INSTRUMENTATION ANDCONTROL SYSTEMS
THIRDEDITION
WILLIAM BOLTON
NewnesisanimprintofElsevier
TheBoulevard,LangfordLane,Kidlington,OxfordOX51GB,UnitedKingdom 50HampshireStreet,5thFloor,Cambridge,MA02139,UnitedStates
Copyright©2021ElsevierLtd.Allrightsreserved.
Nopartofthispublicationmaybereproducedortransmittedinanyformorbyanymeans,electronicormechanical,includingphotocopying, recording,oranyinformationstorageandretrievalsystem,withoutpermissioninwritingfromthepublisher.Detailsonhowtoseekpermission, furtherinformationaboutthePublisher’spermissionspoliciesandourarrangementswithorganizationssuchastheCopyrightClearanceCenterand theCopyrightLicensingAgency,canbefoundatourwebsite: www.elsevier.com/permissions.
ThisbookandtheindividualcontributionscontainedinitareprotectedundercopyrightbythePublisher(otherthanasmaybenotedherein).
Notices
Knowledgeandbestpracticeinthisfieldareconstantlychanging.Asnewresearchandexperiencebroadenourunderstanding,changesinresearch methods,professionalpractices,ormedicaltreatmentmaybecomenecessary.
Practitionersandresearchersmustalwaysrelyontheirownexperienceandknowledgeinevaluatingandusinganyinformation,methods, compounds,orexperimentsdescribedherein.Inusingsuchinformationormethodstheyshouldbemindfuloftheirownsafetyandthesafetyofothers, includingpartiesforwhomtheyhaveaprofessionalresponsibility.
Tothefullestextentofthelaw,neitherthePublishernortheauthors,contributors,oreditors,assumeanyliabilityforanyinjuryand/ordamageto personsorpropertyasamatterofproductsliability,negligenceorotherwise,orfromanyuseoroperationofanymethods,products,instructions,or ideascontainedinthematerialherein.
BritishLibraryCataloguing-in-PublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary.
LibraryofCongressCataloging-in-PublicationData AcatalogrecordforthisbookisavailablefromtheLibraryofCongress. ISBN:978-0-12-823471-6
ForInformationonallNewnespublications visitourwebsiteat https://www.elsevier.com/books-and-journals
Publisher: MaraConner
AcquisitionsEditor: SonniniR.Yura
EditorialProjectManager: AliceGrant
ProductionProjectManager: PrasannaKalyanaraman
CoverDesigner:MatthewLimbert
TypesetbyMPSLimited,Chennai,India
Prefaceix Acknowledgementxiii
1.MeasurementSystems1
1.1Introduction1
1.1.1Systems1
1.2InstrumentationSystems2
1.2.1TheConstituentElementsofan InstrumentationSystem2
1.3PerformanceTerms4
1.3.1Resolution,Accuracy,andError4
1.3.2Range6
1.3.3Precision,Repeatability,andReproducibility7
1.3.4Sensitivity7
1.3.5Stability8
1.3.6DynamicCharacteristics9
1.4Dependability9
1.4.1Reliability10
1.5Requirements11
1.5.1Calibration12
1.5.2SafetySystems13 Problems14
2.InstrumentationSystemElements17
2.1Introduction18
2.2DisplacementSensors18
2.2.1Potentiometer18
2.2.2Strain-GaugedElement19
2.2.3CapacitiveElement20
2.2.4LinearVariableDifferentialTransformer21
2.2.5OpticalEncoders21
2.2.6Moire ´ Fringes22
2.2.7OpticalProximitySensors23
2.2.8MechanicalSwitches24
2.2.9CapacitiveProximitySensor25
2.2.10InductivityProximitySensor26
2.3SpeedSensors26
2.3.1OpticalMethods26
2.3.2IncrementalEncoder26
2.3.3Tachogenerator26
2.4FluidPressureSensors26
2.4.1DiaphragmSensor27
2.4.2PiezoelectricSensor28
2.4.3BourdonTube28
2.5FluidFlow29
2.5.1DifferentialPressureMethods29
2.5.2TurbineMeter31
2.5.3UltrasonicTimeofFlightFlowMeter32
2.5.4VortexFlowRateMethod32
2.5.5CoriolisFlowMeter33
2.6LiquidLevel33
2.6.1Floats34
2.6.2DisplacerGauge34
2.6.3DifferentialPressure34
2.6.4LoadCell34
2.6.5ElectricalConductivityLevelIndicator35
2.6.6CapacitiveLevelIndicator35
2.6.7UltrasonicLevelGauge35
2.6.8NucleonicLevelIndicators36
2.7TemperatureSensors36
2.7.1BimetallicStrips36
2.7.2LiquidinGlassThermometers36
2.7.3ResistanceTemperatureDetectors(RTDs)36
2.7.4Thermistors37
2.7.5Thermocouples37
2.7.6ThermodiodesandTransistors40
2.7.7Pyrometers40
2.8SensorSelection41
2.9SignalProcessing42
2.9.1ResistancetoVoltageConverter42
2.9.2TemperatureCompensation45
2.9.3ThermocoupleCompensation45
2.9.4Protection47
2.9.5Analogue-to-DigitalConversions47
2.9.6Digital-to-AnalogueConversions50
2.9.7MicrocontrollerSystems51
2.9.8Op-Amps52
2.9.9Pressure-to-CurrentConverter56
2.10SignalTransmission56
2.10.1Noise59
2.11SmartSystems60
2.11.1MEMS61
2.12DataPresentationElement62
2.12.1Indicator62
2.12.2IlluminativeDisplays62
2.12.3GraphicalUserInterface(GUI)64
2.12.4DataLoggers64
2.12.5Printers65 Problems66
3.MeasurementCaseStudies71
3.1Introduction71 3.2CaseStudies72
3.2.1ATemperatureMeasurement72
3.2.2AnAbsolutePressureMeasurement73
3.2.3DetectionoftheAngularPositionofa Shaft74
3.2.4AirFlowRateDetermination74
3.2.5FluidLevelMonitoring75
3.2.6MeasurementofRelativeHumidity75
3.2.7DimensionChecking76
3.2.8TemperatureofaFurnace77
3.2.9AutomobileTyrePressureMonitoring77
3.2.10ControlSystemSensorswithAutomobiles77
3.3DataAcquisitionSystems78
3.3.1DataAcquisitionSoftware80
3.3.2DataLoggers80
3.4Testing80
3.4.1Maintenance80
3.4.2CommonFaults81 Problems82
4.ControlSystems85
4.1Introduction85
4.2ControlSystems86
4.2.1Open-andClosed-LoopControl87
4.3BasicElements89
4.3.1BasicElementsofaClosed-LoopSystem89
4.4CaseStudies91
4.4.1ControloftheSpeedofRotationofa MotorShaft91
4.4.2ControlofthePositionofaTool92
4.4.3PowerSteering92
4.4.4ControlofFuelPressure93
4.4.5AntilockBrakes93
4.4.6ThicknessControl94
4.4.7ControlofLiquidLevel95
4.4.8RobotGripper95
4.4.9MachineToolControl97
4.4.10FluidFlowControl97
4.5Discrete-TimeControlSystems98
4.6DigitalControlSystems99
4.7HierarchicalControl100 Problems101
5.ProcessControllers103
5.1Introduction103
5.1.1DirectandReverseActions104
5.1.2DeadTime104
5.1.3Capacitance104
5.2On OffControl104
5.2.1Relays106
5.3ProportionalControl107
5.3.1ProportionalBand107
5.3.2LimitationsofProportionalControl109
5.4DerivativeControl110
5.4.1PDControl110
5.5IntegralControl112
5.5.1PIControl112
5.6PIDControl114
5.6.1PIDProcessController116
5.7Tuning117
5.7.1ProcessReactionTuningMethod118
5.7.2UltimateCycleTuningMethod120
5.7.3QuarterAmplitudeDecay121
5.7.4LambdaTuning121
5.7.5SoftwareTools121
5.7.6AdaptiveControllers122
5.8DigitalSystems122
5.8.1EmbeddedSystems123
5.9FuzzyLogicControl124
5.9.1FuzzyLogic124
5.9.2FuzzyLogicControlSystems126
5.9.3FuzzyLogicController127
5.9.4FuzzyLogicTuningofPIDControllers130
5.10NeuralNetworks130
5.10.1NeuralNetworksforControl132 Problems133
6.CorrectionElements137
6.1Introduction137
6.1.1TheRangeofActuators138
6.2PneumaticandHydraulicSystems138
6.2.1CurrenttoPressureConverter138
6.2.2PressureSources138
6.2.3ControlValves139
6.2.4Actuators140
6.3DirectionalControlValves141
6.3.1Sequencing143
6.3.2ShuttleValve145
6.4FlowControlValves146
6.4.1FormsofPlug147
6.4.2RangeabilityandTurndown149
6.4.3ControlValveSizing149
6.4.4ValvePositioners151
6.4.5OtherFormsofFlowControlValves151
6.4.6Fail-SafeDesign152 6.5Motors152
6.5.1D.C.Motors152
6.5.2BrushlessPermanentMagnetD.C.Motor154
6.5.3StepperMotor154 6.6CaseStudies159
6.6.1ALiquidLevelProcessControlSystem159
6.6.2MillingMachineControlSystem159
6.6.3ARobotControlSystem159 Problems160
7.PLCSystems165
7.1Introduction165
7.2LogicGates166
