基于電容傳感器的塑膜厚度檢測系統(tǒng)的研究

基于電容傳感器的塑膜厚度檢測系統(tǒng)的研究摘要:采用非接觸式電容傳感器、電渦流傳感器組合系統(tǒng)獲取檢測信號,利用基于LabVIEW軟件平臺的計算機實現(xiàn)對塑料薄膜進行在線檢測的工作原理,整個系統(tǒng)由測試單元、電機驅動單元、數(shù)據(jù)采集與處理單元組成。由于測試單元采用了電容、電渦流組合傳感系統(tǒng),信號的后續(xù)處理由基于LabVIEW軟件平臺的計算機自動完成,速了測量過程。采用本系統(tǒng)取代傳統(tǒng)的千分尺測量方法,可以使塑料薄膜厚度測量以完全智能的、實時的、非接觸的方式進行精確測量。最后,通過比較組合測量系統(tǒng)與千分尺測量的測試結果,證明了該系統(tǒng)具有較高的精度,良好的可靠性與可行性。關鍵詞:塑料薄膜;厚度測量;組合傳感系統(tǒng);在線測量目前，塑膜在塑料產(chǎn)品中占有一定的比例。它們廣泛應用于許多領域，比如農(nóng)業(yè)、運輸、醫(yī)學治療、環(huán)境保護和建筑業(yè)。在制造過程中，膜厚是評估產(chǎn)品性質的重要規(guī)范。一般而言，如果一卷中膜厚度的差異大于膜厚度的10%，它將被認為是不合格品。膜的厚度和一致性直接影響產(chǎn)品的物理屬性和機械性能。在使用中，它們也直接影響使用功能、壽命和抵制外部力量的能力。另外，塑膜經(jīng)常以卷分布。不同卷之間的厚度差異影響使用范圍。這聯(lián)系到公司和顧客間的利益。因此，對塑膜的厚度作出精確的測量是很有必要的。全球有幾種可行的方法來測量膜厚，例如機械測定法和重量分析測量法[1]。傳統(tǒng)的測量方法是用千分尺來精確測量，并對成品進行抽樣檢查，因此測量誤差和閱讀誤差是不可避免的。對于一片厚度遠小于100mm的膜來說，測量精確度很低。隨著工農(nóng)制造業(yè)對塑膜的增長需求，傳統(tǒng)的測量方法幾乎不能滿足發(fā)展的需要。在這種情況下，塑膜厚度在線檢測發(fā)展了。主要用于測量和監(jiān)控低密度澆鑄膜和聚丙烯澆鑄膜。就如我們所知，膜由同樣的材料和同樣的工藝制造而成，因此通過上述方法制造的膜的密度是一致的，我們能等距離不間斷地測出日常用膜的寬度為1.4m。同時測試結果能顯示在電腦屏幕上。用電容傳感器測量的基本原理現(xiàn)有幾種類型的非接觸電容傳感器比如極距、極板的面積和兩極板間介質的變化引起電容量的變化，一些傳感器采用微分模式來制造，為了提高靈敏度、改進線性特征、擴大測量范圍和作出誤差補償。最基本的電容傳感器的電容量的變化是由兩極板間的極距的變化引起的。它由一對平行板構成，其中一塊是靜止的或固定的，另一塊是可運動的。電容量的由如下公式可得：（1）公式中：ε0為真空中的介電常數(shù)；ε為兩極板間介質的相對介電常數(shù)[F/m]，當介質是空氣時，其值為1；h為兩極板間的距離[m]；CT為傳感器的電容量[F]一般而言，電容傳感器的測量頭是一個圓柱體，因此電場密度是一致的，電容傳感器能獲得最好的精度。圓柱的直徑為d，底部面積為s=πd2/4從以上公式，我們可以看出當極距h、極板相對面積或電容傳感器介質的變化時，電容量CT將隨著變化。如果被測塑膜置于兩極板間，通過觀察由介質厚度變化引起的電容量的變化，我們可進行厚度測量。從膜厚度獲得的信息電容測量系統(tǒng)由電容傳感器、微測量設備、屏閉導線纜等組成。圖1、是測膜厚度的示意圖。被測膜被置于兩極板間，固定極板的面積為s，極距為h，真空中的介電常數(shù)為ε0，膜厚為t，介質的相對介電常數(shù)為εa，可活動極板和膜間的距離為x，d為傳感器的有效直徑，Cs為固定極板的零溫度系數(shù)，CT為傳感器的電容量，這會被考慮成由三種不同電介質串聯(lián)而成的電容構成。（2）公式中：是上層空間電容量，是膜層空間的電容量，下層空間電容量。在這個測量電路中，有高輸入阻抗和極好線性的一個放大器被用來形成類似工作放大電路的反饋。Vs是20kHz的方波信號，它的振幅是高且有阻尼的，Vo是主放大電路對地的輸出電壓。根據(jù)反饋放大器的原理，當開環(huán)增益Av和輸入阻抗達到足夠大時，輸入和輸出關系表達式為：（3）通過用C1、C2、C3來取代表達式（2）和（3），我們可得：（4）從以上表達式看出，膜厚t和輸出電壓Vo的關系很明顯，就是：（5）在表達式（5）中，k0和m是常量，它們的值是：（6）表達式（5）告訴我們輸出電壓變化量和膜厚成比例關系，因此我們可通過測量輸出電壓[2，3]來獲得膜厚的信息。圖1、用電容傳感器測量膜厚的基本示意圖綜合測量系統(tǒng)在實際測量過程中，把膜平坦地放在固定片或固定極板上，電容傳感器被安在活動片上。