Third LectSATACTIC CHARACTORSTICS OF INSTRUMENT SYSTEM

7/30/2019 Third LectSATACTIC CHARACTORSTICS OF INSTRUMENT SYSTEM

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Third Lecture

Measurement System and ItsStatic Characteristics

Instrumentation and Product Testing


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Introduction to Instrumentation and ProductTesting

Why Need Instrumentation and Product Testing inManufacturing?

The goal of Manufacturing or Production of parts/products

is to produce them economically, in compliance with designspecifications, which assure the proper function andservice of the manufactured product in its expected life.The roles of Instrumentation and Product Testing are:

Quality, Safety and Reliability Control Process Control and Automation Design and Production of Intelligent Products


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Some application examples:

(a) Manual Process Control


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(b) Automatic Inspection for Process Control


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(c) Quality Inspection (d) Indexing for Process Control

Therefore, the basic purpose ofInstrumentation (or EngineeringMeasurement) in Manufacturing is toensure and verify the agreement of theproduct, in various stage ofmanufacturing, with compliance to thespecifications of the design, aiming toachieve and Economical and Efficientproduction of High Quality and Reliable

products.


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(e) Intelligent Products

Toys Automobiles Audio/Video Instruments

Watches Electric Appliance (Rice cooker, Washing

machine, etc.)


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Unit of Measurement

It is a well-known saying that the knowledge about

anything is complete onlywhen if it can bequantified(expressed in numbers) correctly.

Therefore, in conjunction with the above need, for

every kind of quantity measured, there must be aunitto measure it and express it in numbers of thatunit.

Furthermore, in order that this unit is followed byall and not one who is taking the measurements,there must be a universal standardand the variousunits for various parameters of importance must bestandardised.


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What does a Standard mean?

Two meanings:

(1) Documents of regulations orguidelines for standardisation, and

(2) ...


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Standard

Unithas to have some relation to physical world,Therefore,physicalrecords, called standards, are usedto permanently record the size of units.

Definition of standard:

A standardis apermanentor readily reproduciblephysical record of the size of a unit of measurement.

A universal standardmust be one which is reproduciblewith such a degree of accuracy that for all industrialand scientific purposes it may be considered asabsolute.


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Primary Standard and Secondary Standard

Aprimarystandard is used as a fundamental

definition of the size of a physical unit. Primarystandards are specified by the provisions of aninternational treaty.

A secondarystandard is a copy of a primarystandard that can be used routinely for makingmeasurements.

Few of us will ever see/use a primary standard.Rather, we will generally deal with a secondarystandard (say, laboratory standard) that has beencopies from another secondary standard that itselfmay be many steps removed from the primary

standard.


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Error and uncertainty

The difference between a measured value and its truevalue is the error in measurement. The essence of

measurement is not only to know the magnitude of anunknown quantity, but also some knowledge of thedegree of accuracy of the measurement.

Types of errors

Systematic errors are those caused by the measuringprocedure, e.g. errors due to excessive measuringpressure, sine errors etc. Constant errors are those whichaffect all of a series of measurements by the same

amount. e.g. errors in calibration, zero errors, wear ofmeasuring contacts etc.

Random errors (uncertainties) are caused by incapabilityof technology and skill.


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Factors affecting accuracy in measurements

In measurement work, it is important to minimize

errors and thus lead to increased accuracy. Thechoice/design of appropriate method is crucial.

For example, measurement of an angle:

The error becomes very large when measuring anangle close to 90.

h

l

l

hsin

tansec1

hl


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Moreover, it is necessary to identify thevarious factors affecting accuracy. These

can be due to one or more of thefollowing:

Standard S

Workpiece WInstrument IPerson PEnvironment E

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Measurement systems

With a measurement system, physical quantities

are measured so as to obtain data which can betransmitted to recording and display devices.

The measurandis the physical quantity to be

measured, i.e. temperature, pressure, flow rate,

strain, displacement, etc.

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A complete measurement system

All measuring systems include three basic elements:

Transducer Signal

conditioner

Recorder or

Display

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Transducer Signal

conditioner

Recorder or

Display

detect and convert the physical quantity into a more

usable form: a mechanical, pneumatic, hydraulic or

electrical signal.

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Transducer Signal

conditioner

Recorder or

Display

modify the signal to a desirable output:

amplifier, filter, A/D converter, etc.

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Transducer Signal

conditioner

Recorder or

Display

show and record the measurand:

analog or digital

Transmission Path: noise and attenuation

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Example: A Pressure gauge

kPa0

10

20

30

4050

6070

80

90

100

110

Pressurized Gas Pipe

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Bourdon-tube (oval cross-section)

detector transducer stage

Increased pressure

causes movement oftube in this direction

Sector Sector and pinion

are modifying stage

Pressure

source

Pointer and dial

are indicator stage

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Pedometer

Transducer?

Signal Conditioner?

Display?

Mass

Counter

LCD

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4. Characteristics of measurement systems

To choose the one most suited to a particular

measurement application, we have to know

the system characteristics.

The performance characteristics may be

broadly divided into two groups, namely

static and dynamic characteristics.

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Static characteristics

the performance criteria for the measurement

of quantities that remain constant, or vary

only quite slowly.

Dynamic characteristics

the relationship between the system input andoutput when the measured quantity

(measurand) is varying rapidly.

I n practice, the characteristics of the one group may wellinfluence the characteristics of the other. In order to access

overall instrument performance, however, the two groups of

characteristics are normally studied separately and then a

semi-quantitative superposition is carried out.

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Static characteristics(Ref: Principles of Measurement Systems, J.P. Bentley, Longman;

BS 5233, BS 5532, and ISO 3534)

Determination of static characteristics is by calibration:

Element or System

to be calibrated

Standard

instrumentStandard

instrument

Standard

instrument(s)

Input,I

Environmental

Inputs,Ei

Output, O

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Accuracy

This is the closeness with which the measuring

instrument can measure the true value of the

measurand under stated conditions of use, i.e. itsability to tell the truth.

The accuracy of an instrument is quantified by the

difference of its readings and the one given by theultimate or primary standard.

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Frequency

True value, XT Mean of measurement

output, X

Variations, i.e.

random error

Systematic error = X XT

epeatability (that describes

recision) Standard deviation of

measurement data

Basic Concept of Accuracy and Error

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Bias

Bias (constant error) describes a constant error

which exists over the full range of measurement of

an instrument. This error is normally removable by

calibration.

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Precision

Precision describes an instruments degree of

random variations in its output when measuring a

constant quantity.

Precision is often confused with accuracy. High

precision does not imply anything about

measurement accuracy.

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Measuring a fixed target position from

a satellite

31Measuring a fixed target position from


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Measuring a fixed target position from

a satellite

Satellite

Apollo 13:Low

precision,

low

accuracy

Satellite

PolyU 18:High

precision,

low

accuracy

Satellite

CYT 118:High

precision,

High

accuracy

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Thank you