PID控制器毕业设计外文翻译.docx
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PID控制器毕业设计外文翻译.docx
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PID控制器毕业设计外文翻译
附录
一、英文原文
PIDcontroller
Aproportional–integral–derivativecontroller(PIDcontroller)isagenericcontrolloopfeedbackmechanism(controller)widelyusedinindustrialcontrolsystems–aPIDisthemostcommonlyusedfeedbackcontroller.APIDcontrollercalculatesan"error"valueasthedifferencebetweenameasuredprocessvariableandadesiredsetpoint.Thecontrollerattemptstominimizetheerrorbyadjustingtheprocesscontrolinputs.Intheabsenceofknowledgeoftheunderlyingprocess,PIDcontrollersarethebestcontrollers.[1]However,forbestperformance,thePIDparametersusedinthecalculationmustbetunedaccordingtothenatureofthesystem–whilethedesignisgeneric,theparametersdependonthespecificsystem.
ThePIDcontrollercalculation(algorithm)involvesthreeseparateparameters,andisaccordinglysometimescalledthree-termcontrol:
theproportional,theintegralandderivativevalues,denotedP,I,andD.Theproportionalvaluedeterminesthereactiontothecurrenterror,theintegralvaluedeterminesthereactionbasedonthesumofrecenterrors,andthederivativevaluedeterminesthereactionbasedontherateatwhichtheerrorhasbeenchanging.Theweightedsumofthesethreeactionsisusedtoadjusttheprocessviaacontrolelementsuchasthepositionofacontrolvalveorthepowersupplyofaheatingelement.Heuristically,thesevaluescanbeinterpretedintermsoftime:
Pdependsonthepresenterror,Iontheaccumulationofpasterrors,andDisapredictionoffutureerrors,basedoncurrentrateofchange.
BytuningthethreeconstantsinthePIDcontrolleralgorithm,thecontrollercanprovidecontrolactiondesignedforspecificprocessrequirements.Theresponseofthecontrollercanbedescribedintermsoftheresponsivenessofthecontrollertoanerror,thedegreetowhichthecontrollerovershootsthesetpointandthedegreeofsystemoscillation.NotethattheuseofthePIDalgorithmforcontroldoesnotguaranteeoptimalcontrolofthesystemorsystemstability.
Someapplicationsmayrequireusingonlyoneortwomodestoprovidetheappropriatesystemcontrol.Thisisachievedbysettingthegainofundesiredcontroloutputstozero.APIDcontrollerwillbecalledaPI,PD,PorIcontrollerintheabsenceoftherespectivecontrolactions.PIcontrollersarefairlycommon,sincederivativeactionissensitivetomeasurementnoise,whereastheabsenceofanintegralvaluemaypreventthesystemfromreachingitstargetvalueduetothecontrolaction.
Note:
Duetothediversityofthefieldofcontroltheoryandapplication,manynamingconventionsfortherelevantvariablesareincommonuse.
Controlloopbasics
Afamiliarexampleofacontrolloopistheactiontakenwhenadjustinghotandcoldfaucetvalvestomaintainthefaucetwateratthedesiredtemperature.Thistypicallyinvolvesthemixingoftwoprocessstreams,thehotandcoldwater.Thepersontouchesthewatertosenseormeasureitstemperature.Basedonthisfeedbacktheyperformacontrolactiontoadjustthehotandcoldwatervalvesuntiltheprocesstemperaturestabilizesatthedesiredvalue.
Sensingwatertemperatureisanalogoustotakingameasurementoftheprocessvalueorprocessvariable(PV).Thedesiredtemperatureiscalledthesetpoint(SP).Theinputtotheprocess(thewatervalveposition)iscalledthemanipulatedvariable(MV).Thedifferencebetweenthetemperaturemeasurementandthesetpointistheerror(e),thatquantifieswhetherthewateristoohotortoocoldandbyhowmuch.
Aftermeasuringthetemperature(PV),andthencalculatingtheerror,thecontrollerdecideswhentochangethetapposition(MV)andbyhowmuch.Whenthecontrollerfirstturnsthevalveon,theymayturnthehotvalveonlyslightlyifwarmwaterisdesired,ortheymayopenthevalveallthewayifveryhotwaterisdesired.Thisisanexampleofasimpleproportionalcontrol.Intheeventthathotwaterdoesnotarrivequickly,thecontrollermaytrytospeed-uptheprocessbyopeningupthehotwatervalvemore-and-moreastimegoesby.Thisisanexampleofanintegralcontrol.Byusingonlytheproportionalandintegralcontrolmethods,itispossiblethatinsomesystemsthewatertemperaturemayoscillatebetweenhotandcold,becausethecontrollerisadjustingthevalvestooquicklyandover-compensatingorovershootingthesetpoint.
Intheinterestofachievingagradualconvergenceatthedesiredtemperature(SP),thecontrollermaywishtodamptheanticipatedfutureoscillations.Soinordertocompensateforthiseffect,thecontrollermayelecttotempertheiradjustments.Thiscanbethoughtofasaderivativecontrolmethod.
