2003, Evolutionary learning in identification of fuzzy models application to damadics bench

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EVOLUTIONARYLEARNINGINIDENTIFICATIONOFFUZZYMODELS:APPLICATIONTODAMADICSBENCHMARK

ArunasLIPNICKAS*,J´ozefKORBICZInstituteofControlandComputationEngineering,UniversityofZielonaG´ora,ul.Podg´orna50,65-246ZielonaG´ora,Poland,e-mail:{A.Lipnickas;J.Korbicz@issi.uz.zgora.pl}*Permanentemployment:DepartmentofControlTechnology,KaunasUniversityofTechnology,Studentu48,3031Kaunas,Lithuania,e-mail:lipnick@soften.ktu.lt

Abstract:Evolutionarylearningandespeciallyge-neticoptimisationalgorithmshaverecentlyreceivedalotofresearchattentionastoolsforidentifyingfuzzymodelsofthesystems.MostoftenfuzzymodellingemploythefuzzyIF–THENrules.Inthispaper,be-sidesAND–operatortheOR–operatorisalsoconsid-eredinconstructingthepremiserulebase.Ageneticalgorithmisutilisedtofindthepremisestructureoftherules,alsotooptimisefuzzysetmembershipfunc-tionsandtheconsequentmodelstructureoftherulesatthesametime.TheperformanceoftheapproachisdemonstratedontheDAMADICSbenchmarkprob-lem.

Keywords:Evolutionarylearning,fuzzymodel,faultdiagnosis

1.INTRODUCTIONFuzzymodellingutilisingfuzzyIF–THENrules,providesatoolfordesigningqualitativemodelswithoutemployingprecisequantitativeanalysis.However,therearemanysituationswhereexpertdomainknowledge,whichisusuallythebasisfordesigningfuzzymodels,isnotsufficient,duetoincompletenessoftheexistingknowledge,prob-lemscausedbydifferentbiasesofhumanexperts,difficultiesinformingrules,etc.Thatiswhy,methodsfordata–drivenidentificationoffuzzymodelsareofgreatinterest.Mostapproachesproposedinliteratureemphasisethefunctionap-proximationcapabilitiesoffuzzysystems,andlittleattentionispaidtominimisationoftherulebase[3].Ifthenumberofrulesistoolarge,thefuzzymodelishardlyinterpretedbytheman.Thereforeinthisworkwerequirethemodeltobeaccurateandalsotobeassimpleaspossible.Weconsidertheproblemofdata–drivenfuzzyrule–basedmodellingbytheTakagi-Sugeno(TS)type[1,4,7].RulepremisesplayacriticalroleintheTSfuzzysystemsincetheydeterminethestructureofarulebase.Thefuzzyrulesofthefuzzymodelareextractedfromtrainingexamplesbymeansofgenetic–basedpremiselearning.Inordertoconstructasimplefuzzymodelwithahighgen-eralisationcapability,ageneralpremisestructureallowingincompletecompositionsofinputvari-ablesaswellasORoperationofinputtermsisconsidered.Inthispaperageneticalgorithm(GA)isutilisedtooptimisethepremisestructureoftherules,fuzzysetmembershipfunctions,andtheconsequentpartatthesametime.Determina-tionofruleconclusionsisnestedinthepremiselearning,whereconsequencesofindividualrulesaredeterminedunderfixedpreconditions.Dur-ingtherunningofGAtheactualrulenumberisadjustedautomaticallywithinaspecifiedlimit.ThemodellingprocedureutilizeGAtosearchinthecombinatorialspacefortheoptimalstructureofpremisesalsotooptimiseparametersoffuzzysetmembershipfunctionsaswellastofindtheoptimalparametersofconsequentpartsimulta-neously.GAsearchesawidespaceofpossiblesolution,sothereisahighprobabilitythatthefoundsolutionisglobalornearglobal[5].Recently,Xiong[8]introducedageneralpremisestructureallowingnotonlyincompletecompositionofinputvariablesbutalsoORcon-nectivesofinputterms,sothatahighgeneral-isationabilitycanbeachievedbyanindividualrule.Theupperlimitoftherulenumberisprede-terminedbythetechnicianinadvance.Itcanbeconsideredasanestimateofthesufficientamountofrulestoachieveasatisfactoryaccuracy.Whentherule–basepremiseisconstructed,thenthepolynomialmodelsinconsequentpartisfoundbythelocalweightedleastsquaremethod.Theperformanceofthemethodsisdemon-stratedontheDAMADICSbenchmarkproblem.Themulti–disciplinaryandcomplementaryEUResearchTrainingNetworkprojectDAMADICSisfocusedondevelopmentandapplicationofmeth-odsforactuatordiagnosisinindustrialcontrolsystems[2].Thepaperisorganisedasfollows.Inthesecondsection,thebackgroundoffuzzyTakagi–Sugenomodelisgiven.Theevolutionarylearn-ingispresentedinthethirdsection.Inthefourthsection,theDAMADICSbenchmarkisdescribed.Theexperimentalresultsarepresentedinsectionfive.Finally,sectionsixpresentsconclusions.2.TAKAGI–SUGENOMODELTheRiTSruletakesthefollowingform:Ri:IFuisAiTHENyi=fi(u),(1)i=1,2,...,K,whereKisthenumberofrules.UsuallyfuzzyrulesconsistofcanonicalANDconnectionoffuzzytermsinarulepremise.Incomplexprocesseswithmultipleinputvari-ablessuchasystemisnotsuitable,sincethenum-berofrulesincreasesexponentiallybyincreas-inginputdimension.Thereforeruleswithin-completestructurecontainingORconnectionsoffuzzytermsispreferable.Inthisway,thenum-berofrulesinthepremiseisdecreasedbyfind-ingsimilarconsequencepartsfordifferentfuzzyterms.Generally,fi(·)isapolynomialfunctionfromtheinputvariablesu,butinprincipleitcanbeanykindoffunctionaslongasitcanappro-priatelydescribetheoutputofsystemwithinthefuzzyregionsspecifiedbythepremiseoftherule.Asimpleandpracticallyusefulparameterisationistheaffine(linearinparameters)form,yieldingtherules:Ri:IFuisAiTHENyi=aTiU+a0,i,(2)i=1,2,...,K,whereUisthepolynomialinputvectorconstructedfromu,theaiisaparametervectoranda0,iisascalaroffset.WhenwehavetheZ-dimensionalpolynomialofdegree2,thentheparametervec-toraiconsistofLelements:L=(2+Z)!2!Z!−1.(3)Parameteroptimisationcanbeperformedveryfastbyaleastsquaresalgorithm.However,thenumberofparametersgrowsrapidlywithincreasinginputdimensionality.Onewaytodecreasethenumberofparametersistoperformstructureoptimisations.Thestructureoptimisationcanbeperformedbyalinearsubsetselectiontechniquesuchastheorthogonalleastsquaresalgorithm[6].However,thismethodsuf-fersfromthecurseofdimensionality.