro, Universitas Muria Kudus

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Contents

Definition of static Requirement of static characteristics

preservation Static calibration Passive & active sensors

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Definition of Static

The steady state relationship between input and output of an instrument

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

It does not involve differential equations. All the static performance characteristics are

obtained by one form or another via a process called static calibration.

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Contents

Definition of static

Requirement of static characteristics preservation

Static calibration Passive & active sensors

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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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Examples

(a) Low precision,

low accuracy

(b) Precise not accurate

(c) Precise and accurate

Three industrial robots were programmed to place componentsat a particular point on a table. The target point was the centerof a circle shown below. The results are:

static characteristics

Accuracy– a measure of how close the output reading of the

instrument is to the correct value

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Value of measurand

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static characteristics

Precision– a term that describes an instrument’s degree of

freedom from random errors– If a large number of readings are taken of the

same quantity by a high precision instrument, then the spread of readings will be very small

JIM07 - FKM - UTeM 8

static characteristics

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LinearityIt is highly desirable that the measurement system has alinier relationship between input and output means thatthe 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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static characteristics

Linearity– The input and output relationship of a linear transducer can

be represented by the following equation:

– where y is the output of transducer, x is the input of transducer, m is the slope of curve (transfer function), c is the offset.

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

y = mx + c

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static characteristics

Hysteresis – Hysteresis results in predictable error. May be due

to internal friction, freeplay or looseness in the mechanism of an instrument.

– Also in electrical phenomena (relation between the output 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 following figure.

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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=(output)/(input)

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Sensitivity and Non Linearity

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Contents

Definition of static Requirement of static characteristics

preservation

Static calibration Passive & active sensors

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Static Calibration

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

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

The input/output relationships developed in this way comprise a static calibration valid under the stated constant conditions of all other inputs.

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

Ultimate objective is to define measurement accuracy

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Contents

Definition of static Requirement of static characteristics

preservation Static calibration

Passive & active sensors

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Passive & Active Sensors

Sensors are often transducer in that they are devices that convert input energy of one form into output energy of another form.

Categorisation of sensors: depends on how they interact with the environment they are measuring– Passive sensors: they do not add energy as part of the

measurement process but may remove energy in their operation. E.g. thermocouple converts a physical temperature into a voltage signal

– Active sensors: they add energy to the measurement environment as part of the measurement process. E.g. in a radar or sonar system, the distance to some object is measured by actively sending out a radio (radar) or acoustic (sonar) wave to reflect off some object and measure its range from the sensor

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EXERCISE

What is the meaning of the following words:– 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 the instrumentation/measurement system as a result of the measurements made of the physical quantities

– Parameter: Physical quantity within defined (numeric) limits.

– Transducer: A device that converts one form of energy to another

– Actuator: Electronic transducer that converts electrical energy into mechanical energy

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Next Lecture

Analysis on Experimental Data

End of Lecture 2