文献翻译关于自动化双货叉立体仓库系统.docx
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文献翻译关于自动化双货叉立体仓库系统.docx
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文献翻译关于自动化双货叉立体仓库系统
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外文资料翻译
AnAnalysisOfDualShuttleAutomatedStorage
AnAnalysisOfDualShuttle
AutomatedStorage/RetrievalSystems
AdhinarayanKeserla
BrettA.Peters
August1,1994
Thisworkingpaperisnottobecopied,quoted,orcitedwithoutthepermissionoftheauthors.AddresscorrespondencetoBrettA.Peters,Dept.ofIndustrialEngineering,TexasA&MUniversity,CollegeStation,TX77843-3131oremailtobpeters@tamu.edu
Abstract
Thispaperaddressesthethroughputimprovementpossiblewiththeuseofadualshuttleautomatedstorageandretrievalsystem.Withtheuseofsuchasystem,travelbetweentimeinadualcommandcycleisvirtuallyeliminatedresultinginalargethroughputimprovement.Thedualshuttlesystemisthenextendedtoperformanequivalentoftwodualcommandsinonecycleinaquadruplecommandmode(QC).AheuristicthatsequencesretrievalstominimizetraveltimeinQCmodeisdeveloped.MonteCarlosimulationresultsareprovidedforevaluatingtheheuristic'sperformanceandshowthatitperformswell,achievinglargethroughputimprovementscomparedwiththatofthedualcommandcycleoperatingunderthenearestneighborretrievalsequencingheuristic.
Keywords:
AutomatedStorage/RetrievalSystemsDesign;AutomatedStorage/RetrievalSystemsOperation;MaterialHandlingSystems;PerformanceModelingandAnalysis
Introduction
Automatedstorage/retrievalsystems(AS/RS)arewidelyusedinwarehousingandmanufacturingapplications.AtypicalunitloadAS/RSconsistsofstorageracks,S/Rmachines,linkconveyors,andinput/output(I/O)stations.Animportantsystemperformancemeasureisthethroughputcapacityofthesystem.Thethroughputcapacityforasingleaisleistheinverseofthemeantransactiontime,whichistheexpectedamountoftimerequiredfortheS/Rmachinetostoreand/orretrieveaunitload.TheservicetimeforatransactionincludesbothS/Rmachinetraveltimeandpickup/deposittime.ThistimetypicallydependsontheconfigurationofthestoragerackandtheS/Rmachinespecifications.
Hanetal.[2]improvedthethroughputcapacityoftheAS/RSthroughsequencingretrievals.IntelligentlysequencingtheretrievalscanreduceunproductivetravelbetweentimewhentheS/Rmachineistravelingemptyandtherebyincreasethethroughput.TheydevelopanexpressionforthemaximumpossibleimprovementinthroughputiftravelbetweeniseliminatedforanAS/RSthatisthroughputboundandoperatesindualcommandmode.Inessence,thismeansthatiftheS/Rmachinetravelsinasinglecommandpathbutperformsbothastorageandaretrievaloperation,theabovethroughputimprovementcouldbeobtained.
Inthispaper,weanalyzeanalternativedesignoftheS/RmachinethathastwoshuttlesinsteadofoneasinaregularAS/RS.Thenewdesigneliminatesthetravelbetweenthestorageandretrievalpointsandperformsbothastorageandaretrievalatthepointofretrieval,therebyachievingthemaximumthroughputincreasecalculatedbyHanetal.[3].
ThedualshuttleAS/RSisanewdesignaimedatimprovingS/Rmachineperformance.moststudiesonAS/RSsystemshavebeenbasedonasingleshuttledesign.InouranalysisofthedualshuttleAS/RSperformance,webuilduponthesepreviousresearchresults.
AlternativeS/RMachineDesign
Atypicalunit-loadAS/RShasanS/Rmachineoperatingineachaisleofthesystem.TheS/Rmachinehasamastwhichissupportedatthefloorandtheceilingandtravelshorizontallywithintheaisle.Connectedtothismastisashuttlemechanismthatcarriestheunitloadandmovesverticallyupanddownthemast.Theshuttlemechanismalsotransfersloadsinandoutofstoragelocationsintherack.Figure1providesanillustrationofthesingleshuttleS/Rmachine.
Figure1.SingleShuttleS/RMachineDesign
AtypicalsingleshuttleAS/RScanperformasinglecommandcycleoradualcommandcycle.Asinglecommandcycleconsistsofeitherastorageoraretrieval.Forastorage,thetimeconsistsofthetimetopickuptheloadattheI/Opoint,traveltothestoragepoint,deposittheloadatthatpoint,andreturntotheI/Opoint.Thetimeforaretrievalisdevelopedsimilarly.
Adualcommandcycleinvolvesbothastorageandaretrievalinthesamecycle.ThecycletimeinvolvesthetimetopickuptheloadattheI/Opoint,traveltothestoragelocation,placetheloadintherack,travelemptytotheretrievallocation,retrieveaload,returntotheI/Opoint,anddeposittheloadattheI/Opoint.
