Techniques for designingproduction systemsby Mike DoonerOpen University

While individual production technologies are developing rapidly tomeet modern manufacturing objectives, emphasis now needs to bedirected towards finding ways of maximising their effectiveness whenthe various components are combined into systems. In supporting theoptimal designs of complex systems like FMS, new tools are requiredthat not only follow a systems approach but also attempt to prescribea system design. This paper provides some background to FMS design,briefly reviews several supporting design techniques, and highlightsthe need for new tools that will lead to more effective systems.

Introduction

The task of synthesising the many inter-acting component technologies con-stituting a flexible manufacturingsystem (FMS) design into one that satis-fies stringent, sometimes conflicting,manufacturing objectives is undoubt-edly becoming increasingly formidable.Nowadays, manufacturing should notonly be highly productive, it should alsoexhibit other equally importantcharacteristics, such as flexibility — theflexibility to manufacture part families,the flexibility to reconfigure or take onboard new technologies etc. Gettingthe optimal performance from a systemfull of diverse contributing tech-nologies requires an understanding of,first, how the components behave assystems, and secondly how to form orconfigure those that are most effectiveto provide the specified manufacturingobjectives.

A consequence of this increasedcomplexity is that design skills are insuf-ficient to cope, and there is a need forsupport from computer-based tech-nologies or tools to aid in the designprocess. Unfortunately, with severalexceptions, the few available design

tools are neither appropriate to today'srequirements nor able to be readilyunderstood by engineers typicallyinvolved in such activity. By comparisonwith computer-aided design (CAD) forproduct engineering, computer-basedtools for designing production systemsare under-developed. This relativelypoor showing probably reflects a poorlevel of understanding of productionsystem behaviour— knowledge whichis necessary to provide the basis for thecomputer techniques. In many sectionsof industry, production systems designis still treated more like a 'black art' thana science.

Production system components

An operational production system (forexample an FMS) comprises severalmajor components, representing amix of technology, design andmanagement:

• production technologies (forexample computer numerical control,direct numerical control, automatedguided vehicles, robotics)• system configuration (for exampleplant layout, routing)• control (for example cell control

systems, communications)• management.

The aim of system design is to bringtogether these components to form afacility that exhibits certain character-istics — features which can be recog-nised as being valuable to the user.These characteristics, which are gener-ally set by management (manufacturingpolicy), depend both on the individualtechnologies and the way in which theyare combined into systems. Essentially,while production technology focuseson the efficiency of individual pro-cesses, system configuration (forexample plant layout) is an organisa-tional activity which, along with control,may be regarded as integrating ingredi-ents. But whereas plant layout dealswith physical structure, control systemsdeal with logical (or software) structure.

One might speculate that, since pro-duction technologies are tending totake on a more modular form (forexample machining centres and dedi-cated manufacturing cells), it is theknowledge required for the intercon-necting or the configuring of cells orsystems that will surely become moresignificant in the future. For that reason,FMS design needs to be supported byan infra-structure of design tools cover-ing hardware and software structures,enabling effective and appropriate FMSconfigurations to be formed.

Design framework

In the normal sequence of events,manufacturing strategy (or policy)determines in broad terms the majorobjectives of a proposed manufacturing

Fig. 1 Forming part families with GT codinga Mixed parts b Part families

[Photographs courtesy of OIR/Organization for Industrial Research, Bedford, MA, USAJ

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ART PERFORMANCE SUMMARY

STATION 34%

STORAGE 6%

• TRANSPORT

Fig. 2 Process simulationa Trial FMS layout, showing machines, washing station, buffers and conveyor systemb Some historical (quantitative) data or system statistics

IPhotographs courtesy of Citroen Industrie UK Lid., Leamington Spa, England I

facility. In turn, these objectives have tobe translated into design requirements(expressed usually as machine func-tions) and then design details. If a com-pany is involved in small-batchmanufacturing, it might be consideringsome form of FMS cell configuration.Assuming that this manufacturing tech-nology seems generally appropriate,certain parameters then need to beestablished, typically:

• batch size• part types• flexibility (range of part family etc.).