7.2.1Field-ProgrammableGateArrays169
7.3PLCSystem170
7.4PLCProgramming171
7.4.1LogicGates173
7.4.2Latching174
7.4.3InternalRelays174
7.4.4Timers175
7.4.5Counters176
7.5TestingandDebugging178
7.6CaseStudies179
7.6.1SignalLamptoMonitorOperations179
7.6.2CyclicMovementofaPiston179
7.6.3SequentialMovementofPistons179
7.6.4CentralHeatingSystem181 Problems182
8.SystemModels189
8.1Introduction189
8.1.1StaticResponse189
8.1.2DynamicResponse189
8.2Gain190
8.2.1GainofSystemsinSeries191
8.2.2FeedbackLoops191
8.3DynamicSystems193
8.3.1MechanicalSystems193
8.3.2RotationalSystems194
8.3.3ElectricalSystems196
8.3.4ThermalSystems198
8.3.5HydraulicSystems200
8.4DifferentialEquations202
8.4.1First-OrderDifferentialEquations202
8.4.2Second-OrderDifferentialEquations204
8.4.3SystemIdentification205 Problems206
9.TransferFunction209
9.1Introduction209
9.2TransferFunction210
9.2.1TransferFunction211
9.2.2TransferFunctionsofCommonSystem Elements212
9.2.3TransferFunctionsandSystems213
9.3SystemTransferFunctions214
9.3.1SystemsinSeries214
9.3.2SystemswithFeedback215
9.4BlockManipulation216
9.4.1BlocksinSeries217
9.4.2MovingTakeoffPoints217
9.4.3MovingaSummingPoint217
9.4.4ChangingFeedbackandForwardPaths217 9.5MultipleInputs220
9.6Sensitivity221
9.6.1SensitivitytoChangesinParameters221
9.6.2SensitivitytoDisturbances223 Problems223
10.SystemResponse227 10.1Introduction227 10.2Inputs227
10.3DeterminingOutputs228
10.3.1PartialFractions230 10.4First-OrderSystems233
10.4.1First-OrderSystemParameters235 10.5Second-OrderSystems237
10.5.1Second-OrderSystemParameters240 10.6Stability245
10.6.1The s Plane246 10.7Steady-StateError250 Problems252
11.FrequencyResponse257 11.1Introduction257
11.1.1SinusoidalSignals258 11.1.2ComplexNumbers258 11.2SinusoidalInputs260
11.2.1FrequencyResponseFunction260
11.2.2FrequencyResponseforFirst-Order Systems262
11.2.3FrequencyResponseforSecond-Order Systems264 11.3BodePlots265
11.3.1TransferFunctionaConstant K 266
11.3.2TransferFunction1/s n 266
11.3.3TransferFunction s m 267
11.3.4TransferFunction1/(1 1 τ s)267
11.3.5TransferFunction(1 1 τ s)269
11.3.6TransferFunction ω n2/(s 2 1 2ζω ns 1 ω n2)270
11.3.7TransferFunction
n 2 272 11.4SystemIdentification274 11.5Stability276
11.5.1StabilityMeasures277 11.6Compensation278
11.6.1ChangingtheGain278
11.6.2Phase-LeadCompensation280
11.6.3Phase-LagCompensation281 Problems283
12.NyquistDiagrams287 12.1Introduction287 12.2ThePolarPlot287
12.2.1NyquistDiagrams289 12.3Stability290 12.4RelativeStability292 Problems293
13.ControlSystems297
13.1Introduction297 13.2Controllers298
13.2.1ProportionalSteady-StateOffset300
13.2.2DisturbanceRejection301
13.2.3IntegralWind-Up301
13.2.4BumplessTransfer302
13.3FrequencyResponse302
13.4SystemswithDeadTime303
13.5CascadeControl305
13.6FeedforwardControl306
13.7DigitalControlSystems307
13.7.1The z-Transform309
13.7.2TheDigitalTransferFunction G(z)311
13.7.3PIDController314
13.7.4SoftwareImplementationofPID Control317
13.8ControlNetworks317
13.8.1DataTransmission318
13.8.2Networks319
13.8.3ControlAreaNetwork(CAN)320
13.8.4AutomatedAssemblyLines321
13.8.5AutomatedProcessPlantNetworks321
13.8.6PLCNetworks323
13.8.7SupervisoryControlandData Acquisition(SCADA)325
13.8.8TheCommonIndustrialProtocol (CIP)326
13.8.9SecurityIssues326 Problems327 Answers329
AppendixA:Errors341 AppendixB:DifferentialEquations347 AppendixC:LaplaceTransform353 AppendixD:The z-Transform361 Index367
Preface
Thisbookprovidesafirst-levelintroductiontoinstrumentationandcontrolengineeringand assuchissuitablefortheBTECunitsof IndustrialProcessControllers and IndustrialPlantand ProcessControl fortheNationalCertificatesandDiplomasinEngineering,andtheunit Control SystemsandAutomation fortheHigherNationalCertificatesandDiplomasinEngineeringand alsoprovidingabasicintroductiontoinstrumentationandcontrolsystemsforundergraduates.Thebookaimstogiveanappreciationoftheprinciplesofindustrialinstrumentationand aninsightintotheprinciplesinvolvedincontrolengineering.
Thebookintegratesactualhardwarewiththeoryandanalysis,aimingtomakethemathematicsofcontrolengineeringasreadableandapproachableaspossible.
STRUCTUREOFTHEBOOK
Thebookhasbeendesignedtogiveaclearexpositionandguidereadersthroughtheprinciplesinvolvedinthedesignanduseofinstrumentationandcontrolsystems,reviewingbackgroundprincipleswherenecessary.Eachchapterincludesworkedexamples,multiple-choice questionsandproblems;answersaresuppliedtoallquestionsandproblems.Thereare numerouscasestudiesinthetextindicatingapplicationsoftheprinciples.
PERFORMANCEOUTCOMES
Thefollowingindicatetheoutcomesforwhicheachchapterhasbeenplanned.Attheend ofthechaptersthereadershouldbeableto:
Chapter1:Measurementsystems
Readandinterpretperformanceterminologyusedinthespecificationsof instrumentation.
Chapter2:Instrumentationsystemelements
Describeandevaluatesensorscommonlyusedwithinstrumentationusedinthe measurementofposition,rotationalspeed,pressure,flow,liquidlevel,temperatureand thedetectionofthepresenceofobjects.
Describeandevaluatemethodsusedforsignalprocessinganddisplay.
Chapter3:Measurementcasestudies
Explainhowsystemelementsarecombinedininstrumentationforsomecommonly encounteredmeasurements.
Chapter4:Controlsystems
Explainwhatismeantbyopenandclosed-loopcontrolsystems,thedifferencesin performancebetweensuchsystems.
Explaintheprinciplesinvolvedinsomesimpleexamplesofopenandclosed-loopcontrol systems.
Describethebasicelementsofdigitalcontrolsystems.
Chapter5:Processcontrollers
Describethefunctionandterminologyofaprocesscontrollerandtheuseoftwo-step, proportional,derivativeandintegralcontrollaws.
ExplainPIDcontrolandhowsuchacontrollercanbetuned.
Explainwhatismeantbyfuzzylogicandhowitcanbeusedforcontrolapplications.
Explainwhatismeantbyartificialneuralnetworksandhowtheycanbeusedforcontrol applications.
Chapter6:Correctionelements
Describecommonformsofcorrection/regulatingelementsusedincontrolsystems.
Describetheformsofcommonlyusedpneumatic/hydraulicandelectriccorrection elements.
Chapter7:PLCsystems
Describethefunctionsoflogicgatesandtheuseoftruthtables.
DescribethebasicelementsinvolvedwithPLCsystems. DeviseprogramstoenablePLCstocarryoutsimplecontroltasks.
Chapter8:Systemmodels
Explainhowmodelsforphysicalsystemscanbeconstructedintermsofsimplebuilding blocks.
Chapter9:Transferfunction
Definethetermtransferfunctionandexplainhowitisusedtorelateoutputstoinputsfor systems.
Useblockdiagramsimplificationtechniquestoaidintheevaluationoftheoverall transferfunctionofanumberofsystemelements.
Chapter10:Systemresponse
UseLaplacetransformstodeterminetheresponseofsystemstocommonformsofinputs. Usesystemparameterstodescribetheperformanceofsystemswhensubjecttoastep input.
Analysesystemsandobtainvaluesforsystemparameters. Explainthepropertiesdeterminingthestabilityofsystems. Derivethesteady-stateerrorforabasicclosed-loopcontrolsystem.
Chapter11:Frequencyresponse
Explainhowthefrequencyresponsefunctioncanbeobtainedforasystemfromits transferfunction.
ConstructBodeplotsfromaknowledgeofthetransferfunction.
