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外文文献.docx
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外文文献
学校代码:
10128
学号:
200911204088
本科毕业设计外文文献翻译
(
题目:
锅炉内胆水温控制系统设计及仿真
学生姓名:
郭晓晨
学院:
电力学院
系别:
自动化系
专业:
自动化
班级:
自(电)09-1班
指导教师:
王文兰副教授
二〇一三年六月
Classicalcontroltheory
Theclassofcontrolproblemstobeexaminedhereisoneofconsiderableengineeringinterest.weshallconsidersystemswithseveralinputs,someknowascontrolsbecausetheymaybemanipulatedandotherscalledexternaldisturbances,whicharequiteunpredictable.Forexample,inanindustrialfurnacewemayconsiderthefuelflow,theambienttemperature,andtheloadingofmaterialintothefurnacetobeinputs.Ofthere,thefuelflowisaccessibleandcanreadilybecontrolled,whilethelattertwoareusuallyunpredictabledisturbances.
Insuchsituations,oneaspectofthecontrolproblemistodeterminehowthecontrolsshouldbemanipulatedsoastocounteracttheeffectsoftheexternaldisturbancesonthestateofthesystem.onepossibleapproachtothesolutionofthisproblemistouseacontinuousmeasurementofthedisturbances,andfromthisandtheknownsystemequationstodeterminewhatthecontrolinputsshouldbeasfunctionsoftimetogiveappropriatecontrolofthesystemstate.
Adifferentapproachistoconstructafeedbacksystem,thatis,ratherthanmeasurethedisturbancesdirectlyandthencomputetheireffectsonthesystemfromthemodelorsystemequations,wecomparedirectandcontinuousmeasurementsoftheaccessiblesystemstateswithsignalrepresentingtheir”desiredvalues”toformerrorsignal,andusethissigntoproduceinputstothesystemwhichwilldrivetheerrorasclosetozeroaspossible.
Bysomeabuseofterminology,theformerapproachhascometobeknowasopen-loopcontrol,andthelatterasclosed-loopcontrol.Atfirstsight,thetwoapproachesmightappeartobeessentiallyequivalent.
Indeed,onemightsurmisethatanopen-loopcontrolschemeispreferablesinceitisnotnecessarytowaituntilthedisturbanceshaveproducedanundesirablechangeinthesystemstatebeforecorrectiveinputscanbecomputedandapplied.
However,thisadvantageismorethanoutweighedbythedisadvantagesofopen-loopcontrolandtheinherentadvantagesoftheopen-loopcontrolsuggestedabovewouldrequireaverysophisticatedcomputingdevicetodeterminetheinputsrequiredtocounteractthepredicteddisturbanceeffects.Second,afeedbacksystemturnsouttobeinherentlyfarlesssensitivetotheaccuracywithwhichamathematicalmodelofthesystemhasbeendetermined.Putanotherway,aproperlydesignedfeedbacksystemwillstilloperatesatisfactorilyevenwhentheinternalpropertiesofthesystemchangebysignificantamounts.
Anothermajoradvantageofthefeedbackapproachisthatbyplacinga“feedbackloop”aroundasystemwhichinitiallyhasquiteunsatisfactoryperformancecharacteristics,onecaninmanycasesconstructasystemwithsatisfactorybehavior.Consider,forexample,arocketinverticalflight.Thisisessentiallyaninvertedpendulum,balancingonthegasjetproducedbytheengine,andinherentlyunstable.Itcan,however,bekeptstableinverticalflightbyappropriatechangesinthedirectionoftheexhaustjet,whichmaybeachievedbyrotatingtheengineonitsgimbalsmountings.Theonlysatisfactorywayofachievingthesevariationsinjetdirectionistouseafeedbackstrategyinwhichcontinuousmeasurementsoftheangularmotionsoftherocketintwomutuallyperpendicularverticalplanescauseacontrollertomakeappropriateadjustmentstothedirectionoftherocketengine.Stabilizationofaninherentlyunstablesystemcouldnotbeachievedinpracticebyanopen-loopcontrolstrategy.
Themathematicaltoolsrequiredfortheanalysisanddesignoffeedbacksystemsdifferaccordingtothestructuralcomplexityofthestructuralcomplexityofthesystemstobecontrolledandaccordingtotheobjectivesthefeedbackcontrolismeanttoachieve.
Inthesimplestsituation,onecontrolsasingleplantstatevariable,calledtheoutput,bymeansofadjustmentstoasingleplantinput.Theproblemistodesignafeedbacklooparoundthesystemwhichwillensurethattheoutputchangesinresponsetocertainspecifiedtimefunctionsortrajectorieswithanacceptabledegreeofaccuracy.Ineithercase,thetransientswhichareinevitablyexcitedshouldnotbetoo”violent”orpersistfortoolong.
