general measurement system

7/24/2019 General Measurement System

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General MeasurementSystem

Mohd Zaki Nuawi

KMJ2314 Sem 1 Session 20132014


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We make measurements everyday.

Usually, we put little thought into the selectionof instruments for these routinemeasurements.

When the stakes become greater, the

selection of measurement equipment andtechniques, and the interpretation of themeasured data can demand considerableattention.

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ntroduction


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!ntroduction

Why do we need to measure anything?

Because we seek confirmation of our experiences

and of our theories.

Measurement has become a basic tenet of our rational

approach to the expansion of human knowledge.

he scientific method has as its basis the measurement of

phenomena of interest to us in order to de!elop "uantitati!e

descriptions of these.


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" measurement can be de#ned as number withattached units.

$he numerical value of a measurement should re%ectthe sensitivity of the instrument used to make the

measurement. &onsider a bathroom scale, which measurement is

reasonable' ()*.)+ lbs or ()) lbs "n -eact measurement/ does not really eist because

every instrument has some degree of uncertainty. "ninstrument reads only a #nite number of digits thathave meaning.

0very measurement has some degree of uncertainty inthe last decimal place. $he last digit read with aninstrument, with analog readout, is estimated.

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!.ntroduction


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!..ntroduction

#o what is a measure?

$n empirical ob%ecti!e assignment of a number or

symbol to an entity to characteri&e a specific

attribute. '(enton) *++*,

-imension or "uantity reckoned by some standard.'Websters -ictionary,.


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!!ntroduction

his means that a measure is not %ust a number but

characterises a mapping between the manifestation of an

aspect of interest in an element or entity within our uni!erse

of discourse and a mathematical or symbolic system ofranking and comparison. /n so doing) the aspect of interest is

called the attribute) and the mathematical system of ranking

and comparison into which these attributes are mapped is

called a scale. he action of producing the said mapping istermed measurement.

#o a direct or atomicmeasure is a "uantification based on a

mapping into a numerical or symbolic !alue obtained from a

scale of a directly obser!ed aspect of a phenomenon.


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Measurement is an act of assigning a specific value toa physical variabledetected by a sensor.

That physical variable is the measured variable.

A measurement system is a tool used for quantifyingthe measured variable.

It is used to extend the abilities of the human sensesthat, while they can detect and recognise differentdegrees of roughness, length, sound, colour, andsmell, are limited and relative, they are not very adept

at assigning specific values to sensed variables.7

$he 1asics


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0

...$he 1asics

1uantification is therefore the !ery basis of modern science.

Measurement has also had a correspondingly profound impact

on all fields of engineering. /n fact it can be safely asserted

that modern engineering is defined in terms of its scientific

basis) its "uantification of relationships and its measurementbased approaches.


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1asically, a general measurement system

consists of2(. 3ensor4transducer stage

5. 3ignal4conditioning stage

6. 7utput stage

. 8eedback4control stage

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!$he 1asics


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!$he 1asics


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$he sensoris a physical element that

employs some natural phenomenon 9 itsenses the variable being measured.

$he transducerconverts this sensed info intoa detectable signal :can be electrical,mechanical, optical etc;.

$he goalis to convert the sensed info into aform that can easily be quanti#ed.

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!$he 1asics


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KJ2313 & KJ3313 Sem 1 Session 0607 13

!$he 1asics


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3ignal conditioning equipment modi#esthe

transducer signal. 7ptional intermediate stage 9 for eg., increasing

the magnitude of the signal through ampli#cation,removing portions of the signal through some#ltering technique

$he output stageindicates or records thevalue measured 9 for eg., marked scale or arecording device such a computer disk drive.

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!$he 1asics


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n measurement systems that are involved in

process control, there is a fourth stage feedback4control stage.

8or eg., a simple measurement system withcontrol stage is a household furnacethermostat.

$he operator #es the set point for temperatureon thermostat display, and the furnace isactivated as the local temperature at thethermostat, as determined by the sensor withinthe device, rises or falls about the set point.

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!$he 1asics


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$wo basic methods2

(. direct comparison with either aprimary or secondary standard

5. indirect comparison through the use

of a calibrated system.

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Methods of Measurement


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$o measure the length of a rod, for eample, we canuse a ruler. We use a longruler to comparethe above with a standard rod. n this way, we get

a long rodwith a comparison of direct methods. =owever, this method is not always accurate and

adequate. 8or eample, insecuring the rod length above, if the rule used isthe ruler of steel :steel rule;, the accuracy ofmeasurement is the etent ofabout ( mm only. 8or most cases, weneed higher accuracy. 8or that, we need a morecomple measurement system that is not a>ordable bythe ability of the human senses alone.

?irect &omparison


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ndirect comparison requires usto use an intermediary device that calibratedfor more accurate measurement.8or eample, the human senses are not capableof directly measuring thestrain in the material isloaded

for eample. $o #nd the strain before, we needacalibrated system that can detect the strain,then change the output value, and #nally presents

the strain in the form weunderstand. Usually, the earlier strainoutput can beread from a shift in scale, chart or in digital form.

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ndirect &omparison


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" calibration applies a known input value toa measurement system.

n this procedure, a known value is input tothe system under calibration and thesystem output is recorded.

