基于电容传感器的塑膜厚度检测系统的研究

基于电容传感器的塑膜厚度检测系统的研究摘要:采用非接触式电容传感器、电涡流传感器组合系统获取检测信号,利用基于LabVIEW软件平台的计算机实现对塑料薄膜进行在线检测的工作原理,整个系统由测试单元、电机驱动单元、数据采集与处理单元组成。由于测试单元采用了电容、电涡流组合传感系统,信号的后续处理由基于LabVIEW软件平台的计算机自动完成,速了测量过程。采用本系统取代传统的千分尺测量方法,可以使塑料薄膜厚度测量以完全智能的、实时的、非接触的方式进行精确测量。最后,通过比较组合测量系统与千分尺测量的测试结果,证明了该系统具有较高的精度,良好的可靠性与可行性。关键词:塑料薄膜;厚度测量;组合传感系统;在线测量目前，塑膜在塑料产品中占有一定的比例。它们广泛应用于许多领域，比如农业、运输、医学治疗、环境保护和建筑业。在制造过程中，膜厚是评估产品性质的重要规范。一般而言，如果一卷中膜厚度的差异大于膜厚度的10%，它将被认为是不合格品。膜的厚度和一致性直接影响产品的物理属性和机械性能。在使用中，它们也直接影响使用功能、寿命和抵制外部力量的能力。另外，塑膜经常以卷分布。不同卷之间的厚度差异影响使用范围。这联系到公司和顾客间的利益。因此，对塑膜的厚度作出精确的测量是很有必要的。全球有几种可行的方法来测量膜厚，例如机械测定法和重量分析测量法[1]。传统的测量方法是用千分尺来精确测量，并对成品进行抽样检查，因此测量误差和阅读误差是不可避免的。对于一片厚度远小于100mm的膜来说，测量精确度很低。随着工农制造业对塑膜的增长需求，传统的测量方法几乎不能满足发展的需要。在这种情况下，塑膜厚度在线检测发展了。主要用于测量和监控低密度浇铸膜和聚丙烯浇铸膜。就如我们所知，膜由同样的材料和同样的工艺制造而成，因此通过上述方法制造的膜的密度是一致的，我们能等距离不间断地测出日常用膜的宽度为1.4m。同时测试结果能显示在电脑屏幕上。用电容传感器测量的基本原理现有几种类型的非接触电容传感器比如极距、极板的面积和两极板间介质的变化引起电容量的变化，一些传感器采用微分模式来制造，为了提高灵敏度、改进线性特征、扩大测量范围和作出误差补偿。最基本的电容传感器的电容量的变化是由两极板间的极距的变化引起的。它由一对平行板构成，其中一块是静止的或固定的，另一块是可运动的。电容量的由如下公式可得：（1）公式中：ε0为真空中的介电常数；ε为两极板间介质的相对介电常数[F/m]，当介质是空气时，其值为1；h为两极板间的距离[m]；CT为传感器的电容量[F]一般而言，电容传感器的测量头是一个圆柱体，因此电场密度是一致的，电容传感器能获得最好的精度。圆柱的直径为d，底部面积为s=πd2/4从以上公式，我们可以看出当极距h、极板相对面积或电容传感器介质的变化时，电容量CT将随着变化。如果被测塑膜置于两极板间，通过观察由介质厚度变化引起的电容量的变化，我们可进行厚度测量。从膜厚度获得的信息电容测量系统由电容传感器、微测量设备、屏闭导线缆等组成。图1、是测膜厚度的示意图。被测膜被置于两极板间，固定极板的面积为s，极距为h，真空中的介电常数为ε0，膜厚为t，介质的相对介电常数为εa，可活动极板和膜间的距离为x，d为传感器的有效直径，Cs为固定极板的零温度系数，CT为传感器的电容量，这会被考虑成由三种不同电介质串联而成的电容构成。（2）公式中：是上层空间电容量，是膜层空间的电容量，下层空间电容量。在这个测量电路中，有高输入阻抗和极好线性的一个放大器被用来形成类似工作放大电路的反馈。Vs是20kHz的方波信号，它的振幅是高且有阻尼的，Vo是主放大电路对地的输出电压。根据反馈放大器的原理，当开环增益Av和输入阻抗达到足够大时，输入和输出关系表达式为：（3）通过用C1、C2、C3来取代表达式（2）和（3），我们可得：（4）从以上表达式看出，膜厚t和输出电压Vo的关系很明显，就是：（5）在表达式（5）中，k0和m是常量，它们的值是：（6）表达式（5）告诉我们输出电压变化量和膜厚成比例关系，因此我们可通过测量输出电压[2，3]来获得膜厚的信息。图1、用电容传感器测量膜厚的基本示意图综合测量系统在实际测量过程中，把膜平坦地放在固定片或固定极板上，电容传感器被安在活动片上。它的测量头当作一个活动极板。从上我们知道当电容传感器的极距保持不变时，膜厚的变化将会引起电容的变化。只要使用电容传感器，我们必须首先测量传感器测量头和固定片间的初始距离，然后放置塑膜并再次扫描。在膜被放置在片上前将需要一个预先扫描来记录初始距离，然后放置膜片继续扫描，通过减法来获得两值间的差异，这就是我们所需的膜厚。这样活动片可重复性和固定片精确度的需求会要很高甚至是苛求。另外，机械传动导致的误差使系统不能达到要求的精度。为了克服以上问题，涡流传感器被安在电容传感器的每一面上。那么三个传感器组合在一起并被安装在活动片上来形成综合传感器[3]。图2、是综合传感器的结构图。图2、综合传感器结构图该系统由一个电容传感器和两个涡流传感器构成。一个涡流传感器和电容传感器间的距离是l。一般地它是10mm的几倍。当他在某一位置时，假设电容传感器和固定片间的间隙和涡流传感器和固定片间的间隙分别为h1、h2，我们得：h1-h2=Δh（7）这里h2是涡流传感器两个间隙的平均值。因为塑膜是绝缘体，它对涡流传感器的输出信号没有影响。它仅和传感器和固定片间的间隙有关。因而，在扫描和测量过程中，涡流传感器一直测量h2的实际值，但V2的值从涉及到膜厚的电容传感器得到。因此，我们有了如下的方程：v0=k1h1+k2t(8)h1=h2+Δh(9)把式（9）代入式（8），我们可得：v0=k1(h2+Δh)+k2t(10)式中k1和k2是特定系数，Δh两传感器与固定片间距的差值。