自动夹具中英文对照外文翻译文献Word格式文档下载.docx
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AUTOMATICFIXTURESYNTHESISIN3D
KamenPenev
ProgrammableAutomationLaboratory
ComputerScienceDepartmentandInstituteforRoboticsandIntelligentSystems
UniversityofSouthernCalifornia
LosAngeles,CA90089-0781AristidesA.G.Requicha
LosAngeles,CA90089-0781
Abstract
Afixtureisanarrangementoffixturingmodulesthatlocateandholdaworkpartduringamanufacturingoperation.Inthisworkwe.considerfixtureswithfrictionlesspointcontactsandpresentamethodforplacementofcontactpointsonanon-prismatic3Dworkpart.Itisanon-deterministic,potentialfieldalgorithmforcontactpointplacement.Themethodprovidesabasicframeworkfortheintegrationofheterogeneoushigh-levelfixturingagentsthroughaninterfacebasedonzonesofattractionandrepulsionontheworkpartboundary.Thealgorithmmayproduceredundantfixtures,andcanaugmentpartialsolutionstocompleteformclosurefixtures.
1.Introduction
Afixtureisanarrangementoffixturingmodulesthatlocateandholdaworkpartduringamanufacturingoperation,suchasmachining,assemblyandinspection.Fixturingisofessentialimportancetoindustrialmanufacturingandconstitutesasignificantpartofallmanufacturingcosts.Therefore,fixturedesignautomationisveryimportant.Fixturedesigninvolvesagreatvarietyofconsiderations,suchasrestraint,deterministiclocation,loadability,andtoolaccessibility.Efficientalgorithmsthataddressthewholerangeoffixturingissuesforacomprehensivedomainofworkpartsdonotyetexist.Recently,BrostandPeterspublishedanalgorithm[Brost&
Peters1996]thatextendstheearlierclassicworkofBrostandGoldberg[Brost&
Goldberg,1994]tothe3Ddomain.Thisalgorithm,however,requiresverticalandhorizontalplanarsurfacestoconstituteasubstantialpartoftheworkpartboundary.Itgeneratesallpossiblefixturesandthenratesthemaccordinglytocertainmetrics.Thisiscomputationallyexpensive.Wagneretalpresentedanalgorithmthatusessevenmodularstrutsmountedinaboxtofixturepolyhedra[Wagneretal1995].Thisalgorithmisnotcompleteinthesensethatitcannoteffectivelyhandlecertaincases,suchasacubewithfacesparalleltothebox.Italsosuffersfromhighcomputationalcomplexity.WallackandCannysuggestedanothermethodwithan“enumerate-and-rate”flavor[Wallack&
Canny1996].Itcanfixtureprismaticworkpartswithplanarandcylindricalverticalsurfaces.Ponceproposedanalgorithmthatutilizescurvatureeffectstocomputefixtureswithfourfingersforpolyhedralparts[Ponce96].Thereducednumberofcontactsshouldprovideforbettercomplexityofthisalgorithm,butthequalityoftheproducedfixturesseemstobeinferiortotheonesthatutilizemorecontactsandprovideclassicalformclosure.
Inthispaperwepresentanewpotential-fieldalgorithmthatefficientlyproducesqualityfixturedesigns.Ouralgorithmworksforarbitraryworkpartsandprovidesconvenientuniversalmeansforrepresentingvariousfixturingrequirements.Thisalgorithmisadirectgeneralizationofthe2DpotentialfieldfixturingalgorithmofPenevandRequicha[Penev&
Requicha1996].Weconsiderfixtureswithfrictionlesspointcontacts.Ithasbeenproventhatsevencontactsarenecessary.[Somoff,1900]andsufficient[Markenscoffetal,1990]toimmobilizeanyworkpartin3D
Followingaleast-commitmentstrategy,theprocessoffixturesynthesismaybeseparatedintothreestages–fixturingtaskanalysis,contactpointplacement,andfixturelayoutdesign.Inthefixturingtaskanalysisphasetheworkpartgeometryandmanufacturingprocessareanalyzedtoidentifyvariousparametersofthefixturingproblem,suchascuttingforces,inaccessibleorforbiddenareas,andalsotofindfeaturesthatmaybeusefulforapplyingfixturingdevices,suchasmachinedflatsurfaces,horizontalandverticalsurfaces,pairsofparallelsurfaces,pairsofperpendicularsurfaces,etc.
Figure1:
Contactpointplacement
Inthecontactpointplacementphaseanumberofcontactpointsareplacedontheworkpartboundary(Figure1),sothattheresultingconfigurationofcontactssatisfiestheconstraintsidentifiedintheanalysisphaseaswellascertainkinematicrequirementsthatmustbesatisfiedbyanyfixture,suchastotalrestraint.
Figure2:
Fromcontactpointconfigurationtofixturelayoutdesign
Inthelayoutdesignphase“towers”offixturingcomponentsarebuiltandplacedaroundtheworkpartsoastocontactthepartatthepointlocationscomputedinthecontactpointplacementphase.Forexample,acontactpointonahorizontalworkpartsurface(Figure2a)mayleadtotheinstantiationofanoverheadclampthatcontactstheworkpartatthatparticularpoint(Figure2b).Thisisadesign-for-functionproblemconstrainedbythesetofavailablefixturingmodulesandtheirparameters.Thesetofcontactpointsarethefunctionalspecificationandthefixturelayoutisaconfigurationofcomponentsthatachievesit.