$he known value used for the calibration iscalled the standard.

$he calibration procedure establishes thecorrect output scale for the measuringsystem.

$wo types of calibration2 static calibration @dynamic calibration

&alibration


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Most common type of calibration

-3tatic/ implies that the values of thevariables involved remain constantA that is,they do not vary with time or space.

1y applying a range of known input values

and by observing the system output values,a direct calibration curve can be developedfor the measurement system.

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3tatic &alibration


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$he static calibration curve describes the

static input4output relationship for ameasurement system and forms the logic bywhich the indicated output can beinterpreted during an actual measurement.

8or eg, the calibration curve is the basis for#ing the output display scale on thethermometer.

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3tatic &alibration


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KJ2313 & KJ3313 Sem 1 Session 0607 24


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" calibration applies known inputs rangingfrom the minimum to the maimum value forwhich the measurement system is to be

used.$hese limits de#ne the operating range of

the system.

$he input operating range is de#ned as

etending from minto ma.$he input operating range may be epressed

in terms of the di>erence of the limits as

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&alibration Bange

minmax xxri =


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"lso, may specify the output operating rangefrom ymin to yma.

$he output scan or full4scale4operating :837;range is epressed as

&alibration Bange

minmax yyro =


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bl


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CN7WN2 Water at 5Do&

E Ff:&,",dp,;

& F D.+*A ? F ( m

5 G E G (D cmm

8N?2 0pected calibration curve

37HU$7N

Iart of a test matri is to specify the range of theindependent variable and to anticipate the rangeresulting in the dependent variable. n this case, thepressure drop will be measured so that it is the

dependent variable during a static calibration. $oanticipate the output range of the calibration then2

Bearranging the known relation,

dp F :EJ&";5J5

8or F KKL kgJm6 :"ppendi &;, and " F ?5J, we #nd2

E :cmm; dp :NJm5

5 (.)

* (D(D D

$his is plotted below. t is clear that C will not be aconstant as C F f:E;.

Iroblem (.66


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I bl ( 6+


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37HU$7N

ndependent variables

micrometer setting :i.e. the applied displacement;

&ontrolled variablepower supply input

?ependent variable

output voltage measured

0traneous variables

operator set4up, eroing of system, and reading of micrometer

ability to set control variables

&7MM0N$

f you try this you will #nd that the power supply ecitation voltage can have asigni#cant in%uence on the results. $he ability to provide the eact voltageon replication is important in obtaining consistent results in manytransducers. 0ven if you use a regulated laboratory variable power supply,this e>ect can be seen in your data variation on replication as a random

variation. f you use an unregulated source, be prepared to trace thesee>ects as they change from hour to hour or from day to day.

Many H?$ units allow for use of dc power, which is then transformed to acform before being applied to the coil. t is easiest to see the e>ect of powersetting on the results when using this type of transducer.

Iroblem(.6+


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t is important to choose the equipment that

measures a range that includes the valuethat is being measured :eg when measuringa force of up to 6*DN use a load cell thatthat has a range of D4*DN N7$D46*DN;.

"s you approach an etreme themeasurements become unreliable.

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7perating Bange


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$he resolution represents the smallest increment inthe measured value that can be discerned.

n terms of a measurement system, it is quanti#ed by

the smallest scale increment or least count :leastsigni#cant digit; of the output readout indicator.

Besolution

Ruler A .! "#$ %.& cm

Ruler '.!( "#$ %.%( cm


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$he eact value of a variable is called the truevalue.

$he value of the variables as indicated by a

measurement system is called the measuredvalue.

0rror may be de#ned as the di>erence betweenthe measured result and the truevalue of thequantity being measured.

"ccuracy refers to the closeness of agreementbetween the measured value and the true value.

1ut the true value is never knownO 3o, we arealso unsure of the error.

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"ccuracy and 0rror


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Bandom errorF measure of the random variation

found during repeated measurements of a variable.$he repeatabilityof a measurement system refers

to its ability to indicate the same value onrepeated measurements for a speci#c value ofinput.

-precision/ is used as a measure of the repeatabilityof a measurement systemA higher precision infers alower random error, better repeatability or lessvariation between measurements.

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Bandom @ 3ystematic0rrors


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$he portion of the absolute error that

remains constant on repeatedmeasurements is called the systematic error.

With systematic error, there is an o>setorbiasfrom the true value that cannot be

discerned from repeated measurements. 1oth random and systematic errors a>ect

the measure of the system/s accuracy.

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Bandom @ 3ystematicBandom @ 3ystematic

0rrors0rrors


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3uppose a measurement system were used to

measure a variable whose value was kept constantand known eactly, as in a calibration.

8or eg, (D independent measurements were made.

$he variations in the measurements, the observedscatter in the data, would relate to the random errorassociated with the measurement of the variable.

$he o>set between the apparent average of thereadings and the true value is the systematic error.

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n a measurement, the error cannot be

known eactly since the true value is notknown.

1ut based on the results of a calibration,the operator might feel con#dent that the

error is within certain bounds. Uncertaintyrefers to the estimate of the

e>ects of the errors on the result of ameasurement.

Uncertainty

general measurement system

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