电容触摸屏控制设计外文文献及中文翻译

/ALow-Cost,SmartCapacitivePositionSensorAbstractAnewhigh-performance,low-cost,capacitiveposition-measuringsystemisdescribed.Byusingahighlylinearoscillator,shieldingandathree-signalapproach,mostoftheerrorsareeliminated.Theaccuracyamountsto1μmovera1mmrange.Sincetheoutputoftheoscillatorcandirectlybeconnectedtoamicrocontroller,anA/Dconverterisnotneeded.I.INTRODUCTIONThispaperdescribesanovelhigh-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.ElectronicCircuitPElectronicCircuitPersonalComputer ActuatorDisplayFig.1.BlockdiagramofthesystemFig.2.PerspectiveanddimensionsoftheelectrodestructureⅡ.THEELECTRODESTRUCTUREThebasicsensingelementconsistsoftwosimpleelectrodeswithcapacitanceCx,(Fig.2).Thesmallerone(E2)issurroundedbyaguardelectrode.Thankstotheuseoftheguardelectrode,thecapacitanceCxbetweenthetwoelectrodesisindependentofmovements(lateraldisplacementsaswellasrotations)paralleltotheelectrodesurface.TheinfluenceoftheparasiticcapacitancesCpwillbeeliminatedaswillbediscussedinSectionⅢ.AccordingtoHeerens[3],therelativedeviationinthecapacitanceCxbetweenthetwoelectrodescausedbythefiniteguardelectrodesizeissmallerthan:δ<e-π(x/d)(1)wherexisthewidthoftheguardanddthedistancebetweentheelectrodes.Thisdeviationintroducesanonlinearity.Thereforewerequirethatδislessthan100ppm.Alsothegapbetweenthesmallelectrodeandthesurroundingguardcausesadeviation:δ<e-π(d/s)(2)withsthewidthofthegap.Thisdeviationisnegligiblecomparedto(l),whenthegapwidthislessthan1/3ofthedistancebetweentheelectrodes.Anothercauseoferrorsoriginatesfromapossiblefiniteskewangleαbetweenthetwoelectrodes(Fig.3).Assumingthefollowingconditions:thepotentialsonthesmallelectrodeandtheguardelectrodeareequalto0V,thepotentialonthelargeelectrodeisequaltoVvolt,theguardelectrodeislargeenough,itcanbeseenthattheelectricfieldwillbeconcentric.dl/2dl/2l/2Fig.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.ELIMINATIONOFPARASITICCAPACITANCESBesidesthedesiredsensorcapacitanceC,therearealsomanyparasiticcapacitancesintheactualstructure(Fig.2).ThesecapacitancescanbemodeledasshowninFig.4.HereCplrepresentstheparasiticcapacitancesfromtheelectrodeE1andCp2fromtheelectrodeE2totheguardelectrodesandtheshielding.ParasiticcapacitanceCp3resultsfromimperfectshieldingandformsanoffsetcapacitance.WhenthetransducercapacitanceCxisconnectedtoanACvoltagesourceandthecurrentthroughtheelectrodeismeasured,CplandCp2willbeeliminated.Cp3canbeeliminatedbyperforminganoffsetmeasurement.Fig.4.EliminationofparasiticcapacitancesThecurrentismeasuredbytheamplifierwithshuntfeedback,whichhasaverylowinputimpedance.Toobtaintherequiredlinearity,theunity-gainbandwidthfToftheamplifierhastosatisfythefollowingcondition:(9)whereTistheperiodoftheinputsignal.SinceCp2consistsofcablecapacitancesandtheinputcapacitanceoftheopamp,itmayindeedbelargerthanCfandcannotbeneglected.IV.THECONCEPTOFTHESYSTEMThesystemusesthethree-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-PERIODCONVERSIONThesignalswhichareproportionaltothecapacitorvaluesareconvertedintoaperiod,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.PERIODMEASUREMENTWITHAMICROCONTROLLERPerformingperiodmeasurementwithamicrocontrollerisaneasytask.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.EXPERIMENTALRESULTSThesensorisnotsensitivetofabricationtolerancesoftheelectrodes.