从上我们可得膜厚t。图3、给出了在线实验装置。当膜以1～2cm/s的速度纵向运动时，安装在运动片上的传感器通过宽长，因此测量点位呈对角放置。膜厚测量系统的基本组成整个系统由测试单元、数据采集、电机驱动单元与电脑处理单元组成。电容传感器和涡流传感器用于在测试单元完成综合测量。在系统运作中LabVIEW软件平台用于在整个测量过程中[4～6]获得电脑控制。因此测量加快了且数据分析比以前更方便。图4、给出了膜厚测量系统的组成图。在实际测量过程中，电脑用PS-2308图3、在线实验装置芯片来发送指令给步进驱动机。借助于电机驱动器，电动机驱动导螺杆作转动[7]。电容传感器和涡流传感器都安装在螺母（活动片）上。当传感器和螺母水平运动时，他们探测到放置在固定片上的膜而实现重复测量。从传感器获得的信号是微小且微弱的。它们必须被分别放大。这里我们使用电容微测量设备和涡流微测量设备来放大。通过模数转换器（A/D），计算机会处理这些信号最终得出膜厚[8]。图4、膜厚测量系统示意图5、测量结论根据测量需求，测量系统应由步进电动机驱动且不简断地探测来获得被测点的值。所有这些点一般都是等间隙的。采集程序访问每一个点。在访问过程中程序将分别执行多次并记录每次的测量值。然后计算这些值的平均值，平均值将被作为那个点的测量结果。重复周期不要太多和太少，为了最小化附带误差当它静止在被测点时，它要确保测量头能完成采集程序。每个点的测量时间是小于ms。图5、是包含不间断测量获得的20个点的薄膜测试曲线。薄膜的标准尺寸是24μm，平均值是23.025μm。图5、膜厚分布图表1、给出了白色薄膜利用热感光软片制造线条的不同测量仪器测量出的平均值。他们的标准尺寸都是21.5μm，但编号方式是不同的。从表1，我们知道千分尺有最大测量误差和最小可重复性，最大误差达到5.5μm。转矩弹簧比千分尺好，最大误差为3.5μm。电容系统有最大的可重复性且最大误差为1.4μm。这样一个系统的标度由标准胶片来实现，当尺寸范围在15～80μm之中时，结果达到0.2μm，且精度是±2.5μm。表1白色薄膜厚度测量值[μm]结论综合传感器被用于膜厚度检测系统中可实现实时测量。可完全地克服利用单一的电容传感器引起的这个缺点。所以这个测量效率提高了且测量结果在测量在所必需的精确度之内。为了满足其他宽度薄膜的测量需要，我们只需加长或缩短活动片的尺寸。使用这种系统，我们可以智能化地、实时地且高精度不接触地完成厚度测量。就如我们所知，有许多种的薄膜。它们在工农业产品中有广泛的应用。根据不同的应用和它们的使用环境，对塑膜的厚度有不同的需求。因此，很有进一步开的必要。实践已经证明该系统在塑膜测量中有广泛的应用前景。参考文献[1]ChenQ.ChinaPlastics[J].2001,15(1)62-66.[2]ZhengY.ThePrincipleandApplicationofCapacitiveSensors[M].Tianjin:Tianjin[3]QiangX.Sensors[M].(MechanicalIndustriesPublications,Peking1994).[4]ZhangX,HouG,ZhaoW.Measurement&ControlTechnology[J].2000,19(9):22-24.[5]SongX,LiuD,GuoY.MeasurementTechnique[J].2001,1215-17.[6]YangL.LabVIEWProgramDesignandApplication[M].(ElectronicIndustriesPublications,Peking2001)[7]ChenL.StepMotorandApplication[M].(Science&TechnologyPublications,Shanghai1985)[8]YuY.ComputerInterfaceTechnique[M].(ElectronicIndustriesPublications,Peking1996)StudyofPlasticFilmThicknessDetectingSystemBasedonCapacitiveSensorMAYuzhen1,2,WANGXinhua2,DUANFajie1,ZHENGYizhong1,YEShenghua11.StateKeyLaboratoryofPrecisionMeasurementTechnologyandInstruments,TianjinUniversity,Tianjin300072,China;2.SchoolofMechanicalEngineering,JinanUniversity,Jinan250022,ChinaAbstract:ThemainpointisfocusedonprincipleofusingnoncontactcombinationsystemcomposedwithcapacitivesensorandeddycurrentsensorstoacquiremeasurementsignalsandutilizeLabVIEWsoftwareplatformforonlinedataprocessingofplasticfilmthickness.Thewholesystemconsistsoftestunit,motordrivingunit,dataacquisitionandcomputerprocessingunit.Thetestunitdoesmeasurementbymeansofcapacitivesensorandeddycurrentsensors.ThesystemisoperatedontheLabVIEWsoftwareplatformforautomaticcontrolofthewholeprocess,sothemeasurementprocessisaccelerated.Usingcombinationalmeasurementsystemtoreplacetraditionalmicrometer,thethicknessofplasticfilmcanbemeasuredinintelligent,realtime,andnoncontactmodewithhighprecision.Thetestresultsfromthiscombinationalmeasurementsystemarecomparedwiththeresultsfrommicrometer.Itprovesthatthismeasurementsystemhashigherprecisionandbetterrepeatability,alsowithgreaterreliabilityandfeasibility.Keywords:plasticfilm;thicknessmeasurement;combinationalsensorsystem;onlinemeasurementAtpresent,plasticfilmhasagreatproportioninplasticproducts.Theyareappliedextensivelyinmanyfieldssuchasagriculture,package,medicaltreatment,environmentalprotection,andarchitecture.Inmanufactureprocess,filmthicknessisanimportantspecificationtoevaluateproductperformance.Generally,ifthedifferenceoffilmthicknessinarollismorethan10percentofthefilmthickness,itwillbeconsideredasunqualified.Thicknessanduniformityofafilmhaveadirectimpactonphysicalpropertyandmechanicalpropertyoftheproducts.Theyalsohaveanimpactonthefunctionsofusage,longevity,andthecapabilityofrepellingexternalforceactionwhenafilmisinuse.Inaddition,plasticfilmisalwaysdistributedinrolls.Thethicknessdifferencebetweendifferentrollsaffectstheusablearea.Thisislinkedtotheinterestsofthecompanyandconsumer.Therefore,itisveryessentialtomakeaccuratemeasurementforthethicknessofplasticfilm.Thereareseveralmeansavailableworldwidetodetectthefilmthickness,suchasmechanicalmeasurementmethodandweightanalysismeasuremethod[1].Thetraditionaldetectionmethodismakingcontactmeasurementwithfilmbymicrometer,andthesamplinginspectionisalwaystothefinalproducts,sothemeasuringerrorsandreadingerrorsareunavoidable.forafilmwhichislessthanonehundredmicrometersinthickness,andthemeasuringaccuracyisverylow.Withtheincreasingrequirementtoplasticfilmbyindustrialandagriculturalmanufacture,thetraditionalmeasuringmethodscanhardlymeetthedevelopmentaldemand.Inthiscase,aplasticfilmthicknessonlinedetectingsystemisdeveloped.Itismainlyusedtomakemeasurementandmonitorlowdensitycastfilmandpolypropylenecastfilm.Justaswehaveknown,thefilmismanufacturedwiththesamematerialsandthesametechnics,sothedensityofthefilmthroughitsareaiseven,wecanmeasurearoutinefilmwidthof1.4mcontinuouslywithequidistance.Atthesametime,thetestresultscanbedisplayedoncomputerscreen.1TheFundamentalMeasuringPrinciplewithCapacitiveSensorThereareseveraltypesofnoncontactcapacitivesensorsuchasthecapacityvariationcausedbythevariationofpolardistance,polarplateareaandmediabetweentwopolarplates.Somesensorshavebeenmadeindifferentialmodetoadvancethesensitivity,toimprovethelinearcharacteristics,toenlargethemeasurerange,andtomakeerrorcompensation.Thecapacityvariation,themostbasictypeofsensor,iscausedbythepolardistancevariationbetweentwopolarplates.Itiscomposedofacoupleofparallelplates,oneplateisstationaryorfixed,theotheroneismovable.Thecapacityisobtainedbyfollowingequation:(1)whereε0isdielectricconstantinvacuum;εisrelativedielectricconstantofmediabetweentwopolarplates[F/m],wheninair,itsvalueis1;sispolarplatearea[m2];hisdistancebetweentwopolarplates[m];cTisthecapacityofsensor[F].Ingeneral,themeasuringheadofacapacitivesensorisacylinder,sotheelectricfielddensityiseven,thecapacitivesensorcanreachthebestprecision.Itsdiameterisd,theendareaiss=πd2/4,fromaboveequation,wecanseewhenthepolardistanceh,areasorthemediaofacapacitysensorhasvaried,thecapacitycTwillvarywithit.Ifthedetectedplasticfilmisplacedbetweentwopolarplates,byobservingthecapacityvariationcausedbythevariationofmediathickness,wecanmakethethicknessmeasurement[2].2TheInformationPickingupofFilmThicknessThecapacitivemeasurementsystemiscomposedofcapacitivesensor,micromeasuringapparatus,shieldedconductorcablesandsoon.Fig.1isthefilmthicknessmeasuringschematicdiagram.Thedetectedfilmbeingplacedbetweenthetwopolarplates,theareaoffixedpolarplateiss,polardistanceish,thedielectricconstantinvacuumisε0,filmthicknessist,therelativedielectricconstantofmediaisεa,thedistancebetweenmovableplateandfilmisx,distheeffectivediameterofsensor,csisthefixedcapacityofzerotemperaturecoefficient,cTissensorcapacity,whichmaybeconsideredconstitutingwiththreecapacitiesthathavedifferentdielectricmediaconnectinginseries.(2)whereisupperlayerspatialcapacity,isthefilmspatialcapacity,isunderlayerspatialcapacity.Inthismeasuringcircuit,anamplifierthathashighinputimpedanceandfinelinearityisusedtoformfeedbackanalogousoperationalamplificationcircuit,vsis20kHzsquarewaveformsignal,itsamplitudeishighandresisting,v0isoutputvoltageofthemainamplifiertoground.Accordingtotheprincipleoffeedbackamplifier,whentheopenloopgainAvandinputimpedanceapproachgreatenough,theinputoutputrelationshipexpressionis(3)bysubstitutingexpression(2)and(3)withc1、c2、c3,wecanobtain (4)fromaboveexpression,therelationshipbetweenfilmthicknesstandoutputvoltagev0isobvious,whichis(5)inexpression(5),k0andmareconstant,theirvalueare(6)expression(5)tellsusthattheoutputvoltagevariationisproportionaltothefilmthickness,sowecanobtainthefilmthicknessinformationbymeasuringtheoutputvoltage[2,3].Fig.1Basicschematicdiagramoffilmthicknessmeasurementwithcapacitivesensor3CombinationalMeasuringSystemInpracticalmeasuringprocess,placingthefilmplainlyonafixedslideorafixedpolarplate,thecapacitivesensorismountedonamovableslide.Itsmeasuringheadactsasamovablepolarplate.Fromabove,welearnthatwhenthepolardistanceofacapacitivesensorkeepsnochange,thevariationoffilmthicknesswillcausethecapacitivevariation.Ifonlyusingacapacitivesensor,weshouldhavetofirstmeasurethevirgindistancebetweenmeasuringheadofsensorandthefixedslide.Thiswillrequireapriorscanningbeforeafilmbeplacedontheslidetorecordthevirgindistance,thenplacethefilmandscanagain,bysubtractiveoperationtoobtainthedifferenceoftwovalues,itisthefilmthicknessjustwhatweneed.Inthisway,therequirementtorepeatabilityofmotiveslideandaccuracyoffixedslidewillbeveryhighandevenovercritical.Otherwise,theerrorcausedbymechanicaltransmissionmakesthesystemfailtoreachtherequiredaccuracy.Toovercomeaboveproblem,aneddycurrentsensorismountedoneachsideofthecapacitivesensor.Thenthethreesensorsgrouptogetherandaremountedonthemotiveslidetoformthecombinationalsensors[3].Fig.2isdiagramofcombinationalsensors.ThesystemFig.2Diagramofcombinationalsensorsiscomposedofacapacitivesensorandtwoeddysensors.Thedistancebetweenoneeddycurrentsensorandcapacitivesensorisl.Generallyitisseveraltimesoftenmillimeters.Whenitisonacertainposition,assumingtheinterspacebetweenacapacitivesensorandfixedslide,aneddycurrentsensorandfixedslideisseparatelyh1,h2,wehave(7)hereh2isaverageoftwointerspaceofeddycurrentsensors.Becausetheplasticfilmisinsulator,ithasnoeffectonoutputsignalofeddycurrentsensor.Itisonlyrelativetotheinterspacebetweensensorandthefixedslide.So,duringtheprocessofscanninganddetecting,theeddycurrentsensorscanalwaysmeasuretheactualvalueofh2,butthevaluev2obtainedfromcapacitivesensorrelatedtofilmthickness.Sowecanhavethefollowingequationsv0=k1h1+k2t(8)h1=h2+Δh(9)bysubstitutingexpression(8)with(9),wecanobtainv0=k1(h2+Δh)+k2t(10)wherek1andk2arespecifiedcoefficients,Δhisthedifferenceofdistancesbetweentwosensorsandfixedslide.Fromabove,wecanderivethefilmthicknesst.Fig.3showstheexperimentalsetupforonlinemeasurement.Fig.3ExperimentalsetupforonlinemeasurementWhenthefilmmoveslengthwiseatthespeedof1～2cm/s,thesensorsmountedonthemotiveslidemoveacrossthewidth,sothemeasurementpointsarepositioneddiagonally.4TheBasicCompositionofFilmMeasurementSystemThewholesystemiscomposedoftestunit,dataacquisitionunit,motordrivingunit,andcomputerprocessingunit.Thecapacitivesensorandeddycurrentsensorsareusedtoperformcombinationalmeasurementindetectionunit.LabVIEWsoftwareplatformisusedinthesystemoperationtoachievecomputercontrolduringthewholemeasurementprocess[4～6].Sothemeasurementisspeedupanddataanalysisismoreconvenientthanbefore.Fig.4showsthecompositiondiagramoffilmmeasurementsystem.Inpracticalmeasurementprocess,computersendscommandstostepmotorbyPS-2308interfacecard.Withtheaidofmotordriver,themotordrivesleadingscrewtodorotationalmotion[7].Thecapacitivesensorandeddycurrentsensorsareallmountedonthescrewnut(motiveslide).Whilesensorsmovehorizontallywithnut,theydetectthefilmplacedonthefixedslidetorealizerepeatablemessure.Thesignalobtainedfromsensorsistinyandweak.Theymustbeamplifiedrespectively.HereweusecapacitivemicromeasuringapparatusandeddycurrentmicromeasuringapparatustodoFig.4Filmmeasurementsystemdiagramamplification.Byanalogtodigital(A/D)converter,computercanprocessthesesignalsandeventuallyobtainthefilmthickness[8].5MeasurementResultsAccordingtomeasurementrequirement,themeasuringsystemshouldbedrivenbystepmotoranddetectthefilmcontinuouslytoobtainvaluesofmeasuredpoints.Allthesepointsaregenerallyequalinterspaces.Theacquisitionprogramiscalledateverypoint.Ateachcalling,theprogramwillbeexecutedmanytimesrepetiivelyandrecordeverymeasuringvalue,thencalculatestheaverageofthosevalues,thisaverageistakenasthemeasurementresultofthatpoint.Thecycletimesshouldnotbetoomanyandtoofew,itassuresthemeasuringheadisabletocompleteacquisitionprocesswhenitissteadyatmeasuredpointsinordertominimizeaccidentalerror.Themeasurementtimeofeachpointislessthanms.Fig.5isthetestcurveforathinfilmcontaining20pointsobtainedbycontinuousmeasurement.Itsstandardsizeofthethinfilmis24μm,theaveragevalueis23.025μm.Fig.5FilmthicknessscatterdiagramTable1showstheaveragesmeasuredbydifferentmeasuringdeviceonheatsensitivefilmmanufacturinglineforwhitefilms.Theirstandardsizesareall21.5μm,buttheirnumberingaredifferent.FromTable1,welearnthatthemicrometerhasthebiggestmeasuringerrorandtheleastrepeatability,themaximalerrorreachesto5.5μm.Thetorsionspringisbetterthanthemicrometer,themaximalerroris3.5μm.Thecapacitivesystemhasthebestrepeatabilityanditsmaximalerroris1.4μm.Thecalibrationof