IfwecriticallyanalyzethedualcommandcycleoftheS/Rmachine(shownbythesolidlineinFigure2),apotentialopenlocationforafuturestorageiscreatedwhenaretrievalisperformed.Furthermore,ifbotharetrievalandastorageareperformedatthesamepoint,thetravelbetweentime(TB)iseliminated,andthetraveltimewillbeequaltothesinglecommandtraveltime.WiththeexistingAS/RSdesign,thismodeofoperationisnotpossible;therefore,analternativetotheS/Rmachine,adualshuttleR/Smachine,isproposed.
Figure2.DualCommandTravelPathsofS/RandR/SMachines
R/SMachineOperation
ConsideranS/Rmachinewithtwoshuttlemechanismsinsteadofone.ThisnewS/Rmachinecouldnowcarrytwoloadssimultaneously.Eachshuttlemechanismcouldoperateindependentlyoftheother,sothatindividualloadscanstillbestoredandretrieved.AnillustrationofthedualshuttleS/RmachineisshowninFigure3.ThisnewS/Rmachinewouldoperateasdescribedbelow.
Figure3.DualShuttleS/RMachineDesign
TheS/RmachinepicksuptheitemtobestoredfromtheI/Opoint,loadsitintothefirstshuttle,andmovestotheretrievallocation.Afterreachingtheretrievallocation,thesecondshuttleispositionedtopickuptheitemtoberetrieved.Afterretrieval,theS/Rmachinepositionsthefirstshuttleanddepositstheload.TheS/RmachinethenreturnstotheI/Opoint.TheoperationcaneasilybeseenasasinglecommandoperationplusasmalltraveltimeforrepositioningtheS/Rmachinebetweentheretrievalandstorage(aswellastheadditionalpickupanddeposittimeassociatedwiththesecondload).Therefore,theS/RmachinenowoperatesasanR/Smachineperformingaretrievalfirstthenastorageinadualcommandcycle.
SincetheR/Smachinehastwoshuttles,thepositionoftheshuttleshasaroleintheoperationofthesystem.Withtwoshuttles,theR/Smachineisabletoperformadualcommandcycleatonelocationintherack.Thisoperationisaccomplishedbyfirstretrievingtheloadontotheemptyshuttle,transferringthesecondshuttleintoposition,andstoringtheloadintotheemptylocationintherack.However,thechoiceofshuttleconfigurationdoesnotimpacttheanalysisinthispaper.
Toperformtheseoperations,theR/Smachinemustmovethesecondshuttleintopositionafterthefirstshuttlehascompletedtheretrieval.Duetothesmalldistanceinvolved,theR/Smachinewilluseaslowercreepspeedforpositioning,butthistraveltimeisgenerallysmall.Furthermore,anamountofcreeptimeisusuallyincludedinthepickupanddeposittimetoaccountforthisrequiredpositioning.AseconddesigncharacteristicisthatadditionalclearancebeyondthefirstandlastrowandcolumnoftherackmustbeprovidedforovertraveloftheR/Smachinetoaccommodatebothshuttlemechanisms.
ThroughputImprovement
Toestimatethethroughputimprovementbythedualshuttlesystemoverexistingdesigns,weusetheexpressionsforsinglecommandanddualcommandcycletimesdevelopedbyBozerandWhite[1]andthetabulatedvaluesforthenearestneighborheuristicfromHanetal.[4].Indevelopingtheexpressions,theauthorsin[1]and[4]madeseveralassumptions.Thesameassumptionsholdforthenewdesignandincludethefollowing.
1.TherackisconsideredtobeacontinuousrectangularpickfacewheretheI/Opointislocatedatthelowerleft-handcorneroftherack.
2.Theracklengthandheight,aswellastheS/Rmachinevelocityinthehorizontalandverticaldirections,areknown.
3.TheS/Rmachinetravelssimultaneouslyinthehorizontalandverticaldirections.Incalculatingthetraveltime,constantvelocitiesareusedforhorizontalandverticaltravel.Accelerationanddecelerationeffectsareimplicitlyaccountedforineitherareducedtopspeedoranincreasedpickupanddeposittime.Acreepspeedisusedforrepositioningthedualshuttle.
4.Pickupanddeposittimesassociatedwithloadhandlingareassumedconstantand,therefore,thesecouldbeeasilyaddedintothecycletimeexpressions.
5.TheS/Rmachineoperateseitheronasingleordualcommandbasis,i.e.,multiplestopsintheaislearenotallowed.(ThisassumptionislaterrelaxedforthenewR/Smachinetoperformaquadruplecommandcycle.)
6.Forthenearestneighborheuristic,ablockofnretrievalsisavailableforsequencingandthereareminitialopenlocationsintherackface.
DualShuttleS/RSystems
ThenewdesignoftheS/Rmachinehastwoshuttlesandthereforecouldbeoperatedasadualshuttlesystem:
carryingtwoloadsanddepositingthem,retrievingtwoloads,andreturningtotheI/OpointtodeliverthemasshowninFigure4.Theaboveoperationcanbeperformedbystoringandretrievingtheloadsatfourdifferentlocations.Therefore,thetraveltimewouldconsistofthetimeforasinglecommandtravelplusthreetravelbetweentimes.Tomoreefficientlyperformthe4operations,aretrievalandstorageperformedatonelocationisinterspersedwithadualcommandoperation.Thismodeofoperation,termedthequadruplecommand(QC)cycle,eliminatesonetravelbetweenandismoreefficientthanthepreviousmodementionedabove(seeFigure5).TheQCcyclecan
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