Lecture 3_9822fc9acb3cfc586fab43c8285452a9.pdf

Sensor

1. DYNAMICCHARACTERISTICS
Thedynamiccharacteristicsoftheinstrumentareimportant,iftheinstrumentistobeusedfor
varyingordynamicinputs.Inpractice,theparametersmayvaryatdifferentrates.Itisnot
possibletostudyordesigninstrumentsforthesespecificvariations.Hence,certainstandardshave
beenprescribedfordeterminingthedynamicbehaviorofthesystem.Theinputsspecifiedareas
follows:
1.Stepinput.
2.Ramp.
3.Sinusoidalinput.
BY Assist.Lecturer:Truska Kh.Muhammad Salih

2. Thedynamiccharacteristicsofaninstrumentare:
Fidelity:Itisthequalityofindicationbytheinstrumentwithregardtothechangesininput.This
isindicatedasdynamicerror.Dynamicerroristhedifferencebetweenthetruevalueofthe
processvariableandthemeterindication.
Speedofresponse:Itistherapiditywithwhichtheinstrumentrespondstothechangesininput.
Thedelayintheresponseisknownaslag.
BY Assist.Lecturer:Truska Kh.Muhammad Salih

3. Thedynamiccharacteristicsoftheinstrumentsarerepresentedbydifferentialequations.The
orderoftheequationrepresentstheorderoftheinstrument.But,ingeneral,theinstrumentsare
classifiedas
1.Zeroorder.
2.Firstorder.
3.Secondorder.
Thisclassificationappliestoelectromechanical-typeinstrumentswithamovingelement.
Zero-orderinstrument:E.g.Linearpotentiometer.Itisrepresentedbytheequation
�
0�=��
where�
0and�areconstants
�=input
�=output
BY Assist.Lecturer:Truska Kh.Muhammad Salih

4. Theoutputfollowstheinputwithaproportionalityconstantk=�/�
0.
First-orderinstrument:E.g.,Mercurythermometer.Itisrepresentedbythedifferential
equation:
�
1=
??????�
??????�
−�
0�=�
�
Dependinguponthetypeofinput,bxistobemodified.Forastepinputx=xo
Forarampinput�=�
�t
Forasineinput�=�
�sin��
Considerastepinput
�1
�
0
�+1�=
�
�
0
�
0
Thiscanbewrittenintheform�??????−1�=??????�
0
BY Assist.Lecturer:Truska Kh.Muhammad Salih

5. Where
??????=
�
�
0
;??????=
�
1
�
0
Thesolutiontopreviousequationwithinitialconditions,y=0atτ=0,is
�=??????(�
0)(1−�
−�??????
)
Themeasurementlagoftheinstrumentisobtainedfromthegraphbycalculatingthetimetaken
bytheoutputtoreach90%ofthefinalsteady-statevalue.Itisalsocalledthemeasurementlag.
Thoughtheinputchangesinastepfunctionform,theinstrumenttakesadefiniteamountof
time.Theinstrumentsaredesignedusingthesemathematicalequationstomeetthegiven
specifications.ThecorrespondingfigureisshowninFigbelow
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6. Response of the instrument for step input
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7. Second-orderinstrument:E.g.,Springbalanceinthefigurebelow.Thesearerepresentedby
theequation
�
2
??????
2
�
??????
�
2+�
1
??????�
??????
�
+�
0�+��=0
�=input
�=output
�=??????
�
2
�
�
�
2
+??????
�
�
�
�
+??????
��
??????=mass
??????=dampingcoefficient
??????
�=springconstant
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8. Spring balance equivalent circuit
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9. MEASUREMENT AND TRANSDUCER
BY
TRUSKA KH. MUHAMMAD
THIRD STAGE
ERBIL TECHNOLOGY COLLEGE
POLYTECHNIC UNIVERSITY
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10. LECTURE 3
ERRORS IN MEASURMENT INSTRUMENTS
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11. BY Assist.Lecturer:Truska Kh.Muhammad Salih
OUTLINES
1.TYPESOFERRORS
1.1GrossErrors
1.2Systematicerrors.
1.3Randomerrors.

