Non-contact surface charge/voltage …

1 TrekApplicationNoteNumber3002 Non-contactsurfacecharge/ voltage measurementsFieldmeterandvoltmetermethod sDr. chargesandpotentialsusingelectrostatic voltmeteris alsodescribedasanuniquemethodthatcombine sadvantagesofboth broadvarietyofinstrumentsthatcanmeasurea nelectricchargeand/orvoltageona eldmetersandvoltmetersbelongtothecate-go ry eldmetermethods. Bothmeasurementtechniquescomewithmany variationsduetoanextraordinary effort putintodevelopingoflow-cost,accuratedevi ces[1 10].Whichmethodisbetter?Hopefully, theanswercanbefoundin thecomparisoneasier, considera voltmeteranda eldmeter, bothusingtheKelvinvibratingcapacitive parallel-plateca-pacitor. Inthiscon gurationanelectric currentIis beinginducedin thesinusoidallyvibratingsen-sor[11,12].T hiscurrentisproportionaltothevalueofthee lectric potentialpresentonthesur-faceundertest[1 2]:I=U dCdt=U ddt 0AD0+D1 sin(!)

2 T) ==-U 0A D1!cos(!t)[D0+D1sin(!t)]2(1)Uisthediffer enceofpotentialsbetweenthetestedsurfacea ndthevibratingprobe, [V],D0isaconstantrepresentingtheseparati onbetweenelectrodeandthetestedsurfacewhe ntheelectrodeis notvibrating,[m],D1is theamplitudeof vibrations, [m],!is thecircularfrequencyofvibrations,!= 2 f[rad/s],wheref is a frequencyin [Hz],Aisthesurfaceareaofthesensingelectr ode,[m2], istherelative electricpermittivityofthemate-rialbetwee ntheelectrodeandthesurfaceundertest, 1 forair, 0is theelectric permittivityofvacuum, = 10-12[F/m].ThecurrentsignalI isampli edanddemodu-latedusinga phase-sensitive demodulatorcircuit(Figures1 and2)toproducea voltageVpdi-rectlyproportionalto theamplitudeof eldmetersutilize theprocessedsignalVpis eldmetersFigure1 presentsanelectrostatic eldmeter. Afractionof thedetectedandprocessedvoltageVpis invertedandfedback toa uencedby two electric elds:onecreatedby thetestedsurfaceandonegeneratedby , thegreaterthesurfacevoltage, thesetwo voltagescanceleachother.

3 Po-tentiometerP is usedto establisha constantratiobetweenVsandthemeasuredvolt ageVp. WhenTREK,INC. 11601 MapleRidgeRoad Medina,NY14103 Tel:(585)798-3140 Call:1 800 FORTREK FAX:(585)798-3106 of 6 Copyright 2002 trek , 0aTrekApplicationNoteNumber3002 Non-contactsurfacecharge/ voltage measurementsFieldmeterandvoltmetermethod sthesumofthetwo eldsequalszero, thestabil-ityofthesignaldetectedby , thepotentialdif-ferencebetweenthesurface andthesensorcanleadtothedischargeanddama geoftheequip-mentif measuredVsis alsosensitive to highvoltageampli ercircuitto replicatethevoltageonthetestedsurface. Theampli edvoltageis thenappliedtothesensorthusnullifyingthee lectric eldbetweenthetestedsurfaceandthesens-ing electrode. Potentialontheelectrode"follows"thepoten tialonthesurface. Inthiscasethereisnothreatoftheeventualdi schargebetweentheprobeandthesurfaceunder test, atleastwithina certainrangeofD0.

4 If thespanbetweenthesurfaceandsensoris toobig,theprobebe-comesin uencedby otherelectric eldspresentin voltmeterTheAC-feedback voltmeterusesa differenttech-niquetoachievespacingindep endentsurfacevoltage/chargemeasurements[ 6].RatherthancancellingtheKelvincurrentI by useofa feed-back DCvoltagewhichfollowsthesurfacetestvolta geto producezeroelectric eld,theAC feed-back methodutilizesa nullifyingcurrentI' tozerotheKelvincurrentI. ThecurrentI' is producedbyexternalgeneratorcircuittunedt othefrequencyof theKelvinsensoroscillations:I0=C dVtdt(2)Therefore, whencurrentsI andI' canceleachother,I0=IU dCdt=C dVtdt(3)AsbothI andI' currentsareinverselyproportionaltospacin gD0, theratiooftheamplitudeofVttoU (theDCtestsurfacevoltage)remainsconstant overthelargerangeofD0. AsshowninFigure3,theVtsignalis obtainedby ampli cationofthecurrentI convertedtoa voltageatthepreampli- er.

5 AthighgainthecurrentI is beingcancelledto a very presentsa comparisonofmeasurementerrorsforastandar d eldmeterandtheTrekmodel520electrostaticv oltmeter. Thedataindi-catethatit isimportanttokeeptheappropriatespacingbe tweenthe eldmetersensorandthetestedsurfaceinorder toconsiderthemeasure-mentreliable. Table showsa briefcomparisonbe-tween eldmeter, electrostaticvoltmeterandAC-feedback electrostaticvoltmeter. Becauseoftheirprincipleof operation,theelectrostatic eldmetersaresuitableformeasurementscondu ctedonrel-ativelylargeareas. They arealsonotasaccurateaselectrostaticvoltm eters. Sincetheresultspro-videdby the eldmetersdependstronglyontheprobe-to-sur facedistanceD0, it ismoreconve-nienttoreadthemastheelectric eldintensityvalues(thusthename, eldmeter).Magnitudeof eldsmeasuredthisway is usuallyhigh,thereforethereisa riskofdischargesbetweentheprobeandthetes tedsurface.