From this broad initial specification,manufacturing engineering has to inter-pret, develop and evaluate a number ofalternative design proposals, judgedagainst a number of often conflictingcriteria. Major design variables includehow the parts are to be manufactured,the numbers and types of machines, theconfiguration, the materials etc.

In generating the design alternatives,traditionally use is made of existingknow-how and company experience.However, with new technologies likeFMS this activity is far from straightfor-ward, as in-house company expertise isnot likely to be strong. Furthermore,this often innovative period can bestifled by the sheer cost of 'experiment-ing' with expensive investments,because there is considerable pressureto produce 'instant' results. It is surelyin this context that computer systemsneed to play a more significant role,allowing engineering designers toexplore and evaluate new ideas quickly,with the computer systems themselvesgenerating or prescribing solutions.

It is because FMS designs are hugelycomplex, offering many alternativesand comprising many interacting com-

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ponents, that decision making is com-paratively more onerous than whendealing with traditional manufacturingmethods. This added complexity ariseslargely because:

• the component technologies arehighly interactive• there are often many potential sol-utions available from today'stechnology• contrasting objectives have to bebalanced against each other.

If CAD for production systems has arole, it should be to assist the designprocess by restraining the complexity tomanageable proportions, just as CAD isused very effectively for silicon chipdesign. Doubtless, these componentsinteract so extensively that an optimaldesign can only be devised by a systemsapproach. Yet by itself this is not suffi-cient; new models are required thatprescribe a systems design, by gener-ating a set of potential solutions fromwhich engineers can chooseappropriately.

Tools for production system design

Computer-based tools for aiding designcan be classed as 'generative' (prescrip-tive) or 'evaluative' (descriptive). Toolscan, however, combine bothapproaches. Generative tools, on theone hand, produce 'candidate' sol-utions or 'guidelines' from whichappropriate choices should be made.These models can effectively reducemanufacturing complexity by providinggeneral solutions but are not easy totailor for specific needs. Evaluativemodels, on the other hand, act more asa tool for the designer, who, in obtain-ing most from the technique, provides

the ideas and intellectual input. Suchmodels are often used to gain insightinto given complex situations, but donot automatically help a designer con-verge towards an efficient solution.

Historically the computer techniquesthat exist have been developed (in amanner similar to their counterparts inproduct engineering) for specific pur-poses to solve certain sub-problems ofdesign. Some tools, like drafting sys-tems, have been borrowed from com-ponent design and are now used forplant (layout) design, with little modi-fication for that purpose. As a result theuses for such tools are minimal, relyingheavily on the designer (facilities plan-ner) to provide both the ideas and theanalysis — which is usually qualitative.

Some better examples, however, areemerging. Robot cell design systemsare becoming recognised as powerfulsystem layout and programming tools,even though much of the 'evaluated'information is qualitative and potentialsolutions have to be provided by theuser.

What now follows are brief descrip-tions of three significant computer-based design techniques which can beused, directly or indirectly, to aid FMSdesign. The first of these is principally atechnique which is used to form partfamilies, but can provide informationforcell design. The second, a techniquebased on process logic (not geometry),is nowalmosta'standard'tool for evalu-ating process or system configurations.Third, as previously mentioned, is ageometrically based CAD tool for evalu-ating robot cells. Referring to the evol-ution of a production system, the toolsfall, respectively, into different designstages: coding techniques are used forforming manufacturing strategy; pro-cess simulation is used for developing

Computer-Aided Engineering Journal August 1987

strategy; and robot CAD is used fordetail design.

GT (parts) codingParts design coding, based on group

technology (GT) principles, can be usedto form part families, and in turn todesign production cells. One suchsystem, OIR's Multiclass, by exploitingsimilarities in design among a popu-lation of parts (Fig. 1a) through GTcoding helps to establish a number ofproduction cells for manufacturing afamily or families or parts (Fig. 16).Nowadays this mode of manufacturingis referred to as cellular manufactureand is the basis of FMS.

In addition to providing useful datafor FMS, parts coding (or classification)schemes provide input to computer-aided process planning systems — ofthe 'variant' type.

Another technique which canprovide a basis for cell design is produc-tion flow analysis (PFA). Unlike GT cod-ing, which groups parts on the basisof design features, PFA helps to formmanufacturing cells by examining theproduction processes required to makethe parts. Few, if any, commercialimplementations exist, however.

Process simulationThe need to consider the 'dynamics'

of a system configuration or layout hasled to computer simulation becominga powerful tool. Process simulationevaluates the 'process interactions':events such as the arrival of work tomachines, the process logic to copewith given production demands, and'what-if situations — the consequencesof machine breakdowns. Such tools aregenerally used when developing strat-egy, for example the number of pro-cesses required or the sizes of buffers.Thus they are not primarily intended forevaluating layout geometry, and onlystore a crude geometrical model.

Despite this and several other limita-tions, simulation tools are becomingincreasingly widespread, and representone of the few 'practical' aids that assistthe evaluative phase of FMS design.Such systems will not, however, pre-scribe solutions, nor help the engineerconverge on optimal designs, apartfrom providing information on what isnot advisable.

Fig. 2a shows a typical simulator inuse for designing an FMS layout, whileFig. 26 illustrates some of the historicaldata (quantitative) on system perform-ance; both illustrations are taken fromCitroen Industrie's MAST manufactur-ing system design tool.

Robot cell designA more specialised tool is cell design

software or robot simulation software.

Fig. 3 Robot cell design allowing part load/unload sequences to be developedIPhotograph courtesy of BYG Systems Ltd., Nottingham, Englandl

Robot simulators were developed orig-inally from geometric modellers, withadditions for representing robotkinematics, thus allowing motionsequences to be developed. Suchcomputer techniques are interesting inthat they extend a basically geometricalrepresentation to include some degreeof process modelling. Through this typeof facility, cell logic (for example anassembly sequence) can be built up andevaluated. Evaluative aspects includecycle time analysis, collision geometrychecking and robot (timing) workplaceinteractions, as well the evaluation ofthe robot's kinematic performance.

This type of design tool combinesqualitative output (mainly through ani-mation) with quantitative output, asmentioned above. In a manner similarto process simulation, it is engineer-ing's role to provide the 'design' input,while the system evaluates these pro-posals. Consequently it is unlikely thatan optimal design will be achieved inthis way.

Fig. 3 illustrates such a modern robot

cell design system in use to evaluate aturning cell (BYG Systems' GRASP).

The system allows realistic sequencesto be developed, while evalu-ating the robot's ability to produce therequired performance, and producesthe visual effect of a design proposalthrough animation.

Conclusion

This paper has briefly reviewed threetypes of design technique which can beused in different stages of producing anFMS cell or system. Although such toolsare invaluable during such design, andhelp to solve certain sub-problemsassociated with developing the totalsystem, it is argued that these tech-niques by themselves are inappropriatefor solving today's complex problems.New CAD models are needed that willsupport an integrated design method-ology, and the resulting computer toolsshould be capable of prescribing sol-utions until such time as designers nolonger require this form of assistance.

Bibliography

1 HANNAM, R. C : 'Alternatives in the design of flexible manufacturing systems for prismaticparts', Proceedings of IMechE, 1985, 199, (B2)

2 SURI, R.: 'An overview of evaluative models for flexible manufacturing systems', Annals ofOperations Research, 1985, 2, (3), pp. 13-21

3 BROWN, ). et al.: 'An integrated FMS design procedure', Annals of Operations Research,1985, 2, (3), pp. 207-237

4 PELUSI, ). A.: 'Advanced low-volume manufacturing techniques: A CMF case study'.Proceedings of AUTOFACT85 Conference, Detroit, Ml, USA, Nov. 1985

M. Dooner is Lecturer in Design with the Faculty of Technology, Open University, WaltonHall, Milton Keynes MK7 6AA, England.

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