modal testing b&k_模态测试教程

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1、ExperimentalModalAnalysisCopyright©2003Brüel&KjærSound&VibrationMeasurementA/SAllRightsReservedSDOFandMDOFModelsDifferentModalAnalysisTechniquesExcitingaStructureMeasuringDataCorrectlyModalAnalysisPostProcessingf(t)x(t)kcm=++++SimplestFormofVibratingSystemDd=DsinntmkTTi

2、meDisplacementFrequency1TPeriod,Tnin[sec]Frequency,fn=in[Hz=1/sec]1TnDisplacementkmn=2fn=MassandSpringtimem1mIncreasingmassreducesfrequencyMass,SpringandDamperIncreasingdampingreducestheamplitudetimemkc1+c2M=mass(force/acc.)C=damping(force/vel.)K=stiffness(force/disp.)Acce

3、lerationVectorVelocityVectorDisplacementVectorAppliedforceVectorBasicSDOFModelf(t)x(t)kcmSDOFModels—TimeandFrequencyDomainH()F()X()

4、H()

5、1kH()0º–90º–180º0=k/m12m1cf(t)x(t)kcmModalMatrixModalModel(Freq.Domain)X1X2X3X4F3H21H22MDOFModel0°-90°-180°md1+d2d1dF211+2121+2

6、FrequencyFrequencyPhaseMagnitudeWhyBotherwithModalModels?PhysicalCoordinates=CHAOSModalSpace=Simplicityq1112q21q313BearingRotorBearingFoundationDefinitionofFrequencyResponseFunctionFXfffH(f)isthesystemFrequencyResponseFunctionF(f)istheFourierTransformoftheInputf(t)X(f)ist

7、heFourierTransformoftheOutputx(t)F(f)H(f)X(f)BenefitsofFrequencyResponseFunctionF(f)H(f)X(f)FrequencyResponseFunctionsarepropertiesoflineardynamicsystemsTheyareindependentoftheExcitationFunctionExcitationcanbeaPeriodic,RandomorTransientfunctionoftimeThetestresultobtainedwith

8、onetypeofexcitationcanbeusedforpredictingtheresponseofthesystemtoanyothertypeofexcitationComplianceDynamicstiffness(displacement/force)(force/displacement)MobilityImpedance(velocity/force)(force/velocity)InertanceorReceptanceDynamicmass(acceleration/force)(force/acceleration

9、)DifferentFormsofanFRFAlternativeEstimatorsX(f)F(f)H(f)WhichFRFEstimatorShouldYouUse?Definitions:UsercanchooseH1,H2orH3aftermeasurementAccuracyAccuracyforsystemswith:H1H2H3Inputnoise-Best-OutputnoiseBest--Input+outputnoise--BestPeaks(leakage)-Best-Valleys(leakage)Best--SDOFa

10、ndMDOFModelsDifferentModalAnalysisTechniquesExcitingaStructureMeasuringData