Proceedings of the 1996 Winter Simulation Conferenceed. J. M. Charnes, D. J. Morrice, D. T. Brunner, and J. J. 51-vain

MODELING OF CHAIN CONVEYORS AND THEIR EQUIPMENT INTERFACES

Ali K. GunalShigeru Sadakane

Production Modeling CorporationThree Parklane Boulevard, Suite 910 West

Dearborn, Michigan 48126, U.S.A.

ABSTRACT

Chain conveyors are a specific type of conveyor oftenused in a variety of manufacturing and productionapplications, such as bcxly and paint shops. Theseconveyors must typically interface with other types ofconveyors such as cross-transfer conveyors, and alsowith other material-handling equipment such as lifttables and hold tables. Micromodeling of chainconveyors and their equipment interfaces requires closeattention to numerous details. These details include notonly static and operational properties of the chainconveyors themselves, but also the particulars ofdimensional and operational interfaces of the conveyorsand the equipment served by the conveyors, such as lifttables and the conveyor acceleration and decelerationramps.

In this paper, we first delineate the situations inwhich micromodeling of material-handling equipmentis appropriate. We then present an overview ofconveyor types and tenninology. Next, we describe thechallenges of modeling chain conveyors accurately, andour recommendations for meeting these challengeswithin the framework of typical modeling tools andsimulation-study contexts. As an example, we presentdetails of these recommendations relative to theAutoMod modeling tool. In conclusion, we summarizethese recommendations and indicate promisingdirections for further development of modelingtechniques and enhancement of model-building tools.

1 MACRO- VS. MICRO-MODELING

Macro models are, by definition, overview models witha "coarse" level of detail. In contrast, micro modelsincorporate a "fme" (high) level of detail (UIgen, Shore,and Grajo 1994). The appropriate level of detail for aparticular model within a simulation study, and hencethe decision of whether to build a macro or a micromodel, properly depends on the objectives of the study,

1107

Edward J. Williams

206-2 Engineering Computer Center, Mail Drop 3Ford Motor Company

Dearborn, Michigan 48121-2053, U.S.A.

availability of data, credibility concerns, and constraintsof model-development and computer-run time available(Law and McComas 1991). In view of this credibilityconcern, the modeler making the "micro model versusmacro model" decision properly anticipates the task ofvalidation by asking "Will our modeling team be able touse the model - in place of the system - to make thedecisions required by the study objectives?" (Ruch andKellert 1995).

Simulation may be applied to the study of materialhandling systems during any or all of four projectphases: the conceptual phase, the detailed design phase,the launching phase, and the fully-operational phaseClngen and Upendram 1995). In the context ofmodeling material-handling systems, typical indicationscalling for development of a micromodel arerequirements to minimize both global and local work­in-process levels and maximize utilization of material­handling equipment, plus availability of detaileddimensional, cycle-time, and downtime data relative toindividual pieces of equipment, such as the aerialgravity conveyors and motorized roller conveyorscompared in (Cerda 1995). Such modeling is frequentlyrequired due to the mathematical intractibility ofoperational questions involving material-handl ingequipment. For example, (Tsai 1995) verifies the "NP­hard" status of minimizing the likelihood of conveyorstoppage to complete assembly-station work, via mixed­model sequencing, when a conveyor serves even a singlestation with arbitrary processing times. In such acontext, detailed modeling of conveyors becomes vital toaddress decision-making policy relative to both linebalancing and product sequencing. The motivation tomodel conveyors, in particular, at a high level of detailincreases in studies undertaken to assess competingconveyor management strategies, such as dynamicallocation of workpieces to conveyors which alternatelymerge with each other, diverge again, and must servewidely dispersed, dissimilar work cells (Laughery1995). Macro models are frequently analyzed to

1108 Gunal, Sadakane, and 1V"illiams

optimize conveyor and overall system performance viaselection of a workpiece to convey, selection of aconveyor, choice of waiting discipline among conveyors,or choice of waiting discipline among jobs awaiting aconveyor. Micro models of conveyor perfonnancefurnish required input to such macro models (Backersand Steffens 1983).

