MODULAR DISTRIBUTED MANUFACTURING SYSTEMS AND THE IMPLICATIONS FOR INTEGRATED CONTROL

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1 MODULAR DISTRIBUTED MANUFACTURING SYSTEMS AND THE IMPLICATIONS FOR INTEGRATED CONTROL Duncan McFarlane 1 ABSTRACT Driven by the need for more responsive manufacturing processes and as a consequence of increasing complexity in products and production systems, this short paper introduces a number of developments in the area of modular, distributed manufacturing systems. Requirements for the development of such systems are addressed and, in particular, the relevance to current and future integrated control systems is examined. One of the key issues for integrated control systems in the future is the need to provide support for distributed decision-making in addition to existing distributed control capabilities. 1. INTRODUCTION Modularity is typically introduced into a manufacturing operation to increase the flexibility of the operation both in terms of its range of functions and also its ability to be easily reconfigured in the face of changing conditions. Modularity almost always also leads to a distribution of functionality (not necessarily a physical distribution), and this paper examines the role of these two properties in the design and operation of manufacturing production units. Modularity is not a new concept in manufacturing, nor is the necessary distribution of functions or processes. Some existing examples of modular, distributed systems in manufacturing can be found in areas of control systems, equipment design, human resource development and more recently in enterprise management: Control Systems Development -> Modular, distributed control operations through Distributed Control Systems, and/or SCADA managed, bus connected PLC-based control systems Equipment -> Localisation of operations through flexible cells in discrete manufacturing and more recently semi-pipeless process units in process manufacture Human resources -> Increased shop floor autonomy through empowered ( Hoshin ) or Self-Managed Teams Enterprise - > exploiting IT developments through so called Virtual Factory methods Each of these modular, distributed developments contributes a degree of increased flexibility, yet each on their own will not necessarily lead to an improved production performance of the local unit or the overall manufacturing operation. For example, the development of cellular manufacturing methods in the 1970s and 1980s achieved significant improvements in single component manufacturing flexibility and efficiency. However, the limited industrial benefits arising from a number of these initiatives can been attributed to a failure to a) align human resource practices and/or information systems such that they supported the cellular operation or to b) integrate these capabilities within the surrounding production environment. The abstract diagram in Figure 1.1 (see page 2) illustrates a set of necessary components required to support a modular production unit or activity. It is critical that equipment, control/decision processing and human input are all co-ordinated to support the material transformation process and to maintain the functions of the unit. Equally 1 Manufacturing Automation and Control Group, Department of Engineering, University of Cambridge, Mill Lane, Cambridge, CB2 1RX, England, Tel: , Fax: , dcm@eng.cam.ac.uk. 1

2 important is the manner in which these units or activity interconnect, which introduces the issue of integration. INFO & DECISION MATERIALS IN Equipment Control / Decision Processing Human MATERIALS OUT ENERGY/ FLUIDS Figure 1.1 A Modular Manufacturing Unit An emerging set of approaches to the design and operation of manufacturing processes have been identified in which modularity and distribution concepts are being explicitly exploited in an integrated manner to improve production responsiveness to external demands and internal disruptions. This paper describes a number of these approaches, and the implications of such requirements will also be discussed specifically in the context of future control systems architectures and algorithms. 2. RATIONALE - MANUFACTURING RESPONSIVENESS & COMPLEXITY In this section we discuss responsiveness at a production level as a important motivation for the development of more modular, distributed systems. The following working definition clarifies what is meant by production responsiveness: The ability of a production system to respond to dynamic conditions (originating inside or outside the manufacturing organisation) which impact upon production goals. Matson and McFarlane (1998) Hence responsiveness includes, for example, the ability to react to customer demand variations, equipment breakdowns or raw material delays. We also note that responsiveness can involve either adapting production to meet the new condition or absorbing the effect of the change while maintaining the required performance. A range of recent industry studies 2,3,4 and publications (Barclay et al, 1996, Suri et al, 1996, Upton, 1996, Correa et al, 1996, Matson et al, 1998) have revealed the increased need for responsiveness to be an important issue for businesses attempting to compete in a changing market place. This was corroborated in a recent workshop 5 at which a range of industrial speakers confirmed rapid response capabilities as a key competitive mechanism. The full version of this paper will discuss the results of these studies and workshops in greater detail. The studies revealed a need for a range of production requirements. These included a) the need for a higher degree of modularity in order to produce diverse product ranges with standard equipment and to allow for the rapid introduction of new technology into production and b) the need for distribution not only of physical functions but also of production decision making and management in order to allow for dynamic operational structures and to manage rapidly varying production volumes and changing delivery requirements. In summary, the various studies on manufacturing responsiveness have shown that improved solutions are 2 Factory of the Future, Eureka / DTI funded initiative, Leaders for Manufacturing Programme, MIT, Cambridge, MA, USA, Manufacturing Responsiveness in the Face of Market Change, Cambridge University Manufacturing Engineering Group, DTI Overseas Science and Technology Expert Mission, July Responsiveness in Manufacturing, IEE Workshop, February,

