工业工程英文文献及外文翻译.docx
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工业工程英文文献及外文翻译
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英文文献
LineBalancingintheRealWorld
Abstract:
LineBalancing(LB)isaclassic,well-researchedOperationsResearch(OR)optimizationproblemofsignificantindustrialimportance.Itisoneofthoseproblemswheredomainexpertisedoesnothelpverymuch:
whateverthenumberofyearsspentsolvingit,oneiseachtimefacinganintractableproblemwithanastronomicnumberofpossiblesolutionsandnorealguidanceonhowtosolveitinthebestway,unlessonepostulatesthattheoldwayisthebestway.Hereweexplainanapparentparadox:
althoughmanyalgorithmshavebeenproposedinthepast,anddespitetheproblem’spracticalimportance,justonecommerciallyavailableLBsoftwarecurrentlyappearstobeavailableforapplicationinindustriessuchasautomotive.WespeculatethatthismaybeduetoamisalignmentbetweentheacademicLBproblemaddressedbyOR,andtheactualproblemfacedbytheindustry.
Keyword:
LineBalancing,Assemblylines,Optimization
LineBalancingintheRealWorld
EmanuelFalkenauer
OptimalDesign
Av.Jeanne19Abote2,B-1050Brussels,Belgium
+32(0)264610
1Introduction
AssemblyLineBalancing,orsimplyLineBalancing(LB),istheproblemofassigningoperationstoworkstationsalonganassemblyline,insuchawaythattheassignmentbeoptimalinsomesense.EversinceHenryFord’sintroductionofassemblylines,LBhasbeenanoptimizationproblemofsignificantindustrialimportance:
theefficiencydifferencebetweenanoptimalandasub-optimalassignmentcanyieldeconomies(orwaste)reachingmillionsofdollarsperyear.
LBisaclassicOperationsResearch(OR)optimizationproblem,havingbeentackledbyORoverseveraldecades.Manyalgorithmshavebeenproposedfortheproblem.Yetdespitethepracticalimportanceoftheproblem,andtheOReffortsthathavebeenmadetotackleit,littlecommerciallyavailablesoftwareisavailabletohelpindustryinoptimizingtheirlines.Infact,accordingtoarecentsurveybyBeckerandScholl(2004),thereappeartobecurrentlyjusttwocommerciallyavailablepackagesfeaturingbothastateoftheartoptimizationalgorithmandauser-friendlyinterfacefordatamanagement.Furthermore,oneofthosepackagesappearstohandleonlythe“clean”formulationoftheproblem(SimpleAssemblyLineBalancingProblem,orSALBP),whichleavesonlyonepackageavailableforindustriessuchasautomotive.Thissituationappearstobeparadoxical,oratleastunexpected:
giventhehugeeconomiesLBcangenerate,onewouldexpectseveralsoftwarepackagesvyingtograbapartofthoseeconomies.
ItappearsthatthegapbetweentheavailableORresultsandtheirdisseminationinToday’sindustry,isprobablyduetoamisalignmentbetweentheacademicLBproblemaddressedbymostoftheORapproaches,andtheactualproblembeingfacedbytheindustry.LBisadifficultoptimizationproblemevenitssimplestformsareNP-hard–seeGarryandJohnson,1979),sotheapproachtakenbyORhastypicallybeentosimplifyit,inordertobringittoalevelofcomplexityamenabletoORtools.Whilethisisaperfectlyvalidapproachingeneral,intheparticularcaseofLBitledsomedefinitionsoftheproblemhatignoremanyaspectsofthereal-worldproblem.
Unfortunately,manyoftheaspectsthathavebeenleftoutintheORapproachareinfactcrucialtoindustriessuchasautomotive,inthesensethatanysolutionignoring(violating)thoseaspectsbecomesunusableintheindustry.
Inthesequel,wefirstbrieflyrecallclassicORdefinitionsofLB,andthenreviewhowtheactuallinebalancingproblemfacedbytheindustrydiffersfromthem,andwhyasolutiontotheclassicORproblemmaybeunusableinsomeindustries.
2ORDefinitionsofLB
TheclassicORdefinitionofthelinebalancingproblem,dubbedSALBP(SimpleAssemblyLineBalancingProblem)byBeckerandScholl(2004),goesasfollows.Givenasetoftasksofvariousdurations,asetofprecedenceconstraintsamongthetasks,andasetofworkstations,assigneachtasktoexactlyoneworkstationinsuchawaythatnoprecedenceconstraintisviolatedandtheassignmentisoptimal.Theoptimalitycriteriongivesrisetotwovariantsoftheproblem:
eitheracycletimeisgiventhatcannotbeexceededbythesumofdurationsofalltasksassignedtoanyworkstationandthenumberofworkstationsistobeminimized,orthenumberofworkstationsisfixedandthelinecycletime,equaltothelargestsumofdurationsoftaskassignedtoaworkstation,istobeminimized.
AlthoughtheSALBPonlytakesintoaccounttwoconstraints(theprecedenceconstraintsplusthecycletime,ortheprecedenceconstraintsplusthenumberofworkstations),itisbyfarthevariantoflinebalancingthathasbeenthemostresearched.WehavecontributedtothateffortinFalkenauerandDelchambre(1992),whereweproposedaGroupingGeneticAlgorithmapproachthatachievedsomeofthebestperformanceinthefield.TheGroupingGeneticAlgorithmtechniqueitselfwaspresentedindetailinFalkenauer(1998).