Inatypicalsituation,wearegivenasystem,orplant,withcontrolinputu,externaldisturbanced,andoutputy,allscalars,theproblemistodesignafeedbacksystemaroundtheplantconsistingofadevicewhichproducesacontinuousmeasurementymoftheoutput;acomparatorinwhichthissignalissubtractedformareferenceinputyr,representingthedesiredvalueoftheoutput,toproduceanerrorsignale;andacontrollerwhichusestheerrorsignaletoproduceanappropriateinpututotheplant.Weshallcallthisconfigurationasingle-loopfeedbacksystem,atermwhichismeanttoconveytheessentialfeaturethatjustoneofthefeedbacksystemistomaketheoutputy(t)followitsdesiredvalueyr(t)as
closelyaspossibleeveninthepresenceofnonzerodisturbancesd(t).theabilityofasystemtodosoundersteady-stateconditionsisknownasstaticaccuracy.Frequentlyyrisaconstant,inwhichcasewecallthefeedbacksystemaregulatorsystem.
IntroductiontoControlSystems
Asthenamesuggests,acontrolsystemisasysteminwhichsomephysicalquantityiscontrolledbyregulatingenergyinput.Asystemisagroupofphysicalcomponentsassembledtoperformaspecificfunction.Asystemmaybeelectrical,mechanical,hydraulic,pneumatic,thermal,biomedical,oracombinationofanyofthesesystems.Thephysicalquantitymaybeanyphysicalvariable,suchastemperature,pressure,liquidlevel,electricvoltage,ormechanicalposition.Wemayuseanalog,digital,orbothanaloganddigitaltechniquestocontroladesiredphysicalquantity.Generally,thetechniqueemployedisusedtoclassifythecontrolsystemaseither.Analogordigital.Morespecifically,aphysicalquantitytobecontrolledinagivensystemisusedtolabelthatsystem.Forexample,temperatureiscontrolledinatemperaturecontrolsystem,whereasinaliquid-levelsystemapre-assignedliquidlevelismaintained.
Intheearlydaysofcontrolsystems,mostwereanalogsystemsemployinganalogtechniques.Thesesystemswererelativelybulky,complex,andcumbersomebothtodesignandtomaintain.However,withthedevelopmentofdigitaltechnologyandtheinventionofintegratedcircuits,thedesignofcontrolsystemsbecameeasieraswellasmoreeconomical.Presently,mostcontrolsystemsaremicroprocessorcontrolled,primarilybecauseoftheavailabilityofcontrolICsandcheapermemoriesandtremendousadvancementsindata-handlingcapabilities.
OptimaltuningforPIcontroller
1.Introduction
ThePIcontrollerisunquestionablythemostcommonlyusedcontrolalgorithmintheprocesscontrolindustry.Themainreasonisitsrelativelysimplestructure,whichcanbeeasilyunderstoodandimplementedinpractice,andthatmanysophisticatedcontrolstrategies,suchasmodelpredictivecontrol,arebasedonit.Inspiteofitswidespreadusethereexistsnogenerallyaccepteddesignmethodforthecontroller.PIcontrollershavetraditionallybeentunedempirically,e.g.bythemethoddescribedinZiegler&Nichols(1942).Thismethodhasthegreatadvantageofrequiringverylittleinformationabouttheprocess.Thereis,however,asignificantdisadvantagebecausethemethodinherentlygivesverypoordamping.Hang,AstromandHo(1991)proposedtherefinedZiegler-Nicholstuningformula,whichimprovetheperformanceofPIcontroller.
InthispaperwepresentanefficientandpracticalmethodfordesigningPIcontroller.ThedesignisbasedonoptimizationofloaddisturbancerejectionwithconstraintthattheNyquistcurveofthelooptransferfunctionistangenttoalineparalleltotheimagineaxesintheleft-halfofthecomplexplane.Thus,itsatisfiesbothrobustnessandperformancerequirements.Simulationexamplesaregiventoshowtheeffectivenessandflexibilityofthecontrollerinhandingprocessesofdifferentcharacteristics.Furthermore,comparisonwithrefinedZiegler-Nicholsformulashowsthatthenewdesignproceduregivesbetterperformance.
2.TheproposedmethodfordesigningPIcontrollers
APIcontrollerwiththetransferfunction
(1)
isemployedtocontroltheprocess.Hence,
(2)
WithPIcontrolitispossibletomovethegivenpointontheNyquistcurveintwodirectioninthecomplexplane.ThepointAmaybemovedinthedirectionofGp(S)bychangingthegainandintheorthogonaldirectionbychangingtheintegralgain.ThedegreerangeofthemovementofthepointAisfrom0to900.
Robustnessisalwaysofprimaryconcernforprocesscontrolwhenweanalyzeanddesignthecontrolsystems,becausethemodelsusedforthedesignofthecontrollersareusuallyimpreciseandtheparametersofallphysicalsystemsvarywiththeworkingconditionandtime.Thesefactorsusuallymarkthecontrolsystemsunstablesothatthesystemdeviatefromthedesigningrequirement,evenmakethesystemsunstablesothatthesystemscannotworknormally.Sowhendesigningasystem,wealwayshopethemodelingerrorandvariationoftheparametersislessinfluencingontheperformanceofclose-loopsystem.Robustspecificationsusuallyincludegainmargin,phasemarginandsensitivityonwhichmanyformulasforthedesignofPIDandPIcontrollersarebased.
Weproposeanewrobustnessspecification
whichisdefineas
(3)
wherethelooptransferfunctionisG1(j
).Thenewspecificationissimilarwiththegainmarginspecifications.Wehave
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