3 required for: Management of increasing level of change and disturbances in production Management of increasing complexity of processes and variety of products (& computing systems) Cost effective management of shorter product life cycles 3. DEVELOPMENT OF MODULAR, DISTRIBUTED MANUFACTURING SYSTEMS Driven by many of the requirements outlined in the previous section, a number of research, development and protoyping programmes are currently underway world-wide in order to provide support for modular, distributed manufacturing production units which are readily integratable. The focus of much of this work is essentially the development of system capabilities for so called plug and play manufacturing, where production units can be rapidly interchanged, reconfigured and yet be capable of determining their own role in production. In particular, a number of the developments we describe perceive modularity (in operation) as being achieved by increasing the degree of autonomy of a production unit, and that looser distributed co-operative strategies governing unit operation are being pursued in contrast to traditional command-response methods. A number of examples of the developments of modular systems currently underway is given below (the reader is referred to the full paper - McFarlane, for detailed references): (i) Modular, Distributable Equipment Developments - the provision of equipment that is readily reused or reconfigured Reconfigurable Manufacturing (USA) - Programme involving a range of equipment suppliers to the automobile manufacturing industry, developing more readily reconfigurable machinery to cope with design changes and new vehicle models. Intelligent Assembly, Fixturing, Handling (Japan, EU) - Development of intelligent support facilities to enable discrete manufacturing operations to become more autonomous. Development of Self Diagnosis and Self Repair Capabilities (EU/Australia) - Development of facilities for a autonmous production unit to identify, locate and repair failures. (ii) Modular Control System Developments - the provision of distributed control hardware, software, algorithms and interfaces that support local decision making, flexible task allocation, reuse and easy (re)configuration. Holonic Manufacturing (part of International IMS Program) - development of systems components of autonomous, co-operative control systems. Co-operative control is proposed as an alternative to existing command-response control hierarchies which are vulnerable in the face of breakdowns or schedule changes. Next Generation Manufacturing Systems (part of International IMS Program) - integrating autonomous, cooperative production units, with integration based on so called natural or biologic methods. Multi Agent Manufacturing Control systems (projects in EU and other regions) - distributed and highly autonomous software components for supporting distributed, localised resource allocation and task execution (iii) Modular Team Based Developments - support for more autonomous shop floor production teams Hoshin or Self-Empowered Teams - numerous developments and implementations focussing on shared decision-making and requirements for greater autonomy at the shop floor level. (iv) Enterprise Modularity - interconnection methods for establishing dynamic networks of manufacturing plants and companies Virtual Factory - Information, legal, data systems support for companies forming dynamic collaborations (normally linked to specific product developments) 3