Howeverwellresearched,theSALBPishardlyapplicableinindustry,aswewillseeshortly.ThefacthasnotescapedtheattentionoftheORresearches,andBeckerandScholl(2004)definemanyextensionstoSALBP,yieldingacommondenominationGALBP(GeneralizedAssemblyLineBalancingProblem).Eachoftheextensionsreportedintheirauthoritativesurveyaimstohandleanadditionaldifficultypresentinreal-worldlinebalancing.WehavetackledoneofthoseaspectsinFalkenauer(1997),alsobyapplyingtheGroupingGeneticAlgorithm.
ThemajorproblemwithmostoftheapproachesreportedbyBeckerandScholl(2004)isthattheygeneralizethesimpleSALBPinjustoneortwodirections.Therealworldlinebalancing,asfacedinparticularbytheautomotiveindustry,requirestacklingmanyofthosegeneralizationssimultaneously.
3WhatDiffersintheRealWorld
AlthougheventhesimpleSALBPisNP-hard,itisfarfromcapturingthetruecomplexityoftheprobleminitsreal-worldincarnations.Ontheotherhand,smallinstancesoftheproblem,eventhoughtheyaredifficulttosolvetooptimality,areatrickytargetforlinebalancingsoftware,becausesmallinstancesoftheproblemcanbesolvedclosetoptimalitybyhand.Thatishowevernotthecaseintheautomotiveandrelatedindustries(Bus,truck,aircraft,heavymachinery,etc.),sincethoseindustriesroutinelyfeatureAssemblylineswithdozensorhundredsofworkstations,andhundredsorthousandsofOperations.Thoseindustriesarethereforetheprimetargetsforlinebalancingsoftware.
Unfortunately,thosesameindustriesalsoneedtotakeintoaccountmanyoftheGALBPextensionsatthesametime,whichmayexplainwhy,despitetheimpressiveORWorkdoneonlinebalancing;onlyonecommerciallyavailablesoftwareseemstubecurrentlyavailableforthoseindustries.
Weidentifybelowsomeoftheadditionaldifficulties(withrespecttoSALBP)thatmustbetackledinalinebalancingtool,inordertobeapplicableinthoseindustries.
DoNotBalancebutRe-balance
ManyoftheORapproachesimplicitlyassumethattheproblemtobesolvedinvolvesanew,yet-to-be-builtassemblyline,possiblyhousedinanew,yet-to-be-builtfactory.Toouropinion,thisisthegravestoversimplificationoftheclassicORapproach,forinpractice,thisishardlyeverthecase.Thevastmajorityofreal-worldlinebalancingtasksinvolveexistinglines,housedinexistingfactories–infect,thetargetlinetypicallyneedstuberebalancedratherthanbalanced,theneedarisingfromchangesintheproductorthemixofmodelsbeingassembledintheline,theassemblytechnology,theavailableworkforce,ortheproductiontargets.Thishassomefar-reachingimplications,outlinedbelow.
WorkstationsHaveIdentities
Aspointedoutabove,thevastmajorityofreal-worldlinebalancingtasksinvolvesexistinglineshousedinexistingfactories.Inpractice,thisseemingly“uninteresting”observationhasonefar-reachingconsequence,namelythateachworkstationinthelinedoeshaveitsownidentity.Thisidentityisnotduetoany“incapacityofabstraction”onpartoftheprocessengineers,butrathertothefactthattheworkstationsareindeednotidentical:
eachhasitsownspaceconstraints.aworkstationbelowalowceilingcannotelevatethecarabovetheoperators’heads),itsownheavyequipmentthatcannotbemovedsparehugecosts,itsowncapacityofcertainsupplies.compressedair),itsownrestrictionsontheoperationsthatcanbecarriedoutthere.donotplaceweldingoperationsjustbesidethepaintingshop),etc.
CannotEliminateWorkstations
Sinceworkstationsdohavetheiridentity(asobservedabove),itbecomesobviousthatareal-worldLBtoolcannotaimateliminatingworkstations.Indeed,unlesstheeliminatedworkstationswereallinthefrontofthelineoritstail,theireliminationwouldcreategapingholesintheline,byvirtueoftheotherworkstations’retainingoftheiridentities,includingtheirgeographicalpositionsintheworkshop.Also,itsoftensthecasethatmanyworkstationsthatcouldpossiblybeeliminatedbythealgorithmareinfactnecessarybecauseofzoningconstraints.
4Conclusions
Theconclusionsinspection3stemsfromourextensivecontactswithautomotiveandrelatedindustries,andreflectstheirtrueneeds.Other“exotic”constraintsmayapplyinanygivenreal-worldassemblyline,butlinebalancingtoolforthoseindustriesmustbeabletohandleatleastthoseaspectsoftheproblem.Thisisveryfarfromthe“clean”academicSALBP,aswellasmostGALBPextensionsreportedbyBeckerandScholl(2004).Infact,suchatoolmustsimultaneouslysolveseveral-hardproblems:
Findafeasibledefinedreplacem