Makingachangethatistoolargewhentheerrorissmallisequivalenttoahighgaincontrollerandwillleadtoovershoot.Ifthecontrollerweretorepeatedlymakechangesthatweretoolargeandrepeatedlyovershootthetarget,theoutputwouldoscillatearoundthesetpointineitheraconstant,growing,ordecayingsinusoid.Iftheoscillationsincreasewithtimethenthesystemisunstable,whereasiftheydecreasethesystemisstable.Iftheoscillationsremainataconstantmagnitudethesystemismarginallystable.Ahumanwouldnotdothisbecauseweareadaptivecontrollers,learningfromtheprocesshistory;however,simplePIDcontrollersdonothavetheabilitytolearnandmustbesetupcorrectly.Selectingthecorrectgainsforeffectivecontrolisknownastuningthecontroller.
Ifacontrollerstartsfromastablestateatzeroerror(PV=SP),thenfurtherchangesbythecontrollerwillbeinresponsetochangesinothermeasuredorunmeasuredinputstotheprocessthatimpactontheprocess,andhenceonthePV.VariablesthatimpactontheprocessotherthantheMVareknownasdisturbances.Generallycontrollersareusedtorejectdisturbancesand/orimplementsetpointchanges.Changesinfeedwatertemperatureconstituteadisturbancetothefaucettemperaturecontrolprocess.
Intheory,acontrollercanbeusedtocontrolanyprocesswhichhasameasurableoutput(PV),aknownidealvalueforthatoutput(SP)andaninputtotheprocess(MV)thatwillaffecttherelevantPV.Controllersareusedinindustrytoregulatetemperature,pressure,flowrate,chemicalcomposition,speedandpracticallyeveryothervariableforwhichameasurementexists.Automobilecruisecontrolisanexampleofaprocesswhichutilizesautomatedcontrol.
PIDcontrollersarethecontrollersofchoiceformanyoftheseapplications,duetotheirwell-groundedtheory,establishedhistory,simplicity,andsimplesetupandmaintenancerequirements.
History
PIDcontrollersdateto1890sgovernordesign.[1][5]PIDcontrollersweresubsequentlydevelopedinautomaticshipsteering.OneoftheearliestexamplesofaPID-typecontrollerwasdevelopedbyElmerSperryin1911,[6]whilethefirstpublishedtheoreticalanalysisofaPIDcontrollerwasbyRussianAmericanengineerNicolasMinorsky,in(Minorsky1922).MinorskywasdesigningautomaticsteeringsystemsfortheUSNavy,andbasedhisanalysisonobservationsofahelmsman,observingthatthehelmsmancontrolledtheshipnotonlybasedonthecurrenterror,butalsoonpasterrorandcurrentrateofchange;[7]thiswasthenmademathematicalbyMinorsky.TheNavyultimatelydidnotadoptthesystem,duetoresistancebypersonnel.Similarworkwascarriedoutandpublishedbyseveralothersinthe1930s.
Initiallycontrollerswerepneumatic,hydraulic,ormechanical,withelectricalsystemslaterdeveloping,withwhollyelectricalsystemsdevelopedfollowingWorldWarII.
Minorsky'swork
Indetail,Minorsky'sworkproceededasfollows.[8]Hisgoalwasstability,notgeneralcontrol,whichsignificantlysimplifiedtheproblem.Whileproportionalcontrolprovidesstabilityagainstsmalldisturbances,itwasinsufficientfordealingwithasteadydisturbance,notablyastiffgale(duetodroop),whichrequiredaddingtheintegralterm.Finally,thederivativetermwasaddedtoimprovecontrol.TrialswerecarriedoutontheUSSNewMexico,withthecontrollercontrollingtheangularvelocity(notangle)oftherudder.PIcontrolyieldedsustainedyaw(angularerror)of±2°,whileaddingDyieldedyawof±1/6°,betterthanmosthelmsmencouldachieve.
LimitationsofPIDcontrol
WhilePIDcontrollersareapplicabletomanycontrolproblems,andoftenperformsatisfactorilywithoutanyimprovementsoreventuning,theycanperformpoorlyinsomeapplications,anddonotingeneralprovideoptimalcontrol.ThefundamentaldifficultywithPIDcontrolisthatitisafeedbacksystem,withconstantparameters,andnodirectknowledgeoftheprocess,andthusoverallperformanceisreactiveandacompromise–whilePIDcontrolisthebestcontrollerwithnomodeloftheprocess,[1]betterperformancecanbeobtainedbyincorporatingamodeloftheprocess.
Themostsignificantimprovementistoincorporatefeed-forwardcontrolwithknowledgeaboutthesystem,andusingthePIDonlytocontrolerror.Alternatively,PIDscanbemodifiedinmoreminorways,suchasbychangingtheparameters(eithergainschedulingindifferentusecasesoradaptivelymodifyingthembasedonperformance),improvingmeasurement(highersamplingrate,precision,andaccuracy,andlow-passfilteringifnecessary),orcascadingmultiplePIDcontrollers.
PIDcontrollers,whenusedalone,cangivepoorperformancewhenthePIDloopgainsmustbereducedsothatthecontrolsystemdoesnotovershoot,oscillateorhuntaboutthecontrolsetpointvalue.Theyalsohavedifficultiesinthepresenceofnon-linearities,maytradeoffregulationversusresponsetime,donotreacttochangingprocessbehavior(say,theprocesschangesafterithaswarmedup),a
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