PID控制器及智能控制英文文献

PID控制器及智能控制英文文献ArticleID:1000-8152(2000)06-0861-07IntroductionstoPIDControllersandIntelligentControlTANYonghongandDANGXuanju(SchoolofComputerSciences,GuilinInstituteofElectronicTechnologyGuilin,541004,P.R.China)AchielR.VanCauwenberghe(DepartmentofControlEngineeringandAutomation,UniversityofGhentGhent,Belgium)MehrdadSaif(SchoolofEngineeringSciences,SimonFraserUniversityBurnaby,BC,V5A1S6,Canada)Abstract：Industrialautomationlevelhasbecomeameasureofprofessionsmodernizationlevelisanimportantsymbol.Atthesametime,thedevelopmentofthetheoryofcontrolalsoexperiencedtheclassicalcontroltheoryandmoderncontroltheoryandintelligentcontroltheoryofthreephaseautomaticcontrolsystemcanbedividedintotheopenloopcontrolsystemandclosed-loopcontrolsystem.Acontrolsystemincludingthecontroller,sensors,actuators,andtransmitterinputandoutputinterface.Theoutputafteroutputinterface,actuators,addtothecontrolsystem,Controlsystem,theaccused,thetransmitter,throughaftersensorinputinterfacetocontroller.Differentcontrolsystem,itssensorsandactuators,thetransmitterisnotthesame.Atpresent,thePIDcontrolandcontrollerorintelligentinstrumenthasmanyPIDcontrollerintheengineeringpractice,theproductshavebeenwidelyapplied,thereareallsortsofPIDcontrollerproducts,companiesaredevelopedwithPIDparameterself-settingfunctionofintelligentregulator(intelligentregulator),includingthePIDcontrollerisadjustedbyintelligentautomaticadjustmentorcalibration,theadaptivealgorithmisproposedtorealize.HaverealizedusingPIDcontrolpressure,temperature,flow,liquidlevelcontroller,canrealizePIDcontrolfunctionoftheprogrammablelogiccontroller(PLC),andcanrealizePIDcontrolPCsystem,etc.Keywords:intelligentcontrol,PIDcontrolPIDcontrollerscanbestand-alonecontrollers(alsocalledsingleloopcontrollers),controllersinPLCs,embeddedcontrollers,orsoftwareinVisualBasicorC#computerprograms.PIDcontrollersareprocesscontrollerswiththefollowingcharacteristics:_Continuousprocesscontrol_Analoginput(alsoknownas“measurement”or“ProcessVariable”or“PV”)_Analogoutput(referredtosimplyas“output”)_Setpoint(SP)_Proportional(P),Integral(I),and/orDerivative(D)constantsExamplesof“continuousprocesscontrol”aretemperature,pressure,flow,andlevelcontrol.forexample,controllingtheheatingofatank.Forsimplecontrol,youhavetwotemperaturelimitsensors(onelowandonehigh)andthenswitchtheheateronwhenthelowtemperaturelimitsensorturnsonandthenturntheheateroffwhenthetemperaturerisestothehightemperaturelimitsensor.Thisissimilartomosthomeairconditioningheatingthermostats.Incontrast,thePIDcontrollerwouldreceiveinputastheactualtemperatureandcontrolavalvethatregulatestheflowofgastotheheater.ThePIDcontrollerautomaticallyfindsthecorrect(constant)flowofgastotheheaterthatkeepsthetemperaturesteadyatthesetpoint.Insteadofthetemperaturebouncingbackandforthbetweentwopoints,thetemperatureisheldsteady.Ifthesetpointislowered,thenthePIDcontrollerautomaticallyreducestheamountofgasflowingtotheheater.Ifthesetpointisraised,thenthePIDcontrollerautomaticallyincreasestheamountofgasflowingtotheheater.LikewisethePIDcontrollerwouldautomaticallyforhot,sunnydays(whenitishotteroutsidetheheater)andforcold,cloudydays.Theanaloginput(measurement)iscalledthe“processvariable”or“PV”.YouwantthePVtobeahighlyaccurateindicationoftheprocessparameteryouaretryingtocontrol.Forexample,ifyouwanttomaintainatemperatureof＋or－onedegreethenwetypicallystriveforatleasttentimesthatorone-tenthofadegree.Iftheanaloginputisa12bitanaloginputandthetemperaturerangeforthesensoris0to400degreesthenour“theoretical”accuracyiscalculatedtobe400degreesdividedby4,096(12bits)=0.09765625degrees.