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11
ut
ESTIMATION
ALGORITHM
ξt
SYSTEM
MODEL
Bt
ˆ
gt
gt
ˆ
+
εt
Fig.1.2.Blockdiagramofparameterassessment
touserecursiveidentificationprocedures.Hence,inadaptivecontrol,identification
andcontrolprocessesaresimultaneous,andinput-outputsignalscanbemeasured
inaclosed-loopsystem,butthismaycomplicateidentifiabilityconditions.
Accordingtothisconcept,forchosenamodel!sstructure,unknownparameters
(constantortime-varying)shouldbeestimatedon-lineinordertoimplementadap-
tivecontrol.
Twoadaptivecontrolapproachescanbedistinguished:directandindirectcon-
trol.Withtheindirectapproach,thesystemtobecontrolledisidentifieddirectly,i.e.,
theparametersofitsmodelareestimated,andafterwardstheyareusedtodesign
controllaw.Directcontrolreliesondirectidentificationofcontrollaw,i.e.,theesti-
mationofitsparameters,withouttheneedtoestimatemodelparametersofthesys-
temtobecontrolled.Bothtypesofcontrolhaveadvantagesanddisadvantages.The
mainadvantageofdirectcontrolisthepossibilityofusingthesimpleleast-squares
methodtoestimatecontrollawparameters,whereasthemainadvantageofthein-
directmethodistheavailabilityofon-lineknowledgeofthesystemtobecontrolled,
thatcanbeutilisedwheneverachangeofcontrollawisneeded.
Itshouldbenotedthatinbothcasesmentionedabove,identificationhasexplicit
form,i.e.,itprovidescertainvaluesofparameterestimatesofthesystemmodelor
ofthecontrollawmodel.Otheradaptivecontrolapproachescanuseimplicitiden-
tification,forinstance,inadaptivecontrolwithareferencemodel,wheregradient
methodsareusedforadaptationpurposes,e.g.,theMITruleortheLyapunovfunc-
tionmethod.
Ingeneral,itisimpossibletoobtainexactmodelsoftherealsystems.Therefore,
inpractice,onehastoacceptthenecessityofuncertainmodeladoptionincontrol
systemdesign.
Oneimportantquestionconcernsthemeaningofidentification(estimation)ac-
curacy,andhowthisaccuracy,comprehendedinanappropriateway,influencesthe
qualityofadaptivecontrol.Itshouldbenotedthat,inthecaseofadaptivecontrol
withconstantestimatedparameters,itispossibletoabortidentificationafterobtain-
ingtherequiredaccuracyofestimates.Thisresultsinfastercalculation.Moreover,if
thecontrolsystemworksproperlyandtrackingisprovided,thentheidentification
conditionsrelatedtocontrol(input)signalmaynotbefulfilled.Asaresult,contradic-