电容触摸屏控制设计外文文献及中文翻译Word文档格式.docx

电容触摸屏控制设计外文文献及中文翻译Word文档格式.docx

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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<

e-π（x/d）

（1）

wherexisthewidthoftheguardanddthedistancebetweentheelectrodes.Thisdeviationintroducesanonlinearity.Thereforewerequirethatδislessthan100ppm.Alsothegapbetweenthesmallelectrodeandthesurroundingguardcausesadeviation:

e-π（d/s）

（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