UseBodeplotsforfirstandsecond-ordersystemstodescribetheirfrequencyresponse. UsepracticallyobtainedBodeplotstodeducetheformofthetransferfunctionofa system.
Comparecompensationtechniques.
Chapter12:Nyquistdiagrams
DrawandinterpretNyquistdiagrams.
Chapter13:Controlsystems
ExplainthereasonsforthechoicesofP,PI,orPIDcontrollers.
Explaintheeffectofdeadtimeonthebehaviourofacontrolsystem.
Explaintheusesofcascadecontrolandfeedforwardcontrol.
Explaintheprinciplesofdigitalcontrolsystemsandtheuseofthez-transformtoanalyse them.
Describetheprinciplesinvolvedincontrolnetworks.
DescribetheprinciplesinvolvedinFieldbus.
DescribetheprinciplesofCAN,SCADA,DSCandCIPcontrolnetworks.
Identifytheissuesinvolvedinmaintainingasecuresystem.
SOFTWARETOOLS
Detailsofprogramsandmethodssuitablefortheirdevelopmenthavenotbeenincluded inthisbook.ItwasfelttobemoreappropriatetoleavesuchdevelopmenttomorespecialisttextssuchasMATLABandSIMULINKforEngineersbyAgamKumarTyagi(Oxford HigherEducation2011),AGuidetoMATLAB:ForBeginnersandExperiencedUsersby B.R.HuntandR.L.Lipsman(CambridgeUniversityPress2014),Hands-OnIntroduction toLabViewforScientistsandEngineersbyJohnEssick(OxfordUniversityPress2012),and LabviewforEveryone:GraphicalProgrammingMadeEasyandFunbyJeffreyTravis (PrenticeHall,2006).
CHANGESFORTHE3RDEDITION
Themajorchangesintroducedtothethirdeditionareadiscussionofdependabilitythathas beenincludedinChapter1,thediscussionofsmartsystemsextendedandanintroductionto radiotelemetryfordatatransmission.Adiscussionofinteractiveandnon-interactiveformsof PIDcontrolandintegratorwinduphasbeenaddedtoChapter5,anditalsonowincludesa reviseddiscussionofsteady-stateerrorandfuzzylogicandartificialneuralnetworksforcontrolapplications.Chapter10extendsthediscussionofthesteady-stateerror.Chapter13 extendsthediscussionofthe z-transformandbussystemsusedwithcontrolnetworks,introducingtheHARTCommunicationProtocol,FieldbusandCIPcontrolnetworks,andalso extendsthediscussionofsecurityissues.Anappendixhasbeenincludedonthebasicfeatures ofthe z-transform.
W.Bolton
Acknowledgement
Iamgratefultoallthosewhoreviewedthepreviouseditionandmadeveryhelpful suggestionsforthisnewedition.
1.1INTRODUCTION
Thischapterisanintroductiontotheinstrumentationsystemsusedformakingmeasurementsanddealswith thebasicelementsofsuchsystemsandtheterminologyusedtodescribetheirperformanceinuse.
1.1.1Systems
Theterm system willbefreelyusedthroughoutthisbook,andsohereisabriefexplanationofwhatismeant byasystemandhowwecanrepresentsystems.
Ifyouwanttouseanamplifierthenyoumightnotbeinterestedintheinternalworkingoftheamplifierbut whatoutputyoucanobtainforaparticularinput.Insuchasituationwecantalkoftheamplifierbeingasystem anddescribeitbymeansofspecifyinghowtheoutputisrelatedtotheinput.Withanengineeringsysteman engineerisoftenmoreinterestedintheinputsandoutputsofasystemthantheinternalworkingsofthecomponentelementsofthatsystem.
A system canbedefinedasanarrangementofpartswithinsomeboundarywhichworktogethertoprovide someformofoutputfromaspecifiedinputorinputs.Theboundarydividesthesystemfromtheenvironmentand thesysteminteractswiththeenvironmentbymeansofsignalscrossingtheboundaryfromtheenvironmenttothe system,i.e.inputs,andsignalscrossingtheboundaryfromthesystemtotheenvironment,i.e.outputs(Figure1.1).
Ausefulwayofrepresentingasystemisasa blockdiagram.Withintheboundarydescribedbytheboxoutline isthesystem,andinputstothesystemareshownbyarrowsenteringtheboxandoutputsbyarrowsleavingthe box. Figure1.2 illustratesthisforanelectricmotorsystem;thereisaninputofelectricalenergyandanoutputof
System
FIGURE1.1 Asystem. Electric
Inputs
FIGURE1.2 Electricmotorsystem. Input
FIGURE1.3 Amplifiersystem.
Interconnectedsystems.
mechanicalenergy,thoughyoumightconsiderthereisalsoanoutputofwasteheat.Theinterestisintherelationshipbetweentheoutputandtheinputratherthantheinternalscienceofthemotorandhowitoperates.Itis convenienttothinkofthesystemintheboxoperatingontheinputtoproducetheoutput.Thus,inthecaseofan amplifiersystem(Figure1.3)wecanthinkofthesystemmultiplyingtheinput V bysomefactor G,i.e.theamplifiergain,togivetheoutput GV
Oftenweareconcernedwithanumberoflinkedsystems.Forexample,wemighthaveaCDplayersystem linkedtoanamplifiersystem,which,inturn,islinkedtoaloudspeakersystem.Wecanthendrawthisasthree interconnectedboxes(Figure1.4)withtheoutputfromonesystembecomingtheinputtothenextsystem.Indrawingasystemasaseriesofinterconnectedblocks,itisnecessarytorecognisethatthelinesdrawntoconnectboxes indicateaflowofinformationinthedirectionindicatedbythearrowandnotnecessarilyphysicalconnections.
1.2INSTRUMENTATIONSYSTEMS
Thepurposeofan instrumentationsystem usedformakingmeasurementsistogivetheuseranumericalvalue correspondingtothevariablebeingmeasured.Thusathermometermaybeusedtogiveanumericalvaluefor thetemperatureofaliquid.Wemust,however,recognisethat,foravarietyofreasons,thisnumericalvaluemay notactuallybethetruevalueofthevariable.Thus,inthecaseofthethermometer,theremaybeerrorsdueto thelimitedaccuracyinthescalecalibration,orreadingerrorsduetothereadingfallingbetweentwoscalemarkings,orperhapserrorsduetotheinsertionofacoldthermometerintoahotliquid,loweringthetemperatureof theliquidandsoalteringthetemperaturebeingmeasured.Wethusconsiderameasurementsystemtohavean inputofthetruevalueofthevariablebeingmeasuredandanoutputofthemeasuredvalueofthatvariable (Figure1.5). Figure1.6 showssomeexamplesofsuchinstrumentationsystems.
An instrumentationsystem formakingmeasurementshasaninputofthetruevalueofthevariablebeingmeasuredandanoutputofthemeasuredvalue.Thisoutputmightbethenusedinacontrolsystemtocontrolthe variabletosomesetvalue.
1.2.1TheConstituentElementsofanInstrumentationSystem
Aninstrumentationsystemformakingmeasurementsconsistsofseveralelementswhichareusedtocarryout particularfunctions.Thesefunctionalelementsare:
1. Sensor
Thisistheelementofthesystemwhichiseffectivelyincontactwiththeprocessforwhichavariableis beingmeasuredandgivesanoutputwhichdependsinsomewayonthevalueofthevariableandwhichcan
FIGURE1.4
Input: true value of variable
Measurement system
Measurement system InputOutput
Pressure Value for the pressure (A)
Output: measured value of variable
FIGURE1.5 Aninstrumentation/measurementsystem.
Measurement system InputOutput Value for the speed (B) Speed
Measurement system InputOutput Value for the flow rate (C) Flow rate
Sensor: thermocouple
Input: temperature Output: e.m.f. (A)
Sensor: resistance element
Input: temperature Output: resistance change (B)
Input: Output: small e.m.f. larger voltage Wheatstone bridge Amplifier
Amplifier
FIGURE1.6 Examplesofinstrumentation systems:(A)pressuremeasurement,(B)speedometer,(C)flowratemeasurement.
FIGURE1.7 Sensors:(A)thermocouple,(B)resistance thermometer.
Output: FIGURE1.8 Examplesofsignalprocessing.
Input: resistance change Voltage change Larger voltage change (A) (B)
beusedbytherestofthemeasurementsystemtogiveavaluetoit.Forexample,athermocoupleisasensor whichhasaninputoftemperatureandanoutputofasmalle.m.f.(Figure1.7A)whichintherestofthe measurementsystemmightbeamplifiedtogiveareadingonameter.Anotherexampleofasensorisa resistancethermometerelementwhichhasaninputoftemperatureandanoutputofaresistancechange (Figure1.7B).
