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

APracticalApproachtoVibrationDetectionandMeasurementPhysicalPrinciplesandDetectionTechniquesBy:JohnWilson,theDynamicConsultant,LLCThistutorialaddressesthephysicsofvibration；dynamicsofaspringmasssystem；damping；displacement,velocity,andacceleration；andtheoperatingprinciplesofthesensorsthatdetectandmeasuretheseproperties.Vibrationisoscillatorymotionresultingfromtheapplicationofoscillatoryorvaryingforcestoastructure.Oscillatorymotionreversesdirection.Asweshallsee,theoscillationmaybecontinuousduringsometimeperiodofinterestoritmaybeintermittent.Itmaybeperiodicornonperiodic,i.e.,itmayormaynotexhibitaregularperiodofrepetition.Thenatureoftheoscillationdependsonthenatureoftheforcedrivingitandonthestructurebeingdriven.Motionisavectorquantity,exhibitingadirectionaswellasamagnitude.Thedirectionofvibrationisusuallydescribedintermsofsomearbitrarycoordinatesystem(typicallyCartesianororthogonal)whosedirectionsarecalledaxes.Theoriginfortheorthogonalcoordinatesystemofaxesisarbitrarilydefinedatsomeconvenientlocation.Mostvibratoryresponsesofstructurescanbemodeledassingle-degree-of-freedomspringmasssystems,andmanyvibrationsensorsuseaspringmasssystemasthemechanicalpartoftheirtransductionmechanism.Inadditiontophysicaldimensions,aspringmasssystemcanbecharacterizedbythestiffnessofthespring,K,andthemass,M,orweight,W,ofthemass.Thesecharacteristicsdeterminenotonlythestaticbehavior(staticdeflection,d)ofthestructure,butalsoitsdynamiccharacteristics.Ifgistheaccelerationofgravity:F=MAW=MgK=F/d=W/dd=F/K=W/K=Mg/KDynamicsofaSpringMassSystemThedynamicsofaspringmasssystemcanbeexpressedbythesystemsbehaviorinfreevibrationand/orinforcedvibration.FreeVibration.Freevibrationisthecasewherethespringisdeflectedandthenreleasedandallowedtovibratefreely.Examplesincludeadivingboard,abungeejumper,andapendulumorswingdeflectedandlefttofreelyoscillate.Twocharacteristicbehaviorsshouldbenoted.First,dampinginthesystemcausestheamplitudeoftheoscillationstodecreaseovertime.Thegreaterthedamping,thefastertheamplitudedecreases.Second,thefrequencyorperiodoftheoscillationisindependentofthemagnitudeoftheoriginaldeflection(aslongaselasticlimitsarenotexceeded).Thenaturallyoccurringfrequencyofthefreeoscillationsiscalledthenaturalfrequency,fn:ForcedVibration.Forcedvibrationisthecasewhenenergyiscontinuouslyaddedtothespringmasssystembyapplyingoscillatoryforceatsomeforcingfrequency,ff.Twoexamplesarecontinuouslypushingachildonaswingandanunbalancedrotatingmachineelement.Ifenoughenergytoovercomethedampingisapplid,themotionwillcontinueaslongastheexcitationcontinues.Forcedvibrationmaytaketheformofself-excitedorexternallyexcitedvibration.Self-excitedvibrationoccurswhentheexcitationforceisgeneratedinoronthesuspendedmass；externallyexcitedvibrationoccurswhentheexcitationforceisappliedtothespring.Thisisthecase,forexample,whenthefoundationtowhichthespringisattachedismoving.Transmissibility.Whenthefoundationisoscillating,andforceistransmittedthroughthespringtothesuspendedmass,themotionofthemasswillbedifferentfromthemotionofthefoundation.Wewillcallthemotionofthefoundationtheinput,I,andthemotionofthemasstheresponse,R.TheratioR/Iisdefinedasthetransmissibility,Tr:Tr=R/IResonance.Atforcingfrequencieswellbelowthesystemsnaturalfrequency,RI,andTr1.Astheforcingfrequencyapproachesthenaturalfrequency,transmissibilityincreasesduetoresonance.Resonanceisthestorageofenergyinthemechanicalsystem.Atforcingfrequenciesnearthenaturalfrequency,energyisstoredandbuildsup,resultinginincreasingresponseamplitude.Dampingalsoincreaseswithincreasingresponseamplitude,however,andeventuallytheenergyabsorbedbydamping,percycle,equalstheenergyaddedbytheexcitingforce,andequilibriumisreached.Wefindthepeaktransmissibilityoccurringwhenfffn.Thisconditioniscalledresonance.Isolation.Iftheforcingfrequencyisincreasedabovefn,Rdecreases.Whenff=1.414fn,R=IandTr=1；athigherfrequenciesRIandTr1.AtfrequencieswhenR0.1in.,tomakethempractical.Thechangeinintensityorangleofalightbeamdirectedontoareflectivesurfacecanbeusedasanindicationofitsdistancefromthesource.Ifthedetectionapparatusisfastenough,changesofdistancecanbedetectedaswell.Themostsensitive,accurate,andpreciseopticalde