机械毕业设计英文外文翻译55采用遗传算法优化加工夹具定位和加紧位置.docx
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机械毕业设计英文外文翻译55采用遗传算法优化加工夹具定位和加紧位置
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Machiningfixturelocatingandclampingpositionoptimizationusinggeneticalgorithms
Fixturesareusedtolocateandconstrainaworkpieceduringamachiningoperation,minimizingworkpieceandfixturetoolingdeflectionsduetoclampingandcuttingforcesarecriticaltoensuringaccuracyofthemachiningoperation.Traditionally,machiningfixturesaredesignedandmanufacturedthroughtrial-and-error,whichprovetobebothexpensiveandtime-consumingtothemanufacturingprocess.Toensureaworkpieceismanufacturedaccordingtospecifieddimensionsandtolerances,itmustbeappropriatelylocatedandclamped,makingitimperativetodeveloptoolsthatwilleliminatecostlyandtime-consumingtrial-and-errordesigns.Properworkpiecelocationandfixturedesignarecrucialtoproductqualityintermsofprecision,accuracyandfinishofthemachinedpart.
Theoretically,the3-2-1locatingprinciplecansatisfactorilylocateallprismaticshapedworkpieces.Thismethodprovidesthemaximumrigiditywiththeminimumnumberoffixtureelements.Topositionapartfromakinematicpointofviewmeansconstrainingthesixdegreesoffreedomofafreemovingbody(threetranslationsandthreerotations).Threesupportsarepositionedbelowtheparttoestablishthelocationoftheworkpieceonitsverticalaxis.Locatorsareplacedontwoperipheraledgesandintendedtoestablishthelocationoftheworkpieceonthexandyhorizontalaxes.Properlylocatingtheworkpieceinthefixtureisvitaltotheoverallaccuracyandrepeatabilityofthemanufacturingprocess.Locatorsshouldbepositionedasfarapartaspossibleandshouldbeplacedonmachinedsurfaceswhereverpossible.Supportsareusuallyplacedtoencompassthecenterofgravityofaworkpieceandpositionedasfarapartaspossibletomaintainitsstability.Theprimaryresponsibilityofaclampinfixtureistosecurethepartagainstthelocatorsandsupports.Clampsshouldnotbeexpectedtoresistthecuttingforcesgeneratedinthemachiningoperation.
Foragivennumberoffixtureelements,themachiningfixturesynthesisproblemisthefindingoptimallayoutorpositionsofthefixtureelementsaroundtheworkpiece.Inthispaper,amethodforfixturelayoutoptimizationusinggeneticalgorithmsispresented.Theoptimizationobjectiveistosearchfora2Dfixturelayoutthatminimizesthemaximumelasticdeformationatdifferentlocationsoftheworkpiece.ANSYSprogramhasbeenusedforcalculatingthedeflectionofthepartunderclampingandcuttingforces.Twocasestudiesaregiventoillustratetheproposedapproach.
Fixturedesignhasreceivedconsiderableattentioninrecentyears.However,littleattentionhasbeenfocusedontheoptimumfixturelayoutdesign.MenassaandDeVries[1]usedFEAforcalculatingdeflectionsusingtheminimizationoftheworkpiecedeflectionatselectedpointsasthedesigncriterion.Thedesignproblemwastodeterminethepositionofsupports.MeyerandLiou[2]presentedanapproachthatuseslinearprogrammingtechniquetosynthesizefixturesfordynamicmachiningconditions.Solutionfortheminimumclampingforcesandlocatorforcesisgiven.LiandMelkote[3]usedanonlinearprogrammingmethodtosolvethelayoutoptimizationproblem.Themethodminimizesworkpiecelocationerrorsduetolocalizedelasticdeformationoftheworkpiece.RoyandLiao[4]developedaheuristicmethodtoplanforthebestsupportingandclampingpositions.Taoetal.[5]presentedageometricalreasoningmethodologyfordeterminingtheoptimalclampingpointsandclampingsequenceforarbitrarilyshapedworkpieces.LiaoandHu[6]presentedasystemforfixtureconfigurationanalysisbasedonadynamicmodelwhichanalysesthefixture–workpiecesystemsubjecttotime-varyingmachiningloads.Theinfluenceofclampingplacementisalsoinvestigated.LiandMelkote[7]presentedafixturelayoutandclampingforceoptimalsynthesisapproachthataccountsforworkpiecedynamicsduringmachining.Acombinedfixturelayoutandclampingforceoptimizationprocedurepresented.Theyusedthecontactelasticitymodelingmethodthataccountsfortheinfluenceofworkpiecerigidbodydynamicsduringmachining.Amaraletal.[8]usedANSYStoverifyfixturedesignintegrity.Theyemployed3-2-1method.TheoptimizationanalysisisperformedinANSYS.Tanetal.[9]describedthemodeling,analysisandverificationofoptimalfixturingconfigurationsbythemethodsofforceclosure,optimizationandfiniteelementmodeling.
Mostoftheabovestudiesuselinearornonlinearprogrammingmethodswhichoftendonotgiveglobaloptimumsolution.Allofthefixturelayoutoptimizationproceduresstartwithaninitialfeasiblelayout.Solutionsfromthesemethodsaredependingontheinitialfixturelayout.Theydonotconsiderthefixturelayoutoptimizationonoverallworkpiecedeformation.