2. Signalprocessor
Thiselementtakestheoutputfromthesensorandconvertsitintoaformwhichissuitablefordisplayor onwardtransmissioninsomecontrolsystem.Inthecaseofthethermocouplethismaybeanamplifierto makethee.m.f.bigenoughtoregisteronameter(Figure1.8B).Thereoftenmaybemorethananitem, perhapsanelementwhichputstheoutputfromthesensorintoasuitableconditionforfurtherprocessingand thenanelementwhichprocessesthesignalsothatitcanbedisplayed.Theterm signalconditioner isusedfor anelementwhichconvertstheoutputofasensorintoasuitableformforfurtherprocessing.Thusinthecase oftheresistancethermometertheremightbeasignalconditioner,suchasaWheatstonebridge,which transformstheresistancechangeintoavoltagechange,thenanamplifiertomakethevoltagebigenoughfor display(Figure1.8B)orforuseinasystemusedtocontrolthetemperature.
3. Datapresentation
Thispresentsthemeasuredvalueinaformwhichenablesanobservertorecogniseit(Figure1.9).Thismay beviaadisplay,e.g.apointermovingacrossthescaleofameterorperhapsinformationonavisualdisplay unit(VDU).Alternatively,oradditionally,thesignalmayberecorded,e.g.inacomputermemory,or transmittedtosomeothersystemsuchasacontrolsystem.
Figure1.10 showshowthesebasicfunctionalelementsformameasurementsystem.
Display
Input:Output: signal from system signal in observable form
FIGURE1.9 Adatapresentationelement.
Input
Display Record Transmit True value of variable
Sensor Signal processor
Output Measured value of the input variable
Theterm transducer isoftenusedinrelationtomeasurementsystems.Transducersaredefinedasanelement thatconvertsachangeinsomephysicalvariableintoarelatedchangeinsomeotherphysicalvariable.Itisgenerallyusedforanelementthatconvertsachangeinsomephysicalvariableintoanelectricalsignalchange.Thus sensorscanbetransducers.However,ameasurementsystemmayusetransducers,inadditiontothesensor,in otherpartsofthesystemtoconvertsignalsinoneformtoanotherform.
EXAMPLE
Witharesistancethermometer,elementAtakesthetemperaturesignalandtransformsitintoresistancesignal,elementB transformstheresistancesignalintoacurrentsignal,elementCtransformsthecurrentsignalintoadisplayofamovement ofapointeracrossascale.Whichoftheseelementsis(a)thesensor,(b)thesignalprocessor,(c)thedatapresentation?
ThesensoriselementA,thesignalprocessorelementB,andthedatapresentationelementisC.Thesystemcanbe representedby Figure1.11.
Sensor Signal processor Data presentation
FIGURE1.10 Measurementsystemelements. ABC
Temperature signal Resistance change Current change Movement of pointer across a scale
FIGURE1.11 Example.
1.3PERFORMANCETERMS
Thefollowingaresomeofthemorecommontermsusedtodefinetheperformanceofmeasurementsystems andfunctionalelements.
1.3.1Resolution,Accuracy,andError
Resolution isthesmallestamountofaninputsignalchangethatcanbereliablydetectedbyaninstrument. Resolutionasstatedinamanufacturer’sspecificationsforaninstrumentisusuallytheleast-significantdigit (LSD)oftheinstrumentorinthecaseofasensorthesmallestchangethatcanbedetected.Forexample,the OMRONZX-Edisplacementsensorhasaresolutionof1 μm.
Accuracy istheextenttowhichthevalueindicatedbyameasurementsystemorelementmightbewrong. Forexample,athermometermayhaveanaccuracyof 6 0.1 C.Accuracyisoftenexpressedasapercentageofthefull
rangeoutputorfull-scaledeflection(f.s.d).Forexample,asystemmighthaveanaccuracyof 6 1%off.s.d.Ifthe full-scaledeflectionis,say,10A,thentheaccuracyis 6 0.1A.Theaccuracyisasummationofallthepossibleerrors thatarelikelytooccur,aswellastheaccuracytowhichthesystemorelementhasbeencalibrated.Asanillustration, theaccuracyofadigitalthermometerisquotedinthespecificationas:fullscaleaccuracy betterthan2%.
Theterm error isusedforthedifferencebetweentheresultofthemeasurementandthetruevalueofthequantitybeingmeasured,i.e.
Error 5 Measuredvalue Truevalue
Thusifthemeasuredvalueis10.1whenthetruevalueis10.0,theerroris 1 0.1.Ifthemeasuredvalueis9.9 whenthetruevalueis10.0,theerroris 0.1.
SeeAppendixAforadiscussionofhowtheaccuracyofavaluedeterminedforsomequantitycanbecomputed fromvaluesobtainedfromanumberofmeasurements,e.g.theaccuracyofthevalueofthedensityofsomematerial whencomputedfrommeasurementsofitsmassandvolume,boththemassandvolumemeasurementshavingerrors.
Errorscanariseinanumberofwaysandthefollowingdescribessomeoftheerrorsthatareencounteredin specificationsofinstrumentationsystems.
1. Hysteresiserror
Theterm hysteresiserror (Figure1.12)isusedforthedifferenceinoutputsgivenfromthesamevalueof quantitybeingmeasuredaccordingtowhetherthatvaluehasbeenreachedbyacontinuouslyincreasing changeoracontinuouslydecreasingchange.Thus,youmightobtainadifferentvaluefromathermometer usedtomeasurethesametemperatureofaliquidifitisreachedbytheliquidwarminguptothemeasured temperatureoritisreachedbytheliquidcoolingdowntothemeasuredtemperature.
2. Non-linearityerror
Theterm non-linearityerror (Figure1.13)isusedfortheerrorthatoccursasaresultofassumingalinear relationshipbetweentheinputandoutputovertheworkingrange,i.e.agraphofoutputplottedagainstinput isassumedtogiveastraightline.Fewsystemsorelements,however,haveatrulylinearrelationshipandthus errorsoccurasaresultoftheassumptionoflinearity.Linearityerrorisusuallyexpressedasapercentage erroroffullrangeorfullscaleoutput.Asanillustration,thenon-linearityerrorfortheOMRONZX-E displacementsensorisquotedas 6 0.5%.Asafurtherillustration,aloadcellisquotedinitsspecificationas having:non-linearityerror 6 0.03%offullrange,hysteresiserror 6 0.02%offullrange.
3. Insertionerror
Whenacoldthermometerisputintoahotliquidtomeasureitstemperature,thepresenceofthecold thermometerinthehotliquidchangesthetemperatureoftheliquid.Theliquidcoolsandsothethermometer endsupmeasuringalowertemperaturethanthatwhichexistedbeforethethermometerwasintroduced.Theact ofattemptingtomakethemeasurementhasmodifiedthetemperaturebeingmeasured.Thiseffectiscalled loading andtheconsequenceasan insertionerror.Ifwewantthismodificationtobesmall,thenthethermometer shouldhaveasmallheatcapacitycomparedwiththatofthe liquid.Asmallheatcapacitymeansthatverylittleheat isneededtochangeitstemperature.Thustheheattakenfromtheliquidisminimisedandsoitstemperature littleaffected.
Loadingisaproblemthatisoftenencounteredwhenmeasurementsarebeingmade.Forexample,whenan ammeterisinsertedintoacircuittomakeameasurementofthecircuitcurrent,itchangestheresistanceofthe circuitandsochangesthecurrentbeingmeasured(Figure1.14).Theactofattemptingtomakesucha
FIGURE1.12
FIGURE1.13
FIGURE1.14 Loadingwithanammeter:(A)circuitbeforemeter introduced,(B)extraresistanceintroducedbymeter.
FIGURE1.15 Loadingwithavoltmeter:(A)beforemeter,(B) withmeterpresent.
FIGURE1.16 Deadspace.
measurementhasmodifiedthecurrentthatwasbeingmeasured.Iftheeffectofinsertingtheammeteristobeas smallaspossibleandfortheammetertoindicatetheoriginalcurrent,theresistanceoftheammetermustbevery smallwhencomparedwiththatofthecircuit.
Whenavoltmeterisconnectedacrossaresistortomeasurethevoltageacrossit,thenwhatwehavedoneis connectedaresistance,thatofthevoltmeter,inparallelwiththeresistanceacrosswhichthevoltageistobe measured.Iftheresistanceofthevoltmeterisnotconsiderablyhigherthanthatoftheresistor,thecurrent throughtheresistorismarkedlychangedbythecurrentpassingthroughthemeterresistanceandsothevoltage beingmeasuredischanged(Figure1.15).Theactofattemptingtomakethemeasurementhasmodifiedthe voltagethatwasbeingmeasured.Iftheeffectofinsertingthevoltmeterinthecircuitistobeassmallaspossible, theresistanceofthevoltmetermustbemuchlargerthanthatoftheresistanceacrosswhichitisconnected.Only thenwillthecurrentbypassingtheresistorandpassingthroughthevoltmeterbeverysmallandsothevoltage notsignificantlychanged.
EXAMPLE
Twovoltmetersareavailable,onewitharesistanceof1kΩ andtheother1MΩ.Whichinstrumentshouldbeselectedif theindicatedvalueistobeclosesttothevoltagevaluethatexistedacrossa2kΩ resistorbeforethevoltmeterwas connectedacrossit?
The1MΩ voltmetershouldbechosen.Thisisbecausewhenitisinparallelwith2kΩ,lesscurrentwillflowthroughit thanifthe1kΩ voltmeterhadbeenusedandsothecurrentthroughtheresistorwillbeclosertoitsoriginalvalue.Hence theindicatedvoltagewillbeclosertothevaluethatexistedbeforethevoltmeterwasconnectedintothecircuit.