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).ThisiscomparabletotheaccuracyandrangeofthesystembasedonaPSDasdescribedin[2].Asaresultoftheseexperiments,itwasfoundthattheresolutionamountstoapproximately20aF.Thisresultwasachievedbyaveragingover256oscillatorperiods.Afurtherincreaseoftheresolutionbylengtheningthemeasurementtimeisnotpossibleduetothel/fnoiseproducedbythefirststagesinboththeintegratorandtheComparator.Theabsoluteaccuracycanbederivedfromthepositionaccuracy.Sincea1mmdisplacementcorrespondstoachangeincapacitanceof50fF,theabsoluteaccuracyof1μminthepositionamountstoanabsoluteaccuracyof50aF.CONCLUSIONAlow-cost,high-performancedisplacementsensorhasbeenpresented.Thesystemisimplementedwithsimpleelectrodes,aninexpensivemicrocontrollerandalinearcapacitance-to-periodconverter.Whenthecircuitryisprovidedwithanaccuratereferencecapacitor,thecircuitcanalsobeusedtoreplaceexpensivecapacity-measuringsystems.REFERENCES[1]G.C.M.MeijerandR.Schner,“Alinearhigh-performancePSDdisplacementtransducerwithamicrocontrollerinterfacing,”SensorsandActuators,A21-A23,pp.538-543,1990.[2]J.vanDrecht,G.C.M.Meijer,andP.C.deJong,“ConceptsforthedesignofsmartsensorsandsmartsignalprocessorsandtheirapplicationtoPSDdisplacementtransducers,”DigesrofTechnicalPapers,Transducers’91.[3]W.C.Heerens,“Applicationofcapacitancetechniquesinsensordesign,”Phys.E:Sci.Insfrum.,vol.19,pp.897-906,1986.[4]K.Martin,‘‘Avoltage-controlledswitched-capacitorrelaxationoscillator,”IEEEJ.,vol.SC-16,pp.412-413,1981.一种低成本智能式电容位置传感器摘要本文描述了一种新的高性能，低成本电容位置测量系统。通过运用高线性振荡器，屏蔽和三信号通道，大部分误差被消退。其精确度在1毫米范围内达1微米。由于振荡器的输出可干脆连接到微限制器，所以无需用A/D转换器。Ⅰ.导言本文介绍了一种新型高性能，低成本的电容位移测量系统，特点如下：1毫米测量范围1微米精确度0.1s总测量时间对应到电容域，规格相当于：1皮法的变更范围；只有这个范围的50fF（fF是法拉乘以10的负15次方。f是femto的缩写）用于位移传感器。50aF确定电容测量误差。梅耶尔和施里尔[1]以及最近的范德雷赫特河，梅耶尔，和德容[2]提出了位移测量系统，接受一个PSD（位置敏感探测器）作为传感元件。和本文描述的电容传感器元件相比，运用PSD的缺点是，PSD和LED（发光二极管）有更高的成本和功率消耗。运用[2]中所提概念的信号处理器，被接受到电容元件的运用中。通过广泛运用屏蔽，智能A/D转换，该系统能够将高精确度和低成本结合。换能器产生可以选择和干脆由微限制器读出的三段调制信号。微限制器，相应的，计算位移及发送此值到主机电脑（图1）或显示或驱动执行器。电子电路上位机 电子电路上位机 执行器演示图1该系统的框图金属屏蔽电极屏蔽金属屏蔽电极屏蔽图2电极结构的尺寸和透视图Ⅱ.电极结构基本传感元件包含电容为Cx的两个简洁电极(图2）。较小的一个（E2）是由屏蔽电极包围。由于运用屏蔽电极，两电极间的电容Cx可平行于电极表面独立运动（横向平移以及旋转）。寄生电容Cp的影响可被消退，将在第3节探讨。据Heerens[3]，由有限屏蔽电极大小造成的两个电极之间电容Cx的相对偏差小于：δ<e-π(x/d)(1)其中x是屏蔽的宽度，d是电极之间的距离。这种偏差引入了非线性。因此，我们规定δ小于100ppm。此外小电极和四周屏蔽之间的间距产生一个偏差：δ<e-π(d/s)(2)S是间距的宽度。当间距宽度小于电极之间距离的1/3时，这偏差和（1）相比是微乎其微的。另一个误差的缘由可能源自两个电极之间的有限倾斜角α（图3）。假设符合下列条件：小电极和屏蔽电极上的电势等于0V大型电极电势等于V伏屏蔽电极足够大可以看出，电场将同心。dl/2dl/2l/2图3倾斜角度α的电极为了使计算简洁，我们将假设电极在一个方向无限大。问题就成为一个二维问题，可以用极坐标（Υ，φ）方法解决。在这种状况下，电场可以表述为：(3)为了计算小电极的损耗，我们设定φ为0，整定Υ：(4)Bl是小电极的左侧边界：(5)Br是右边界：(6)求解（4）结果：(7)对小α的近似：(8)选择比d小的l似乎是可行的，因此该误差将只确定于角度α。在这种状况下，0.6°的角度变更，将产生小于100ppm的误差。对参数εo和l是常数的设计，两个电极之间的电容将仅仅取决于电极之间的距离d。Ⅲ.寄生电容的消退除了志向传感器电容Cx，在实际结构中还有许多寄生电容（图2）。这些电容可以建模，如图4所示。这里Cpl代表电极El的寄生电容，Cp2是从电极E2到屏蔽电极和屏蔽层的。寄生电容Cp3造成不完善屏蔽，形成一个偏移电容。当传感器电容Cx连接到AC电压源，通过电极的电流可测，Cpl和Cp2，将被消退。Cp3可通过偏移测量消退。图4消退寄生电容电流通过并联反馈放大器测量，它具有特殊低的输入阻抗。要获得所需的线性度，放大器的单位增益带宽fT必需符合下列条件：(9)T是在此期间的输入信号。由于Cp2包括电缆电容和运算放大器的输入电容，它很可能大于Cf而不行忽视。Ⅳ.本系统的概念该系统接受了[2]提出的三信号的概念，它是基于以下原则。当我们用线性系统测量电容Cx，得到一个值：(10)其中m是未知的增益,Moff是未知偏移。以相同的方式，通过测量参考量Cref，测量偏移Moff，使m=0,参数m和Moff被抵消。最终的测量结果P定义为：(11)在我们的例子中，传感器的电容Cx为：(12)其中Ax，是电极面积，do是它们之间最初的距离，ε是介电常数，△d是要测量的位移。对于参考电极，它为：