2 DEFINITIONS AND TERMINOLOGY OFCONVEYORS AND INTERFACE EQUIPMENT

Conveyors can transport a high volume of items over afixed path at adjustable speed with little manualintervention. Many varieties of conveyors are in use,such as belt conveyors (endless belt), chute conveyors(metal slides), screw conveyors (large spiral containedin confining trough or tube), chain conveyors (endlesschain), and roller conveyors (load carried on transverserollers which are either gravity- or power-driven) (Sule1988). For example, belt or roller conveyors can bereadily configured to transport small, odd-shaped items(Gould 1994). Chain-driven conveyors with rollersurfaces, the type of conveyor considered in this paper,are occasionally non-powered (gravity) or, more usually,powered (live). Such conveyors are relativelyinexpensive, readily assembled and adjusted, and wellsuited for a wide range of loads, provided the materialsbeing transported have, or are mounted on, a rigidriding surface (Allegri 1992). These chain-drivenconveyors may accumulate material by use of slip orclutch mechanisms built into the rollers. The "rollerflight" variety typically uses two parallel sections ofchain supporting rollers on non-rotating shafts. Thoserollers can turn under the material, pennitting itsaccumulation (Gould 1993). Additionally, accelerationand deceleration sections may be appended to poweredroller conveyors when precise material control isrequired, as at interface points, through inclines ordeclines, or around curves (K. W. Tunnell Company,Incorporated 1995). Recent advances in designs andmaterials have markedly increased speeds and decreasedoperating noise of powered roller conveyors, henceincreasing their economic appeal (Witt 1995).

These conveyors characteristically interact with othermaterial-handling equipment such as lift tables and holdtables. Lift tables provide a working or material­transfer surface at heights and positions chosen forergonomic and operational advantage (Tompkins andWhite 1984). Hold tables are a passive variant of lifttables; unlike a lift table, a hold table cannot move aload perpendicularly to the direction of travel of adownstream conveyor to align the load for placement onthat conveyor.

3 CHALLENGES OF MODELING CHAINCONVEYORS

As predicted in (Muth and White 1979), work inconveyor modeling comprises both analytical models(limited to a low level of complexity but quantifyingfundamental relationships among important conveyorparameters) and simulation models (allowing tradeoffstudies during design and analysis of operationalproblems of existing conveyors). Numerous studiesillustrate the wide applicability of simulation to theanalysis of conveyor systems, such as designing andimplementing a power-and-free conveyor system (Goodand Bauner 1984), comparative macro evaluations ofaccumulating and non-accumulating conveyor systems(Henriksen and Schriber 1986), layout and flow pathanalysis of overhead conveyors (Foote et al. 1988),micromodeling of conveyors transporting extremelyfragile workpieces forbidden to touch one another(Bopings 1988), and rapid assessments of proposedmodifications to a power and free conveyor system in anautomotive paint shop (Graehl 1992).

Chain conveyors are widely used in automotiveassembly plants. High volume production requires cycletimes in the order of a few seconds. For example, for aconveyor running at fifteen feet per minute and forpallets of length fifteen feet, a time loss of two secondsamounts to two jobs per hour and forty jobs per day overtwo ten-hour shifts. Depending on the sales price of acar, such a loss might be somewhere from $500,000 to$1,200,000 in lost revenue per day. On the other hand,the types of material handling equipment required tosupport such production might be very costly to install,operate, and maintain. Therefore, the system designmust strive for balance between high throughputcapacity and low redundancy in material handlingequipment requirements. Consequently, it is necessarythat simulation models of such material handlingsystems should represent sufficient detail of the partsmovement to make accurate assessments of theadequacy of those systems and the overall productionsystems of which they are a component. Output resultsfrom micromodels of conveyor configuration andperformance can then guide experimentation withmacromodels of the manufacturing system.

A typical vehicle assembly plant would have anabundance of various kinds of conveyors. Chainconveyors constitute one of the most commonly usedtypes of material handling equipment in many assemblyplants. Modeling of those conveyors can be achallenging task. The following is a discussion of someof the issues in modeling of chain conveyors and thesupporting material handling equipment that can refound in a typical assembly plant.