1.3.2Range
The range ofvariableofsystemisthelimitsbetweenwhichtheinputcanvary.Forexample,aresistancethermometersensormightbequotedashavingarangeof 200 Cto 1 800 C.Theterm deadband or deadspace is usedifthereisarangeofinputvaluesforwhichthereisnooutput. Figure1.16 illustratesthis.Forexample, bearingfrictioninaflowmeterusingarotormightmeanthatthereisnooutputuntiltheinputhasreacheda particularflowratethreshold.
1.3.3Precision,Repeatability,andReproducibility
Theterm precision isusedtodescribethedegreeoffreedomofameasurementsystemfromrandomerrors. Thus,ahighprecisionmeasurementinstrumentwillgiveonlyasmallspreadofreadingsifrepeatedreadings aretakenofthesamequantity.Alowprecisionmeasurementsystemwillgivealargespreadofreadings.For example,considerthefollowingtwosetsofreadingsobtainedforrepeatedmeasurementsofthesamequantity bytwodifferentinstruments:
20.1mm,20.2mm,20.1mm,20.0mm,20.1mm,20.1mm,20.0mm 19.9mm,20.3mm,20.0mm,20.5mm,20.2mm,19.8mm,20.3mm
Theresultsofthemeasurementgivevaluesscatteredab outsomevalue.Thefirstsetofresultsshowsasmallerspreadofreadingsthanthesecondandindicatesahigherdegreeofprecisionfortheinstrumentusedfor thefirstset.
Thetermsrepeatabilityandreproducibilityarewaysoftalkingaboutprecisioninspecificcontexts.Theterm repeatability isusedfortheabilityofameasurementsystemtogivethesamevalueforrepeatedmeasurementsof thesamevalueofavariable.Commoncausesoflackofrepeatabilityarerandomfluctuationsintheenvironment, e.g.changesintemperatureandhumidity.Theerrorarisingfromrepeatabilityisusuallyexpressedasapercentageofthefullrangeoutput.Forexample,apressuresensormightbequotedashavingarepeatabilityof 6 0.1% offullrange.Thuswitharangeof20kPa,thiswouldbeanerrorof 6 20Pa.Theterm reproducibility isused describetheabilityofasystemtogivethesameoutputwhenusedwithaconstantinputwiththesystemor elementsofthesystembeingdisconnectedfromitsinputandthenreinstalled.Theresultingerrorisusually expressedasapercentageofthefullrangeoutput.
Notethatprecisionshouldnotbeconfusedwithaccuracy.Highprecisiondoesnotmeanhighaccuracy. Ahighprecisioninstrumentcouldhavelowaccuracy. Figure1.17 illustratesthis.
1.3.4Sensitivity
The sensitivity indicateshowmuchtheoutputofaninstrumentsystemorsystemelementchangeswhenthe quantitybeingmeasuredchangesbyagivenamount,i.e.theratiooutput/input.Forexample,athermocouple mighthaveasensitivityof20 μV/ Candsogiveanoutputof20 μVforeach1 Cchangeintemperature.Thus,if wetakeaseriesofreadingsoftheoutputofaninstrumentforanumberofdifferentinputsandplotagraphof outputagainstinput(Figure1.18),thesensitivityistheslopeofthegraph.Forexample,aniron constantan thermocouplemightbequotedashavingasensitivityat0 Cof0.05mV/ C.
Thetermisalsofrequentlyusedtoindicatethesensitivitytoinputsotherthanthatbeingmeasured,i.e.environmentalchanges.Forexample,thesensitivityofasystemorelementmightbequotedtochangesintemperatureorperhapsfluctuationsinthemainsvoltagesupply.Thusapressuremeasurementsensormightbequoted ashavingatemperaturesensitivityof 6 0.1%ofthereadingper Cchangeintemperature.
FIGURE1.17 Precisionandaccuracy.
FIGURE1.18 Sensitivityasslopeofinput outputgraph.
Asanillustrationofthetypeofinformationavailableinaspecification,acommercialpressuremeasurement systemisquotedinthemanufacturer’sspecificationashaving:
Range0to10kPa
SupplyVoltage 6 15Vdc
Linearityerror 6 0.5%FS
Hysteresiserror 6 0.15%FS
Sensitivity5Vdcforfullrange
Thermalsensitivity 6 0.02%/ C
Thermalzerodrift0.02%/ CFS
Temperaturerange0to50 C
EXAMPLE
Aspringbalancehasitsdeflectionmeasuredforanumberofloadsandgavethefollowingresults.Determineitssensitivity.
Loadinkg0 1234
Deflectioninmm010203040
Figure1.19 showsthegraphofoutputagainstinput.Thegraphhasaslopeof10mm/kgandsothisisthesensitivity.
FIGURE1.19 Example.
EXAMPLE
Apressuremeasurementsystem(adiaphragmsensorgivingacapacitancechangewithoutputprocessedbyabridgecircuit anddisplayedonadigitaldisplay)isstatedashavingthefollowingcharacteristics.Explainthesignificanceoftheterms:
Range:0to125kPaand0to2500kPa
Accuracy: 6 1%ofthedisplayedreading
Temperaturesensitivity: 6 0.1%ofthereadingper C
Therangeindicatesthatthesystemcanbeusedtomeasurepressuresfrom0to125kPaor0to2500kPa.Theaccuracyis expressedasapercentageofthedisplayedreading,thusiftheinstrumentindicatesapressureof,say,100kPathentheerror willbe 6 1kPa.Thetemperaturesensitivityindicatesthatifthetemperaturechangesby1 Cthedisplayedreadingwillbein errorby 6 0.1%ofthevalue.Thusforapressureof,say,100kPatheerrorwillbe 6 0.1kPafora1 Ctemperaturechange.
1.3.5Stability
The stability ofasystemisitsabilitytogivethesameoutputwhenusedtomeasureaconstantinputovera periodoftime.Theterm drift isoftenusedtodescribethechangeinoutputthatoccursovertime.Thedriftmay beexpressedasapercentageofthefullrangeoutput.Theterm zerodrift isusedforthechangesthatoccurin outputwhenthereiszeroinput.
Steady-state reading
1.3.6DynamicCharacteristics
Thetermsgivenaboverefertowhatcanbetermedthe staticcharacteristics.Thesearethevaluesgivenwhen steady-stateconditionsoccur,i.e.thevaluesgivenwhenthesystemorelementhassettleddownafterhaving receivedsomeinput.The dynamiccharacteristics refertothebehaviourbetweenthetimethattheinputvalue changesandthetimethatthevaluegivenbythesystemorelementsettlesdowntothesteady-statevalue.For example, Figure1.20 showshowthereadingofananalogueammetermightchangewhenthecurrentisswitched on.Themeterpointeroscillatesbeforesettlingdowntogivethesteady-statereading.
Thefollowingaretermscommonlyusedfordynamiccharacteristics.
1. Responsetime
Thisisthetimewhichelapsesaftertheinputtoasystemorelementisabruptlyincreasedfromzerotoa constantvalueuptothepointatwhichthesystemorelementgivesanoutputcorrespondingtosome specifiedpercentage,e.g.95%,ofthevalueoftheinput.
2. Risetime
Thisisthetimetakenfortheoutputtorisetosomespecifiedpercentageofthesteady-stateoutput.Often therisetimereferstothetimetakenfortheoutputtorisefrom10%ofthesteady-statevalueto90%or95%of thesteady-statevalue.
3. Settlingtime
Thisisthetimetakenfortheoutputtosettletowithinsomepercentage,e.g.2%,ofthesteady-statevalue.
1.4DEPENDABILITY
Theterm dependability (seethepaper DependabilityandItsThreats:ATaxonomy byAlgurdisAvizienis,JeanClaudeLaprieandBrianRandell freelyavailableon-line)ishereusedtodescribetheabilityofasystemto deliveraservicethatcanbetrusted,servicebeingasystem’sbehaviourasperceivedbytheuser.OtherdefinitionsthathavebeenusedfordependabilityincludetheISOdefinitionasavailabilityperformanceanditsinfluencingfactors,namelyreliabilityperformance,maintainability,performanceandmaintenancesupport performance.AnIECdefinitioninvolvestheextenttowhichthesystemcanbereliedupontoperformexclusivelyandcorrectlythesystemtasksunderdefinedoperationalandenvironmentalconditionsoveradefined periodoftimeoratagiventime.
Dependabilityencompassesthefollowingattributes:
• Availability,i.e.readinessforcorrectservice;
• Reliability,i.e.theabilitytocontinuewithcorrectservice;
• Safety,i.e.theabilitytodeliveraservicewhichissafetotheuserandtheenvironment;
• Maintainability,i.e.theabilitytoundergorepairssuchastheremovaloffaultycomponents,preventive maintenanceandmodifications;and
• Integrity,i.e.theabsenceofimpropersystemalterations.
FIGURE1.20 Oscillationsofameterreading.
Thedependabilityspecificationforasystemneedstoincludetherequirementsfortheaboveattributesin termsoftheacceptablefrequencyandseverityoffailuresforthespecifieduseenvironment.