6 Fieldmetersarelessex-pensive thanelectrostaticvoltmeters, sincetheydonotrequirehighvoltagecircuitr y toproduceproperfeedback ,INC. 11601 MapleRidgeRoad Medina,NY14103 Tel:(585)798-3140 Call:1 800 FORTREK FAX:(585)798-3106 of 6 Copyright 2002 trek , 0aTrekApplicationNoteNumber3002 Non-contactsurfacecharge/ voltage measurementsFieldmeterandvoltmetermethod svibratingKelvinsensorsurfaceunder testphasesensitivedemodulatorVPintegrato roscillatoramplifierpre-amplifierbootstr appedpower supplyFigure1:Electrostatic eldmeter[13].vibratingKelvinsensorsurfac eunder testphasesensitivedemodulatorVintegrator oscillatoramplifierpre-amplifiercircuitc ommonbootstrappedpower supplyFigure2:Electrostaticvoltmeter(vol tagefollower)[13].vibratingKelvinsensors urfaceunder testV+-oscillatoramplifier 1pre-amplifieramplifier 2 VtFigure3:AC-feedback electrostaticvoltmeter[6,10]. trek ,INC. 11601 MapleRidgeRoad Medina,NY14103 Tel:(585)798-3140 Call:1 800 FORTREK FAX:(585)798-3106 of 6 Copyright 2002 trek , 0aTrekApplicationNoteNumber3002 Non-contactsurfacecharge/ voltage measurementsFieldmeterandvoltmetermethod s510152025020406080100120140160180200dis tance from the surface under test, [mm]error, [%]tested with 1 [kV]applied to the 310 [cm2] circular plateTrek Model 520 ESVM fieldmeterFigure4:Comparisontestbetweene lectrostaticvoltmeterand ,INC.

7 11601 MapleRidgeRoad Medina,NY14103 Tel:(585)798-3140 Call:1 800 FORTREK FAX:(585)798-3106 of 6 Copyright 2002 trek , 0aTrekApplicationNoteNumber3002 Non-contactsurfacecharge/ voltage measurementsFieldmeterandvoltmetermethod sothertypesof eldmetersavailable, forexampleradioactive eldmeters, rotatingvaneunits. Eventhoughtheirconstructionandprincipleo fopera-tionarerelativelysimple, they sufferfromdisad-vantagessuchaspresenceof theradioactive ma-terial,pooraccuracyandhighpowerconsum ptionby thedrive motorof , particularlythevoltagefollowers, canbeemployedfortestsofrelativelysmallch argedareas- they have muchbetterres-olutionthan eldmeters. Voltmetersarealsoveryaccurateovera , , voltmeteris a low-costalternative doesnothave highvoltagecircuitry andis accuratewithina certainspeci edrangeofdistancesD0. Forex-ampleTrek's model520holdsthe5%accuracyoverthedistanc ebetween3 and30[mm][10,14].

8 Thereis a riskofdischargesbetweentheprobeandthetes tedsurface, sotheresolutionoftheAC-feedback voltmeteris limitedby thedistanceD0. Table1 summarizesfeaturesanddisadvan-tagesofthe electrostatic eldmetersandDCandAC-feedback eldmeterDC-feedback ESVMAC-feedback ESVM general recommendationfortestsof largesurfaceslargeandsmallsurfaceslargea ndsmallsurfacesmeasuredvariableelectric eldintensityvoltagevoltagecostlowhighmed iumspatialresolutionpoorvery goodgoodaccuracygoodat thelargeprobe-to-surfacedistanceexcellen tat thesmallprobe-to-surfacedistancevery goodwithinthespeci edprobe-to-surfacedistanceprobepotential ground(possibilityof arcing)potentialof thetestedsurfaceground(possibilityof arcing)distanceindependentnowithina certain,speci edrange(dependsontheprobetype)withina broadrange(dependsontheprobetype)Table 1:Overview of featuresReferences[1] voltmeter. U. S. ,1970.

9 [2] eldmeasurementapparatushavinga ca- trek ,INC. 11601 MapleRidgeRoad Medina,NY14103 Tel:(585)798-3140 Call:1 800 FORTREK FAX:(585)798-3106 of 6 Copyright 2002 trek , 0aTrekApplicationNoteNumber3002 Non-contactsurfacecharge/ voltage measurementsFieldmeterandvoltmetermethod spacitordetectorwithfeedback todrive S. patentno. 3611127,1971.[3] non-contactingdy-namicvoltagefollowerfor voltagemeasure-mentof electrostaticallychargedsurfaces. patentno. 3729675,1973.[4]B. T. Williams. Highspeedelectrostaticvolt-meter. U. S. patentno. 4205267,1980.[5]B. T. Williams. Low impedanceelectrostaticdetector. U. S. patentno. 4370616,1983.[6]B. T. S. ,1989.[7]F Rossi, Opat, Mod-i , 63(7):3736 3743,1992.[8]S. , 5:478 480,1972.[9] U. S. patentno. 4106869,1978.[10]D. Feedback-based , pagesS43 S45,November1995.[11]W. , 3:367 368,1932.[12]FoordT.

10 Useofa ca-pacitive (J. Phys. E),2(2):411 413,1969.[13] , volumeIIB, chapterElectro-staticchargemeasurements, pages440 ,2ndedition,1987.[14]D. eldmetermeasurementsonGMRrecordingheads. InEO/ESDS ymposiumProceedings,volumeEOS-22,pages49 9 504,Ananheim,CA, ,INC. 11601 MapleRidgeRoad Medina,NY14103 Tel:(585)798-3140 Call:1 800 FORTREK FAX:(585)798-3106 of 6 Copyright 2002 trek , 0a