Computer-Aided.Design.Engineering.and.Manufacturing P2

2 Computer-Integrated Assembly for Cost Effective Developments Rinaldo C. Michelini University of Genova Gabriella M. Acaccia University of Genova Massimo Callegari University of Genova Rezia M. Molfino University of Genova Roberto P. Razzoli University of Genova 2.1 Introduction 2.2 Assembly in Intelligent Manufacturing Market-Driven Trends in Factory Automation · Cost Effectiveness by Means of Flexibility · The Technology of the Assembly Process 2.3 Effectiveness Through Flexibility Issues Assessment of the Flexibility Requirements · Decision Supports and Simulation · Example Developments 2.4 Reconfigurable Set-Ups Assembly Facilities Modular Assembly Transfer Lines · Modularity of Assembly Lines with Buffers and By-Passes 2.5 High-Versatility Self-Fixturing Facilities Robot-Operated Assembling Set-Ups · Assembling by Integrated Control-and-Management 2.6 Concluding Comments · Off-Process Setting of a Customer-Driven Mass Production Assembly Facility · Exploiting Recovery Flexibility with Adaptive Modular Assembly Facilities · Programming Optimal Assembly for One-of-a-Kind Products · 2.7 Acknowledgments 2.1 Introduction For many manufacturing enterprises, assembly is an important portion of the final costs. Effectiveness was traditionally hunted for by reducing complex schedules into unit tasks (scientific work organization) and by enabling sequential assembly lines (vertical flow-shop). The approach leads to the highest pro-ductivity, and it is prised for mass production. Flow-lines and fixed schedules, however, require amorti-sation plans based on steady programmes on duty horizons corresponding to product volumes exceeding some minimal threshold. Market saturation and trade instability look for quick updating of the offered items, properly adapted to wider classes of buyers’ needs, possibly, down to the limit situation of one-of-a-kind customised quality. Worldwide enterprises looking for purchasers’ oriented supply are, thus, concerned by time-varying artefacts; extended mixes of items have to be processed in parallel and delivered with short time-to-market. The emphasis toward customised quality, product variety, frequently up-dated offers, quick delivery dates, etc. needs a new approach to effectiveness, exploiting knowledge-intensive set-ups, by schedules complexity preservation (intelligent work organization) and robotised assembly cells (distributed versatility job-shop). © 2001 by CRC Press LLC The return on investments deals with leanness, namely on checking each addition or modification on its actual usefulness to increase item’s quality; and with economy of scope, namely on carefully monitoring aims and tasks on their ability of granting a positive value-chain, while avoiding unnecessary accom-plishments and useless equipments. These new trends move to intelligent manufacturing set-ups, supporting: recoveryflexibility, as option instead of set-apart resources; tactical flexibility, to approach optimal schedules assembly process; and strategic flexibility, for processing variable product mixes. Computer-integrated assembly becomes a cost effective opportunity, whether exploited to draw out actual benefits from the technological versatility of the new resources. Different tracks are considered by the present chapter, namely: · modularity, to make possible the setting and the resetting of the assembly facility with due account of artefacts evolution, by the off-process management of versatility; and · robotics, to enable the functional versatility so that several product mixes are processed together, under supervised standard testing operations according to total quality. The two techniques are each other subsidiary, have different application range, and will be discussed in the following, distinguishing the levels of productivity and of flexibility that each time are needed; sample cases are used for explanatory purposes. Computer-integrated assembly is a relevant aid to fully exploit flexible automation by enabling the process-driven adaptivity, once the visibility on every relevant quantity affecting the processing progression is provided and the transparency of the current decision logic is acknowledged. Such visibility is the preliminary requirement to enable the economy of scope and can actually be reached by expanding the factory automation software with inclusion of proper expert simulation codes. The present chapter is organised as follows: · a section recalls the basic options of intelligent manufacturing to support effective assembly processes; namely: the market-driven trends in factory automation, towards computer-integrated assembly, from scientific job allotment to intelligent task assessment; the options of flexibility for achieving cost effective issues; and the basic technologies of the assembly process, from product-oriented assembly lines endowed with special purpose equipment, to operation-oriented func-tional units based on modular layout or built with robot technology; · a section considers, at the shop floor level, the characterising features to maximise the return on investments in flexible automation, with focus on assembly problems; i.e., the basic references for evaluating flexibility effects, from the analysis of characteristic features to judgmental