激光测振仪

激光测振仪ReferencebeamMeasurementbeamDetectorLightSourcexDLSignalattheDetectorIfthedistancextothetargetchangescontinuously,thenthelightintensityIatthedetectorvariesinaperiodicmanner.Theperiodicityisequivalenttoachangeinxofl/2orthephasefof2p.IntensityIDisplacementxl/2ThephotodetectorisonlysensitivetotheintensityIofthelight.Thesignalatthedetectorhastheform:I=Imax/2{1+cos(Dj)}ThephasedifferenceDjisafunctionoftheopticalpathdifferenceDLaccordingto:Dj=4p(DL/l)TheDopplerFrequencyWhentheobjectmovesataconstantvelocityv,theopticalpathdifferenceDLbecomesafunctionofthetimet.DL=DL(t)Theintensityatthedetectorchangesperiodically.Thelengthofthisperiodisl/2.ThefrequencythatisproducedasafunctionofvelocityiscalledDopplerfrequencyfD.fD=2*|v|lAccordingtothetheoryofrelativitylighttheonlymovementwhichcanberecognizedistherelativemotionbetweenthetargetandtheinterferometer.Thereisnodirectionalsensitivity.|v|l/2=DirectionDetectionHeterodyneInaheterodyneinterferometerthelightinonearmoftheinterferometerisfrequencyshiftedwithrespecttotheother.Braggcellf0+fDf0f0+40MHzSignalatthedetectorhastheformI=Imax/2{1+cos(2p(fB+fDt)}SignalDemodulationSensorheadFrequencyVelocitydecoderjmodfmodDfVV(t)PhaseDisplacementdecoder(optional)DjVX(t)f0j0X(t)V(t)=dx/dtThesignalprocessorofaPolytecLDVcancontainvelocityand/ordisplacementoutputs.Theinterfertometercomparesphasejmodandfrequencyfmodoftheobjectbeamwiththoseoftheinternalreferencebeam(f0andj0).Displacementisderivedbymeasuringthephasechangeandvelocitybymeasuringthefrequency.Singlepoint,out-of-planevibrometersDifferentialout-ofplanevibrometerIn-planevibrometersScanningvibrometersRotationalvibrometer3Dvibrometer