Ingeneral,themeanstoattaindependabilityinclude:
• Faultprevention,i.e.theabilitytopreventtheoccurrenceorintroductionoffaults;
• Faulttolerance,i.e.themeanstoavoidservicefailuresinthepresenceoffaults;
• Faultremoval,i.e.themeanstoreducethenumberandseverityoffaults;and
• Faultforecasting,i.e.themeanstoestimatethefutureoccurrenceandconsequencesoffaults.
Faultpreventionandfaulttoleranceaimsinvolvethegivingtothesystemoftheabilitytodeliveraservice thatcanbetrustedwhilefaultremovalandfaultforecastingaimtogiveconfidenceinthatabilityandthatthe dependabilityspecificationsareadequateandthesystemislikelytomeetthem.Faultscanariseduringthe developmentofthesystemorduringitsoperationandmaybeinternalfaultswithinthesystemorresultfrom faultsexternaltothesystemwhichpropagateserrorsintothesystem.Faultsmayoriginateinthehardwareof thesystemorbefaultsthataffectsoftwareusedwiththesystem.Thecauseofafaultmaybearesultofhuman actions,possiblymaliciousorsimplyomissionssuchaswrongsettingofparameters.Maliciousactionscanbe designedtodisruptserviceoraccessconfidentialinformationandinvolvesuchelementsasaTrojanhorseor virus.Thepaperreferredtoearlier,i.e. DependabilityandItsthreats:ATaxonomy,givesaclassificationoffaults thatcanoccuras:
• Thephaseofsystemlifeduringwhichfaultsoccurduringthedevelopmentofthesystem,duringmaintenance whenitisinuse,andproceduresusedtooperateormaintainthesystem;
• Thelocationoffaults:internaltothesystemorexternal;
• Thephenomenologicalcauseofthefaults:naturalfaultsthatnaturallyoccurwithouthumanintervention,and human-madefaultsasaresultofhumanactions;
• Thedimensioninwhichfaultsoccurinhardwareorsoftware;
• Howthefaultswereintroduced:maliciousornon-malicious;
• Theintentofthehumanorhumanswhointroducedthefaults:deliberateornon-deliberate;
• Howthehumanintroducedthefaults:accidentalorincompetence;and
• Thepersistenceofthefaults:permanentortransient.
Maintainabilityforasysteminvolvesbothcorrectivemaintenancewithrepairsfortheremovaloffaultsand preventativemaintenanceinwhichrepairsarecarriedoutinanticipationoffailures.Maintenancealsoinvolves adjustmentsinresponsetoenvironmentalchangesandaugmentationofthesystem’sfunction.
1.4.1Reliability
Ifyoutossacointentimesyoumightfind,forexample,thatitlandsheadsuppermostsixtimesoutofthe ten.If,however,youtossthecoinforaverylargenumberoftimesthenitislikelythatitwilllandheadsuppermosthalfofthetimes.Theprobabilityofitlandingheadsuppermostissaidtobehalf.The probability ofaparticulareventoccurringisdefinedasbeing
Probability 5
Numberofoccurencesoftheevent
Totalnumberoftrials
Whenthetotalnumberoftrialsisverylarge.Theprobabilityofthecoinlandingwitheitheraheadsortails uppermostislikelytobe1,sinceeverytimethecoinistossedthiseventwilloccur.Aprobabilityof1meansa certaintythattheeventwilltakeplaceeverytime.Theprobabilityofthecoinlandingstandingonedgecanbe consideredtobezero,sincethenumberofoccurrencesofsuchaneventiszero.Theclosertheprobabilityisto 1themorefrequentaneventwilloccur;thecloseritistozerothelessfrequentitwilloccur.
Reliabilityisanimportantrequirementofameasurementsystem.The reliability ofameasurementsystem,or elementinsuchasystem,isdefinedasbeingtheprobabilitythatitwilloperatetoanagreedlevelofperformance,foraspecifiedperiod,subjecttospecifiedenvironmentalconditions.Theagreedlevelofperformance mightbethatthemeasurementsystemgivesaparticularaccuracy.Thereliabilityofameasurementsystemis likelytochangewithtimeasaresultofperhapsspringsslowlystretchingwithtime,resistancevalueschanging
asaresultofmoistureabsorption,wearoncontactsandgeneraldamageduetoenvironmentalconditions.For example,justafterameasurementsystemhasbeencalibrated,thereliabilityshouldbe1.However,afterperhaps 6monthsthereliabilitymighthavedroppedto0.7.Thusthesystemcannotthenbereliedontoalwaysgivethe requiredaccuracyofmeasurement,ittypicallyonlygivestherequiredaccuracyseventimesintenmeasurements,seventytimesinahundredmeasurements.
Ahighreliabilitysystemwillhavealowfailurerate. Failurerate isthenumberoftimesduringsomeperiodof timethatthesystemfailstomeettherequiredlevelofperformance,i.e.:
Numberoffailures
Failurerate 5
Numberofsystemsobserved 3 Timeobserved
Afailurerateof0.4peryearmeansthatinoneyear,iftensystemsareobserved,4willfailtomeetthe requiredlevelofperformance.If100systemsareobserved,40willfailtomeettherequiredlevelofperformance. Failurerateisaffectedbyenvironmentalconditions.Forexample,thefailurerateforatemperaturemeasurement systemusedinhot,dusty,humid,corrosiveconditionsmightbe1.2peryear,whileforthesamesystemusedin dry,cool,non-corrosiveenvironmentitmightbe0.3peryear.
Failureratesaregenerallyquantifiedbygivingthe meantimebetweenfailures (MTBF).Thisisastatistical representationofthereliabilityin thatwhileitdoesnotgivethetimetofailureforaparticularexampleof thesystemitdoesrepresentthetimetofailurewhen thetimesforalotoftheexamplesofthatsystemare considered.
Withameasurementsystemconsistingofanumberofelements,failureoccurswhenjustoneoftheelements failstoreachtherequiredperformance.Thusinasystemforthemeasurementofthetemperatureofafluidin someplantwemighthaveathermocouple,anamplifier,andameter.Thefailurerateislikelytobehighestfor thethermocouplesincethatistheelementincontactwiththefluidwhiletheotherelementsarelikelytobein thecontrolledatmosphereofacontrolroom.Thereliabilityofthesystemmightthusbemarkedlyimprovedby choosingmaterialsforthethermocouplewhichresistattackbythefluid.Thusitmightbeinastainlesssteel sheathtopreventfluidcomingintodirectcontactwiththethermocouplewires.
EXAMPLE
ThefailurerateforapressuremeasurementsystemusedinfactoryAisfoundtobe1.0peryearwhilethesystemused infactoryBis3.0peryear.Whichfactoryhasthemostreliablepressuremeasurementsystem?
Thehigherthereliabilitythelowerthefailurerate.ThusfactoryAhasthemorereliablesystem.Thefailurerateof 1.0peryearmeansthatif100instrumentsarecheckedoveraperiodofayear,100failureswillbefound,i.e.onaverage eachinstrumentisfailingonce.Thefailurerateof3.0meansthatif100instrumentsarecheckedoveraperiodofayear, 300failureswillbefound,i.e.instrumentsarefailingmorethanonceintheyear.
1.5REQUIREMENTS
Themainrequirementofameasurementsystemis fitnessforpurpose.Thismeansthatif,forexample,alength ofaproducthastobemeasuredtoacertainaccuracythatthemeasurementsystemisabletobeusedtocarry outsuchameasurementtothataccuracy.Forexample,alengthmeasurementsystemmightbequotedashaving anaccuracyof 6 1mm.Thiswouldmeanthatallthelengthvaluesitgivesareonlyguaranteedtothisaccuracy, e.g.forameasurementwhichgavealengthof120mmtheactualvaluecouldonlybeguaranteedtobebetween 119and121mm.Iftherequirementisthatthelengthcanbemeasuredtoanaccuracyof 6 1mmthenthesystem isfitforthatpurpose.If,however,thecriterionisforasystemwithanaccuracyof 6 0.5mmthenthesystemis notfitforthatpurpose.
Inordertodelivertherequiredaccuracy,themeasurementsystemmusthavebeencalibratedtogivethataccuracy. Calibration istheprocessofcomparingtheoutputofameasurementsystemagainststandardsofknownaccuracy.Thestandardsmaybeothermeasurementsystemswhicharekeptspeciallyforcalibrationdutiesorsome meansofdefiningstandardvalues.Inmanycompaniessomeinstrumentsanditemssuchasstandardresistorsand cellsarekeptinacompanystandardsdepartmentandusedsolelyforcalibrationpurposes.
1.5.1Calibration
Calibration shouldbecarriedoutusingequipmentwhichcanbetraceablebacktonationalstandardswitha separatecalibrationrecordkeptforeachmeasurementinstrument.Thisrecordislikelytocontainadescription oftheinstrumentanditsreferencenumber,thecalibrationdate,thecalibrationresults,howfrequentlytheinstrumentistobecalibrated,andprobablydetailsofthecalibrationproceduretobeused,detailsofanyrepairsor modificationsmadetotheinstrument,andanylimitationsonitsuse.