TheGAshasbeenproventobeusefultechniqueinsolvingoptimizationproblemsinengineering[10–12].Fixturedesignhasalargesolutionspaceandrequiresasearchtooltofindthebestdesign.FewresearchershaveusedtheGAsforfixturedesignandfixturelayoutproblems.Kumaretal.[13]haveappliedbothGAsandneuralnetworksfordesigningafixture.Marcelin[14]hasusedGAstotheoptimizationofsupportpositions.Vallapuzhaetal.[15]presentedGAbasedoptimizationmethodthatusesspatialcoordinatestorepresentthelocationsoffixtureelements.FixturelayoutoptimizationprocedurewasimplementedusingMATLABandthegeneticalgorithmtoolbox.HYPERMESHandMSC/NASTRANwereusedforFEmodel.Vallapuzhaetal.[16]presentedresultsofanextensiveinvestigationintotherelativeeffectivenessofvariousoptimizationmethods.TheyshowedthatcontinuousGAyieldedthebestqualitysolutions.LiandShiu[17]determinedtheoptimalfixtureconfigurationdesignforsheetmetalassemblyusingGA.MSC/NASTRANhasbeenusedforfitnessevaluation.Liao[18]presentedamethodtoautomaticallyselecttheoptimalnumbersoflocatorsandclampsaswellastheiroptimalpositionsinsheetmetalassemblyfixtures.KrishnakumarandMelkote[19]developedafixturelayoutoptimizationtechniquethatusestheGAtofindthefixturelayoutthatminimizesthedeformationofthemachinedsurfaceduetoclampingandmachiningforcesovertheentiretoolpath.Locatorandclamppositionsarespecifiedbynodenumbers.Abuilt-infiniteelementsolverwasdeveloped.
Someofthestudiesdonotconsidertheoptimizationofthelayoutforentiretoolpathandchipremovalisnottakenintoaccount.Someofthestudiesusednodenumbersasdesignparameters.
Inthisstudy,aGAtoolhasbeendevelopedtofindtheoptimallocatorandclamppositionsin2Dworkpiece.DistancesfromthereferenceedgesasdesignparametersareusedratherthanFEAnodenumbers.FitnessvaluesofrealencodedGAchromosomesareobtainedfromtheresultsofFEA.ANSYShasbeenusedforFEAcalculations.Achromosomelibraryapproachisusedinordertodecreasethesolutiontime.DevelopedGAtoolistestedontwotestproblems.Twocasestudiesaregiventoillustratethedevelopedapproach.Maincontributionsofthispapercanbesummarizedasfollows:
(1)developedaGAcodeintegratedwithacommercialfiniteelementsolver;
(2)GAuseschromosomelibraryinordertodecreasethecomputationtime;
(3)realdesignparametersareusedratherthanFEAnodenumbers;
(4)chipremovalistakenintoaccountwhiletoolforcesmovingontheworkpiece.
GeneticalgorithmswerefirstdevelopedbyJohnHolland.Goldberg[10]publishedabookexplainingthetheoryandapplicationexamplesofgeneticalgorithmindetails.Ageneticalgorithmisarandomsearchtechniquethatmimicssomemechanismsofnaturalevolution.Thealgorithmworksonapopulationofdesigns.Thepopulationevolvesfromgenerationtogeneration,graduallyimprovingitsadaptationtotheenvironmentthroughnaturalselection;fitterindividualshavebetterchancesoftransmittingtheircharacteristicstolatergenerations.
Inthealgorithm,theselectionofthenaturalenvironmentisreplacedbyartificialselectionbasedonacomputedfitnessforeachdesign.Thetermfitnessisusedtodesignatethechromosome’schancesofsurvivalanditisessentiallytheobjectivefunctionoftheoptimizationproblem.Thechromosomesthatdefinecharacteristicsofbiologicalbeingsarereplacedbystringsofnumericalvaluesrepresentingthedesignvariables.
GAisrecognizedtobedifferentthantraditionalgradientbasedoptimizationtechniquesinthefollowingfourmajorways[10]:
1.GAsworkwithacodingofthedesignvariablesandparametersintheproblem,ratherthanwiththeactualparametersthemselves.
2.GAsmakesuseofpopulation-typesearch.Manydifferentdesignpointsareevaluatedduringeachiterationinsteadofsequentiallymovingfromonepointtothenext.
3.GAsneedsonlyafitnessorobjectivefunctionvalue.Noderivativesorgradientsarenecessary.
4.GAsuseprobabilistictransitionrulestofindnewdesignpointsforexplorationratherthanusingdeterministicrulesbasedongradientinformationtofindthesenewpoints.
Inmachiningprocess,fixturesareusedtokeepworkpiecesinadesirablepositionforoperations.Themostimportantcriteriaforfixturingareworkpiecepositionaccuracyandworkpiecedeformation.Agoodfixturedesignminimizesworkpiecegeometricandmachiningaccuracyerrors.Anotherfixturingrequirementisthatthefixturemustlimitdeformationoftheworkpiece.Itisimportanttoconsiderthecuttingforcesaswellastheclampingforces.Withoutadequatefixturesupport,machiningoperationsdonotconformtodesignedtolerances.Finiteelementanalysisisapowerfultoolintheresolutionofsomeofthe