无接触式电阻率测试仪厚度测试原理

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1、CapacitanceSensingTheoryCapacitivesensorscanbedividedintotwocategoriesbasedupontheirperformanceandintendeduse.Highresolutionsensorsaretypicallyusedindisplacementandpositionmonitoringapplicationswherehighaccuracy,stabilityandlowtemperaturedriftarerequired.Quitefrequentlythesesensorsareusedinprocessm

2、onitoringandclosed-loopfeedbackcontrolsystems.Proximitytypecapacitivesensorsaremuchlessexpensiveandaretypicallyusedtodetectthepresenceofapartorusedincountingapplications.Thefollowingpaperdescribescharacteristicsofhighresolutionsystems,theiroperatingprincipleandapplication.Figure1:TypicalCapacitance

3、SensorThesesensorsareusedthroughoutavarietyofindustriestoprovidehighlystable,accuratemeasurementsofdisplacement,vibration,position,thicknessandrunout.Mostprobesarepassivebydesign,allowingthemtobeoperatedinhighshockandvibrationenvironments,andatextremetemperatures.Capacitanceprobesaretypicallymodele

4、dasaparallelplatecapacitor.Iftwoconductivesurfacesareseparatedbyadistanceandavoltageisappliedtooneofthesurfaces,anelectricfieldiscreated.Thisoccursduetothedifferentchargesstoredoneachofthesurfaces.Capacitancereferstotheabilityofthesurfacestoholdacharge.Inatypicalsensorsystemtheprobeisoneoftheplates

5、andthetargetbeingmeasuredistheotherplate.Ifaconstantcurrentisapplied,thecapacitancechangecanbemonitoredasalinearvoltagechargerelatedtothedistancebetweentheplates.Thisdistance,orgap,isafunctionoftheareaofthecapacitancesensoraccordingtothefollowingformula:Capacitance=AreaXDielectric/GapFromthisrelati

6、onship,youcanseethatcapacitanceisdirectlyproportionaltotheareaofthesensorandthedielectricpropertyofthematerialbetweenthesensorandtarget(typicallyair).Thegreatertheareaofthecapacitancesensorthelargerthemeasurementrange,orgap.Ifweassumetheareaandthedielectricbetweentheplatesremainconstantforaspecific

7、probe,anychangeincapacitanceisinverselyproportionaltothechangeindistancebetweentheprobeandtargetbeingmeasured.ThischangeisprocessedwithinMTII’sAccumeasure™capacitanceamplifierandconvertedtoavoltageformonito