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外文翻译--电容式传感器.doc

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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CapacitiveSensorOperationPart1TheBasicsPart1ofthistwopartarticlereviewstheconceptsandtheoryofcapacitivesensingtohelptooptimizecapacitivesensorperformance.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/100µm.Thatmeansthatforevery100µmchangeindistance,theoutputvoltagechangesexactly1.0V.Withthiscalibration,a2Vchangeintheoutputmeansthatthetargethasmoved200µmrelativetotheprobe.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.Itisthemeasurementoftheworstcaseabsoluteerrorinthecalibratedrange.Theerrorbandiscalculatedbycomparingtheoutputvoltagesatspecificgapstotheirexpectedvalue.Theworstcaseerrorfromthiscomparisonislistedasthesystemserrorband.InFigure7,theworstcaseerroroccursfora0.50mmgapandtheerrorbandinboldis–0.010.GapmmExpectedValueVDCActualValueVDCErrormm0.50–10.000–9.800–0.0100.75–5.000–4.900–0.0051.000.0000.0000.0001.255.0005.0000.0001.5010.00010.1000.005Figure7.ErrorvaluesBandwidthisdefinedasthefrequencyatwhichtheoutputfallsto–3dB,afrequencythatisalsocalledthecutofffrequency.A–3dBdropinthesignallevelisanapproximately30decrease.Witha15kHzbandwidth,achangeof±1Vatlowfrequencywillonlyproducea±0.7Vchangeat15kHz.Widebandwidthsensorscansensehighfrequencymotionandprovidefastrespondingoutputstomaximizethephasemarginwhenusedinservocontrolfeedbacksystemshowever,lowerbandwidthsensorswillhavereducedoutputnoisewhichmeanshigherresolution.Somesensorsprovideselectablebandwidthtomaximizeeitherresolutionorresponsetime.Resolutionisdefinedasthesmallestreliablemeasurementthatasystemcanmake.Theresolutionofameasurementsystemmustbebetterthanthefinalaccuracythemeasurementrequires.Ifyouneedtoknowameasurementwithin0.02µm,thentheresolutionofthemeasurementsystemmustbebetterthan0.02µm.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,thenithasanoutputnoiseof0.000,2mm0.2µm.Thismeansthatatanyinstantintime,theoutputcouldhaveanerrorof0.2µm.Theamountofnoiseintheoutputisdirectlyrelatedtobandwidth.Generallyspeaking,noiseisdistributedoverawiderangeoffrequencies.Ifthehigherfrequenciesarefilteredbeforetheoutput,theresultislessnoiseandbetterresolutionFigures8,9.Whenexaminingresolutionspecifications,itiscriticaltoknowatwhatbandwidththespecificationsapply.CapacitiveSensorOperationPart2SystemOptimizationPart2ofthistwopartarticlefocusesonhowtooptimizetheperformanceofyourcapacitivesensor,andtounderstandhowtargetmaterial,shape,andsizewillaffectthesensorsresponse.EffectsofTargetSizeThetargetsizeisaprimaryconsiderationwhenselectingaprobeforaspecificapplication.Whenthesensingelectricfieldisfocusedbyguarding,itcreatesaslightlyconicalfieldthatisaprojectionofthesensingarea.Theminimumtargetdiameterisusually130ofthediameterofthesensingarea.Thefurthertheprobeisfromthetarget,thelargertheminimumtargetsize.RangeofMeasurementTherangeinwhichaprobeisusefulisafunctionofthesizeofthesensingarea.Thegreaterthearea,thelargertherange.Becausethedriverelectronicsaredesignedforacertainamountofcapacitanceattheprobe,asmallerprobemustbeconsiderablyclosertothetargettoachievethedesiredamountofcapacitance.Ingeneral,themaximumgapatwhichaprobeisusefulisapproximately40ofthesensingareadiameter.Typicalcalibrationsusuallykeepthegaptoavalueconsiderablylessthanthis.Althoughtheelectronicsareadjustableduringcalibration,thereisalimittotherangeofadjustment.MultipleChannelSensingFrequently,atargetismeasuredsimultaneouslybymultipleprobes.Becausethesystemmeasuresachangingelectricfield,theexcitationvoltageforeachprobemustbesynchronizedortheprobeswillinterferewitheachother.Iftheywerenotsynchronized,oneprobewouldbetryingtoincreasetheelectricfieldwhileanotherwastryingto

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