它的測量頭當作一個活動極板。從上我們知道當電容傳感器的極距保持不變時，膜厚的變化將會引起電容的變化。只要使用電容傳感器，我們必須首先測量傳感器測量頭和固定片間的初始距離，然后放置塑膜并再次掃描。在膜被放置在片上前將需要一個預先掃描來記錄初始距離，然后放置膜片繼續(xù)掃描，通過減法來獲得兩值間的差異，這就是我們所需的膜厚。這樣活動片可重復性和固定片精確度的需求會要很高甚至是苛求。另外，機械傳動導致的誤差使系統(tǒng)不能達到要求的精度。為了克服以上問題，渦流傳感器被安在電容傳感器的每一面上。那么三個傳感器組合在一起并被安裝在活動片上來形成綜合傳感器[3]。圖2、是綜合傳感器的結構圖。圖2、綜合傳感器結構圖該系統(tǒng)由一個電容傳感器和兩個渦流傳感器構成。一個渦流傳感器和電容傳感器間的距離是l。一般地它是10mm的幾倍。當他在某一位置時，假設電容傳感器和固定片間的間隙和渦流傳感器和固定片間的間隙分別為h1、h2，我們得：h1-h2=Δh（7）這里h2是渦流傳感器兩個間隙的平均值。因為塑膜是絕緣體，它對渦流傳感器的輸出信號沒有影響。它僅和傳感器和固定片間的間隙有關。因而，在掃描和測量過程中，渦流傳感器一直測量h2的實際值，但V2的值從涉及到膜厚的電容傳感器得到。因此，我們有了如下的方程：v0=k1h1+k2t(8)h1=h2+Δh(9)把式（9）代入式（8），我們可得：v0=k1(h2+Δh)+k2t(10)式中k1和k2是特定系數(shù)，Δh兩傳感器與固定片間距的差值。從上我們可得膜厚t。圖3、給出了在線實驗裝置。當膜以1～2cm/s的速度縱向運動時，安裝在運動片上的傳感器通過寬長，因此測量點位呈對角放置。膜厚測量系統(tǒng)的基本組成整個系統(tǒng)由測試單元、數(shù)據(jù)采集、電機驅動單元與電腦處理單元組成。電容傳感器和渦流傳感器用于在測試單元完成綜合測量。在系統(tǒng)運作中LabVIEW軟件平臺用于在整個測量過程中[4～6]獲得電腦控制。因此測量加快了且數(shù)據(jù)分析比以前更方便。圖4、給出了膜厚測量系統(tǒng)的組成圖。在實際測量過程中，電腦用PS-2308圖3、在線實驗裝置芯片來發(fā)送指令給步進驅動機。借助于電機驅動器，電動機驅動導螺桿作轉動[7]。電容傳感器和渦流傳感器都安裝在螺母（活動片）上。當傳感器和螺母水平運動時，他們探測到放置在固定片上的膜而實現(xiàn)重復測量。從傳感器獲得的信號是微小且微弱的。它們必須被分別放大。這里我們使用電容微測量設備和渦流微測量設備來放大。通過模數(shù)轉換器（A/D），計算機會處理這些信號最終得出膜厚[8]。圖4、膜厚測量系統(tǒng)示意圖5、測量結論根據(jù)測量需求，測量系統(tǒng)應由步進電動機驅動且不簡斷地探測來獲得被測點的值。所有這些點一般都是等間隙的。采集程序訪問每一個點。在訪問過程中程序將分別執(zhí)行多次并記錄每次的測量值。然后計算這些值的平均值，平均值將被作為那個點的測量結果。重復周期不要太多和太少，為了最小化附帶誤差當它靜止在被測點時，它要確保測量頭能完成采集程序。每個點的測量時間是小于ms。圖5、是包含不間斷測量獲得的20個點的薄膜測試曲線。薄膜的標準尺寸是24μm，平均值是23.025μm。圖5、膜厚分布圖表1、給出了白色薄膜利用熱感光軟片制造線條的不同測量儀器測量出的平均值。他們的標準尺寸都是21.5μm，但編號方式是不同的。從表1，我們知道千分尺有最大測量誤差和最小可重復性，最大誤差達到5.5μm。轉矩彈簧比千分尺好，最大誤差為3.5μm。電容系統(tǒng)有最大的可重復性且最大誤差為1.4μm。這樣一個系統(tǒng)的標度由標準膠片來實現(xiàn)，當尺寸范圍在15～80μm之中時，結果達到0.2μm，且精度是±2.5μm。表1白色薄膜厚度測量值[μm]結論綜合傳感器被用于膜厚度檢測系統(tǒng)中可實現(xiàn)實時測量?？赏耆乜朔脝我坏碾娙輦鞲衅饕鸬倪@個缺點。所以這個測量效率提高了且測量結果在測量在所必需的精確度之內(nèi)。為了滿足其他寬度薄膜的測量需要，我們只需加長或縮短活動片的尺寸。使用這種系統(tǒng)，我們可以智能化地、實時地且高精度不接觸地完成厚度測量。就如我們所知，有許多種的薄膜。它們在工農(nóng)業(yè)產(chǎn)品中有廣泛的應用。根據(jù)不同的應用和它們的使用環(huán)境，對塑膜的厚度有不同的需求。因此，很有進一步開的必要。實踐已經(jīng)證明該系統(tǒng)在塑膜測量中有廣泛的應用前景。參考文獻[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