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外文翻译--对振动侦查和测量的一种实用方法物理原则和侦查技术 英文版.pdf

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外文翻译--对振动侦查和测量的一种实用方法物理原则和侦查技术 英文版.pdf

APracticalApproachtoVibrationDetectionandMeasurementPhysicalPrinciplesandDetectionTechniquesByJohnWilson,theDynamicConsultant,LLCThistutorialaddressesthephysicsofvibrationdynamicsofaspringmasssystemdampingdisplacement,velocity,andaccelerationandtheoperatingprinciplesofthesensorsthatdetectandmeasuretheseproperties.Vibrationisoscillatorymotionresultingfromtheapplicationofoscillatoryorvaryingforcestoastructure.Oscillatorymotionreversesdirection.Asweshallsee,theoscillationmaybecontinuousduringsometimeperiodofinterestoritmaybeintermittent.Itmaybeperiodicornonperiodic,i.e.,itmayormaynotexhibitaregularperiodofrepetition.Thenatureoftheoscillationdependsonthenatureoftheforcedrivingitandonthestructurebeingdriven.Motionisavectorquantity,exhibitingadirectionaswellasamagnitude.ThedirectionofvibrationisusuallydescribedintermsofsomearbitrarycoordinatesystemtypicallyCartesianororthogonalwhosedirectionsarecalledaxes.Theoriginfortheorthogonalcoordinatesystemofaxesisarbitrarilydefinedatsomeconvenientlocation.Mostvibratoryresponsesofstructurescanbemodeledassingledegreeoffreedomspringmasssystems,andmanyvibrationsensorsuseaspringmasssystemasthemechanicalpartoftheirtransductionmechanism.Inadditiontophysicaldimensions,aspringmasssystemcanbecharacterizedbythestiffnessofthespring,K,andthemass,M,orweight,W,ofthemass.Thesecharacteristicsdeterminenotonlythestaticbehaviorstaticdeflection,dofthestructure,butalsoitsdynamiccharacteristics.IfgistheaccelerationofgravityFMAWMgKF/dW/ddF/KW/KMg/KDynamicsofaSpringMassSystemThedynamicsofaspringmasssystemcanbeexpressedbythesystemsbehaviorinfreevibrationand/orinforcedvibration.FreeVibration.Freevibrationisthecasewherethespringisdeflectedandthenreleasedandallowedtovibratefreely.Examplesincludeadivingboard,abungeejumper,andapendulumorswingdeflectedandlefttofreelyoscillate.Twocharacteristicbehaviorsshouldbenoted.First,dampinginthesystemcausestheamplitudeoftheoscillationstodecreaseovertime.Thegreaterthedamping,thefastertheamplitudedecreases.Second,thefrequencyorperiodoftheoscillationisindependentofthemagnitudeoftheoriginaldeflectionaslongaselasticlimitsarenotexceeded.Thenaturallyoccurringfrequencyofthefreeoscillationsiscalledthenaturalfrequency,fnForcedVibration.Forcedvibrationisthecasewhenenergyiscontinuouslyaddedtothespringmasssystembyapplyingoscillatoryforceatsomeforcingfrequency,ff.Twoexamplesarecontinuouslypushingachildonaswingandanunbalancedrotatingmachineelement.Ifenoughenergytoovercomethedampingisapplid,themotionwillcontinueaslongastheexcitationcontinues.Forcedvibrationmaytaketheformofselfexcitedorexternallyexcitedvibration.Selfexcitedvibrationoccurswhentheexcitationforceisgeneratedinoronthesuspendedmassexternallyexcitedvibrationoccurswhentheexcitationforceisappliedtothespring.Thisisthecase,forexample,whenthefoundationtowhichthespringisattachedismoving.Transmissibility.Whenthefoundationisoscillating,andforceistransmittedthroughthespringtothesuspendedmass,themotionofthemasswillbedifferentfromthemotionofthefoundation.Wewillcallthemotionofthefoundationtheinput,I,andthemotionofthemasstheresponse,R.TheratioR/Iisdefinedasthetransmissibility,TrTrR/IResonance.Atforcingfrequencieswellbelowthesystemsnaturalfrequency,RI,andTr1.Astheforcingfrequencyapproachesthenaturalfrequency,transmissibilityincreasesduetoresonance.Resonanceisthestorageofenergyinthemechanicalsystem.Atforcingfrequenciesnearthenaturalfrequency,energyisstoredandbuildsup,resultinginincreasingresponseamplitude.Dampingalsoincreaseswithincreasingresponseamplitude,however,andeventuallytheenergyabsorbedbydamping,percycle,equalstheenergyaddedbytheexcitingforce,andequilibriumisreached.Wefindthepeaktransmissibilityoccurringwhenfffn.Thisconditioniscalledresonance.Isolation.Iftheforcingfrequencyisincreasedabovefn,Rdecreases.Whenff1.414fn,RIandTr1athigherfrequenciesR0.1in.,tomakethempractical.Thechangeinintensityorangleofalightbeamdirectedontoareflectivesurfacecanbeusedasanindicationofitsdistancefromthesource.Ifthedetectionapparatusisfastenough,changesofdistancecanbedetectedaswell.Themostsensitive,accurate,andpreciseopticaldeviceformeasuringdistanceordisplacementisthelaserinterferometer.Withthisapparatus,areflectedlaserbeamismixedwiththeoriginalincidentbeam.Theinterferencepatternsformedbythephasedifferencescanmeasuredisplacementdownto100

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