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VisualFeedbackControlofaMicroLatheHirotakaOJIMA1,KatsuhiroSAITO1,LiboZHOU1,JunSHIMIZU1,HiroshiEDA11IbarakiUniversityKeywordsMicrolath,Visualfeedback,PositioncontrolAbstractMicromachiningprogressesrapidlyinrecentyears.Inthisresearch,amicrolathewhichisinstallableandoperationalinsideSEMvacuumchamberhasbeendesignedanddeveloped.Asafirststep,visuallyguidedmicrolathesystemisdevelopedwithimageofCCDcameradeviceinsteadofSEMimage.UnliketheconventionalfeedbackcontrolwhichpositionstheXYtableonly,thisschemeoffersadirectcontroloftheposition,pathandspeedofthetooltip.Usingproposedmethod,cuttingexperimentwasachieved,anditisconfirmedthatdevelopedmicrolathesystemiseffectivetodocutting.1IntroductionRecently,thesystemcapableofproducingthemicropartsarerequestedalongwiththeminiaturization1.Micromachiningprogressesrapidlyinrecentyears.Theexploratoryresearchhasapproachedtoalevelofaccessingasinglemoleculeoratom.Asadrivingforce,MEMSmicroelectronicmechanicalsystemhasbeenplayingamajorroleinmakingmicrocomponentsanddevices.However,MEMSisbasedonthephotolithographytechnologyandtherebyapplicableintolimitedmaterialssuchassiliconmonocrystalline.Inordertomeetthedemandsofminiaturizationinelectronicandopticalapplications,alternativemicromachiningtechnologywhichisabletoaccessavarietyofmaterialsina3dimensionalwayisrequired2.MicroMesoMechanicalManufacturingM4offersaccessibilitytodifferentkindsofmaterialaccordingtoeachobjective,andattainshighrepeatabilityandaccuracywiththelatestultraprecisionmeans.Thereare,however,manyscientificandtechnologicalbarriersencounteredinpragmaticimplementationofM4.Oneofthemisthesurfacechemistryeffects.Whenmachiningpartsareatmicroscale,itisrecognizedthatthesurfaceareatovolumeratiowillbeincreasedinbothchipsandtheresultingpartascomparedtoconventionalmacromachiningprocess.Anotherproblemisthedirectmotionandpositioncontrol.Sensorsthatarecapableofdirectlymeasuringtherelativedisplacementbetweenthetoolandworkpiecearenotyetavailable.Inthisresearch,amicrolathewhichisinstallableandoperationalinsideSEMvacuumchamberhasbeendesignedanddeveloped3.Fig.1showstheconceptsofthedevelopedmicrolathe.Atsuchoxygenfreecondition,cuttingtestsareconductedtounderstandsurfacechemistryeffectsonmicromachining.However,sincedevelopedmicrolatheissmallinsize,rigidityofthelatheislow.ThusthepositionofthetoolofthelatheisnotabletobecontrolledaccuratelywithaconventionalmethodwhichcontrolsXYtableonly.Therefore,thevisionguidedcontrolmethodisproposed.TheimagefromtheSEMscanningelectronmicroscopeisdigitizedbyCCDintopixelswith8bitgrayscale.Sinceeachpixelcontains2Dpositionalinformation,thevisionsystemthusoffersanorthogonalcoordinatehereafterreferredasthepixelcoordinateforobjectsinviewtoreferto.ThepixelcoordinateisfreefromthemechanicalinaccuracyandoffersadirectmeasurementofSEMCCDMicrolatheFig.1.ConceptofthedevelopedmicrolatheˆŠŠH.Ojima,K.Saito,L.Zhou,J.Shimizu,H.Edatherelativepositionoftoolandworkpiece.TheresolutionincreasestogetherwiththemagnificationofthemicroscopeandthenumberofCCDpixels.Inthisresearch,avisioncontrolschemehasbeenproposedandimplementedforfeedbackcontrolofthetoolmovements.UnliketheconventionalfeedbackcontrolwhichpositionstheXYtableonly,thisschemeoffersadirectcontroloftheposition,pathandspeedofthetooltip.Asafirststep,visuallyguidedmicrolathesystemisdevelopedwithimageofCCDcameradeviceinsteadofSEMimage.2OverviewofsystemActuatingmoduleSensingmoduleProcessingmoduleImageinformationActuatorsignalCaptureboardMicrolatheXZstageAMPCPUDiamondtoolCCDWorkpieceD/AboardFig.2.BlockdiagramofsystemTable1.SpecificationofsystemSizeofmicrolatheWDH909042mmSpindlerotationalspeed08000rpmDepthofcutTraversefeed1010mmCenterhighadjustment30PmToolDiamondNoseangle/noseradius40°/2PmScanningrate20frame/sTotalpixels0.3megapixelShowninFig.2istheblockdiagramofdevelopedmicrolathesystem,whichconsistsofthreemainmodulestheactuatingmodulethatdrivesmicrolathe,thesensingmodulethatimportsimagesandtheprocessingmodulethatimplementsfeedbackcontrol.Eachmoduleisresponsiblefordifferentfunction.Theactuatingmoduleisthecoreelementwherethecuttingoperationiscarriedout.ThesensingmoduleimportsimagesfromCCDimagedevice,andobtainsthepositionofthetoolandtheworkpiece.Theothertasksincludingtheimageprocessingandfeedbackcontrolareexecutedbytheprocessingmodule.UpperpictureofFig.1showstheoverallappearanceofthesystem.Table.1showsthespecificationsofthesystem.TheactuatingmodulefurtherincorporatesadiamondtoolwithaXZlinearstage,andthesensingmoduleincludesahighresolutionCCDimagedevice.Throughsensingmodule,theappearanceoftheworkingareaisnotonlydisplayedonthemonitortothegivetheoperatorthevisualinformation,butalsoconvertedintodigitalsignalforsubsequentprocessing.AsthecontroldiagramshowinFig.2,themovementsofthediamondtoolaregovernedwiththevisualfeedbackcontrol.Thesensingmodulefirstabstractsthepositionsofthetoolandworkpiecebycomparingthepreregisteredtemplateswiththecapturedvisualinformation.Correspondingtotherelativepositionsoftoolandworkpiece,thetoolpathandspeedarecalculatedandconvertedintoappropriatepulsetrain.egfhefghacbdabcdFig.4.DrivingprincipleofXZstageXYZCenterhighadjustmentDCmotorSpindleXZstageMicrolatheFig.3.XZstageandmicrolathe3ActuatingmoduleThedevelopedmicrolatheisshownrightwardinFig.3.ThislatheconsistsofthemainspindlewiththecolletchuckwiththeDCmotor,thecenterhighadjustmentusingapiezoelectricactuatorandXZstagewhichperformsbothdepthofcutXaxisandtraversefeedZaxis.TheXZstageisdrivenbytheinertialsliding,andiscomposedofapiezoelectricactuatorandthelinearguide.XZstageisshownleftwardinFig.3.AnaccuratetoolpositioningisachievedbydrivingtheXZstageprecisely.ImportantpointsofdrivingtheXZstagearethecontrolofthedrivingdirection,distanceandvelocity.Figure4showstheinertialslidingmechanismbythesawtoothwave.Thedirectionofthemovementisdecidedbytherising/trailingedgeofthesawtoothwaveasshowninFig.4.Forexample,ˆŠ‹VisualFeedbackControlofaMicroLathethemechanismintherightdirectionisexplainedasfollows.Thevoltagegraduallyrises,andapiezoelectricactuatorstretchesmostin1.Theactuatorshrinksbasedonthecentroidin2byfallingrapidlyofthevoltage.Onlythesidewherethefrictionalforceissmallmovesastheactuatorstretchesgraduallywiththeascentofthevoltagein3.Theactuatorisstretchesagainin4,andadvancestowardtherightdirection.Theactuatorsimilarlyadvancesalsotowardtheleftdirectionifareversepulsetrainisgiven.00.10.20.30.40200400600FrequencyAíHzSpeedAímm/s±80V±40VFig.5.VelocitychangedependingonfrequencyandvoltageNext,thevelocitycontrolofthismechanismisdescribed.AsshowninFig.5,thevelocityisproportionaltobothfrequencyofthepulsetrainanddrivingvoltage.Finally,drivingdistancecanbecontrolledaccordingtothenumberofpulses,becausethedrivingdistancebyoneplusisabout500Pmat±80Vor250Pmat±40V.500,420140,420500,60140,60XZ320,2404123Fig.8.ExperimentalconditionoflinearpathcontrolXpixelZpixelcount210121011000200300400countFig.7.RecognitionaccuracyoftooltipXZstageDiamondtoolCCDWorkpieceX,ZZXFig.6.Visualsensingsystem4SensingmoduleThediamondtoolismountedonXZstage,whichusespiezoelectricactuatortodrivetool.Thosemechanicalinaccuracies,mainlycausedbythermalexpansion,hysteresis/driftinactuatorsandmisalignmentoforthogonalaxis,maydirectlydeliveranegativeeffecttothesystemperformance.Tosolvetheseproblems,avisioncontrolschemeasshowninFig.6isdeveloped.TheleftpictureinFig.6showsthemicrolatheandCCDimagedevicelocatedinYaxis.FromtherightpictureinFig.6,theincomingvisualinformationfromtheCCDisdigitizedintopixelswith8bitgrayscalebythesensingmodule.Aseachpixelbears2Dpositionalinformation,thevisionsystemthusoffersanorthogonalcoordinatereferredasthepixelcoordinateforobjectsinviewtoreferto.ThepixelcoordinateisfreefromthemechanicalinaccuracyanditsresolutionincreasestogetherwiththemagnificationoftheCCD.Ata480640pixelframeusedinthecurrentresearch,forexample,theresolutionofthepixelcoordinateisabout6PmwhentheviewoftheCCDistwofoldmagnified.WhentheCCDisalignedalongYaxis,thepositionofthetooltipandworkpieceisprojectedintoa2DpixelcoordinateXZwhichiscommonlysharedbytheXZstageandworkpiece.Drivenandcontrolledbythepixelcoordinate,thetoolisabletobepositionedandmovedattheaccuracyofpixelresolutionwithnoeffectbythemechanicalinaccuracy.Inaddition,iftherigiditybetweenXZstageandtoolislow,positioningoftooltipisnotachievedbydrivingXZstageaccurately.Thus,moreimportantly,thisoperationisaneffectivemethodofpositioningforthemicrolathewithalowrigidity.Figure7showstherecognitionaccuracythatismadebyuseofshapebasedpatternmatching4torecognizetheactualtooltiprepeatedly500times.Wecomprehendfromthegraphthat88.5reliabilitycanbeachievedwithinthelimesof±1pixel6Pm.5ProcessingmoduleForthesystemwhichisconsistedoftheactuatingandsensingmoduleinprevioussection3and4,thevisualˆŠŒH.Ojima,K.Saito,L.Zhou,J.Shimizu,H.Edafeedbackcontrolmethodisdescribedinthissection.ThetooltipisdrivenbyvisualfeedbackcontrolmethodwithpositionsofthetooltipandtargetsfromCCDimagedevice.Asafirststep,weexaminedlinearpathcontrolandcircularpathcontrolofthetooltip.Inthesepathcontrols,drivingfrequencyis300Hz162Pm/s.Atfirst,linerpathcontroloftooltipisdescribed.AsshowninFig.8,thetargetpositionisdefinedas320,240whichisthecenteroftheimagefromCCD,andfourkindsofpathcontrolareexamined.Inthecaseoflinerpathcontrol,theangleformedbythetargetpositionandthepresentpositionofthetooltipisfedbacktoachievethepathcontrol.Figure9ashowstheresultantpathofthetooltipwithoutfeedbackcontrol,andbshowsthatwithfeedbackcontrol.Inthecaseofthepathwithoutfeedback,finalerrorsoffourpathsarebetween5pixels30Pmand15pixels90Pm.Ontheotherhand,thepathwithfeedbackfollowsalongthetargetpath,andfinalerroriswithin2pixels12Pm.Next,thecircularpathcontrolwhichismultiaxialinterpolationisdescribed.TheconditionofthecircularpathcontrolisshowninFig.10.Thecenterofthetargetcircularpathisdefinedas320,240whichisthecenteroftheimagefromCCD,andtheradiusofthetargetpathis100pixels600Pm,moreoverthetooltipisdrivenfromstartingpoint220,240alongcounterclockwisedirectionrepeated3times.Inthecaseofcircularpathcontrol,weconsidertofeedbacknotonlytheangleformedthecenterofthetargetcircularpathandthepresenttoolposition,butalsothedeviationoftheradiuswhichistheerrorbetweentheradiusofthetargetcircularpathandthedistancefromthecenterofthetargetpathtothepresenttoolposition.Inthecaseofthedrivingthepathwithoutfeedbackcontrol,thetoolisdrivenbytheanglespreparedinadvance.Figure11ashowstheresultantpathofthetooltipwithoutfeedbackcontrol,andbshowsthepathwithfeedbackcontroloftheangleonly,andcshowsthepathwithfeedbackcontroloftheangleandradius.Figure11ashowsthattheresultantpathdepartedfromtargetpath,andthecenterandtheradiusofthepatharedeflectedfromthoseofthetargetpath.Figure11bshowsthecenteroftheresultantpathmatchesthecenterofthetargetpath,butextendstheradiusoftheresultantpathasthepathgoesaround.Moreover,Fig.11cshowsthattheresultantpath0100200300400500100200300400500600XpixelZpixelgoal1234Zpixelawithoutfeedbackcontrol0100200300400500100200300400500600XpixelZpixelgoal1234ZpixelbwithfeedbackcontrolFig.9.Experimentalresultsoflinearpathcontrol220,240320,240XZCCWFig.10.Experimentalconditionofcircularpathcontrol0100200300400100200300400500XpixelZpixel0100200300400100200300400500XpixelZpixel0100200300400100200300400500XpixelZpixeltooltargettooltargettooltargetZpixelZpixelZpixelgetawithoutfeedbackcontrolbwithfeedbackcontroloftheanglecwithfeedbackcontroloftheangleandradiusFig.11.ExperimentalresultsofcircularpathcontrolˆŠ