电容触摸屏控制设计外文文献及中文翻译.docx
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电容触摸屏控制设计外文文献及中文翻译.docx
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电容触摸屏控制设计外文文献及中文翻译
ALow-Cost,SmartCapacitivePositionSensor
Abstract
Anewhigh-performance,low-cost,capacitiveposition-measuringsystemisdescribed.Byusingahighlylinearoscillator,shieldingandathree-signalapproach,mostoftheerrorsareeliminated.Theaccuracyamountsto1μmovera1mmrange.Sincetheoutputoftheoscillatorcandirectlybeconnectedtoamicrocontroller,anA/Dconverterisnotneeded.
I.INTRODUCTION
Thispaperdescribesanovelhigh-performance,low-cost,capacitivedisplacementmeasuringsystemfeaturing:
1mmmeasuringrange,
1μmaccuracy,
0.1stotalmeasuringtime.
Translatedtothecapacitivedomain,thespecificationscorrespondto:
apossiblerangeof1pF。
only50fFofthisrangeisusedforthedisplacementtransducer。
50aFabsolutecapacitance-measuringinaccuracy.
MeijerandSchrier[l]andmorerecentlyVanDrecht,Meijer,andDeJong[2]haveproposedadisplacement-measuringsystem,usingaPSD(PositionSensitiveDetector)assensingelement.SomedisadvantagesofusingaPSDarethehighercostsandthehigherpowerconsumptionofthePSDandLED(Light-EmittingDiode)ascomparedtothecapacitivesensorelementsdescribedinthispaper.
Thesignalprocessorusestheconceptspresentedin[2],butisadoptedfortheuseofcapacitiveelements.Bytheextensiveuseofshielding,guardingandsmartA/Dconversion,thesystemisabletocombineahighaccuracywithaverylowcost-price.Thetransducerproducesthree-period-modulatedsignalswhichcanbeselectedanddirectlyreadoutbyamicrocontroller.Themicrocontroller,inreturn,calculatesthedisplacementandcansendthisvaluetoahostcomputer(Fig.1)oradisplayordriveanactuator.
Fig.1.Blockdiagramofthesystem
Fig.2.Perspectiveanddimensionsoftheelectrodestructure
Ⅱ.THEELECTRODESTRUCTURE
ThebasicsensingelementconsistsoftwosimpleelectrodeswithcapacitanceCx,(Fig.2).Thesmallerone(E2)issurroundedbyaguardelectrode.Thankstotheuseoftheguardelectrode,thecapacitanceCxbetweenthetwoelectrodesisindependentofmovements(lateraldisplacementsaswellasrotations)paralleltotheelectrodesurface.TheinfluenceoftheparasiticcapacitancesCpwillbeeliminatedaswillbediscussedinSectionⅢ.
AccordingtoHeerens[3],therelativedeviationinthecapacitanceCxbetweenthetwoelectrodescausedbythefiniteguardelectrodesizeissmallerthan:
δ (1) wherexisthewidthoftheguardanddthedistancebetweentheelectrodes.Thisdeviationintroducesanonlinearity.Thereforewerequirethatδislessthan100ppm.Alsothegapbetweenthesmallelectrodeandthesurroundingguardcausesadeviation: δ (2) withsthewidthofthegap.Thisdeviationisnegligiblecomparedto(l),whenthegapwidthislessthan1/3ofthedistancebetweentheelectrodes. Anothercauseoferrorsoriginatesfromapossiblefiniteskewangleαbetweenthetwoelectrodes(Fig.3).Assumingthefollowingconditions: thepotentialsonthesmallelectrodeandtheguardelectrodeareequalto0V, thepotentialonthelargeelectrodeisequaltoVvolt, theguardelectrodeislargeenough, itcanbeseenthattheelectricfieldwillbeconcentric. Fig.3.Electrodeswithangleα. Tokeepthecalculationssimple,wewillassumetheelectrodestobeinfinitelylargeinonedirection.Nowtheproblemisatwo-dimensionalonethatcanbesolvedbyusingpolar-coordinates(r,φ).Inthiscasetheelectricalfieldcanbedescribedby: (3) Tocalculatethechargeonthesmallelectrode,wesetφto0andintegrateoverr: (4) withBltheleftborderofthesmallelectrode: (5) andBrtherightborder: (6) Solving(4)resultsin: (7) Forsmallα'sthiscanbeapproximatedby: (8) Itappearstobedesirabletochooselsmallerthand,sotheerrorwilldependonlyontheangleα.Inourcase,achangeintheangleof0.6°willcauseanerrorlessthan100ppm. Withaproperdesigntheparametersεoandlareconstant,andthenthecapacitancebetweenthetwoelectrodeswilldependonlyonthedistancedbetweentheelectrodes. Ⅲ.ELIMINATIONOFPARASITICCAPACITANCES BesidesthedesiredsensorcapacitanceC,therearealsomanyparasiticcapacitancesintheactualstructure(Fig.2).ThesecapacitancescanbemodeledasshowninFig.4.HereCplrepresentstheparasiticcapacitancesfromtheelectrodeE1andCp2fromtheelectrodeE2totheguardelectrodesandtheshielding.ParasiticcapacitanceCp3resultsfromimperfectshieldingandformsanoffsetcapacitance.WhenthetransducercapacitanceCxisconnectedtoanACvoltagesourceandthecurrentthroughtheelectrodeismeasured,CplandCp2willbeeliminated.Cp3canbeeliminatedbyperforminganoffsetmeasurement. Fig.4.Eliminationofparasiticcapacitances Thecurrentismeasuredbytheamplifierwithshuntfeedback,wh
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