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外文翻译--在独自领域最佳多功能夹具布置的设计.doc

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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2MULTIOBJECTIVEOPTIMALFIXTURELAYOUTDESIGNINADISCRETEDOMAINDianaPelinescuandMichaelYuWangDepartmentofMechanicalEngineeringUniversityofMarylandCollegePark,MD20742USAEmailyuwangeng.umd.eduAbstractThispaperaddressesamajorissueinfixturelayoutdesigntoevaluatetheacceptablefixturedesignsbasedonseveralqualitycriteriaandtoselectanoptimalfixtureappropriatewithpracticaldemands.Theperformanceobjectivesconsideredarerelatedtothefundamentalrequirementsofkinematiclocalizationandtotalfixturingformclosureandaredefinedastheworkpiecelocalizationaccuracyandthenormanddistributionofthelocatorcontactforces.Anefficientinterchangealgorithmisuaedinamultiplecriteriaoptimizationprocessfordifferentpracticalcases,leadingtopropertradeoffstrategiesforperformingfixturesynthesis.I.INTRODUCTIONProperfixturedesigniscrucialtoproductqualityintermsofprecisionandaccuracyinpartfabricationandassembly.Fixturingsystems,usuallyconsistingofclampsandlocators,mustbecapabletoassurecertainqualityperformances,besidesofpositioningandholdingtheworkpiecethroughoutallthemachiningoperations.Althoughthereareafewdesignguidelinessuchas321rule,automatedsystemsfordesigningfixturesbasedonCADmodelshavebeenslowtoevolve.Thisarticledescribesaresearchapproachtoautomateddesignofaclassoffixturesfor3Dworkpieces.Thepartsconsideredtobefixturedpresentanarbitrarycomplexgeometry,andthedesignedfixturesarelimitedtotheminimumnumberofelementsrequired,i.e.sixlocatorsandaclamp.Furthermore,thefixelsaremodeledasnonfrictionalpointcontactsandarerestrictedtobeappliedwithinagivencollectionofdiscretecandidatelocations.Ingeneral,thesetoffixturelocationsavailableisassumedtobeapotentiallyverylargecollectionforexample,thelocationsmightbegeneratedbydiscretizingtheexteriorsurfacesoftheworkpiece.Thegoalofthefixturedesignistodeterminefirst,fromtheproposeddiscretedomain,thefeasiblefixtureconfigurationsthatsatisfytheformclosureconstraint.Secondly,thesetsofacceptablefixturedesignsareevaluatedon3severalcriteriaandoptimalfixturesareselected.Theperformancemeasuresconsideredinthisworkarethelocalizationaccuracy,andthenormanddistributionofthelocatorcontactforces.Theseobjectivescoverthemostcriticalerrorsourcesencounteredinafixturedesign,thepositionerrorsandtheunwantedstressinthepartfixtureelementsduetoanoverloadedorunbalancedforcesystem.Theoptimalfixturedesignapproachisbasedonaconceptofoptimumexperimentdesign.Thealgorithmdevelopedevaluatesefficientlytheadmissibledesignsexploitingtherecursivepropertiesinlocalizationandforceanalysis.Thealgorithmproducestheoptimalfixturedesignthatmeetsasetofmultipleperformancerequirements.II.RELATEDWORKLiteratureongeneralfixturingtechniquesissubstantial,e.g.,1.Theessentialrequirementoffixturingisthecenturyoldconceptofformclosure2,whichhasbeenextensivelystudiedinthefieldofroboticsinrecentyears3,4.Thereareseveralformalmethodsforanalyzingperformanceofagivenfixturebasedonthepopularscrewtheory,dealingwithissuessuchaskinematicclosure5,contacttypesandfrictioneffects6.Adifferentanalysisapproachbasedonthegeometricperturbationtechniquewasreportedin7.Anautomaticmodularfixturedesignprocedurebasedonthismethodwasdevelopedin8toincludegeometricaccessconstraintsinadditiontokinematicclosure.Theproblemofdesigningmodularfixturesgainedmoreattentionlately9.Therehasalsobeenextensiveresearchinfixturedesigns,focusingonworkpieceandfixturestructuralrigidity6,toolaccessibilityandpathclearance7.Theproblemoffixturesynthesishasbeenlargelystudiedforthecaseofafixednumberoffixtureelementsorfixels8,10,particularlyintheapplicationtoroboticmanipulationandgraspingforitsobviouseasons3,4.Thisarticleaimstobeanextensionoftheresultsonthefixturedesignissuespreviouslyreportedin14.III.FIXTUREMODELThefundamentalperformanceofafixtureischaracterizedbythekinematicconstraintsimposedontheworkpiecebeingheldbythefixture.Thekinematicconditionsarewellunderstood3,4,5,7,12.Forafixtureofnlocatorsi1,2,,n,thefixturecanberepresentedbyyGTqwheredefinesmallperturbationsinthelocatorpositionsandthelocationoftheworkpiecerespectively.Thefixturedesignisdefinedbythelocatormatrixiwhereandniandridenotethesurfacenormalandpositionattheithcontactpointontheworkpiecesurface.Theproblemoffixturedesignrequiresthesynthesisofafixturingschemetomeetagivensetofperformancerequirements.IV.QUALITYPERFORMANCECRITERIAFORAFIXTUREA.AccurateLocalizationAnessentialaspectoffixturequalityistopositionwithprecisiontheworkpieceintothefixturingsystem.Ingeneraltheworkpiecepositionalerrorsareduetothegeometric4variabilityofthepartandthelocatorssetuperrors.Thispaperwillfocusonlyontheworkpiecepositionalerrorsduetothelocatorpositioningerrors.Asanextensionofthefixturemodelequationeq.1,thelocatorpositioningerrorsycanberelatedwiththeworkpiecelocalizationerrorqasfollowsClearly,forgivensourceerrorstheworkpiecepositionalaccuracydependsonlyonthelocatorlocationsbeingindependentfromtheclampingsystem,theFisherinformationmatrixMGGTcharacterizingcompletelythesystemerrors.Ithasbeenshown12thatasuitablecriteriontoachievehighlocalizationaccuracyistomaximizethedeterminantoftheinformationmatrixDoptimality,i.e.,maxdetM.B.MinimalLocatorContactForcesAnotherobjectiveinplanningafixturelayoutmightbetominimizeallsupportforcesatthelocatorcontactregionsthroughoutalltheoperationswithcompletekinematicrestraintorforceclosure.LocatorcontactforcesinresponsetotheclampingactionaregivenasNormalizingtheseforceswithrespecttotheclampingintensityweobtainTheforceclosureconditionrequirestheseforcestobealwayspositiveforeachlocatoriofasetofnlocatorsComputingthenormofthelocatorcontactforcesleadstoanappropriatedesignobjective,i.e.minNotethatthisobjectiveindicatesbothlocatorandclamppositionstobedeterminedintheoptimizationprocess.C.BalancedLocatorContactForcesAnothersignificantissueindesigningafixtureisthatthetotalforceactingontheworkpiecehavetobedistributedasuniformlyaspossibleamongthelocatorcontactregions.Ifprepresentsthemeanreactiveforceinresponsetotheclampaction,thenwedefinethedispersionofthelocatorcontactforcesasTherefore,minimizingthedefineddispersionrepresentsanobjectiveforabalancedforceclosuremind.V.OPTIMALFIXTUREDESIGNWITHINTERCHANGEALGORITHMS

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