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tionoccursbetweentheneedofdisturbancecompensationandparameterestimation
whichisexceptionallyimportantincaseswheretheparametersaretime-varyingand
thedisturbancesarecorrelated.
Themeasureofaccuracy,oftenusedforidentificationinthecaseofstochasticdis-
turbances,isthecovariancematrix,P,ofmodelparameterassessmentor,connected
byCramér-Raoinequality,theFisherinformationmatrixM(seeChapter3).Quality
indicesarethendefinedbyscalarfunctionsofmatrixPorM,e.g.,determinants:
detP,detM,ortraces:trP,trM.Appropriateindicescanbealsodefinedfor
disturbancesdifferentthanstochastic.Itisverydifficulttoderivetheanaliticalor
eventhenumericaldependenceofadaptivecontrolqualityonestimationaccuracy,
determinedbygivenindices.
Often,inaclosed-loopsystem,convergenceoftheassessmentofmodelparame-
terstotherealparameters,i.e.,consistency,cannotbeguaranteed.Theproblemwith
thecertaintyequivalencepropertyiswellknown,e.g.,inadaptivelinear-quadratic
(LQ)controlorinlinear-quadratic-Gaussian(LQG)control.Itemergesfromthelack
ofconvergenceofassessmenttotherealparameters,whenthegeneralquadratic
formofcontrolqualityindexisusedthatincludesthecostsofcontrol(input)values.
TheproblemisexplainedinmoredetailinChapter6.
Besidesaccuracy,theequivalenceofidentification,i.e.,identifiability,whichpar-
ticularlyconcernsparameters,isanimportantquestion.ThisisdiscussedinChap-
ters2–4.
1020Classificationofidentificationmethods
Methodsandalgorithmsusedforlinearsystemidentificationcanbeclassified
withtheaidofmanycriteria.Oneoftheseisatypeofdisturbanceinput(adoptedin
thisbook)forwhichtheidentificationofadeterministicsystem,astochasticsystem
orasystemwithboundednoisecanbedistinguished.Inthesecasestheappropri-
atemethodformodelparametersestimationshouldbeused,andthemodelcanbe
parametricornonparametric.Parametricmodels,usuallyintheformoftransfer
functions,basicallyhavealowernumberofparametersincomparisonwithnon-
parametricmodels,whicharemainlysteporimpulseresponses.
Anothercriterionisconnectedtothetypeofmodelusedinsystemidentifica-
tion(continuous-timeordiscrete-timemodelscanbedistinguished).Mostofthe
currentidentificationmethodsconcerndiscrete-timemodelsbecauseofthediscrete-
time(samples)forminwhichsignalsaregathered,andbecauseofcomputerpro-
cessingofsignals.However,itisworthnotingthatitispossibletousediscrete-time
methodsforidentificationofsystemsdescribedbycontinuous-timemodels.
Thedifferentcriterionconcernsthedomaininwhichtheidentificationmethodis
operating,i.e.,thetimedomainorthefrequencydomain.Forbothdomains,sepa-
ratetoolboxesinMatlabareprepared.Identificationcanbeappliedtosingle-input/
single-output(SISO)systemsortomultiple-input/multiple-output(MIMO)systems.
Anotherissueisnonlinearsystemidentification.Thisbookmainlyconcernsthese-
lectedparametricidentificationmethodsforlinearSISOdynamicsystemswhichare
describedinthediscrete-timedomain.