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大连交通大学2010届本科毕业生毕业设计(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/100µm.Thatmeansthatforevery100µmchangeindistance,theoutputvoltagechangesexactly1.0V.Withthiscalibration,a2Vchangeintheoutputmeansthatthetargethasmoved200µmrelativetotheprobe.FocusingtheElectricFieldWhenavoltageisappliedtoaconductor,theelectricfieldemanatesfromeverysurface.Inacapacitivesensor,thesensingvoltageisappliedtothesensingareaoftheprobe.Foraccurate大连交通大学2010届本科毕业生毕业设计(论文)外文翻译measurements,theelectricfieldfromthesensingareaneedstobecontainedwithinthespacebetweentheprobeandthetarget.Iftheelectricfieldisallowedtospreadtootheritems—orotherareasonthetarget—thenachangeinthepositionoftheotheritemwillbemeasuredasachangeinthepositionofthetarget.Atechniquecalled"guarding"isusedtopreventthisfromhappening.Tocreateaguard,thebackandsidesofthesensingareaaresurroundedbyanotherconductorthatiskeptatthesamevoltageasthesensingareaitself.Whenthevoltageisappliedtothesensingarea,aseparatecircuitappliestheexactsamevoltagetotheguard.Becausethereisnodifferenceinvoltagebetweenthesensingareaandtheguard,thereisnoelectricfieldbetweenthem.Anyotherconductorsbesideorbehindtheprobeformanelectricfieldwiththeguardinsteadofwiththesensingarea.Onlytheunguardedfrontofthesensingareaisallowedtoformanelectricfieldwiththetarget.DefinitionsSensitivityindicateshowmuchtheoutputvoltagechangesasaresultofachangeinthegapbetweenthetargetandtheprobe.Acommonsensitivityis1V/0.1mm.Thismeansthatforevery0.1mmofchangeinthegap,theoutputvoltagewillchange1V.Whentheoutputvoltageisplottedagainstthegapsize,theslopeofthelineisthesensitivity.Asystem'ssensitivityissetduringcalibration.Whensensitivitydeviatesfromtheidealvaluethisiscalledsensitivityerror,gainerror,orscalingerror.Sincesensitivityistheslopeofaline,sensitivityerrorisusuallypresentedasapercentageofslope,acomparisonoftheidealslopewiththeactualslope.Offseterroroccurswhenaconstantvalueisaddedtotheoutputvoltageofthesystem.Capacitivegaugingsystemsareusually"zeroed"duringsetup,eliminatinganyoffsetdeviationsfromtheoriginalcalibration.However,shouldtheoffseterrorchangeafterthesystemiszeroed,errorwillbeintroducedintothemeasurement.Temperaturechangeistheprimaryfactorinoffseterror.Sensitivitycanvaryslightlybetweenanytwopointsofdata.Theaccumulatedeffectofthisvariationiscalledlinearityerro.Thelinearityspecificationisthemeasurementofhowfartheoutputvariesfromastraightline.Tocalculatethelinearityerror,calibrationdataarecomparedtothestraightlinethatwouldbestfitthepoints.Thisstraightreferencelineiscalculatedfromthecalibrationdatausingleastsquaresfitting.Theamountoferroratthepointonthecalibrationlinefurthestawayfromthisideallineisthelinearityerror.Linearityerrorisusuallyexpressedintermsofpercentoffullscale(%/F.S.).Iftheerrorattheworstpointis0.001mmandthefullscalerangeofthecalibrationis1mm,thelinearityerrorwillbe0.1%.大连交通大学2010届本科毕业生毕业设计(论文)外文翻译Notethatlinearityerrordoesnotaccountforerrorsinsensitivity.Itisonlyameasureofthestraightnessofthelineratherthantheslopeoftheline.Asystemwithgrosssensitivityerrorscanstillbeverylinear.Errorbandaccountsforthecombinationoflinearityandsensitivityerrors.Itisthemeasurementoftheworst-caseabsoluteerrorinthecalibratedrange.Theerrorbandiscalculatedbycomparingtheoutputvoltagesatspecificgapstotheirexpectedvalue.Theworst-caseerrorfromthiscomparisonislistedasthesystem'serrorband.InFigure7,theworst-caseerroroccursfora0.50mmgapandtheerrorband(inbold)is–0.010.Gap(