[1]Wesay“theoretical”becauseitwouldassumetherewasnonoiseanderrorinourtemperaturesensor,wiring,andanalogconverter.Thereareotherassumptionssuchaslinearity,etc..Thepointbeing－with1/10ofadegree“theoretical”accuracy－evenwiththeusualamountofnoiseandotherproblemsonedegreeofaccuracyshouldeasilybeattainable.Theanalogoutputisoftensimplyreferredtoas“output”.Oftenthisisgivenas0~100percent.Inthisheatingexample,itwouldmeanthevalveistotallyclosed(0%)ortotallyopen(100%).Thesetpoint(SP)issimply－whatprocessvaluedoyouwant.Inthisexample－whattemperaturedoyouwanttheprocessat?ThePIDcontroller’sjobistomaintaintheoutputatalevelsothatthereisnodifference(error)betweentheprocessvariable(PV)andthesetpoint(SP).InFig.16.1,thevalvecouldbecontrollingthegasgoingtoaheater,thechillingofacooler,thepressureinapipe,theflowthroughapipe,thelevelinatank,oranyotherprocesscontrolsystem.PIDcontrollerWhatthePIDcontrollerislookingatisthedifference(or“error”)betweenthePVandtheSP.Itlooksattheabsoluteerrorandtherateofchangeoferror.Absoluteerrormeans－isthereabigdifferenceinthePVandSPoralittledifference?Rateofchangeoferrormeans－isthedifferencebetweenthePVorSPgettingsmallerorlargerastimegoeson.Whenthereisa“processupset”,meaning,whentheprocessvariableorthesetpointquicklychanges－thePIDcontrollerhastoquicklychangetheoutputtogettheprocessvariablebackequaltothesetpoint.[2]Ifyouhaveawalk-incoolerwithaPIDcontrollerandsomeoneopensthedoorandwalksin,thetemperature(processvariable)couldriseveryquickly.ThereforethePIDcontrollerhastoincreasethecooling(output)tocompensateforthisriseintemperature.OncethePIDcontrollerhastheprocessvariableequaltothesetpoint,agoodPIDcontrollerwillnotvarytheoutput.Youwanttheoutputtobeverysteady(notchanging).Ifthevalve(motor,orothercontrolelement)isconstantlychanging,insteadofmaintainingaconstantvalue,thiscouldcausemorewearonthecontrolelement.Sotherearethesetwocontradictorygoals.Fastresponse(fastchangeinoutput)whenthereisa“processupset”,butslowresponse(steadyoutput)whenthePVisclosetothesetpoint.Notethattheoutputoftengoespast(overshoots)thesteady-stateoutputtogettheprocessbacktothesetpoint.Forexample,acoolermaynormallyhaveitscoolingvalveopen34%tomaintainzerodegrees(afterthecoolerhasbeenclosedupandthetemperaturesettleddown).Ifsomeoneopensthecooler,walksin,walksaroundtofindsomething,thenwalksbackout,andthenclosesthecoolerdoo－rthePIDcontrollerisfreakingoutbecausethetemperaturemayhaveraised20degrees!Soitmaycrankthecoolingvalveopento50,75,oreven100percent－tohurryupandcoolthecoolerbackdown－beforeslowlyclosingthecoolingvalvebackdownto34percent.[3]Let’sthinkabouthowtodesignaPIDcontroller.Wefocusonthedifference(error)betweentheprocessvariable(PV)andthesetpoint(SP).Therearethreewayswecanviewtheerror.ThismeanshowbigisthedifferencebetweenthePVandSP.IfthereisasmalldifferencebetweenthePVandtheSP－thenlet’smakeasmallchangeintheoutput.IfthereisalargedifferenceinthePVandSP－thenlet’smakealargechangeintheoutput.Absoluteerroristhe“proportional”(P)componentofthePIDcontroller.ThesumoferrorsovertimeGiveusaminuteandwewillshowwhysimplylookingattheabsoluteerror(proportional)onlyisaproblem.Thesumoferrorsovertimeisimportantandiscalledthe“integral”(I)componentofthePIDcontroller.EverytimewerunthePIDalgorithmweaddthelatesterrortothesumoferrors.InotherwordsSumofErrorS=Error1+Error2+Error3+Error4+....ThedeadtimeDeadtimereferstothedelaybetweenmakingachangeintheoutputandseeingthechangereflectedinthePV.Theclassicalexampleisgettingyourovenattherighttemperature.Whenyoufirstturnontheheat,ittakesawhilefortheovento“heatup”.Thisisthedeadtime.Ifyousetaninitialtemperature,waitfortheoventoreachtheinitialtemperature,andthenyoudeterminethatyousetthewrongtemperature－thenitwilltakeawhilefortheoventoreachthenewtemperaturesetpoint.Thisisalsoreferredtoasthe“derivative”(D)componentofthePIDcontroller.Thisholdssomefuturechangesbackbecausethechangesintheoutputhavebeenmadebutarenotreflectedintheprocessvariableyet.AbsoluteError/ProportionalOneofthefirstideaspeopleusuallyhaveaboutdesigninganautomaticprocesscontrolleriswhatwecall“proportional”.Meaning,ifthedifferencebetweenthePVandSPissmall－thenlet’smakeasmallcorrectiontotheoutput.IfthedifferencebetweenthePVandSPislargethenlet’smakealargercorrectiontotheoutput.Thisideacertainlymakessense.WesimulatedaproportionalonlycontrollerinMicrosoftExcel.Fig.16.2isthechartshowingtheresultsofthefirstsimulation(DEADTIME=0,proportionalonly):ProportionalandIntegralControllersTheintegralportionofthePIDcontrolleraccountsfortheoffsetprobleminaproportionalonlycontroller.WehaveanotherExcelspreadsheetthatsimulatesaPIDcontrollerwithproportionalandintegralcontrol.Here(Fig.16.3)isachartofthefirstsimulationwithproportionalandintegral(DEADTIME=0,proportional=0.4).Asyoucantell,thePIcontrollerismuchbetterthanjustthePcontroller.However,deadtimeofzero(asshowninthegraph)isnotcommon.ThesimulationchartDerivativeControlDerivativecontroltakesintoconsiderationthatifyouchangetheoutput,thenittakestimeforthatchangetobereflectedintheinput(PV).Forexample,let’stakeheatingoftheoven.ThesimulationchartIfwestartturningupthegasflow,itwilltaketimefortheheattobeproduced,theheattoflowaroundtheoven,andforthetemperaturesensortodetecttheincreasedheat.Derivativecontrolsortof“holdsback”thePIDcontrollerbecausesomeincreaseintemperaturewilloccurwithoutneedingtoincreasetheoutputfurther.SettingthederivativeconstantcorrectlyallowsyoutobecomemoreaggressivewiththePIconstants.Intelligenceandintelligentsystemscanbecharacterizedinanumberofwaysandalonganumberofdimensions.Therearecertainattributesofintelligentsystems,commoninmanydefinitions,whichareofparticularinteresttothecontrolcommunity.Inthefollowing,severalalternativedefinitionsandcertainessentialcharacteristicsofntelligentsystemsarefirstdiscussed.Abriefworkingdefinitionofintelligentsystemsthatapturestheircommoncharacteristicsisthenpresented.Inmoredetail,westartwitharathereneraldefinitionofintelligentsystems,wediscusslevelsofintelligence,andweexplaintherolefcontrolinintelligentsystemsandoutlineseveralalternativedefinitions.[1]Wethendiscussdaptationandlearning,autonomyandthenecessityforefficientcomputationalstructuresinntelligentsystems,todealwithcomplexity.Weconcludewithabriefworkingcharacterizationfintelligent(control)systems.Westartwithageneralcharacterizationofintelligentsystems:Anintelligentsystemhastheabilitytoactappropriatelyinanuncertainenvironment,wherenappropriateactionisthatwhichincreasestheprobabilityofsuccess,andsuccessisthechievementofbehavioralsubgoalsthatsupportthesystem’sultimategoal.[2]Inorderforaman-madeintelligentsystemtoactappropriately,itmayemulatefunctionsofivingcreaturesandultimatelyhumanmentalfaculties.Anintelligentsystemcanbeharacterizedalonganumberofdimensions.Therearedegreesorlevelsofintelligencethatcanemeasuredalongthevariousdimensionsofintelligence.Ataminimum,intelligencerequiresheabilitytosensetheenvironment,tomakedecisionsandtocontrolaction.Higherlevelsofntelligencemayincludetheabilitytorecognizeobjectsandevents,torepresentknowledgeinaorldmodel,andtoreasonaboutandplanforthefuture.Inadvancedforms,intelligencerovidesthecapacitytoperceiveandunderstand,tochoosewisely,andtoactsuccessfullyunderlargevarietyofcircumstancessoastosurviveandprosperinacomplexandoftenhostilenvironment.[3]Intelligencecanbeobservedtogrowandevolve,boththroughgrowthinomputationalpowerandthroughaccumulationofknowledgeofhowtosense,decideandactincomplexandchangingworld.Theabovecharacterizationofanintelligentsystemisrathergeneral.Accordingtothis,areatnumberofsystemscanbeconsideredintelligent.Infact,accordingtothisdefinition,evenathermostatmaybeconsideredtobeanintelligentsystem,althoughoflowlevelofintelligence.Itiscommon,however,tocallasystemintelligentwheninfactithasaratherhighlevelofintelligence.Thereexistanumberofalternativebutrelateddefinitionsofintelligentsystemsandinthefollowingwementionseveral.Theyprovidealternative,butrelatedcharacterizationsofintelligentsystemswithemphasisonsystemswithhighdegreesofintelligence.Thefollowingdefinitionemphasizesthefactthatthesysteminquestionprocessesinformation,anditfocusesonman-madesystemsandintelligentmachines:Machineintelligenceistheprocessofanalyzing,organizingandconvertingdataintoknowledge;where(machine)knowledgeisdefinedtobethestructuredinformationacquiredandappliedtoremoveignoranceoruncertaintyaboutaspecifictaskpertainingtotheintelligentmachine.Thisdefinitionleadstotheprincipleofincreasingprecisionwithdecreasingintelligence,whichclaimsthat:applyingmachineintelligencetoadatabasegeneratesaflowofknowledge,lendingananalyticformtofacilitatemodelingoftheprocess.Next,anintelligentsystemischaracterizedbyitsabilitytodynamicallyassignsubgoalsandcontrolactionsinaninternalorautonomousfashion:Manyadaptiveorlearningcontrolsystemscanbethoughtofasdesigningacontrollawtomeetwell-definedcontrolobjectives.Thisactivityrepresentsthesystem’sattempttoorganizeororderits“knowledge”ofitsowndynamicalbehavior,sotomeetacontrolobjective.Theorganizationofknowledgecanbeseenasoneimportantattributeofintelligence.Ifthisorganizationisdoneautonomouslybythesystem,thenintelligencebecomesapropertyofthesystem,ratherthanofthesystem’sdesigner.Thisimpliesthatsystemswhichautonomously(self)-organizecontrollerswithrespecttoaninternallyrealizedorganizationalprincipleareintelligentcontrolsystems.[5]Aproceduralcharacterizationofintelligentsystemsisgivennext:Intelligenceisapropertyofthesystemthatemergeswhentheproceduresoffocusingattention,combinatorialsearch,andgeneralizationareappliedtotheinputinformationinordertoproducetheoutput.Onecaneasilydeducethatonceastringoftheaboveproceduresisdefined,theotherlevelsofresolutionofthestructureofintelligencearegrowingasaresultoftherecursion.Havingonlyonelevelstructureleadstoarudimentaryintelligencethatisimplicitinthethermostat,ortoavariable-structureslidingmodecontroller.Theconceptsofintelligenceandcontrolarecloselyrelatedandtheterm“IntelligentControl”hasauniqueanddistinguishablemeaning.Anintelligentsystemmustdefineandusegoals.Controlisthenrequiredtomovethesystemtothesegoalsandtodefinesuchgoals.Consequently,anyintelligentsystemwillbeacontrolsystem.Conversely,intelligenceisnecessarytoprovidedesirablefunctioningofsystemsunderchangingconditions,anditisnecessarytoachieveahighdegreeofautonomousbehaviorinacontrolsystem.Sincecontrolisanessentialpartofanyintelligentsystem,theterm“IntelligentControlSystems”issometimesusedinengineeringliteratureinsteadof“IntelligentSystems”or“IntelligentMachines”.Theterm“IntelligentControlSystem”simplystressesthecontrolaspectoftheintelligentsystem.Below,onemorealternativecharacterizationofintelligent(control)systemsisincluded.Accordingtothisview,acontrolsystemconsistsofdatastructuresorobjects(theplantmodelsandthecontrolgoals)andprocessingunitsormethods(thecontrollaws):Anintelligentcontrolsystemisdesignedsothatitcanautonomouslyachieveahighlevelgoal,whileitscomponents,controlgoals,plantmodelsandcontrollawsarenotcompletelydefined,eitherbecausetheywerenotknownatthedesigntimeorbecausetheychangedunexpectedly.Thereareseveralessentialpropertiespresentindifferentdegreesinintelligentsystems.Onecanperceivethemasintelligentsystemcharacteristicsordimensionsalongwhichdifferentdegreesorlevelsofintelligencecanbemeasured.[6]Belowwediscussthreesuchcharacteristicsthatappeartoberatherfundamentalinintelligentcontrolsystems.AdaptationandLearning.Theabilitytoadapttochangingconditionsisnecessaryinanintelligentsystem.Althoughadaptationdoesnotnecessarilyrequiretheabilitytolearn,forsystemstobeabletoadapttoawidevarietyofunexpectedchangeslearningisessential.Sotheabilitytolearnisanimportantcharacteristicof(highly)intelligentsystems.AutonomyandIntelligence.Autonomyinsettingandachievinggoalsisanimportantcharacteristicofintelligentcontrolsystems.Whenasystemhastheabilitytoactappropriatelyinanuncertainenvironmentforextendedperiodsoftimewithoutexternalintervention,itisconsideredtobehighlyautono