外文翻译--在独自领域最佳多功能夹具布置的设计.doc

1外文翻译专业机械设计制造及其自动化学生姓名班级学号指导教师2MULTI-OBJECTIVEOPTIMALFIXTURELAYOUTDESIGNINADISCRETEDOMAINDianaPelinescuandMichaelYuWangDepartmentofMechanicalEngineeringUniversityofMarylandCollegePark,MD20742USAE-mail:AbstractThispaperaddressesamajorissueinfixturelayoutdesign:toevaluatetheacceptablefixturedesignsbasedonseveralqualitycriteriaandtoselectanoptimalfixtureappropriatewithpracticaldemands.Theperformanceobjectivesconsideredarerelatedtothefundamentalrequirementsofkinematiclocalizationandtotalfixturing(form-closure)andaredefinedastheworkpiecelocalizationaccuracyandthenormanddistributionofthelocatorcontactforces.Anefficientinterchangealgorithmisuaedinamultiple-criteriaoptimizationprocessfordifferentpracticalcases,leadingtopropertrade-offstrategiesforperformingfixturesynthesis.I.INTRODUCTIONProperfixturedesigniscrucialtoproductqualityintermsofprecisionandaccuracyinpartfabricationandassembly.Fixturingsystems,usuallyconsistingofclampsandlocators,mustbecapabletoassurecertainqualityperformances,besidesofpositioningandholdingtheworkpiecethroughoutallthemachiningoperations.Althoughthereareafewdesignguidelinessuchas3-2-1rule,automatedsystemsfordesigningfixturesbasedonCADmodelshavebeenslowtoevolve.Thisarticledescribesaresearchapproachtoautomateddesignofaclassoffixturesfor3Dworkpieces.Thepartsconsideredtobefixturedpresentanarbitrarycomplexgeometry,andthedesignedfixturesarelimitedtotheminimumnumberofelementsrequired,i.e.sixlocatorsandaclamp.Furthermore,thefixelsaremodeledasnon-frictionalpointcontactsandarerestrictedtobeappliedwithinagivencollectionofdiscretecandidatelocations.Ingeneral,thesetoffixturelocationsavailableisassumedtobeapotentiallyverylargecollection；forexample,thelocationsmightbegeneratedbydiscretizing3theexteriorsurfacesoftheworkpiece.Thegoalofthefixturedesignistodeterminefirst,fromtheproposeddiscretedomain,thefeasiblefixtureconfigurationsthatsatisfytheform-closureconstraint.Secondly,thesetsofacceptablefixturedesignsareevaluatedonseveralcriteriaandoptimalfixturesareselected.Theperformancemeasuresconsideredinthisworkarethelocalizationaccuracy,andthenormanddistributionofthelocatorcontactforces.Theseobjectivescoverthemostcriticalerrorsourcesencounteredinafixturedesign,thepositionerrorsandtheunwantedstressinthepart-fixtureelementsduetoanoverloadedorunbalancedforcesystem.Theoptimalfixturedesignapproachisbasedonaconceptofoptimumexperimentdesign.Thealgorithmdevelopedevaluatesefficientlytheadmissibledesignsexploitingtherecursivepropertiesinlocalizationandforceanalysis.Thealgorithmproducestheoptimalfixturedesignthatmeetsasetofmultipleperformancerequirements.II.RELATEDWORKLiteratureongeneralfixturingtechniquesissubstantial,e.g.,1.Theessentialrequirementoffixturingisthecentury-oldconceptofformclosure2,whichhasbeenextensivelystudiedinthefieldofroboticsinrecentyears3,4.Thereareseveralformalmethodsforanalyzingperformanceofagivenfixturebasedonthepopularscrewtheory,dealingwithissuessuchaskinematicclosure5,contacttypesandfrictioneffects6.Adifferentanalysisapproachbasedonthegeometricperturbationtechniquewasreportedin7.Anautomaticmodularfixturedesignprocedurebasedonthismethodwasdevelopedin8toincludegeometricaccessconstraintsinadditiontokinematicclosure.Theproblemofdesigningmodularfixturesgainedmoreattentionlately9.Therehasalsobeenextensiveresearchinfixturedesigns,focusingonworkpieceandfixturestructuralrigidity6,toolaccessibilityandpathclearance7.Theproblemoffixturesynthesishasbeenlargelystudiedforthecaseofafixednumberoffixtureelements(orfixels)8,10,particularlyintheapplicationtoroboticmanipulationandgraspingforitsobviouseasons3,4.Thisarticleaimstobeanextensionoftheresultsonthefixturedesignissuespreviouslyreportedin14.III.FIXTUREMODELThefundamentalperformanceofafixtureischaracterizedbythekinematicconstraintsimposedontheworkpiecebeingheldbythefixture.Thekinematicconditionsarewellunderstood3,4,5,7,12.Forafixtureofnlocators(i=1,2,n),thefixturecanberepresentedby:y=GTqwheredefinesmallperturbationsinthelocatorpositionsandthelocationoftheworkpiecerespectively.Thefixturedesignisdefinedbythelocatormatrixiwhereandniandridenotethesurfacenormalandpositionattheithcontactpointontheworkpiecesurface.Theproblemoffixturedesignrequiresthesynthesisofafixturingschemetomeetagivensetofperformancerequirements.IV.QUALITYPERFORMANCECRITERIAFORAFIXTURE4A.AccurateLocalizationAnessentialaspectoffixturequalityistopositionwithprecisiontheworkpieceintothefixturingsystem.Ingeneraltheworkpiecepositionalerrorsareduetothegeometricvariabilityofthepartandthelocatorsset-uperrors.Thispaperwillfocusonlyontheworkpiecepositionalerrorsduetothelocatorpositioningerrors.Asanextensionofthefixturemodelequation(eq.1),thelocatorpositioningerrorsycanberelatedwiththeworkpiecelocalizationerrorqasfollows:Clearly,forgivensourceerrorstheworkpiecepositionalaccuracydependsonlyonthelocatorlocationsbeingindependentfromtheclampingsystem,theFisherinformationmatrixMGGTcharacterizingcompletelythesystemerrors.Ithasbeenshown12thatasuitablecriteriontoachievehighlocalizationaccuracyistomaximizethedeterminantoftheinformationmatrix(Doptimality),i.e.,maxdetM.B.MinimalLocatorContactForcesAnotherobjectiveinplanningafixturelayoutmightbetominimizeallsupportforcesatthelocatorcontactregionsthroughoutalltheoperationswithcompletekinematicrestraintorforce-closure.Locatorcontactforcesinresponsetotheclampingactionaregivenas:Normalizingtheseforceswithrespecttotheclampingintensityweobtain:Theforce-closureconditionrequirestheseforcestobealwayspositiveforeachlocatoriofasetofnlocators:Computingthenormofthelocatorcontactforces:leadstoanappropriatedesignobjective,i.e.minNotethatthisobjectiveindicatesbothlocatorandclamppositionstobedeterminedintheoptimizationproc