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Amicroplanetarygearmechanismfeaturingahighgearreductionratiowithcompactnessinsizeispresentedinthispaper.SUMMiTVisemployedforthefabricationmethodsothattheredundancyofassemblingpartsiseliminated.Thedesignrulesofwhichhasalsobeenchecked.Tomakefulluseofthebenefitsofthesurface-micro-machining,theplanetaryreductiongearisdesignedtowardusingtheon-chipmicro-engine.Theexpectedgearreductionratioiscalculatedandcomparedwiththeconventionalchaingearmechanism.Themicroplanetarygearmechanismpresentedinthispaperisexpectedtohave162:

1reductionratioutilizinglessspaceconsumption.Thisisanorderofmagnitudehigherthanthepreviouslyreporteddesigninasinglereductiongeartrain.

Keywords:

MEMS,planetarygear,reductiongearsurface-micromachining,SUMMiTVprocess

Nomenclature

asungear

bplanetgears

cinternalgear（fixed）

dinternalgear（rotary）

nthenumberofunitsofgeartrain

Ddiameterofthepitchcircle

Nnumberofteeth

Pnumberofplanets

angularvelocity

Introduction

Thegearmechanismsinmicroelectromechanicalsystems（MEMS）arecommonlyexpectedtogeneratehightorqueintheconfinedmicro-sizesystems.However,itisgenerallydifficultforthemicro-scalesystemstohavesuchahightorquewithouthavingmultiplereductionsystems.

Thedesignofthereductiongeardrivebasedonaplanetaryparadoxgearmechanismcanincreasethetorquewithinacompactarea,sincethemicroplanetarygearsystemhasanadvantageofhighreductionratioperunitvolume[1].Howeveritsmechanismissocomplicatedthatrelativelyfewattemptshavebeenmadetominiaturizethegearsystems[2-3].Suzumorietal.[2]usedthemechanicalparadoxplanetarygearmechanismtodrivearobotfor1-inpipesforwardorbackward.Theyemployedasinglemotortodrivethegearmechanismswithhighreductionratio.Precisegearfabricationwasenabledbymicrowireelectricaldischargemachining（micro-EDM）.Theseparts,however,shouldbeassembledbeforethedrivemotorisattachedtothegearbox.Takeuchiet.al.[3]alsousedmicro-EDMtofabricatethemicroplanetarygears.TheysuggestedspecialcermetsorHighCarbonSteelforpossiblematerials.Whilethedesigncanachieveareductionratioof200,thegearsshouldalsobeassembledandmotordriven.Toenablethedrivingoftheplanetarygearbyonchipmeans,SandiaUltra-planarMulti-levelMEMSTechnology（SUMMiT-V）process[4]forplanetarygearfabricationisadoptedinthisstudy.TheSUMMiT-Vprocessistheonlyfoundryprocessavailablewhichutilizesfourlayersofreleasablepolysilicon,foratotaloffivelayers（includingagroundplane）[5].Duetothisfact,itisfrequentlyusedincomplicatedgearmechanismsbeingdrivenbyon-chipelectrostaticactuators[5].However,inmanycases,themicroenginesmaynotproduceenoughtorquetodrivethedesiredmechanicalload,sincetheirelectrostaticcombdrivestypicallyonlygenerateafewtensofmicronewtonsofforce.Fortunately,theseenginescaneasilybedrivenattensofthousandsofrevolutionsperminutes.Thismakesitveryfeasibletotradespeedfortorque[7].Rodgersetal.[7]proposedtwoduallevelgearswithanoverallgearreductionratioof12:

1.Thussixofthesemodulartransmissionassembliescanhavea2,985,984:

1reductionratioatthecostofthehugespace.

Withthedesireforsizecompactnessandatthesametime,highreductionratios,theplanetarygearsystemispresentedinthispaper.Itwillbethefirstplanetarygearmechanismusingsurfacemicromachining,totheauthorsknowledge.Theprinciplesofoperationsoftheplanetarygearmechanism,fabrication,andtheexpectedperformanceoftheplanetarygearsystemsaredescribedinthispaper.

Principlesofoperation

Analternativewayofusinggearstotransmittorqueistomakeoneormoregears,i.e.,planetarygears,rotateoutsideofonegear,i.e.sungear.Mostplanetaryreductiongears,atconventionalsize,areusedaswell-knowncompactmechanicalpowertransmissionsystems[1].TheschematicoftheplanetarygearsystememployedisshowninFigure

SinceSUMMiTVdesignsarelaidoutusingAutoCAD2000,theFigure1isgeneratedautomaticallyfromthelayoutmasks（Appendix[1]）.Oneunitoftheplanetarygearsystemiscomposedofsixgears:

onesungear,a,threeplanetarygears,b,onefixedringgear,c,onerotatingringgear,d,andoneoutputgear.Thenumberofteethforeachgearisdifferentfromoneanotherexceptamongtheplanetarygears.Aninputgearisthesungear,a,drivenbythearmconnectedtothemicro-engine.Therotatingringgear,d,isservedasanoutputgear.Forexample,ifthearmdrivesthesungearintheclockwisedirection,theplanetarygears,b,willrotatecounter-clockwiseattheirownaxisandatthesametime,thosewillrotateaboutthesungearinclockwisedirectionresultinginplanetarymotion.Duetotherelativemotionbetweentheplanetarygears,b,andthefixedringgear,c,therotatingringgear,d,willrotatecounterclockwisedirection.Thisissocalleda3Kmechanicalparadoxplanetarygear[1].