Modeling of Chain Conve,yors and Their Equipment Interfaces 1109

3.1 Production and Transfer Conveyors

These conveyor sections move pallets throughproduction operations. They are also used for bufferstorage purposes, holding pallets between productiondepartments or during off-shift activities such as clean­ups. Furthennore, pallets are accumulated in a bank ofconveyors before they are sequenced for the nextoperation.

There are essentially two types of chain conveyors:accumulating and non-accumulating. Accumulatingconveyors, also referred to as roller flight conveyors,have the ability to hold pallets without stopping thepower chain. A non-accumulating conveyor allows thepallets to stop only when the entire conveyor chainstops. In a typical setup, non-accumulating conveyorsare used to move pallets through production processes(paint booths, wash rooms, etc.). Accumulatingconveyors are used for other purposes mentionedpreviously (e.g., temporary storage, delivery, andresequencing). Regardless of the accumulationcapabilities, most conveyors have special head and tailsections (speed-up sections) that are used to accelerateand decelerate pallets, respectively. Those sections areneeded to adjust the speed of pallets as they move ontoand off the conveyors, which typically have much lowerchain speeds than the equipment with which theyinterface.

Pallets move on chain conveyors with a specifiedminimum distance between them. Depending on thespeed of the conveyor, this distance may be different onvarious conveyors of a typical production setup. On anaccumulating conveyor, the spacing between palletsmight be different in accumulation from that betweenmoving pallets. Also, once in accumulation, a palletdoes not start moving again until the preceding palletmoves away to a distance defined as the moving space.

Most simulation software provides built-in constructsto model accumulating and non-accumulatingconveyors. Simulation software that does not providesuch constructs might require substantial amounts ofoverhead code to account for the operationalcharacteristics of either type of conveyors for a typicalproduction system. Even with built-in conveyorconstructs, some of the operational characteristics mightbe challenging modeling tasks. A speed-up section, forexample, consists of several smaller sections of the sameconveyor that run at different speeds (from low to highspeed or vice versa along the direction of travel). Apallet changes its speed when its center of gravity shiftsfrom one portion of the speed-up section to the next. Asthere might be pallets of different lengths movingthrough the system, the time to get onto and off aconveyor differs among pallets. Some simulation

software (such as AutoMod) allows precise calculationof these time delays by providing mechanisms to definesuch small sections of conveyor, the pallet size, andlocations for speed changes. In other packages, thesame effect can be achieved by calculating such delaysprior to simulation and by explicitly representing themin the model. A further complication to modelingspeed-up sections is the requirement that noaccumulation be allowed on those sections, even onaccumulating conveyors.

3.2 Cross Transfer Conveyors

Cross transfers are fast-moving chain conveyors with noaccumulation capability. These conveyors are used tomove pallets between different sections of productionand transfer conveyors. A cross transfer conveyortypically interfaces several production and transferconveyors. Incoming pallets are moved to one ofseveral conveyors served by the cross transfer conveyor.There is a series of hold and lift tables along thoseconveyor sections to compensate for the lack ofaccumulation capability. Hold tables are used only fortemporary storage purposes. Lift tables also act as theinterface between the cross transfer and productionconveyors, as they have the capability to push the palletsperpendicularly. Pallets move sideways on a crosstransfer. A pallet moves along a cross transfer from onehold/lift table to another, stopping as necessary. Apallet stops on a table if the next table is occupied byanother pallet. Once stopped at a table, the pallet israised off (disengaged from) the conveyor chain toprevent its nloving further (as the chain runscontinuously on a cross transfer). Once the next tablebecomes available, the table lowers the pallet back onto(into reengagement with) the chain and the pallet startsmoving along the conveyor again. Only after a palletreaches the next table or clears a limit switch placed at adistance from the table, can another pallet move ontothe table. Such control is required because crosstransfer conveyors typically move at higher speeds andserve several chain conveyors. Typical simulationsoftware constructs for conveyors provide no built-insupport for such control logic. There is a clear need tomodel such logic to represent cross transfer conveyorsaccurately.

