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ControlSystemDesignGuideThirdEditionUsingYour ComputertoUnderstandandDiagnoseFeedback ControllersGeorgeEllisDigitalInstantDownload
Author(s):GeorgeEllis
ISBN(s):9780122374616,0122374614
Edition:3
FileDetails:PDF,14.46MB
Year:2004
Language:english
ImportantSafetyGuidelinesforReaders
Thisbookdiscussesthenormaloperation,commissioning,andtroubleshootingof controlsystems.Operationofindustrialcontrollerscanproducehazards,suchas:
. Largeamountsofheat
. High-voltagepotentials
. Movementofobjectsormechanismsthatcancauseharm
. Flowofharmfulchemicals
. Flames
. Explosionsorimplosions
Unsafeoperationmakesitmorelikelyforaccidentstooccur.Accidentscancause personalinjurytoyou,yourcoworkers,andotherpeople.Accidentscanalsodamage ordestroyequipment.Byoperatingcontrolsystemssafely,youmakeitlesslikelythat anaccidentwilloccur. Alwaysoperatecontrolsystemssafely!
Youcanenhancethesafetyofcontrolsystemoperationbytakingthefollowing steps:
1.Allowonlypeopletrainedinsafety-relatedworkpracticesandlock-out/tag-out procedurestoinstall,commission,orperformmaintenanceoncontrolsystems.
2.Alwaysfollowmanufacturer-recommendedprocedures.
3.Alwaysfollownational,state,local,andprofessionalsafetycoderegulations.
4.Alwaysfollowthesafetyguidelinesinstitutedattheplantwheretheequipment willbeoperated.
5.Alwaysuseappropriatesafetyequipment,forexample,protectiveeyewear, hearingprotection,safetyshoes,andotherprotectiveclothing.
6.Neverattempttooverridesafetydevicessuchaslimitswitches,emergencystop switches,lightcurtains,orphysicalbarriers.
7.Alwayskeepclearfrommachinesorprocessesinoperation.
8.Providereliableprotection,suchasmechanicalstopsandemergencyoff switches,sothatunanticipatedbehaviorfromthecontrollercannotharmyou oranyoneelseandcannotdamageequipment.
Rememberthatanychangeofsystemparameters(forexample,tuninggains), components,wiring,oranyotherfunctionofthecontrolsystemmaycauseunexpected results,suchassysteminstabilityoruncontrolledsystemexcitation.
Rememberthatcontrollersandothercontrolsystemcomponentsaresubjectto failure.Forexample,amicroprocessorinacontrollermayexperiencecatastrophic failureatanytime.Leadstoorwithinfeedbackdevicesmayopenorshorttogetherat anytime.Failureofacontrollermaycauseunexpectedresults,suchassystem instabilityoruncontrolledsystemexcitation.
Thisbookpresentsobservers,theuseofwhichwithincontrolsystemsposescertain risks,includingthattheobservermaybecomeunstableorotherwisefailtoobserve signalstoanaccuracynecessaryforpropersystemoperation.Ensurethatobservers behaveproperlyinalloperatingconditions.
Ifyouhaveanyquestionsconcerningthesafeoperationofequipment,contactthe equipmentmanufacturer,plantsafetypersonnel,orlocalgovernmentalofficials,such astheOccupationalHealthandSafetyAdministration. Alwaysoperatecontrolsystemssafely!
IntroductiontoControls
Controltheoryisusedforanalysisanddesignoffeedbacksystems,suchasthose thatregulatetemperature,fluidflow,motion,force,voltage,pressure,tension,and current.Skillfullyused,controltheorycanguideengineersineveryphaseofthe productandprocessdesigncycle.Itcanhelpengineerspredictperformance,anticipate problems,andprovidesolutions.
Collegesteachcontrolswithlittleemphasisonday-to-dayproblems.Theacademic communityfocusesonmathematicalderivationsandonthedevelopmentofadvanced controlschemes;itoftenneglectsthemethodsthatarecommonlyappliedinindustry. Studentscancompleteengineeringprogramsthatincludecoursesoncontrolsandstill remainuntutoredonhowtodesign,model,build,tune,andtroubleshootabasic controlsystem.Theunfortunateresultisthatmanyworkingengineerslayaside analysiswhentheypracticetheirprofession,relyinginsteadoncompanyhistoryand trial-and-errormethods.