The nationalstandards aredefinedbyinternationalagreementandaremaintainedbynationalestablishments, e.g.theNationalPhysicalLaboratoryinGreatBritainandtheNationalBureauofStandardsintheUnitedStates. Therearesevensuch primarystandards,andtwo supplementary ones,theprimaryonesbeing:
1. Mass
ThekilogramisdefinedbysettingPlanck’sconstant h toexactly662607015 3 10 34 Jsgiventhedefinitions ofthemetreandsecond.Then1kgis h/(662607015 3 10 34).
2. Length
Thelengthstandard,themetre,isdefinedasthedistancetravelledbylightinavacuumin1/(299792458) second.
3. Time
Thetimestandard,thesecond,isdefinedasatimedurationof9192631770periodsofoscillationofthe radiationemittedbythecaesium-133atomunderpreciselydefinedconditionsofresonance.
4. Current
Thecurrentstandard,theampere,isdefinedastheflowof1/(602176634 3 10 19)timestheelementary charge e persecond.
5. Temperature
Thekelvin(K)istheunitoftemperatureandisdefinedbysettingthenumericalvalueoftheBoltzmann constant k tobe1380649 3 10 23 J/Kgiventhedefinitionsofthekilogram,metreandsecond.
6. Luminousintensity
Thecandelaisdefinedastheluminousintensity,inagivendirection,ofaspecifiedsourcethatemits monochromaticradiationoffrequency540 3 1012 Hzandthathasaradiantintensityof1/683wattperunit steradian(aunitsolidangle,seelater).
7. Amountofsubstance
Themoleisdefinedastheamountofsubstanceofexactly602214076 3 1023 elementaryentities.
The supplementarystandards are:
1. Planeangle
Theradianistheplaneanglebetweentworadiiofacirclewhichcutsoffonthecircumferenceanarcwith alengthequaltotheradius(Figure1.21).
2. Solidangle
Thesteradianisthesolidangleofaconewhich,havingitsvertexinthecentreofthesphere,cutsoffan areaofthesurfaceofthesphereequaltothesquareoftheradius(Figure1.22).
Primarystandardsareusedtodefinenationalstandards,notonlyintheprimaryquantitiesbutalsoinother quantitieswhichcanbederivedfromthem.Forexample,aresistancestandardofacoilofmanganinwireis definedintermsoftheprimaryquantitiesoflength,mass,time,andcurrent.Typicallythesenationalstandards
inturnareusedtodefinereferencestandardswhichcanbeusedbynationalbodiesforthecalibrationof standardswhichareheldincalibrationcentres.
Theequipmentusedinthecalibrationofaninstrumentineverydaycompanyuseislikelytobe traceable back tonationalstandardsinthefollowingway:
1. Nationalstandardsareusedtocalibratestandardsforcalibrationcentres.
2. Calibrationcentrestandardsareusedtocalibratestandardsforinstrumentmanufacturers.
3. Standardisedinstrumentsfrominstrumentmanufacturersareusedtoprovidein-companystandards.
4. In-companystandardsareusedtocalibrateprocessinstruments.
Thereisasimpletraceabilitychainfromtheinstrumentusedinaprocessbacktonationalstandards (Figure1.23).Inthecaseof,say,aglassbulbthermometer,thetraceabilitymightbe:
1. Nationalstandardoffixedthermodynamictemperaturepoints.
2. Calibrationcentrestandardofaplatinumresistancethermometerwithanaccuracyof 6 0.005 C.
3. Anin-companystandardofaplatinumresistancethermometerwithanaccuracyof 6 0.01 C.
4. Theprocessinstrumentofaglassbulbthermometerwithanaccuracyof 6 0.1 C.
1.5.2SafetySystems
Statutorysafetyregulationslaydowntheresponsibilitiesofemployersandemployeesforsafetyinthe workplace.Theseincludeforemployersthedutyto:
• Ensurethatprocessplantisoperatedandmaintainedinasafewaysothatthehealthandsafetyofemployees isprotected.
• Provideamonitoringandshutdownsystemforprocessesthatmightresultinhazardousconditions.
Employeesalsohavedutiesto:
• Takereasonablecareoftheirownsafetyandthesafetyofothers.
• Avoidmisusingordamagingequipmentthatisdesignedtoprotectpeople’ssafety. Thus,inthedesignofmeasurementsystems,dueregardhastobepaidtosafetybothintheirinstallationand operation.Thus:
• Thefailureofanysinglecomponentinasystemshouldnotcreateadangeroussituation.
• Afailurewhichresultsincableopenorshortcircuitsorshortcircuitingtogroundshouldnotcreatea dangeroussituation.
FIGURE1.23 Traceabilitychain.
• Foreseeablemodesoffailureshouldbeconsideredforfail-safedesignsothat,intheeventoffailure,the systemperhapsswitchesoffintoasafecondition.
• Systemsshouldbeeasilycheckedandreadilyunderstood.
Themainrisksfromelectricalinstrumentationareelectrocutionandthepossibilityofcausingafireorexplosion asaconsequenceofperhapscablesorcomponentsoverheatingorarcingsparksoccurringinanexplosiveatmosphere.Thusitisnecessarytoensurethatanindividualcannotbecomeconnectedbetweentwopointswithapotentialdifferencegreaterthanabout30Vandthisrequiresthecarefuldesignofearthingsothatthereisalwaysan adequateearthingreturnpathtooperateanyprotectivedeviceintheeventofafaultoccurring.
PROBLEMS
Questions1to5havefouransweroptions:A.B,C,andD Choosethecorrectanswerfromtheansweroptions
1. DecidewhethereachofthesestatementsisTrue(T)orFalse(F).
Sensorsinameasurementsystemhave:
i. Aninputofthevariablebeingmeasured.
ii. Anoutputofasignalinaformsuitableforfurtherprocessinginthemeasurementsystem.
WhichoptionBESTdescribesthetwostatements?
A. (i)T(ii)T
B. (i)T(ii)F
C. (i)F(ii)T
D. (i)F(ii)F
2. Thesignalconditionerelementinameasurementsystem:
A. Givesanoutputsignaldependentonthetemperature.
B. Changesthetemperaturesignaltoacurrentsignal.
C. Takestheoutputfromthesensorandmakesitbigger.
D. Givesanoutputdisplay.
3. DecidewhethereachofthesestatementsisTrue(T)orFalse(F). Thediscrepancybetweenthemeasuredvalueofthecurrentinanelectricalcircuitandthevaluebeforethe measurementsystem,anammeter,wasinsertedinthecircuitisbiggerthelarger: i. Theresistanceofthemeter.
ii. Theresistanceofthecircuit.
WhichoptionBESTdescribesthetwostatements?
A. (i)T(ii)T
B. (i)T(ii)F
C. (i)F(ii)T
D. (i)F(ii)F
4. DecidewhethereachofthesestatementsisTrue(T)orFalse(F). Ahighlyreliablemeasurementsystemisonewherethereisahighchancethatthesystemwill: i. Haveahighmeantimebetweenfailures.
ii. Haveahighprobabilityoffailure.
WhichoptionBESTdescribesthetwostatements?
A. (i)T(ii)T
B. (i)T(ii)F
C. (i)F(ii)T
D. (i)F(ii)F
5. DecidewhethereachofthesestatementsisTrue(T)orFalse(F). Ameasurementsystemwhichhasalackofrepeatabilityisonewheretherecouldbe: i. Randomfluctuationsinthevaluesgivenbyrepeatedmeasurementsofthesamevariable. ii. Fluctuationsinthevaluesobtainedbyrepeatingmeasurementsoveranumberofsamples. WhichoptionBESTdescribesthetwostatements?
A. (i)T(ii)T
B. (i)T(ii)F
C. (i)F(ii)T
D. (i)F(ii)F
6. Listandexplainthefunctionalelementsofameasurementsystem.
7. Explaintheterms(a)reliabilityand(b)repeatabilitywhenappliedtoameasurementsystem.
8. Explainwhatismeantbycalibrationstandardshavingtobetraceabletonationalstandards.
9. Explainwhatismeantby‘fitnessforpurpose’whenappliedtoameasurementsystem.
10. Thereliabilityofameasurementsystemissaidtobe0.6.Whatdoesthismean?
11. Themeasurementinstrumentsusedinthetoolroomofacompanyarefoundtohaveafailurerateof0.01per year.Whatdoesthismean?
12. Determinethesensitivityoftheinstrumentsthatgavethefollowingreadings: (a)
Loadkg0 2468
Deflectionmm018365472
(b)
Temperature C010203040
VoltagemV00.591.191.802.42
(c)