3.3 Lift and Hold Tables

These special tables are used along cross transferconveyors. The main purpose of a lift table is to movepallets onto and off the cross transfer. Ali ft table haspowered rolls to move pallets in a directionperpendicular to the direction of travel on the cross

1110 Gunal, Sadakane, and WilliaIlls

transfer. These rolls are used to move the palletsfrom/to production and transfer conveyors. A hold tableis typically used to provide temporary storage on theconveyor to allow accumulation (cross transfersotherwise have no accumulation capability). Either typeof table raises pallets from the conveyor chain to stopthem from moving. Once the pallet is ready to moveforward, the table lowers the pallet back onto the chain,and upon contact with the chain, the pallet startsmoving again, as described in the previous subsection.Pallets do not stop at a table if they can continue tomove forward (Le., if the next table on the chain isavailable) and do not experience the cycle of going upand down at that table.

Availability of a table is usually signaled by the lastpallet to visit the table. There is a clear limit switchplaced at a distance from the table. This switch may beon the cross transfer conveyor, on an adjacent table, oron a production/transfer conveyor served by the crosstransfer. Once the pallet reaches a clear switch, it setsthe switch "off," indicating that it is now safe for itssuccessor pallet to move onto the conveyor.Accordingly, any pallet waiting to get clearance to moveonto the table sets the switch "on," flagging that thetable is now assigned to its use.

In a simulation model, cross transfer conveyors canbe treated as non-accumulating conveyors with discretesegments between hold/lift tables. Simulation packageswith built-in conveyor constructs provide adequatesupport to model the movement between tables.However, additional steps should be taken to account forthe up-down cycle of tables. Also, the model shouldreplicate the logic to control the access to the tables.This effect can be achieved by treating the tables asresources and by capturing and releasing them atappropriate points.

3.4 Powered Roll Beds

Powered-roll beds are relatively short conveyor sectionswhich use powered rolls for high-speed transfer ofpallets. They are typically used betweenproduction/transfer conveyors and other higher-speedtransfer equipment such as lift tables, turntables, andlifts (Sims 1992). They also provide buffer storagespace for one pallet at a time. Those conveyors run athigher speeds than the others described above.Consequently, during transfers from/to power-roll beds,pallets experience speed changes. As indicated earlier,due to different sizes of pallets, the time to clear apower-roll bed differs among pallets. Consequently, asimulation model needs to represent the clearance timeby accounting for the travel distances and the length ofthe pallets. With simulation languages such as

AutoMod, it is possible to address such detail byutilizing built-in capabilities. With some otherlanguages, up-front calculations are required for eachpower roll bed to determine the time it takes to moveonto and off that power roll bed.

3.5 Turntables

Turntables are short conveyor tread sections mounted ona bearing surface; they can rotate around a vertical axisto reorient pallets in transit (Cahill 1985). Someproduction processes require the parts to be in a certainorientation. Movement on conveyors and cross transfersbetween them changes the pallet orientation. Also,transfer/production conveyor segments are sometimes atangles to each other. Consequently, in a typicalproduction setup utilizing chain conveyors, there wouldbe several turntables with different amounts of rotation(e.g., 45, 90, 180 degree turns). Once a pallet movesonto it, a turntable rotates and aligns itself with the newpath that the pallet will follow. In most cases, aturntable does not start its rotation back to its loadingposition before the pallet clears a limit switch. Once thepallet clears the limit switch, the table rotates back tothe loading position. The rotation time is typicallyproportional to the angle of rotation. Clearly, asimulation model should make sure that a pallet doesnot attempt to move onto a turntable before the turntablereturns to the loading position. Turntables can bemodeled in most simulation languages by usingresources and by capturing and releasing thoseresources at appropriate points in time.

3.6 Lifts

In a typical production facility, there will be conveyorsand production processes at different floors. Lifts(elevators) are used for transferring pallets verticallybetween different floors. Apart from the vertical traveltime, there are delays in loading and unloading of lifts(Hudson 1954). Also, in most cases, there will beclearance limits for enabling/ disabling access to lifts.Due to various pallet sizes, those delays wi 11 differamong pallets. Consequently, the model should haveeither an explicit (in-travel distances, pallet sizes, andlocations of clear limits) or an implicit (in-time delaysand signals) representation of the delays that occur inthe load-travel-unload-return cycle of lifts.

4 AN AUTOMOD EXAMPLE OF CHAINCONVEYORS

AutoMod is an industrial simulation system combiningthe convenience of CAD-like drawing tools, an

Modeling of Chain Conve:l/ors and Tlleir Equipment Interfaces 1111

engineering-oriented modeling language, and accuratecapture of distances, sizes, speeds, and accelerations asaids to building accurate simulation models supportedby three-dimensional animation (Rohrer 1994a).

The example model is a small portion of a large bodyshop simulation built for an automobile manufacturer.Although small in scale, this example demonstratessome of the components mentioned above. Figure 1(next page) depicts a CAD layout of the portion of thebody shop, including three accumulating chainconveyors, two cross transfer conveyors, and several liftand hold tables.

AutoMod has detailed built-in constructs formodeling conveyors. The CAD drawing of the layout isimported into AutoMod as a static background and aconveyor system is superimposed upon it. Figure 2(next page) displays the conveyor system used for thisportion of the system. As Figure 2 shows, to capture thedetails of transfers between equipment running atdifferent speeds, smaller conveyor pieces were slightlyextended over the gaps between the actual conveyors.An "x" represents locations where loads stop and!oreither set or reset clear switches. Also, the speed-upsections on chain conveyors are represented as separateconveyor pieces appended to the end/tail of conveyorsegments. Furthermore, multiple stations were definedto represent the locations where the speed of a palletchanges. The stations in AutoMod are defined with aload alignment attribute that determines when a load isconsidered to be at that location. For example, if astation has a "trailing edge" alignment, then the loadwill be considered to have arrived at that station whenthe trailing edge of the load reaches it. Consequently,travel speeds and distances can be accurately capturedin the model by appropriately laying out stations andselecting their alignment attributes accordingly.

The lift tables interfacing with conveyor segments arerepresented as smaller conveyor sections attacheddirectly to the end of a chain conveyor segment. Thecross transfer conveyors are also represented as separateconveyor segments laid atop the segments that representliftJhold tables. To model transitions between lift/holdtables and the cross transfer conveyors, two stations arecreated at the same coordinates. One of the stations isthen attached to the conveyor segment that representsthe hold/lift table; the other, to the conveyor segmentrepresenting the cross transfer line. Then, by using the"move" command at appropriate points, the loadrepresenting the pallet is instantaneously transferredfrom one segment to the other, displaying a smoothmovement in animation. The time delays in loweringand raising the load to/from the chain are explicitlyrepresented in the model by using the "wait" command.

Clear limit switches are represented in the model asconveyor stations. Once a load reaches a stationrepresenting a clear switch, it releases a resource thatcorresponds to the area that the switch controls. Clearly,most of those stations have a htrailing edge" alignment.Loads trying to enter the area wait to capture theresource and start moving as soon as the previous palletclears the area. For each lift table and each hold table,there is a corresponding resource in the model.

5 ANALYSIS AND RESULTS OF AUTOMODEXAMPLE

The complete simulation model of the body shopcontained many more conveyors, processing times atstations, and downtimes associated wi th processes andsome of the material handling equipment. Analysis ofthis model was conveniently undertaken with AutoStat.AutoStat, working in conjunction with AutoMod,provides detennination of initial transients,management of scenarios as a database, and a 44Designof Experiments" feature (Rohrer 1994b).

Analyses performed with the base model indicatedthat the system as designed could not meet the targetproduction. Detailed analyses showed that the portionof the system shown in Figure I was one of severa]problem areas. Part of the problem was due to thedistance between the first and second lift tables on thecross transfer (right side of drawing). As indicatedearlier, on a cross transfer conveyor, pallets do not moveuntil the next lifUhold table is cleared by the previouspallet. In this case, because of the long distance, eachpallet experienced a delay exceeding the maximumallowable cycle time. The problem was remedied byplacing a clear limit switch at a distance sufficientlyremoved from the lift table to avoid collisions.Consequently, the cycle time at the table wasconsiderably reduced, thereby increasing thethroughput.

Another part of the problem was due to the flow ofpallets through the area. Normal flow of palletsrequired that the pallets go up on the cross transfer tothe first conveyor, and proceed to the cross transfer(right side of Figure 1). The top two conveyors were tobe used as temporary storage only when the palletscould not be sent (e.g., due to equipment down time) tothe next set of conveyors that are beyond the top part ofthe drawing. However, simulations showed that thesystem would back up faster because once the firstconveyor is blocked, there was no other way to uti1izethe middle conveyor for temporary storage. Analternative to this routing scheme was to send the palletsto the last conveyor at the top of the surge as the normalflow and then to accumulate pallets on the bottom two

1112 Gunal, Sadakane, and Williams

00°Ii~!

'! .

L..-.jo-~~--cross-transfer

lift table

~lr~!r"T accumulating chain conveyor p::

cross-transfer-+{} !spee -up roll I: :

hold table -------J rr.:rj .,

i~i

turntable with powered rolls ~ l~JI

Figure 1: Actual Conveyor System, CAD Layout

small conveyor pieces extendedX over gaps (speed-up sections)

X < X

X > X

X < X

A

Xx load stop to (re)set clear switches< or > direction of motion

X

Figure 2: Model Conveyor System, AutoMod Representation

Modeling of Chain Conve.yors and Their Equipnlcnt Interfaces 111:3

conveyors. Again, the model runs showed that thisrouting scheme worked better than the original andimproved the overall throughput of the system.

Similarly, the complete model helped to identifyseveral other problems with the conveyor system. Byfacilitating quick evaluation of the alternatives, thesimulation model not only played a significant role inthe detailed design of the system, but also providedquantitative support for managerial capital-investmentdecisions constrained by a tight project timetable.

In all these ways, results from micromodeling ofconveyors, when included in macromodels of theproduction system, helped assess the degree to whichmaterial handling improvements spawnedimprovements in overall system performance.

6 SUMMARY

Particularly at the micromodeling level of detail, theaccurate representation of conveyors and the equipmentinterfacing with them comprises numerous challenges.Meeting these challenges requires close attention tooperational specifications and detail of the equipment,plus keen awareness of the capabilities of the simulationmodeling tool chosen for use. This paper surveys thesechallenges and specific examples of them pertinent to amodel including chain conveyors, cross transferconveyors, lift and hold tables, and clear switches.

ACKNOWLEDGMENTS

Dr. Onur M. Ulgen, Professor, Department of Industrial& Systems Engineering, University of Michigan ­Dearborn, and president, Production ModelingCorporation, has made valuable suggestions helpful tothe presentation, organization, and clarity of this paper.Additionally, the cogent criticisms of two anonymousreferees were especially helpful in these regards.

A preliminary version of this paper was presented atthe June 1996 AutoSimulations annual users' groupmeeting (AutoSimulations Symposium '96 MountainRendezvous Proceedings, pages 313-319).

APPENDIX: TRADEMARKS

AutoMod and AutoStat are registered trademarks ofAutoSimulations, Incorporated.

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AUTHOR BIOGRAPHIES

ALI K. GUNAL is a Systems Consultant at ProductionModeling Corporation. He received his Ph.D. degree inIndustrial Engineering from Texas Tech University in1991. Prior to joining PMC, he worked as anOperations Research Specialist for the State ofWashington, where he developed a simulation systemfor modeling and analysis of civil lawsuit litigation. AtPMC, he is involved in consulting services for theanalysis of manufacturing systems using simulation andother Industrial Engineering tools. He is familiar withseveral simulation systems including AutoMod,ARENA, QUEST, ROBCAD, and IGRIP. He is amember of the Institute for Operations Research and theManagement Sciences [INFORMS], the Institute ofIndustrial Engineers [lIE], and the Society ofManufacturing Engineers [SME].

SHIGERU SADAKANE holds a bachelor's degree inIndustrial & Systems Engineering (University ofMichigan - Dearborn, 1994). He is an Appl icationsEngineer at Production Modeling Corporation, andhighly familiar with simulation and facilities-layoutoptimization systems including AutoMod, WITNESS,LayOPT, QUEST, IGRIP, and SIMAN. He is a memberof the Institute of Industrial Engineers [lIE] and theSociety of Manufacturing Engineers [SME].

EDWARD J. WILLIAMS holds bachelor's andmaster's degrees in mathematics (Michigan StateUniversity, 1967; University of Wisconsin, 1968). From1969 to 1971, he did statistical programming andanalysis of biomedical data at Walter Reed ArmyHospital, Washington, D.C. He joined Ford in 1972,where he works as a computer software analystsupporting statistical and simulation software. Since1980, he has taught evening classes at the University ofMichigan, including both undergraduate and graduatesimulation classes using GPSS/H, SLAM II, or SIMAN.He is a member of the Association for ComputingMachinery [ACM] and its Special Interest Group inSimulation [SIGSIM], the Institute of Electrical andElectronics Engineers [IEEE], the Society for ComputerSimulation [SCS], and the Society of ManufacturingEngineers [SME]. He serves on the editorial board ofthe International Journal of Industrial Engineering ­Applications and Practice.

Proceedings of the 1996 Winter Sirnulation C'onferenceed. J. M. Charnes, D. J. Morrice, D. T. Brunner, and J. J. Snrain

A SIMULATION TESTBED FOR INVESTIGATION OF TANDEM AGV SYSTEMS

Leon Bing-Shiun ChuangJoseph A. Heim

Industrial Engineering, 352650University of Washington

Seattle, Washington 98195, USA

ABSTRACT

A tandem, one vehicle per loop, approach toAutomated Guided Vehicle System (AGVS) guidepath design has been proposed as a more efficientand flexible alternative to traditional AGV systemdesign. The tandem design can substantially simplifythe design task while providing even furtherflexibility for changes in production system layoutand operation. Tandem guide paths, however, havepotential disadvantages because vehicle break-down,or even scheduled maintenance of vehicles, results inloop inaccessibility.

Previous work comparing the efficacy oftraditional guide path configurations and tandem loopconfigurations provided only limited flexibility forexperimentation. A tandem AGV system simulationtestbed has been developed to facilitate investigationand analysis of a variety of loop reconfigurationstrategies and algorithms. A particular focus of ourcurrent investigations is the concept of Real-TimeLoop Reconfiguration (RTLR), a means ofresponding to single vehicle failures in tandem AGVsystems. The RTLR can solve the potential loopinaccessibility problem and also enhance theflexibility of the overall AGV system design.

Results from initial studies used to verify thecorrect operation of the testbed are presented, and abrief outline of future applications for the tandemloop reconfiguration model are discussed.

INTRODUCTION

The automated guided vehicle (AGV) has thepotential for improving the perfonnance of thematerial handling tasks required in advancedmanufacturing systems. However, good design ofthe AGV system plays an essential role in using thetechnology efficiently. The AGV system designeffort is comprised of the route configuration, AGVdispatching rules selection, traffic control, andobviously, vehicle selection. In this paper we will

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focus on the design of the guide paths rather than themechanical or electrical components of the systems.

The tandem guide path design approach has beenrecognized as a more efficient and flexible alternativeto traditional guide path design, because itsignificantly reduces the complexity of the overallAGV system design effort (Bozer and Srinivasan1991). The tandem AGV system configuration asillustrated in Figure 1, however, does presentpotential disadvantages: a loop (i.e., one closedsegment of the AGV system guide path set) maybecome inaccessible because the AGV assigned to asingle loop is unavailable. The loss of an AGV isusually a result of vehicle break-down or scheduledmaintenance. But there are also opportunities toenhance the flexibility of the production systemlayout with tandem guide paths.

Previous ways of responding to the loss of vehicle­services has been to provide a stand-by AGV.Providing "spare" vehicles, however, can be anexpensive way to provide continued materialhandling services to all loops. And, in someinstances, this approach may not be practical,because vehicle configurations may not be consistentacross all loops.

To compensate for this potential problem, we havedeveloped what we call Real-Time LoopReconfiguration (RTLR). RTLR is a means ofresponding to the loss of one or more vehicles (i.e.,loop accessibility) and a way to dynamically changethe control strategies for the AGV system based onproduct mix or production machinery availability.

The complex nature of AGV system design in thissituation provides several opportunities to combineanalytical and simulation-based methodologies. Inthis paper we will report on the simulation aspects ofthe project. The complex nature of AGV systemdesign in this situation provides several opportunitiesto combine analytical and simulation-basedmethodologies. In this paper we will report on thesimulation aspects of the project. Discrete eventsimulation techniques have been frequently used todesign and evaluate alternative AGV systems, often