Thisbookavoidsthematerialandorganizationofmostcontroltheorytextbooks. Forexample,designguidelinesarepresentedthroughout;theseguidelinesarea combinationofindustry-acceptedpracticesandwarningsagainstcommonpitfalls. Nontraditionalsubjects,suchasfiltersandmodeling,arepresentedherebecausethey areessentialtounderstandingandimplementingcontrolsystemsintheworkplace. Thefocusofeachchapteristoteachhowtousecontrolstoimproveaworking machineorprocess.
Thewideavailabilityofpersonalcomputersandworkstationsisanimportant advanceforcontrolsystemdesigners.Manyoftheclassicalcontrolmethods,suchas therootlocusmethod,aregraphicalratherthananalytical.Theircreatorssoughtto avoidwhatwasthentheoverwhelmingnumberofcomputationsrequiredforanalyticalmethods.Fortunately,thesecalculationsnolongerpresentabarrier.Virtually everypersonalcomputercanexecutethecalculationsrequiredbyanalyticalmethods. Withthisinmind,theprinciplesandmethodspresentedhereinareessentiallyanalytical,andthearithmeticismeanttobecarriedoutbyacomputer.
1.1 VisualModelQ SimulationEnvironment
Mostengineersunderstandthefoundationsofcontroltheory.Conceptssuchas transferfunctions,blockdiagrams,the s-domain,andBodeplotsarefamiliartomost ofus.Buthowshouldworkingengineersapplytheseconcepts?Asinmostdisciplines, theymustdevelopintuition,andthisrequiresfluencyinthebasics.Inordertobe fluent,youmustpractice.
Whenstudyingcontrolsystemtechniques,findingequipmenttopracticeonisoften difficult.Asaresult,designersoftenrelyoncomputersimulations.Tothisend,the authordeveloped,asacompaniontothisbook, VisualModelQ,astand-alone, graphical,PC-basedsimulationenvironment.Theenvironmentprovidestime-domain andfrequency-domainanalysisofanaloganddigitalcontrolsystems. VisualModelQ isanenhancementoftheoriginal ModelQ,inthat VisualModelQ allowsreadersto viewandbuildmodelsgraphically.Dozensof VisualModelQ modelsweredeveloped forthisbook.Thesemodelsareusedextensivelyinthechaptersthatfollow.Readers canruntheseexperimentstoverifyresultsandthenmodifyparametersandother conditionstoexperimentwiththeconceptsofcontrolsystems.
VisualModelQ iswrittentoteachcontroltheory.Itmakesconvenientthose activitiesthatarenecessaryforstudyingcontrols.Controllawgainsareeasyto change.Plotsoffrequency-domainresponse(Bodeplots)arerunwiththepressofa button.Themodelsin VisualModelQ runcontinuously,justasreal-timecontrollers do.Themeasurementequipmentrunsindependently,soyoucanchangeparameters andseetheeffectsimmediately.
1.1.1Installationof VisualModelQ
VisualModelQ isavailableatwww.qxdesign.com.Theunregisteredversionisavailablefreeofcharge.Althoughtheunregisteredversionlacksseveralfeatures,itcan executeallthemodelsusedinthisbook.Readersmayelecttoregistertheircopiesof VisualModelQ atanytime;seewww.qxdesign.comfordetails.
VisualModelQ runsonPCsusingWindows95,Windows98,Windows2000, WindowsNT,andWindowsXP.Downloadandruntheexecutablefile setup.exe for VisualModelQV6.0 orlater. VisualModelQ installswithbothaUser’sManualanda ReferenceManual.Afterinstallation,readtheUser’sManual.Notethatyoucan accesstheReferenceManualviaInternetExplorerbypressingtheF1key.Finally, checktheWebsitefromtimetotimeforupdatedsoftware.
1.1.2Errata
Checkwww.qxdesign.comforerrata.Itistheauthor’sintentiontoregularlyupdate theWebpageascorrectionsbecomeknown.
1.2TheControlSystem
Thegeneralcontrolsystem,asshowninFigure1-1,canbedividedintothecontroller andthemachine.Thecontrollercanbedividedintothecontrollawsandthepower converter.Themachinemaybeatemperaturebath,amotor,or,asinthecaseof apowersupply,aninductor/capacitorcircuit.Themachinecanalsobedividedinto twoparts:theplantandthefeedbackdevice(s).Theplantreceivestwotypesofsignals: acontrolleroutputfromthepowerconverterandoneormoredisturbances.Simply put,thegoalofthecontrolsystemistodrivetheplantinresponsetothecommand whileovercomingdisturbances.
