外文翻译电容式传感器.docx

外文翻译电容式传感器.docx

《外文翻译电容式传感器.docx》由会员分享，可在线阅读，更多相关《外文翻译电容式传感器.docx（12页珍藏版）》请在冰点文库上搜索。

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