Fabricationprocedureandteststructures

ThefeaturesoftheSUMMiTVprocessofferfourlevelsofstructuralpolysiliconlayersandanelectricalpolylevel,andalsoemploytraditionalintegratedcircuitprocessingtechniques[4].TheSUMMiTVtechnologyisespeciallysuitableforthegearmechanism.Theplanetarygearmechanismcanbedrivenbytheon-chipengineandthusisanotherreasonofusingtheSUMMiTVprocess.

SincetheSandiaprocessissuchawell-knownprocedure[5-7],onlybriefexplanationispresented.Figure2representsthecross-sectionalviewofFigure1,andalsowasgeneratedfromtheAutoCADlayoutmasks（Appendix[1]）.Thediscontinuityinthecross-sectionisfortheetchholes.Thepoly1（gray）isusedforthehubsandalsopatternedtomakethefixedringgear,i.e.,c,thesungear,i.e.,a,therotatingringgear,i.e.,c,andtheoutputgearispatternedinthepoly2.Sincetheplanetarygearneedstocontactboththefixedringandrotatingringgear,poly2isaddedtopoly3,wherethegearteethareactuallyformed.Thepoly4layerisusedforthearmthatdrivesthesungear.Afterthereleaseetch,theplanetarygearswillfalldownsothatthosewillengageboththeringgears.

ThefiguresfortheteststructuresarepresentedinAppendix[2].Sincetheaimofthispaperistosuggestagearreductionmechanism,theplanetarygearsystemisdecomposedtoseveralgearunitstoverifyitsperformance.Thefirstteststructureisaboutthearm,whichrotatesthesungear,connectedtotheon-chipengine.Theangularvelocityofthearmdependsontheengineoutputspeed.Thesecondteststructuredescribesthepointatwhichthesungearandplanetarygearsareengagedtothefixedringgear.Becauseofthefactthattheringgearisfixed,theplanetarygearisjusttransmittingthetorquefromthesungeartothefixedringgearwithoutplanetmotion,e.g.,rotatingitsownaxisnotaroundthesungear.Whentherotatingringgearismountedontopofthefixedringgear,i.e.,thethirdteststructure,theplanetarygearsbegintorotatearoundthesungearsothattheplanetmotionareenabled.Therefore,onceoneoutputgearisattachedtotherotatingringgear,i.e.,thefinalteststructure,thewholereductionunitiscompleted.Dismantlingthe

Planetarygearintothreeteststructuresallowsthepinpointingofpossibleerrorsinthegearsystem.

Solutionsprocedureandexpectedperformance

Thereductionratioisdefinedastheratiobetweentheangularvelocityofthedrivergearandthatofthedrivengear.Highreductionratiosindicatetradingspeedfortorque.Forexample,a10:

1gearreductionunitcouldincreasetorqueanorderofmagnitude.Sincethegearsintheplanetarysystemshouldbemeshedtooneanother,thedesignofgearmoduleshouldfollowarestriction.Forexample,thenumberofteethforthesungearpluseitherthatofthefixedringgearorthatoftherotatingringgearshouldbethemultipleofthenumberofplanets,P（equation1）.Equation2,whichrepresentthereductionratio,shouldobservetheequation1first.TheNisthenumberoftheteethforcorrespondinggear.

Gears,a,b,c,dintheplanetarygearsystemhaveatoothmoduleof4m,whichisacomparablesizeofthecurrentgearreductionunits[5],andthetoothnumbersare12,29,69,and72respectively.Thereforetheoverallreductionratiois162:

1fromequation

（2）.Rodgersetal.[7]reporteda12:

1reductionunitusingsurfacemicromachining,whichislessthanorderofmagnitudeforthegearreductionratiooftheplanetarygearsystem.AlthoughthereductionfromRodgersetal.[7]needstobeoccupiedinapproximately0.093mm2,theplanetarygearsystemonlyutilizesanareaofapproximately0.076mm2.Thus,thisplanetaryreductiondesigncanachieveanorderofmagnitudehigherreductionratiowithlessspace.Sincethereductionmoduleiscomposedofseveralreductionunits,theadvantageofusingaplanetarygearsystemisselfevidentinFigure3.

Figure3showsthecomparisonofreductionratiosbetweentheproposedplanetarygearmechanismi.e.162n,andtheSandiagearsystem[7],i.e.12n,asafunctionofthenumberofunits,i.e.,n.Theordinateisdrawninlogscalesothattheordersofmagnitudedifferencesbetweentwomodulesareevident.Forexample,inamodulewithfivenumbersofunits,thereductionratiodifferencebetweentwoisapproximatelysixordersofmagnitudes.Furthermore,theplanetarygearsystemcansave8500m2insuchafiveunitreductionsystem.

Concl