外文翻译--电容式传感器.doc

南昌航空大学科技学院学士学位（论文）外文翻译CapacitiveSensorOperationPart1:TheBasicsPart1ofthistwo-partarticlereviewstheconceptsandtheoryofcapacitivesensingtohelptooptimizecapacitivesensorperformance.Part2ofthisarticlewilldiscusshowtoputtheseconceptstowork.Noncontactcapacitivesensorsmeasurethechangesinanelectricalpropertycalledcapacitance.Capacitancedescribeshowtwoconductiveobjectswithaspacebetweenthemrespondtoavoltagedifferenceappliedtothem.Avoltageappliedtotheconductorscreatesanelectricfieldbetweenthem,causingpositiveandnegativechargestocollectoneachobjectCapacitivesensorsuseanalternatingvoltagethatcausesthechargestocontinuallyreversetheirpositions.Themovementofthechargescreatesanalternatingelectriccurrentthatisdetectedbythesensor.Theamountofcurrentflowisdeterminedbythecapacitance,andthecapacitanceisdeterminedbythesurfaceareaandproximityoftheconductiveobjects.Largerandcloserobjectscausegreatercurrentthansmallerandmoredistantobjects.Capacitanceisalsoaffectedbythetypeofnonconductivematerialinthegapbetweentheobjects.Technicallyspeaking,thecapacitanceisdirectlyproportionaltothesurfaceareaoftheobjectsandthedielectricconstantofthematerialbetweenthem,andinverselyproportionaltothedistancebetweenthemasshown.:Intypicalcapacitivesensingapplications,theprobeorsensorisoneoftheconductiveobjectsandthetargetobjectistheother.(Usingcapacitivesensorstosenseplasticsandotherinsulatorswillbediscussedinthesecondpartofthisarticle.)Thesizesofthesensorandthetargetareassumedtobeconstant,asisthematerialbetweenthem.Therefore,anychangeincapacitanceisaresultofachangeinthedistancebetweentheprobeandthetarget.Theelectronicsarecalibratedtogeneratespecificvoltagechangesforcorrespondingchangesincapacitance.Thesevoltagesarescaledtorepresentspecificchangesindistance.Theamountofvoltagechangeforagivenamountofdistancechangeiscalledthesensitivity.Acommonsensitivitysettingis1.0V/100m.Thatmeansthatforevery100mchangeindistance,theoutputvoltagechangesexactly1.0V.Withthiscalibration,a2Vchangeintheoutputmeansthatthetargethasmoved200mrelativetotheprobe.FocusingtheElectricFieldWhenavoltageisappliedtoaconductor,theelectricfieldemanatesfromeverysurface.In南昌航空大学科技学院学士学位（论文）外文翻译acapacitivesensor,thesensingvoltageisappliedtothesensingareaoftheprobe.Foraccuratemeasurements,theelectricfieldfromthesensingareaneedstobecontainedwithinthespacebetweentheprobeandthetarget.Iftheelectricfieldisallowedtospreadtootheritemsorotherareasonthetargetthenachangeinthepositionoftheotheritemwillbemeasuredasachangeinthepositionofthetarget.Atechniquecalledguardingisusedtopreventthisfromhappening.Tocreateaguard,thebackandsidesofthesensingareaaresurroundedbyanotherconductorthatiskeptatthesamevoltageasthesensingareaitself.Whenthevoltageisappliedtothesensingarea,aseparatecircuitappliestheexactsamevoltagetotheguard.Becausethereisnodifferenceinvoltagebetweenthesensingareaandtheguard,thereisnoelectricfieldbetweenthem.Anyotherconductorsbesideorbehindtheprobeformanelectricfieldwiththeguardinsteadofwiththesensingarea.Onlytheunguardedfrontofthesensingareaisallowedtoformanelectricfieldwiththetarget.DefinitionsSensitivityindicateshowmuchtheoutputvoltagechangesasaresultofachangeinthegapbetweenthetargetandtheprobe.Acommonsensitivityis1V/0.1mm.Thismeansthatforevery0.1mmofchangeinthegap,theoutputvoltagewillchange1V.Whentheoutputvoltageisplottedagainstthegapsize,theslopeofthelineisthesensitivity.Asystemssensitivityissetduringcalibration.Whensensitivitydeviatesfromtheidealvaluethisiscalledsensitivityerror,gainerror,orscalingerror.Sincesensitivityistheslopeofaline,sensitivityerrorisusuallypresentedasapercentageofslope,acomparisonoftheidealslopewiththeactualslope.Offseterroroccurswhenaconstantvalueisaddedtotheoutputvoltageofthesystem.Capacitivegaugingsystemsareusuallyzeroedduringsetup,eliminatinganyoffsetdeviationsfromtheoriginalcalibration.However,shouldtheoffseterrorchangeafterthesystemiszeroed,errorwillbeintroducedintothemeasurement.Temperaturechangeistheprimaryfactorinoffseterror.Sensitivitycanvaryslightlybetweenanytwopointsofdata.Theaccumulatedeffectofthisvariationiscalledlinearityerro.Thelinearityspecificationisthemeasurementofhowfartheoutputvariesfromastraightline.Tocalculatethelinearityerror,calibrationdataarecomparedtothestraightlinethatwouldbestfitthepoints.Thisstraightreferencelineiscalculatedfromthecalibrationdatausingleastsquaresfitting.Theamountoferroratthepointonthecalibrationlinefurthestawayfromthisideallineisthelinearityerror.Linearityerrorisusuallyexpressedinterms南昌航空大学科技学院学士学位（论文）外文翻译ofpercentoffullscale(%/F.S.).Iftheerrorattheworstpointis0.001mmandthefullscalerangeofthecalibrationis1mm,thelinearityerrorwillbe0.1%.Notethatlinearityerrordoesnotaccountforerrorsinsensitivity.Itisonlyameasureofthestraightnessofthelineratherthantheslopeoftheline.Asystemwithgrosssensitivityerrorscanstillbeverylinear.Errorbandaccountsforthecombinationoflinearityandsensitivityerrors.Itisthemeasurementoftheworst-caseabsoluteerrorinthecalibratedrange.Theerrorbandiscalculatedbycomparingtheoutputvoltagesatspecificgapstotheirexpectedvalue.Theworst-caseerrorfromthiscomparisonislistedasthesystemserrorband.InFigure7,theworst-caseerroroccursfora0.50mmgapandtheerrorband(inbold)is0.010.Gap(mm)ExpectedValue(VDC)ActualValueVDC)Error(mm)0.5010.0009.8000.0100.755.0004.9000.0051.000.0000.0000.0001.255.0005.0000.0001.5010.00010.1000.005Figure7.ErrorvaluesBandwidthisdefinedasthefrequencyatwhichtheoutputfallsto3dB,afrequencythatisalsocalledthecutofffrequency.A3dBdropinthesignallevelisanapproximately30%decrease.Witha15kHzbandwidth,achangeof1Vatlowfrequencywillonlyproducea0.7Vchangeat15kHz.Wide-bandwidthsensorscansensehigh-frequencymotionandprovidefast-respondingoutputstomaximizethephasemarginwhenusedinservo-controlfeedbacksystems；however,lower-bandwidthsensorswillhavereducedoutputnoisewhichmeanshigherresolution.Somesensorsprovideselectablebandwidthtomaximizeeitherresolutionorresponsetime.Resolutionisdefinedasthesmallestreliablemeasurementthatasystemcanmake.Theresolutionofameasurementsystemmustbebetterthanthefinalaccuracythemeasurementrequires.Ifyouneedtoknowameasurementwithin0.02m,thentheresolutionofthemeasurementsystemmustbebetterthan0.02m.Theprimarydeterminingfactorofresolutioniselectricalnoise.Electricalnoiseappearsintheoutputvoltagecausingsmallinstantaneouserrorsintheoutput.Evenwhentheprobe/targetgapisperfectlyconstant,theoutputvoltageofthedriverhassomesmall南昌航空大学科技学院学士学位（论文）外文翻译butmeasurableamountofnoisethatwouldseemtoindicatethatthegapischanging.Thisnoiseisinherentinelectroniccomponentsandcanbeminimized,butnevereliminated.Ifadriverhasanoutputnoiseof0.002Vwithasensitivityof10V/1mm,thenithasanoutputnoiseof0.000,2mm(0.2m).Thismeansthatatanyinstantintime,theoutputcouldhaveanerrorof0.2m.Theamountofnoiseintheoutputisdirectlyrelatedtobandwidth.Generallyspeaking,noiseisdistributedoverawiderangeoffrequencies.Ifthehigherfrequenciesarefilteredbeforetheoutput,theresultislessnoiseandbetterresolution(Figures8,9).Whenexaminingresolutionspecifications,itiscriticaltoknowatwhatbandwidththespecificationsapply.CapacitiveSensorOperationPart2:SystemOptimizationPart2ofthistwo-partarticlefocusesonhowtooptimizetheperformanceofyourcapacitivesensor,andtounderstandhowtargetmaterial,shape,andsizewillaffectthesensorsresponse.EffectsofTargetSizeThetargetsizeisaprimaryconsiderationwhenselectingaprobeforaspecificapplication.Whenthesensingelectricfieldisfocusedbyguarding,itcreatesaslightlyconicalfieldthatisaprojectionofthesensingarea.Theminimumtargetdiameterisusually130%ofthediameterofthesensingarea.Thefurthertheprobeisfromthetarget,thelargertheminimumtargetsize.Rangeo