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1、APPLICATIONNOTEHowtoDeterminetheModalParametersofSimpleStructuresbySvendGade,HenrikHerlufsenandHansKonstantin-Hansen,Brüel&Kjær,DenmarkThemodalparametersofsimplestructurescanbeeasilyestablishedbytheuseofPULSE™,theMulti-analyzerSystemType3560.Thisapplicationnotedescribeshowtomeasurethemodalfrequ
2、enciesbyinspectionoffrequencyresponsefunctions,howtodeterminethemodaldampingwiththeaidofthefrequencyweightingfunctionincludedintheanalyzer,andhowtoestablishthemodeshapesbyexaminingthevalueoftheimaginarypartofthefrequencyresponsefunction.Thefrequencyresponsefunctionofastructurecanbeseparatedinto
3、aIdentificationofModesFrequencysetofindividualmodes.ByusingadB
4、H
5、dB
6、H
7、PULSEMulti-analyzerSystemType3560,eachmodecanbeidentifiedintermsoffrequency,dampingandmodeshape.f1f2=205Hzf3Hzf2=204.25HzHzModeShapeDampingdB
8、H
9、·W(f)Imag.HHzdBh(t)2ndBendingMode8.7dBHzms1t=30.76msz2==0.0254t·2pf2880212/1eIntr
10、oductionInpractice,nearlyallvibrationproblemsarerelatedtostructuralweaknesses,associ-atedwithresonancebehaviour(thatisnaturalfrequenciesbeingexcitedbyoperationalforces).Itcanbeshownthatthecompletedynamicbehaviourofastructure(inagivenfrequencyrange)canbeviewedasasetofindividualmodesofvibration,e
11、achhavingacharacteristicnaturalfrequency,damping,andmodeshape.Byusingtheseso-calledmodalparameterstomodelthestructure,problemsatspecificresonancescanbeexaminedandsubsequentlysolved.Thefirststageinmodellingthedynamicbehaviourofastructureistodeterminethemodalparametersasintroducedabove:❍Theresona
12、nce,ormodal,frequency❍Thedampingfortheresonance–themodaldamping❍Themodeshape3560Themodalparameterscanbedeterminedfromasetoffrequencyresponsemeasure-mentsbetweenareferencepointandanumberofmeasurementpoints.Suchameas-urementpoint,asintroducedhere,isusuallycalledaDegree-of-Freedom(DOF).Themodalfre
13、quenciesanddampingscanbefoundfromallfrequencyresponsemeasure-mentsonthestructure(exceptthoseforwhichtheexcitationorresponsemeasurementisinanodalposition,thatis,wherethedisplacementiszero).Thesetwoparametersarethereforecalled“Globa