电容触摸屏控制设计外文文献及中文翻译.docx

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,whichhasaverylowinputimpedance.Toobtaintherequiredlinearity,theunity-gainbandwidthfToftheamplifierhastosatisfythefollowingcondition:

（9）

whereTistheperiodoftheinputsignal.

SinceCp2consistsofcablecapacitancesandtheinputcapacitanceoftheopamp,itmayindeedbelargerthanCfandcannotbeneglected.

IV.THECONCEPTOFTHESYSTEM

Thesystemusesthethree-signalconceptpresentedin[2],whichisbasedonthefollowingprinciples.WhenwemeasureacapacitorCxwithalinearsystem,weobtainavalue:

（10）

wheremistheunknowngainandMoff,theunknownoffset.ByperformingthemeasurementofareferencequantityCref,inanidenticalwayandbymeasuringtheoffset,Moff,bymakingm=0,theparametersmandMoffareeliminated.ThefinalmeasurementresultPisdefinedas:

（11）

Inourcase,forthesensorcapacitanceC,itholdsthat:

（12）

whereAxistheareaoftheelectrode,doistheinitialdistancebetweenthem,εisthedielectricconstantand△disthedisplacementtobemeasured.Forthereferenceelectrodesitholdsthat:

（13）

withAreftheareaanddrefthedistance.Substitutionof（12）and（13）into（10）andtheninto（11）yields:

（14）

Here,Pisavaluerepresentingthepositionwhilea1anda0areunknown,butstableconstants.Theconstanta1=Aref/Axisastableconstantprovidedthereisagoodmechanicalmatchingbetweentheelectrodeareas.Theconstantao=（Arefd0/（Axdref）willalsobeastableconstantprovidedthatdoanddrefareconstant.Theseconstantscanbedeterminedbyaone-timecalibration.Inmanyapplicationsthiscalibrationcanbeomitted;whenthedisplacementsensorispartofalargersystem,anoverallcalibrationisrequiredanyway.Thisoverallcalibrationeliminatestherequirementforaseparatedeterminationofa1anda0.

V.THECAPACITANCE-TO-PERIODCONVERSION

Thesignalswhichareproportionaltothecapacitorvaluesareconvertedintoaperiod,usingamodifiedMartinoscillator[4]（Fig.5j.

WhenthevoltageswingacrossthecapacitorisequaltothatacrosstheresistorandtheNANDgatesareswitchedoff,thisoscillatorhasaperiodToff:

Toff=4RCoff.（15）

Sincethevalueoftheresistoriskeptconstant,theperiodvariesonlywiththecapacitorvalue.Now,byswitchingontherightNANDport,thecapacitanceCXcanbeconnectedinparalleltoCoff.Thentheperiodbecomes:

Tx=4R（Coff+Cx）=4RCx+Toff（16）

TheconstantsRandToffareeliminatedinthewaydescribedinSectionIV.

In[2]itisshownthatthesystemisimmuneformostofthenonidealitiesoftheopampandthecomparator,likeslewing,limitationsofbandwidthandgain,offsetvoltages,andinputbiascurrents.Thesenonidealitiesonlycauseadditiveormultiplicativeerrorswhichareeliminatedbythethree-signalapproach.

VI.PERIODMEASUREMENTWITHAMICROCONTROLLER

Performingperiodmeasurementwithamicrocontrollerisaneasytask.Inourcase,anINTEL87C51FAisused,whichhas8kByteROM,256ByteRAM,andUARTforserialcommunication,andthecapabilitytomeasureperiodswitha333nsresolution.Eventhoughthecountersare16bwide,theycaneasilybeextendedinthesoftwareto24bormore.

Theperiodmeasurementtakesplacemostlyinthehardwareofthemicrocontroller.Therefore,itispossibletolettheCPUofthemicrocontrollerperformothertasksatthesametime（Fig.6）.Forinstance,simultaneouslywiththemeasurementofperiodTx,periodTrefandperiodToff,therelativecapacitancewithrespecttoCrefiscalculatedaccordingto（11）,andtheresultistransferredthroughtheUARTtoapersonalcomputer.

Fig.5.ModifiedMartinoscillatorwithmicrocontrollerandelectrodes.

Fig.6.Periodmeasurementasbackgroundprocess.

Fig.7.Positionerrorasfunctionofthepositionandestimateofthenonlinearity.

VII.EXPERIMENTALRESULTS

Thesensorisnotsensitivetofabricationtolerancesoftheelectrodes.Thereforeinourexperimentalsetupweusedsimpleprintedcircuitboardtechnologytofabricatetheelectrodes,whichhaveaneffectiveareaof12mm×12mm.Theguardelectrodehasawidthof15mm,whilethedistancebetweentheelectrodesisabout5mm.Whenthedistancebetweentheelectrodesisvariedovera1mmrange,thecapacitancechangesfrom0.25pFto0.3pF.Thankstothechosenconcept,evenasimpledualopamp（TLC272AC）andCMOSNAND’scouldbeused,allowingasingle5Vsupplyvoltage.Thetotalmeasurementtimeamountstoonly100ms,wheretheoscillatorwasrunningatabout10kHz.

Thesystemwastestedinafullyautomatedsetup,usinganelectricalXYtable,thedescribedsensorandapersonalcomputer.Toachievetherequiredmeasurementaccuracythesetupwasautozeroedeveryminute.Inthiswaythenonlinearity,long-termstabilityandrepeatabilityhavebeenfoundtobetterthan1μmoverarangeof1mm（Fig.7）