1.2.1TheController
Thecontrollerincorporatesbothcontrollawsandpowerconversion.Controllaws, suchasproportional-integral-differential(PID)control,arefamiliartocontrolengineers.Theprocessoftuning—settinggainstoattaindesiredperformance—amounts toadjustingtheparametersofthecontrollaws.Mostcontrollersletdesignersadjust gains;themostflexiblecontrollersallowthedesignertomodifythecontrollaws themselves.Whentuning,mostcontrolengineersfocusonattainingaquick,stable commandresponse.However,insomeapplications,rejectingdisturbancesismore importantthanrespondingtocommands.Allcontrolsystemsshoulddemonstrate robustperformancebecauseevennearlyidenticalmachinesandprocessesvarysomewhatfromonetotheother,andtheychangeovertime.Robustoperationmeans controllawsmustbedesignedwithenoughmargintoaccommodatereasonable changesintheplantandpowerconverter.
Virtuallyallcontrollershavepowerconverters.Thecontrollawsproduceinformation,butpowermustbeappliedtocontroltheplant.Thepowerconvertercanbe drivenbyanyavailablepowersource,includingelectric,pneumatic,hydraulic,or chemicalpower.
1.2.2TheMachine
Themachineismadeoftwoparts:theplantandthefeedback.Theplantistheelement orelementsthatproducethesystemresponse.Plantsaregenerallypassive,andthey usuallydissipatepower.Examplesofplantsincludeaheatingelementandamotor coupledtoitsload.
Controlsystemsneedfeedbackbecausetheplantisrarelypredictableenoughtobe controlled openloop —thatis,withoutfeedback.Thisisbecausemostplantsintegrate thepowerconverteroutputtoproducethesystemresponse.Voltageisappliedto inductorstoproducecurrent;torqueisappliedtoinertiatoproducevelocity;pressure isappliedtoproducefluidflow.Inallthesecases,thecontrolsystemcannotcontrol theoutputvariabledirectlybutmustprovidepowertothemachineasphysicsallows andthenmonitorthefeedbacktoensurethattheplantisontrack.
1.3TheControlsEngineer
Thefocaltaskofmanycontrolsengineersissystemintegrationandcommissioning. Themostfamiliarpartofthisprocessistuningthecontrolloops.Thisprocesscanbe intimidating.Oftendozensofparametersmustbefine-tunedtoensurethatthesystem livesuptothespecification.Sometimesthatspecificationisentirelyformal,butmore oftenitisacombinationofformalrequirementsandknow-howgainedwithyearsof experience.Usuallyonlythemostseniorengineersinacompanyarecapableof judgingwhenasystemisperformingwellenough.
Forsomecontrolsystems,eachinstallationmayrequiredaysorweekstobe correctlycommissioned.Inacomplexmachinesuchasarollingmill,thatprocess cantakemonths.Eachpieceofthemachinemustbecarefullytunedatthesite.Soeven afterthedesignofthemachineiscomplete,theexpertiseofacontrolsengineeris requiredeachtimeaunitisinstalled.
Althoughmostcontrolsengineersfocusoninstallation,theirjobshouldbeginwhen themachineisdesigned.Manycompaniesfailtotakeadvantageoftheircontrols expertiseearlyinaproject;thisisshortsighted.Acontrolsengineermaysuggestan improvedfeedbackdeviceorenhancementstoamachinethatwillhelpovercomea stubbornproblem.Ideally,theprojectmanagerwillsolicitthisinputearly,because changesofthisnatureareoftendifficulttomakelater.
Thecontrolsengineershouldalsocontributetotheselectionofthecontroller. Therearemanycontrols-orientedfactorsthatshouldbetakenintoaccount.Does thecontrollerimplementfamiliarcontrollaws?Fordigitalcontrollers,istheprocessor fastenoughfortheneedsoftheapplication?Istheunitappropriateforthesupport teamandforthecustomerbase?Theselectionandspecificationofacontrolleroften involveinputfrommanypeople,butsomequestionscanbeansweredbestbyaskilled controlsengineer.
Whatistheroleforcontroltheoryinthedailytasksofcontrolsengineers?Atits root,controltheoryprovidesunderstandingand,withthat,intuition.Shouldthe
companypurchasethecontrollerthatrunsfourtimesfastereventhoughitcosts25% more?Shouldtheyinvestinmachinechanges,andwhatwillbetheexpectedimprovementfromthoseefforts?Howmuchwillthebetterfeedbackdevicehelpanoise problem?Understandingcontrolsdoesn’tguaranteethattheengineerwillhavethe correctanswer.Butafirmgrasponthepracticalsideofcontrolswillprovidecorrect answersmoreoftenandthuspositionthecontrolsengineertoprovideleadershipin processandproductdevelopmentandsupport.