4 Next Generation Manufacturing Systems (part of International IMS Program) - also dealing with aspects of enterprise integration under the assumption of a repeatable (Fractal) architecture of the different levels of the business. Globeman 21 (part of International IMS Program) - Focussed on the information and knowledge linkages (and their standards) between self-contained companies or sections of companies. The project lists described here are by no means comprehensive, and it should be emphasised that the above programmes are anchored clearly by the need for effective integration of the developments with the existing factory operations. More details will be provided in McFarlane (1998). 4. IMPLICATIONS FOR INTEGRATED CONTROL The modular developments described in the previous section, and in particular those outlined in (ii), can be related to specific requirements for future industrial integrated control systems. The key issue emerging for the support of modular (autonomous) and distributed production is that control systems must not only provide for local (feedback) control capabilities but also for a degree of local decision making. This would support the unit in determining prioritised task allocation or optimal set point strategies, for example, under dynamic operating conditions. We examine control requirements under the categories of hardware, software, algorithms and commercial issues, noting that in many cases, at least partial solutions already exist for the requirements given. 4.1 Implications for Control - Hardware In order to provide for local decision support capabilities, the main requirement on control hardware is that it be capable of supporting an increased level of information processing and possibly process modelling at a local level. This will require greater processing power than is currently available in many PLC systems, and possible PLC to PC real-time connectivity. Because historical information will be required, fast database access is likely to be an essential component. Co-operative control (e.g. holonic manufacturing, multi agent systems) requires a more sophisticated form of communications than command-response, and support is required for multi directional, complex traffic and also the provision of human interaction at the production unit level. 4.2 Implications for Control - Software Object Oriented programming was developed with the intention of modular software development and this would appear to be the likely future candidate for future control programming standards. Generally, software support for those methods emerging from the academic field of Artificial Intelligence (e.g. capabilities for autonomous agent system development, expert systems etc. ) will be required in order to develop manufacturing units with a degree of intelligence about their own state and capabilities at a given point in time. In order to ensure effective migration from existing methods it will be necessary for some adaptation of existing coding standards (e.g. Function Blocks - IEC TC65/WG6, Programmable Controller Languages - IEC ) 4

5 4.3 Implications for Control - Algorithms To date much of the research into algorithms in the control community has focussed on more effective design methods for feedback controllers. While there remains scope for new innovations in this area in the manufacturing domain, from the perspective of modular, distributed systems development the following issues appear to be most critical for algorithmic development: Reasoning Capability - develop of local reasoning / situation assessment tools which do not rely heavily on historical data alone Co-operative mechanisms - mechanism which support stable interaction between production units and guaranteed performance when determining task allocation, executing tasks and monitoring process conditions Embedding models - a simple replica or model of a production units behaviour within its own control system appears critical for any form of autonomous behaviour. Diagnosis/Error Recovery Routines - routines which are able to effectively integrate structural (model based) information and operator experience (rule based) information On-line Learning Capabilities - support for on line parameter adaptation, and learning of improved control and decision making strategies. 4.4 Implications for Control - Commercial A final implication for control systems within a modular manufacturing environment is that control functions will only form part of the solution for any given manufacturing requirement. The need to produce an effective integrated production unit may lead to a greater level of partnerships between control and equipment manufacturers, software houses and systems integrators and end-users. 5. CONCLUDING ISSUES This paper has introduced the concept of modular, distributed manufacturing systems and assess some of the future requirements for industrial control to be able to support these systems. By way of conclusion, Figure 5.1 contrasts some current and future issues relating to this work: NOW Distributed Control Automated Operation Control / Equipment / HR System Design User Friendly Interfaces FUTURE Distributed Decision Making & Support Autonomous Operations Manufacturing Unit Design Human Integrated Operation Figure 5.1 Current and Future Issues in Control System Development 6. ACKNOWLEDGEMENTS The author is would like to gratefully acknowledge support from the EPSRC (UK), the EU ESPRIT Programme, and DTI OSTEMs Unit for their funding of work reported in this paper. 5

6 7. REFERENCES Barclay, I., J. Poolton and Z. Dann, Improving Competitive Responsiveness Via The Virtual Environment, IEEE IEMC 96, pp Correa, H.L. and N. Slack, Framework to analyse flexibility and unplanned change in manufacturing systems, Computer Integrated Manufacturing Systems, Vol. 9, No. 1, 1996, pp Suri, R, R Veeramani and Joe Church, Quick Response Manufacturing, IIE Solutions, November 1995, pp Upton, D., The Management of Manufacturing Flexibility, Calif. Mgt Review, Vol 36, No 2, McFarlane, D., Modular Distributed Manufacturing Systems And The Implications For Integrated Control, manuscript under preparation,