外文翻译电容式传感器.docx
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外文翻译电容式传感器
CapacitiveSensorOperationPart1:
TheBasics
Part1ofthistwo-partarticlereviewstheconceptsandtheoryofcapacitivesensingtohelptooptimizecapacitivesensorperformance.Part2ofthisarticlewilldiscusshowtoputtheseconceptstowork.
Noncontactcapacitivesensorsmeasurethechangesinanelectricalpropertycalledcapacitance.Capacitancedescribeshowtwoconductiveobjectswithaspacebetweenthemrespondtoavoltagedifferenceappliedtothem.Avoltageappliedtotheconductorscreatesanelectricfieldbetweenthem,causingpositiveandnegativechargestocollectoneachobject
Capacitivesensorsuseanalternatingvoltagethatcausesthechargestocontinuallyreversetheirpositions.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.
FocusingtheElectricField
Whenavoltageisappliedtoaconductor,theelectricfieldemanatesfromeverysurface.Inacapacitivesensor,thesensingvoltageisappliedtothesensingareaoftheprobe.Foraccuratemeasurements,theelectricfieldfromthesensingareaneedstobecontainedwithinthespacebetweentheprobeandthetarget.Iftheelectricfieldisallowedtospreadtootheritems—orotherareasonthetarget—thenachangeinthepositionoftheotheritemwillbemeasuredasachangeinthepositionofthetarget.Atechniquecalled"guarding"isusedtopreventthisfromhappening.Tocreateaguard,thebackandsidesofthesensingareaaresurroundedbyanotherconductorthatiskeptatthesamevoltageasthesensingareaitself.Whenthevoltageisappliedtothesensingarea,aseparatecircuitappliestheexactsamevoltagetotheguard.Becausethereisnodifferenceinvoltagebetweenthesensingareaandtheguard,thereisnoelectricfieldbetweenthem.Anyotherconductorsbesideorbehindtheprobeformanelectricfieldwiththeguardinsteadofwiththesensingarea.Onlytheunguardedfrontofthesensingareaisallowedtoformanelectricfieldwiththetarget.
Definitions
Sensitivityindicateshowmuchtheoutputvoltagechangesasaresultofachangeinthegapbetweenthetargetandtheprobe.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%.
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
–0.010
0.75
–5.000
–4.900
–0.005
1.00
0.000
0.000
0.000
1.25
5.000
5.000
0.000
1.50
10.000
10.100
0.005
Figure7.Errorvalues
Bandwidthisdefinedasthefrequencyatwhichtheoutputfallsto–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.
Ifadriverhasanoutputnoiseof0.002Vwithasensitivityof10V/1mm,thenithasanoutputnoiseof0.000,2mm(0.2µm).Thismeansthatatanyinstantintime,theoutputcouldhaveanerrorof0.2µm.
Theamountofnoiseintheoutputisdirectlyrelatedtobandwidth.Generallyspeaking,noiseisdistributedoverawiderangeoffrequencies.Ifthehigherfrequenciesarefilteredbeforetheoutput,theresultislessnoiseandbetterresolution(Figures8,9).Whenexaminingresolutionspecifications,itiscriticaltoknowatwhatbandwidththespecificationsapply.
CapacitiveSensorOperationPart2:
SystemOptimization
Part2ofthistwo-partarticlefocusesonhowtooptimizetheperformanceofyourcapacitivesensor,andtounderstandhowtargetmaterial,shape,andsizewillaffectthesensor'sresponse.
EffectsofTargetSize
Thetargetsizeisaprimaryconsiderationwhenselectingaprobeforaspecificapplication.Whenthesensingelectricfieldisfocusedbyguarding,itcreatesaslightlyconicalfieldthatisaprojectionofthesensingarea.Theminimumtargetdiameterisusually130%ofthediameterofthesensingarea.Thefurthertheprobeisfromthetarget,thelargertheminimumtargetsize.
RangeofMeasurement
Therangeinwhichaprobeisusefulisafunctionofthesizeofthesensingarea.Thegreaterthearea,thelargertherange.Becausethedriverelectronicsaredesignedforacertainamountofcapacitanceattheprobe,asmallerprobemustbeconsiderablyclosertothetargettoachievethedesiredamountofcapacitance.Ingeneral,themaximumgapatwhichaprobeisusefulisapproximately40%ofthesensingareadiameter.Typicalcalibrationsusuallykeepthegaptoavalueconsiderablylessthanthis.Althoughtheelectronicsareadjustableduringcalibration,thereisalimittotherangeofadjustment.
MultipleChannelSensing
Frequently,atargetismeasuredsimultaneouslybymultipleprobes.Becausethesystemmeasuresachangingelectricfield,theexcitationvoltageforeachprobemustbesynchronizedortheprobeswillinterferewitheachother.Iftheywerenotsynchronized,oneprobewouldbetryingtoincreasethe