Guided Vehicle(AGV) Systems Based on Just-in-time(

Simulationofautomatedguidedvehicle(AGV)systemsbasedonjust-in-time(JIT)philosophyinajob-shopenvironmentSaadettinErhanKesen*,O¨merFarukBaykoc¸FacultyofEngineeringandArchitecture,DepartmentofIndustrialEngineering,GaziUniversity,Maltepe06570,Ankara,TurkeyReceived5December2005;accepted7November2006Availableonline10January2007AbstractAutomatedGuidedVehicle(AGV)systemshavebeenfrequentlyusedasmaterialhandlingequipmentinmanufacturingsystemssincethelasttwodecades.TheuseofAGVsystemshastakenattentionofpractitionersandresearchers.AlthoughtherearenumerousstudiesconcerningwithAGVsystemsinliterature,afewofthemdealwiththeadaptationofthesesystemsintoJITsystems.Moreover,thefacilitylayoutsconsideredinthosestudieshaveflow-shopenvironment.Inthispaper,asimulationmodelofahypotheticalsystemwhichhasajobshopenvironmentandwhichisbasedonJITphilos-ophywasdeveloped.Inaddition,adispatchingalgorithmforvehiclesmovingthroughstationswaspresentedinordertoimprovetransportationefficiency.Ingivenlayout,multiplevehiclescanmoveandtheirbi-directionalflowisallowed.Afterthemodelhadbeenestablished,itwasmentionedhowtoperformsimulationoutputanalysis.Thefactorswhichmaybeimportantforthesystemweredeterminedbyoutputanalysis.Anexperimentplanwaspreparedbytakingintoaccountthesefactors.Inthisplan,twolevelswereselectedforeachfactorandanexperimentaldesignwasconducted.Theeffectofeachfactoroneachperformancemeasureandtheinteractionofthesefactorswereexamined.2006ElsevierB.V.Allrightsreserved.Keywords:AGVsystems;JIT;Simulation;Bi-directionalflow;ExperimentalDesign1.IntroductionConventionalapproachesusedinmodelingmanufacturingsystemscanbedividedintotwotypes:Analy-ticalmodelsandsimulationmodels.Bothtechniqueshaveadvantagesanddisadvantages.Analyticaltech-niquedefinesthesystemasmathematicalequationsanditcanoptimizethesystem.However,sincemodelingasystemasmathematicalequationsrequiresalotofassumptionsaboutthesystemandthemoreassumptionsareconsideredthemorethesystembecomesunreal.Ontheotherhand,simulationtechniquemaynotgiveoptimumsolutions.Itinvestigatesasystem’slong-termbehavior.Inspiteofthefactthatgen-eratingasolutionaboutasystemismoretime-consuminginsimulationtechnique,itismoreconvenientinmodelingcomplexsystemsthananalyticaltechnique.1569-190X/$-seefrontmatter2006ElsevierB.V.Allrightsreserved.doi:10.1016/j.simpat.2006.11.002*Correspondingauthor.Tel.:[phone].E-mailaddresses:[e-mail](S.E.Kesen),[e-mail](O¨.F.Baykoc¸).SimulationModellingPracticeandTheory15(2007)272–284[1],Free-pathtransporterscanmovethroughasystemwithoutconcernfordelayscausedbyothervehicles.Guidedtransportersmovealongafixedpathandmaycontendwitheachotherforspacealongthatpath.Especially,iftherearemultiplevehiclestravelingthroughasystemandbi-directionalpathsareavailable,vehicleroutingproblememerges.Vehicleroutingisknownasadifficultprobleminliter-ature.Tosolvethisproblem,differentpoliciesshouldbedeveloped.Alsoinachievinganacceptablesolutiontothisproblem,thestructureofconsideredsystemplaysacrucialrole.AccordingtoKimuraandTerada[7],multi-stageproductionprocessescanbeclassifiedintotwotypes:pushsystemsandpullsystems.MostoftheAmericanandEuropeanproductionsystemsemploypushsystemswhereastheJapaneseJITsystemsemploypullsystems.AspointedoutinBaykocandErol[2]themaindif-ferencebetweenthetwosystemsisthatinthepullsystem,thematerialisroutedfromtheprecedingstagetothesucceedingstageaccordingtotheconsumptionrateofsucceedingstage.AlthoughtherearenumerousstudiesexaminingandinvestigatingbehaviorsofAGVsystems,afewoftheminvestigatetherelationshipbetweenAGVsystemsandJITproductionphilosophy.Atthosestudies,facilitylayoutsofsystemshaveaflow-shopenvironment.Suchajob-shopenvironment,thefactorswhichmaybeimportantweredetermined.Thispaperaimstoexaminehowthesefactorsaffectperformancemeasuresspec-ifiedinthejob-shopenvironment.Toaccomplishthis,experimentaldesign,oneofthestatisticalprocedures,wasapplied.Bytheaidofexperimentaldesign,mainandinteractioneffectsamongfactorsandthestatisticalsignifi-cancesoftheseeffectswereclearlyconsidered.Inotherwords,itwasdesiredtoshowthathowthissystemreactedunderdifferentfactorsettings.Insimulationmodelofconsideredsystem,demandarrivalsarestochas-ticand,processingtimeofeachjoboneachstationisdeterministic.Theeffectsoffourfactorsonthesystemperformancemeasures,whicharetimeinsystem(flow-time),timebetweenconsecutivejobs,throughputrateandnumberofjobswaitingonAGVqueue,aretested.SubsequentsectionconsiderssomesimulationstudiesrelatedtoAGVsystemsinliterature.2.AshortliteraturereviewInthisliteraturesurvey,severalsimulationstudiesconcerningwithAGVsystemsarediscussed.KooandJang[8]presentedstochasticvehicletraveltimemodelsforAGVbasedmaterialhandlingsystemswithemphasisontheemptytraveltimesofvehiclesunderflexiblemanufacturingsystem.Asimulationexper-imentisusedtoevaluateanddemonstratethepresentedmodels.JeongandRandhawa[5]consideredthedispatchingproblemassocia