LoadN 01234
ChargepC 036912
13. Calibrationofavoltmetergavethefollowingdata.Determinethemaximumhysteresiserrorasapercentage ofthefull-scalerange.
Increasinginput:
StandardmV01.02.03.04.0
VoltmetermV01.01.92.94.0
Decreasinginput:
StandardmV4.03.02.01.00
VoltmetermV4.03.02.11.10
InstrumentationSystemElements
OUTLINE
2.1Introduction18
2.2DisplacementSensors18
2.2.1Potentiometer18
2.2.2Strain-GaugedElement19
2.2.3CapacitiveElement20
2.2.4LinearVariableDifferentialTransformer21
2.2.5OpticalEncoders21
2.2.6Moire ´ Fringes22
2.2.7OpticalProximitySensors23
2.2.8MechanicalSwitches24
2.2.9CapacitiveProximitySensor25
2.2.10InductivityProximitySensor26
2.3SpeedSensors26
2.3.1OpticalMethods26
2.3.2IncrementalEncoder26
2.3.3Tachogenerator26
2.4FluidPressureSensors26
2.4.1DiaphragmSensor27
2.4.2PiezoelectricSensor28
2.4.3BourdonTube28
2.5FluidFlow29
2.5.1DifferentialPressureMethods29
2.5.2TurbineMeter31
2.5.3UltrasonicTimeofFlightFlowMeter32
2.5.4VortexFlowRateMethod32
2.5.5CoriolisFlowMeter33
2.6LiquidLevel33
2.6.1Floats34
2.6.2DisplacerGauge34
2.6.3DifferentialPressure34
2.6.4LoadCell34
2.6.5ElectricalConductivityLevelIndicator35
2.6.6CapacitiveLevelIndicator35
2.6.7UltrasonicLevelGauge35
2.6.8NucleonicLevelIndicators36
2.7TemperatureSensors36
2.7.1BimetallicStrips36
2.7.2LiquidinGlassThermometers36
2.7.3ResistanceTemperatureDetectors(RTDs)36
2.7.4Thermistors37
2.7.5Thermocouples37
2.7.6ThermodiodesandTransistors40
2.7.7Pyrometers40
2.8SensorSelection41
2.9SignalProcessing42
2.9.1ResistancetoVoltageConverter42
2.9.2TemperatureCompensation45
2.9.3ThermocoupleCompensation45
2.9.4Protection47
2.9.5Analogue-to-DigitalConversions47
2.9.6Digital-to-AnalogueConversions50
2.9.7MicrocontrollerSystems51
2.9.8Op-Amps52
2.9.9Pressure-to-CurrentConverter56
2.10SignalTransmission56 2.10.1Noise59
2.11SmartSystems60 2.11.1MEMS61
2.12DataPresentationElement62
2.12.1Indicator62
2.12.2IlluminativeDisplays62
2.12.3GraphicalUserInterface(GUI)64
2.12.4DataLoggers64
2.12.5Printers65 Problems66
2.1INTRODUCTION
Thischapterdiscussesthesensors,signalprocessors,anddatapresentationelementscommonlyusedinengineering.Theterm sensor isusedforanelementwhichproducesasignalrelatingtothequantitybeingmeasured. Theterm signalprocessor isusedfortheelementthattakestheoutputfromthesensorandconvertsitintoaform whichissuitablefordatapresentation. Datapresentation iswherethedataisdisplayed,recorded,ortransmitted tosomecontrolsystem.
2.2DISPLACEMENTSENSORS
Adisplacementsensorishereconsideredtobeonethatcanbeusedto:
1. Measurealineardisplacement,i.e.achangeinlinearposition.Thismight,forexample,bethechangein lineardisplacementofasensorasaresultofachangeinthethicknessofsheetmetalemergingfromrollers.
2. Measureanangulardisplacement,i.e.achangeinangularposition.Thismight,forexample,bethechange inangulardisplacementofadriveshaft.
3. Detectmotion,e.g.thismightbeaspartofanalarmorautomaticlightsystem,wherebyanalarmis soundedoralightswitchedonwhenthereissomemovementofanobjectwithinthe‘view’ofthesensor.
4. Detectthepresenceofsomeobject,i.e.aproximitysensor.Thismightbeinanautomaticmachiningsystem whereatoolisactivatedwhenthepresenceofaworkpieceissensedasbeinginposition.
Displacementsensorsfallintotwogroups:thosethatmakedirectcontactwiththeobjectbeingmonitored,by springloadingormechanicalconnectionwiththeobject,andthosewhicharenon-contacting.Forthoselinear displacementmethodsinvolvingcontact,thereisusuallyasensingshaftwhichisindirectcontactwiththeobject beingmonitored,thedisplacementofthisshaftisthenbeingmonitoredbyasensor.Thisshaftmovementmay beusedtocausechangesinelectricalvoltage,resistance,capacitance,ormutualinductance.Forangulardisplacementmethodsinvolvingmechanicalconnection,therotationofashaftmightdirectlydrive,throughgears,the rotationofthesensorelement,thisperhapsgeneratingane.m.f.Non-contactingproximitysensorsmightconsist ofabeamofinfraredlightbeingbrokenbythepresenceoftheobjectbeingmonitored,thesensorthengivinga voltagesignalindicatingthebreakingofthebeam,orperhapsthebeambeingreflectedfromtheobjectbeing monitored,thesensorgivingavoltageindicatingthatthereflectedbeamhasbeendetected.Contactingproximity sensorsmightbejustmechanicalswitcheswhicharetrippedbythepresenceoftheobject,thetermlimitswitch beingused.Thefollowingareexamplesofdisplacementsensors.
2.2.1Potentiometer
A potentiometer consistsofaresistanceelementwithaslidingcontactwhichcanbemovedoverthelengthof theelementandconnectedasshownin Figure2.1.Withaconstantsupplyvoltage Vs,theoutputvoltage Vo betweenterminals1and2isafractionoftheinputvoltage,thefractiondependingontheratiooftheresistance R12 betweenterminals1and2comparedwiththetotalresistance R oftheentirelengthofthetrackacrosswhich thesupplyvoltageisconnected.Thus Vo/Vs 5 R12/R.Ifthetrackhasaconstantresistanceperunitlength,the outputisproportionaltothedisplacementofthesliderfromposition1.Arotarypotentiometerconsistsofacoil
FIGURE2.1 Potentiometer.
ofwirewrappedroundintoacirculartrack,oracircularfilmofconductiveplasticoraceramic metalmix termedacermet,overwhicharotatableslidingcontactcanberotated.Henceanangulardisplacementcanbe convertedintoapotentialdifference.Lineartrackscanbeusedforlineardisplacements.
Withawire-woundtracktheoutputvoltagedoesnotcontinuouslyvaryasthesliderismovedoverthetrack butgoesinsmalljumpsastheslidermovesfromoneturnofwiretothenext.Thisproblemdoesnotoccurwith aconductiveplasticorthecermettrack.Thus,thesmallestchangeindisplacementwhichwillgiverisetoa changeinoutput,i.e.theresolution,tendstobemuchsmallerforplastictracksthanwire-woundtracks.Errors duetonon-linearityofthetrackforwiretrackstendtorangefromlessthan0.1%toabout1%ofthefullrange outputandforconductiveplasticscanbeaslowasabout0.05%.Thetrackresistanceforwire-woundpotentiometerstendstorangefromabout20 Ω to200kΩ andforconductiveplasticfromabout500 Ω to80kΩ. Conductiveplastichasahighertemperaturecoefficientofresistancethanwireandsotemperaturechangeshave agreatereffectonaccuracy.Theresolutionofsuchasensordependsonitsconstruction.Ifitisawire-wound coilwitharotatableslidingcontactthenthefinerthewirethehighertheresolution.Thusasensorwith25turns permmwouldhavearesolutionof 6 40 μm.Suchasensorhasafastresponsetimeandalowcost.
Thefollowingisanexampleofpartofthespecificationofacommerciallyavailabledisplacementsensorusing aplasticconductingpotentiometertrack:
Rangesfrom0to10mmto0to2m
Non-linearityerror 6 0.1%offullrange
Resolution 6 0.02%offullrange
Temperaturesensitivity 6 120partspermillion/ C
Resolution 6 0.02%offullrange
Anapplicationofapotentiometeristosensethepositionoftheacceleratorpositioninanautomobileandfeed theinformationtotheenginecontrolsystem.Anotherpotentiometermightbeusedasthethrottleposition sensor.
2.2.2Strain-GaugedElement
Straingauges consistofametalfoilstrip(Figure2.2A),flatlengthofmetalwire(Figure2.2B),orastripofsemiconductormaterialwhichcanbestuckontosurfaceslikeapostagestamp.Whenthewire,foil,strip,orsemiconductorisstretched,itsresistance R changes.Thefractionalchangeinresistance ΔR/R isproportionaltothe strain ε,i.e.:
where G,theconstantofproportionality,istermedthe gaugefactor.
Metalstraingaugestypicallyhavegaugefactorsoftheorderof2.0.Whensuchastraingaugeisstretchedits resistanceincreases,andwhencompresseditsresistancedecreases.Strainis‘changeinlength/originallength’ andsotheresistancechangeofastraingaugeisameasurementofthechangeinlengthofthegaugeandhence thesurfacetowhichthestraingaugeisattached.Thusadisplacementsensormightbeconstructedbyattaching straingaugestoacantilever(Figure2.3),thefreeendofthecantileverbeingmovedasaresultofthelinear
FIGURE2.2 Straingauges.
FIGURE2.3 Strain-gaugedcantilever.
Another random document with no related content on Scribd:
