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Simulation of a generic flexible assembly systemN. F. EDMONDSON and A. H. REDFORDAbstract. During the early 1980s, the concept of a flexibleassembly machine was first suggested by Hounsfield (1983).Following this, a variety of research projects have beenconducted in an attempt to develop a flexible assembly systemthat is functional and economically viable. The most significantcontribution has been made by the EURICA EU 321 FAMOS INFACT1project. However, despite the develop-ments made during these projects, no industrial based systemexists today. This paper presents a simulation model of a novelconcept for a multi-station flexible automatic assemblymachine, and examines the application of a rule based controlstrategy for the control of a materials handling system used insuch a system.1. IntroductionBefore discussing flexible assembly it is importantto understand the limitations of traditional dedicatedassembly. Dedicated assembly is a mass productiontechnology that was developed in the early 1900s byHenry Ford, to assemble a unique product in verylarge volumes, and this led to a very cost effectivesolution. Dedicated assembly automates the assemblytask by breaking it down into simple operations thatcan be conducted by a series of workheads, theassembly being built up as it passes down the line.Parts are supplied in bulk, placed in individual partsfeeders and presented to automatic workheads, whichinsert them into the part assembly at high speed. Thisform of assembly can achieve cycle times of as little as1 second per assembly.As dedicated assembly machines are only suitablefor a single product, any significant product designchange will result in considerable assembly machineredesign costs, and lengthy reconfiguration time. It isalso clear that such equipment can only be justified forlarge production volumes, as the equipment cost isspread over the life of a single product. For this reason,the application of dedicated assembly has traditionallybeen restricted to high volume production. Further-more, the world market is demanding greater productvariety, consistent high quality, shorter lead times,competitively priced products and rapid new productintroduction. In Scandinavian countries, these factorsare now accompanied by increasing labour costs.In the main, product assembly has remained amanual process, being subject to quality variations,fluctuations in productivity, fluctuations in labour ratesand health and safety issues. In an attempt to reducethe cost of product assembly, many European compa-nies have moved their assembly plants to lower costregions. However, this is not always the ideal solution asit increases transportation costs, places a physicalbarrier between design and production and suffersfrom quality variations.The introduction of semi-automatic assembly (seefigure 1) has been one approach adopted by industry tocounter the problems associated with manual assembly.Semi-automatic assembly automates critical parts of theassembly sequence, such as screwing or push-fit opera-tions whilst an operator performs the part feeding andpositioning tasks. This enables the manual assemblytasks that traditionally suffer from quality variations tobe controlled using automation and the costly partfeeding and manipulation tasks to be performed usinglow cost labour. However, semi-automatic assembly stillrequires a significant investment in dedicated toolingand remains subject to fluctuations in production rates,and fluctuations in labour rates.The advantages of assembling in low cost regionscan be eliminated if sufficient automation can beintroduced into the assembly system (Fieldman et al.1996), as the production system is no longer reliant onlarge numbers of people. The assembly plant can thenbe placed close to the customer market to reducelogistical costs.INT. J. COMPUTER INTEGRATED MANUFACTURING, 2003, VOL. 16, NO. 3, 157172Authors: N. F. Edmondson, University of Loughborough, UK and Grundfos A/SPoul Due Jensens Vej 7, DK-8850 Bjerringbro, Denmark; A. H. Redford,University of Salford, UK1http:/www.3.eureka.be/home/The driving factor behind the design and develop-ment of flexible assembly systems is economics. Aspreviously stated, it is not economically viable to build adedicated assembly machine for small batch productionquantities (30 000 to 500 000 units/year), as the piecepart cost of assembly will be too high. The main goal,therefore, behind the development of a flexibleassembly machine is the minimization of specialpurpose equipment, i.e. the equipment that can beamortized against the product. This will allow morethan just one product type to be assembled on themachine and the machine cost to be spread over theproduction of more products.Flexible assembly utilizes assembly robots andflexible part feeders in order to create a hybrid ofmanual, semi-automatic and dedicated assembly that iscapable of small batch, large product variety produc-tion. The flexible assembly machine can be comparedwith a CNC machining station (see figure 2). Partprograms, fixtures, tools and raw components are thesystem input, and finished products are the result.There are three basic automatic flexible assemblysystem configurations, Single Station, Multiple Stationand Automatic Flexible Assembly Line. Figure 3 shows aSingle Station system, the assembly robot (manipulator)is located at the centre of the system and the partsfeeders are located at the perimeter of the manipulatorwork zone. As a single manipulator performs all of theassembly tasks in series, the assembly time can becomelong if the assembly has many parts. If the number ofparts in an assembly becomes too large, it may not bepossible to fit them around the manipulator perimeterand a larger manipulator will have to overcome theseproblems, a Multiple Station layout can be used (seefigure 4). The assembly operation is broken down intosmall groups of tasks performed at a number ofassembly stations, the manipulator visits each of theassembly stations progressively. In order to make thispossible, a fixture transfer system is required thatincreases the system cost. This approach enablesN. F. Edmondson and A. H. Redford158Figure 1. A semi-automatic assembly station.Figure 2. The CNC assembly machine.Gantry manipulatorPalletPalletPalletPalletAssembly ZonePart feedersFigure 3. Schematic of single station automatic flexibleassembly cell.Gantry manipulatorParts feedersParts feedersPalletPalletPalletPaPalletAssemblystationAssemblystationAssemblystationAssemblystationAssestaFigure 4. Schematic of multiple station flexible ducts with more components to be assembled withgreater speed than a single station system.To date the only commercially successful imple-mentation of flexible assembly has been Flexible LineAssembly (see figure 5), where a number of manip-ulators are used to replace the dedicated workheadsused in dedicated assembly lines, each manipulatorperforming a few assembly tasks at each assemblystation positioned along an indexing transfer system.Such assembly systems are capable of high volumeproduction of a single product having many variants,for example, the Sony Walkman or the assembly ofvideo cameras (Whitney, 1999). The commercialsuccess of these systems is due to the high productionvolumes of a single product, making the cost per unit ofproduction economically acceptable.The development of a generic flexible assemblysystem involves the design, selection and integration ofa number of different mechanical systems in order todevelop an assembly system that is capable of assem-bling a wide variety of products having an unknownspecification. A specific system configuration is depen-dent on a variety of factors, such as product size, weight,component insertion direction, and manipulator geo-metry.The concept of flexible assembly was first intro-duced by Hounsfield (1983). It was argued that suchsystems would be used to assemble the middle rangeof production volume between manual assembly anddedicated assembly (Lotter 1986). Twenty years later,no such systems exist as commercially availablesystems, and low volume assembly remains a manualor semi-automatic process, despite the rise in the costof labour by 50%, a 70% reduction in the cost of robottechnology and a significant improvement in theperformance of robotic technology (Delgado 2001).Edmondson and Redford (2001a) identified thatthe reason why flexible assembly was never implemen-ted during the 1980s was because the cost of thetechnology was too high. Edmondson and Redford(2001a) also identified that, owing to the increase inlabour costs and reduction in the cost of robots, flexibleassembly can now offer significant savings over semi-automatic assembly and, in some cases, manualassembly for low and medium volume assembly.Based on the above findings a novel concept (PatentApplication No PA 2001 00045) for a multi-stationflexible automatic assembly machine has been devel-oped (Edmondson and Redford 2001b, c, d, e and f).An isometric view of the multi-station flexible automaticassembly station can be seen in figure 6.This paper examines the application of a rule-basedcontrol strategy for the control of the materialshandling system used in the multi-station flexibleautomatic assembly system.2. Assembly workspace layoutEdmondson and Redford (2001c) identified thatthe most suitable layout for the various assembly cellelements is as shown in figure 7.Two assembly fixtures are used in the assembly cellso that the manipulator can move directly to thesecond assembly fixture when all of the assembly taskson the first assembly fixture have been completed,whilst the materials handling system removes andreplaces the completed fixture of assemblies. Thisallows the first assembly fixture to be removed andreplaced by the materials handling system without theneed for the manipulator to stop working, hencemaximizing the manipulator utilization. Furthermore,the time the manipulator spends performing gripperand tool changes is minimized by assembling inmultiples of products on each fixture; in this way, thetime taken to perform a gripper change is distributedSimulation of a generic flexible assembly system159Part pallet transport systemPallet storage areaSCARA robot Assembly fixture Fixture transport systemFigure 5. Schematic of flexible assembly line.across a number of products as opposed to a singleproduct, and the materials handling system is notrequired to replace an assembly fixture for eachproduct assembled.In order to achieve an appropriate production rate,a number of cells can be linked together to form anassembly line (see figure 8). Each assembly cell has aself-contained materials handling system, which inter-acts with the other assembly systems via a transfermechanism. The result is that no increase in demandon the material handling system is experienced when aseries of cells are linked together, and it is unlikely thatthe manipulators will have to wait for the handlingsystem to supply parts.N. F. Edmondson and A. H. Redford160Figure 6. Isometric view of multi-station flexible automatic assembly system.K4DK61K6EK69K70K75K6CK61K74K6FK72K77K6FK72K6BK73K70K61K63K65K50K61K72K74K70K61K6CK6CK65K74K46K6CK65K78 K66K65K65K64K65K72K73K54K6FK6FK6C K6DK61K67K61K7AK69K6EK65K73K50K61K72K74K70K61K6CK6CK65K74K50K61K72K74K70K61K6CK6CK65K74K50K61K72K74K70K61K6CK6CK65K74K46K69K6EK69K73K68K65K64K70K72K6FK64K75K63K74K70K61K6CK6CK65K74K46K69K78K74K75K72K65K46K69K78K74K75K72K65Circular workspace layoutK46K6CK65K78 K66K65K65K64K65K72K73Figure 7. Assembly cell layout.Figure 8. Flexible assembly cell line.3. Materials handling systemThe multi-station flexible automatic assembly ma-chine can be considered as two basic mechanicalsystems which operate in parallel; the anthropomorphicmanipulator, which performs the actual assembly task,and the materials handling equipment, which ensuresthat the manipulator is fed with the correct parts,fixtures and tools at the correct