Inthisresearchwefocusoncontactpointplacementanditsintegrationwithpartandtaskanalysis.Anarrangementofcontactpointsmustsatisfycertainkinematicconditionsinordertobeabasisforagoodfixture.Inparticular,itmustprovideformclosure,deterministiclocation,clampingstability,detachabilityandloadability[Asada&
By].
Thealgorithmusesadiscretizationoftheworkpartboundary,similartothemeshesusedinFEA.However,unlikeFEA,ourattentionisonthemeshnodes,ratherthanonthemeshelements.Discretizationwaschosenforthefollowingreasons:
First,wecanhandleworkpartswitharbitrarygeometry,aslongasthepart’sboundaryisacollectionofsmoothsurfaceswhichweknowhowtomesh.ThisrequirementissatisfiedbyallsurfacesusedinmodernCADsystems.Second,discretizationisnecessaryinordertoavoidanexpensivecomputationofgeodesiccurves.Third,discretizationshouldnotsignificantlyaffecttheresults,aslongasthenumberofdiscretecandidatelocationsontheboundaryismuchlargerthanthenumberofsurfaces.Inourimplementationthediscretizedboundaryconsistsofseveralhundredpointsonly.Experimentalevidenceindicatesthatthisissufficientforrealisticworkparts.
Weintroduceapotentialfieldontheworkpartboundarydefinedbyzonesofattractionandrepulsion,whichwecallP-zones.Thecontactsaremodeledaschargedparticlesthatmoveontheboundarydrivenbythispotentialfield.Thecontactsarealsosubjecttomutualrepulsionbasedonthedistancebetweeneachtwocontactsinthewrenchvectorspace.Thealgorithmexecutesaseriesofsimulationepochs.Eachepochstartswitharandomconfiguration,proceedsthroughacertainnumberofstepstowardlowerpotentialenergyandendswithatestforkinematicconditions(formclosure).Thealgorithmterminateswhenanepochproducessatisfactoryconfiguration.
Tospreadthecontactpointsontheboundarywesimulaterepulsionbetweeneachpairofthem.Theintensityofrepulsionbetweentwocontactpointsdependsonthedistancebetweentheircorrespondingwrenchesinthewrenchvectorspace.Oursimulationproceedsinalimitednumberofstepsoruntilequilibriumisreached.Theresultingplacementshouldhaveagoodchanceofleadingtoagoodfixture.Sucharandomizedmethodassumesthatthesetofn-tuplesofcontactpoints(forngreaterthanthree)thatsatisfythekinematicrequirementshasmeasuregreaterthanzeroandisrelativelylarge.Thatis,thesolutionspaceislarge.Althoughwehavenotbeenabletoprovethishypothesismathematically,ourexperimentshaveconfirmedit.Moreover,themeasureincreaseswiththenumberofcontactpoints,e.g.itiseasiertofindaformclosurearrangementwitheightpointsthanwithseven.
Thenotionofrepulsionisessentialinourmethodasitallowsotherconsiderationstobeaccommodatedeasily.Wecanputadditionalrepulsionspotsontheworkpartboundarytorepresentforbiddenregions.Wecanalsointroducecentersofattraction.Thesecorrespondtoareasthatwererecommendedbytheanalysisphaseasdesirableforplacingcontactpoints,e.g.datumsurfaces.Thus,weproposeapotentialfieldforuniformlyrepresentingheterogeneousfixturinginformation.Regionsofrepulsioncorrespondtoareaswithpositivepotential.Negativepotentialisassociatedwithattraction.Zeropotentialcorrespondstoneutralareas.Theinitialrandomlyselectedcontactpointsareregardedasparticlesthatarebeingattractedorrepelledbyapotentialfieldthatincludesapairwiserepulsion.Thegoalofthesystemofcontactpointsistominimizeitstotalpotentialenergy.
2TheInput
TheinputtoouralgorithmconsistsofCADmodelsoftheworkpartboundaryandasetofsolidP-zones.EachP-zonedefinesapotential-fieldinfluencingregionwithnon-zerocharge.
3DiscretizingtheWorkpartBoundary
Thefirststepinourmethodistodiscretizetheboundaryoftheworkpart,thuscreatingthecandidatecontactpointlocationswhichwecallnodes.Discretizationisdonebyinvokingastandardfaceterembeddedinthegeometricmodelerweuse.Thediscretizationisstoredinanorientedgraphdatastructure.Eachnodeofthegraphcorrespondstoanodeonthemesh.Theedgesofthegraphcorrespondtoedgesofthemeshconnectingneighboringnodes.Ateachnodethescrewrepresentingthepointcontactiscomputedandstored.Ascrewisaconciseandconvenientrepresentationofthesurfacenormalandthelocationofthenode.Itisusedinallkinematictestsbasedonscrewtheory.
4ComputingthePo