hints for the setting of the flexibility figures; the decision tools, supporting the choice of the efficiency setting/fitting figures, exploiting computer-simulation as consultation aid and process-driven gov-ern as controller-manager of shop floor operations; · a section presents cost effective issues in computer-integrated assembly for situations requiring mass production with sudden switching to new artefacts within short time-to-market terms; namely: the off-process setting of versatility by reconfigurable modular facilities; and the adaptive fitting (recovery flexibility) of buffered modular assembly facilities with (limited) physical resources redundancy; and · a section presents robot assembly facilities, aimed at exploiting the options of flexibility for customers-driven artefacts; in particular: the on-process setting (strategic flexibility) of robotised assembly facilities; and the efficient fitting (tactical flexibility) by integration of control and management; both situations characterised by the functional redundancy of the knowledge inten-sive solutions provided by intelligent manufacturing. The example cases of sections 2.4 and 2.5 have been developed by the Industrial Robot Design Research Group at the University of Genova, Italy, in front of diversified industrial applications at shop floor level, aiming at govern for flexibility issues, according to the basic ideas summarised in sections 2.2 and 2.3. © 2001 by CRC Press LLC 2.2 Assembly in Intelligent Manufacturing Efficient manufacturing of industrial artefacts is conditioned by assembly. Product and process reengi-neering is positively concerned by setting up cost effective facilities. The return on investment is, however, a critical issue; to obtain the right layout, the effectiveness of the assembly section has to be assessed against actual potentialities. The study needs, in general, consider the entire enterprise’s organization, from the design to the selling of the artefacts and the degree of automation in both material and data processing has to be acknowledged. At the front-end level one typically deals with: · fixed assembly stands: the components (suitably assorted and fed) are joined to the (principal) workpieces at properly fixtured stations by, typically, job-shop logistic; and · transfer assembly lines: the (main) workpieces are transferred by flow shop logistic (with convey-ors, belts, etc.) and sequentially joined to the (concurrently fed) parts. Intermediate solutions, aiming at best compromising effectiveness and adaptivity are, as well, used, e.g., · cell shops, performing group technology subassemblies by means of segmented carousels inter-connected by adaptive dispatching; and · transfer sections, joining varying mixes (for adaptive processing, job enrichment, etc.) and enabling several assembly cycles through parts rerouting. Performance depends on organization and equipment. Productivity (assessed as nominal net production on the reference time horizon) actually reaches the highest figures with flow shop and transfer assembly, based on specially fixtured units (readily adapted tofixed automation) aiming at low cost mass production. Flexibility (related to the property of modifying process abilities to accept varying product mixes) requires technological versatility; job-shop organization with general purpose workstations is prised, in connection to robotics, for on-process and on-line adaptivity. The emphasis toward product variety, constant quality, higher reliability, frequently updated design, shorter time-to-market, and the likes, forces concurrent enterprise approach, aiming at integrated solu-tions, from design and development to assembly and delivering stages. Computer integration is the main factor in simultaneously achieving the said goals by knowledge intensive set-ups. Thus, special attention is, for instance, reserved to: · assembly planning [BAL91], [BOO82], [DeW88], [DEF89], [HEN90], [HoL91], [HoS89], [KoL87], [LeG85], [MAT90], [Mul87], [RoL87], [Wol90]; · design for assembly [AND83], [Bjo89], [BoD84], [BoA92], [Hoe89], [NeW78], [TUR87]; and · similar options and methods, improving the exploitation of process-embedded knowledge. Conversely, the layout of the assembly equipment lags behind in flexible automation; as a result, on the final products the related costs percentually appear to increase. For several applications, indeed, robotics in assembly provides meagre benefits, since: · robots magnify the operation-driven constraints of dedicated equipment (fixtures, jigs, grippers, feeders, etc.) and require a considerable amount of propriety software; as a result, side costs are four to five times the robot cost; and · manipulation architecture supports poorly optimised motion for any particular task; even if sophisticated path planning and dynamics shaping options are provided, the duty-cycle time and position accuracy are worse than the ones of dedicated units. An alternative suggests that assembly equipment, built from modular units has to be considered [ACA96a], [Dre82], [GIU91], [MIL93], [Rog93], [TaA81]: · productivity preserves the figures of special purpose assembly lines; · reuse of the selected fixed assets into differently configured layouts makes possible amortisation plans based on sequences of product mixes. © 2001 by CRC Press