time and place, whilstperforming other functions such as finished productremoval from the assembly area.Redford (1991) lists the total material handlingrequirements as follows:The handling of pieceparts into the system. Pieceparts arecategorized into two groups; those which can behandled using traditional small parts feeders, e.g.vibratory bowl feeders, and those which cannot besupplied using small parts feeders.The handling of pallets, fixtures and tools. Apart fromfeeding pieceparts to the assembly system, the materialshandling system will also be required to handle palletsof parts, assembly fixtures and the transfer of tools inand out of the assembly machine.The removal of the completed product from the system.Finished assemblies need to be removed from theassembly fixture, and this function is performed by themanipulator picking the product from the fixture andtransferring it to the material removal system. This cantake the form of a simple output shoot that deposits theproduct into a bin of other finished products in apseudo-random manner. However, in most cases, theproduct has to be handled by some other form ofequipment, e.g. test, processing or packaging; hence, itwould be logical to keep the products position andorientation. This can be performed using some form ofmechanical transfer device, which moves the productdirectly to the next process. Alternatively, if theproceeding process is not in close proximity to theassembly system or a storage buffer is required, theproducts can be placed in some form of packaging, e.g.palletized or magazined, so that the next process canautomatically unload the packaging.The accommodation of operations external to the assembly cell.Many electromechanical products require assemblytasks or processes that cannot be incorporated intothe assembly machine due to economic, or technicalreasons. If these products are to be assembledautomatically, some form of transfer system needs tobe incorporated to enable the transfer of partly finishedassemblies to and from the external processes.The transportation of partially finished products to and fromrework. It is inevitable that faults will occur occasionallyduring the assembly operation. It is generally acceptedthat there are three basic corrective actions that can beperformed.(1) Stop the equipment and wait for manual assistance.(2) Attempt automatic recovery.(3) Remove the partially completed product from thesystem, carry out reparation work offline andreturn the reworked product to the system forcompletion.The first two activities, being in-cell activities, placeno demand on the materials handling system, while thethird solution would require the application of thematerials handling system.Redford (1991) also suggested that the materialshandling function should be performed using twosystems; flexible small parts feeders for pieceparts(Edmondson and Redford 2001f) and a pallet systemfor all other handling operations due to the common-ality and frequency of material handling motions intoand out of the assembly system.To meet these requirements it was identified(Edmondson and Redford 2001d) that the materialshandling system must be capable of removing andreplacing one pallet at a time without moving any otherpallets or fixtures. For example, when a pallet is emptyit must be replaced without interrupting activities beingconducted on other pallets. It was found that thisfunction could be achieved using a cylindrical manip-ulator mounted beneath the anthropomorphic manip-ulator, see figure 9.Parts supplied to the system on pallets are fed frompallet magazines located around the circumference ofthe articulated robots workspace. Pallets are removedfrom the base of the pallet stack and fed to thecylindrical robot, which places them in the requiredpallet location. A buffer belt can be added to the palletmagazine, which automatically reloads the magazineenabling longer periods of unmanned production.Empty pallets are fed to empty pallet magazines locatedaround the assembly cell or at the middle belt joiningthe two robot cells. The same system can also be used tostore and automatically change the assembly fixtures.4. Strategy for operating the materials handling systemBased on the development of a flexible assemblymachine using two gantry manipulators to performassembly tasks, and a single pallet shuttle runningunder the length of the gantry manipulators work-Simulation of a generic flexible assembly system161space, Redford and Dailami (1998) proposed that thepallet shuttle could be controlled using a schedulingmethod that would schedule the materials handlingtasks so that they would occur during non-productiveperiods of the assembly manipulators work cycle, forinstance during tool and gripper changes. It wasassumed that the pallet shuttle would run out ofcapacity and be unable to meet the material handlingrequirements of the two manipulators. Hence, it wassuggested that the control logic of the pallet shuttleshould be reprogrammed for each new product themachine was to assemble, based on the output of thescheduling exercise, in an attempt to optimize thepallet shuttles utilization.The multi-station flexible automatic assembly systemproposed by Edmondson and Redford (2001d) pro-poses the use of a single pallet shuttle for each assemblymanipulator in the assembly system, as opposed to asingle pallet shuttle for all manipulators in the system,as a means of avoiding the lack of materials handlingcapacity. This paper examines the possibility of con-trolling the pallet shuttle using a set of standard rulesfor all products. The rules determining the reaction ofthe pallet shuttle are in response to signals generatedduring the assembly process, i.e. the material handlingsystem does not have a fixed operating cycle butperforms tasks based on the requirements of theassembly process. The rules were developed iterativelyduring the construction of the simulation model, andare listed as follows in order of priority.(1) Empty the mid