mm)ExpectedValue(VDC)ActualValueVDC)Error(mm)0.50–10.000–9.800–0.0100.75–5.000–4.900–0.0051.000.0000.0000.0001.255.0005.0000.0001.5010.00010.1000.005Figure7.ErrorvaluesBandwidthisdefinedasthefrequencyatwhichtheoutputfallsto–3dB,afrequencythatisalsocalledthecutofffrequency.A–3dBdropinthesignallevelisanapproximately30%decrease.Witha15kHzbandwidth,achangeof±1Vatlowfrequencywillonlyproducea±0.7Vchangeat15kHz.Wide-bandwidthsensorscansensehigh-frequencymotionandprovidefast-respondingoutputstomaximizethephasemarginwhenusedinservo-controlfeedbacksystems;however,lower-bandwidthsensorswillhavereducedoutputnoisewhichmeanshigherresolution.Somesensorsprovideselectablebandwidthtomaximizeeitherresolutionorresponsetime.Resolutionisdefinedasthesmallestreliablemeasurementthatasystemcanmake.Theresolutionofameasurementsystemmustbebetterthanthefinalaccuracythemeasurementrequires.Ifyouneedtoknowameasurementwithin0.02µm,thentheresolutionofthemeasurementsystemmustbebetterthan0.02µm.Theprimarydeterminingfactorofresolutioniselectricalnoise.Electricalnoiseappearsintheoutputvoltagecausingsmallinstantaneouserrorsintheoutput.Evenwhentheprobe/targetgapisperfectlyconstant,theoutputvoltageofthedriverhassomesmallbutmeasurableamountofnoisethatwouldseemtoindicatethatthegapischanging.Thisnoiseisinherentinelectroniccomponentsandcanbeminimized,butnevereliminated.大连交通大学2010届本科毕业生毕业设计(论文)外文翻译Ifadriverhasanoutputnoiseof0.002Vwithasensitivityof10V/1mm,thenithasanoutputnoiseof0.000,2mm(0.2µm).Thismeansthatatanyinstantintime,theoutputcouldhaveanerrorof0.2µm.Theamountofnoiseintheoutputisdirectlyrelatedtobandwidth.Generallyspeaking,noiseisdistributedoverawiderangeoffrequencies.Ifthehigherfrequenciesarefilteredbeforetheoutput,theresultislessnoiseandbetterresolution(Figures8,9).Whenexaminingresolutionspecifications,itiscriticaltoknowatwhatbandwidththespecificationsapply.CapacitiveSensorOperationPart2:SystemOptimizationPart2ofthistwo-partarticlefocusesonhowtooptimizetheperformanceofyourcapacitivesensor,andtounderstandhowtargetmaterial,shape,andsizewillaffectthesensor'sresponse.EffectsofTargetSizeThetargetsizeisaprimaryconsiderationwhenselectingaprobeforaspecificapplication.Whenthesensingelectricfieldisfocusedbyguarding,itcreatesaslightlyconicalfieldthatisaprojectionofthesensingarea.Theminimumtargetdiameterisusually130%ofthediameterofthesensingarea.Thefurthertheprobeisfromthetarget,thelargertheminimumtargetsize.RangeofMeasurementTherangeinwhichaprobeisusefulisafunctionofthesizeofthesensingarea.Thegreaterthearea,thelargertherange.Becausethedriverelectronicsaredesignedforacertainamountofcapacitanceattheprobe,asmallerprobemustbeconsiderablyclosertothetargettoachievethedesiredamountofcapacitance.Ingeneral,themaximumgapatwhichaprobeisusefulisapproximately40%ofthesensingareadiameter.Typicalcalibrationsusuallykeepthegaptoavalueconsiderablylessthanthis.Althoughtheelectronicsareadjustableduringcalibration,thereisalimittotherangeofadjustment.MultipleChannelSensingFrequently,atargetismeasuredsimultaneouslybymultipleprobes.Becausethesystemmeasuresachangingelectricfield,theexcitationvoltageforeachprobemustbesynchronizedortheprobeswillinterferewitheachother.Iftheywerenotsynchronized,oneprobewouldbetryingtoincreasetheelectricfieldwhileanotherwastryingtodecreaseit;theresultwouldbeafalsereading.Driverelectronicscanbeconfiguredasmastersorslaves;themastersetsthesynchronizationfortheslavesinmultichannelsystems.
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