LLC The modular approach presumes the interfaces consistency based on mechanical and electronic standards. Then, work cycles are analysed into sets of process primitives having each function performed by a modular unit. The flexibility is managed off-process by reconfiguring the facility as soon as the plans for the mass production of new artefacts are fixed. The opportunity will be considered and example appli-cations are recalled in section 1.4, as issues leading to mass production, while supporting short time-to-market for new artefacts by means of reconfigurability. Market-Driven Trends in Factory Automation The availability on the market of comparable offers requires continuous adaptation of current delivery to users’ satisfaction, to win new buyers and preserve/expand the trading position of the enterprise. The course turns to become more relevant as the number of specifications is increased to better adapt the products to lifecycle standards on safety, anti-pollution, etc. or on recycling and dismantling rules according to prescriptions aiming at sustainable development promulgated by every industrialised coun-try [AlJ93], [AlL95], [BoA92], [Eba94], [JOV93], [SEL94], [Wie89], [ZUS94]. Effectiveness is dealt by balanced and integrated views: customers’ responsiveness, simultaneous product-and-process design, productive decentralisation for technology adaptivity, and the likes. Each offered artefact is, thereafter, endowed by quality ranges attributes covering multitudes of users’ requests. Leaving up the mass pro-duction aims, the actual trend is to propose (once again after handicrafts time) one-of-a-kind products purposely adapted to individual whims with, however, quality figures granted by standard tolerances, as compared to craftworks, Figure 2.1. Customised artefact quality is consistent with intelligent manufacturing by means of flexible speciali-sation.Assembly is a critical step; on-line manual operators are common practice when product variability makes uneasy the facing of changing tasks with high productivity levels. Fully robotised assembly cells Craft manufacturing Mass manufacturing Customised manufacturing Enterprise return Work organisation Technical specification Decision structure Motivation style Knowledge features economy of skill master to apprentice indenture design while manufacturing craftsmen commitment individual creativity non replaceable personal contribution economy of scale scientific job-allotment off-process optimal assessment hierarchical specialisation division of competencies addition of sectorialised team work economy of scope intelligent task assessment simultaneous product/process design decentralised responsibility collaborative reward distributed cooperative processing FIGURE 2.1 Trends in market-driven and technology-pushed manufacturing. © 2001 by CRC Press LLC are, indeed, endowed by extended versatility so that the mix of items jointly processed can be quite large, but productivity is far from the capability of special purpose assembly facilities. For mass delivery, fixed automation solutions are, therefore, preferred; the switching to new sets of artefacts cannot be done, unless the different special purpose devices, properly matching the requested changes, are enabled. The compression of the time-to-market is sought by simultaneous engineering, namely, by developing prod-ucts (with design-for-assembly, etc. rules) and processes (with modular configurability or function programmability). The ameliorations have been related to the automatic preparation of the assembly sequences [ArG88], [Bon90], [Boo87], [JIA88], [Koj90], [MIC88], [Tip69], [WAR87] with attention focused on the process modelling, aiming at plant fitting for granting the visibility of every relevant effect. Formal descriptions have been likely proposed to design the assembly facilities [ACA96B], [ArG88], [Hoe89], [LeG85], [Moi88], [ONO93], [ONO94b], [SEK83], [Van90], [WIE91] liable to be translated into functional models. The computer integration is a powerful contrivance; centrality, however, shall be left to the manufacturing flow, according to requirements of leanness, which say that non-necessary options are directly (since not fully exploited) and indirectly (because redundant accomplishment) nuisances. Summing up, important improvements are expected from the integrated control and management of the processing operations [ACA88c], [ACA89c], [ACA92b], [MIC92b], [MIC94d], with account of flex-ibility of the physical (the set-up) and the logical (the fit-out) resources. One should look for: · the effective set-up of assembly sections, tailored to product mixes included by the enterprise strategic planning; · the proper fit-out of assembly schedules, adapted to production agendas within the enterprise tactical planning. Off-process and on-process adaptivity, Figure 2.2, happens to become a market-driven request; it has to be tackled over at the proper level: · setting is concerned by the structural frames, Components, facility-configuration and control: CFC the set-up of CFC frames presents as everlasting activity; choices provide reference for identifying current process set-ups all along the life of the facility; and FIGURE 2.2 Flexibility setting/fitting by controllers/managers. © 2001 by CRC Press LLC ... - tailieumienphi.vn