12. BY Assist.Lecturer:Truska Kh.Muhammad Salih
TYPESOFERRORS
Differenttypesoferrorsthatoccurinmeasurementshavebeenclassifiedintothreecategories.
Theseinclude
1.Grosserrors.
2.Systematicerrors.
3.Randomerrors.
Thiswillenableuserstoavoidtheseerrorsandthusproveandincreasethecorrectnessin
measurement.

13. BY Assist.Lecturer:Truska Kh.Muhammad Salih
1.GrossErrors
Thesearebasicallyhumanerrorscausedbytheoperatororpersonusingtheinstrument.The
instrumentmaybegoodandmaynotgiveanyerrorbutstillthemeasurementmaygowrong
duetotheoperator.Thedifferenttypesofgrosserrorsare:
(i)Takingwrongreadings.
(ii)Readingwithparallaxerror(incorrectpositionofeyes).
(iii)Incorrectadjustmentsofzeroandfull-scaleadjustments.
(iv)Improperapplicationsofinstruments:Usinga0–100Vvoltmetertomeasure0.1V,etc.
(v)Wrongcomputation:Whenpoweristobedetermined,‘V’and‘I’aremeasured.Ifthe
computationgoeswrong,eventhough‘V’and‘I’havebeenmeasuredcorrectly,the
measurementofpowerwillbewrong.Thus,wrongcomputationcanresultinerror.

14. BY Assist.Lecturer:Truska Kh.Muhammad Salih
2.SystematicErrors
Thesearedividedintotwocategories:
(i)Instrumentalerrors:Duetoshortcomingsoftheinstruments.
(ii)Environmentalerrors:Duetoexternalconditionsaffectingtheinstrument.
2.1.Instrumentalerrors.
Evenifhumanerrorsareavoidedorpropercareistakentoseethatsucherrorsdonotoccur,
errorscanstilloccurinmeasurementsduetotheinstrument.

15. BY Assist.Lecturer:Truska Kh.Muhammad Salih
Thepossiblereasonscanbeasfollows:
(a)Frictioninbearingsofvariousmovingcomponentscancauseincorrectreadings.
(b)Irregularspringtensioninanalogmeters.
(c)Calibrationerrorsduetoaging(differencesbetweenvaluedindicatedbyaninstrumentand
thoseareactual.
(d)Zerosettingnotadjustedproperly.
(e)Full-scalesettingnotadjustedproperly.
(f)Faultydisplaycircuitindigitalinstruments.
Theseerrorscanbeavoidedby
•Applyingcorrectionfactors.
•Selectingsuitableinstruments.
•Calibratingtheinstruments.

16. BY Assist.Lecturer:Truska Kh.Muhammad Salih
2.2.Environmentalerrors.
Ambientparameterssuchastemperature,pressure,humidity,magneticandelectrostaticfields,dust,
andothersuchexternalparameterscanaffecttheperformanceoftheinstrument.Improperhousingof
theinstrumentalsocangivewrongreadings.Sucherrorscanbeavoidedbyair-conditioning,magnetic
shielding,cleaningtheinstruments,andhousingtheinstrumentsproperlydependingonthe
applicationandtypeoftheinstrument.
3.RandomErrors
Thoughgrosserrorsandsystematicerrorscanbeavoidedbytakingpropercare,someothererrorscan
alsooccurinmeasurements.Nospecificreasoncanbeassignedandprecautioncouldbetakento
avoidtheseerrors.Sucherrorsarecategorizedasrandomerrors.Noisethatisimpracticablecancause
randomerrorsinmeasurements.Toavoidtheseerrors,frequencyofmeasurementistobeincreased,
i.e.,thesameparameteristobemeasuredoften.Errorinsuchmeasurementscanbeestimatedby
statisticalanalysis.