MOTIVATION AND HOLONIC MANUFACTURING SYSTEMS
The following are well-recognized facts by manufacturing experts [2,3]:
a. A major part of manufacturing will gradually shift from mass production to the manufacturing of semi-customized products to meet the increasingly diverse demand.
b. The "made-in-house" mind set will gradually shift to a more collaborative mentality, and various entities will team up with others to speed up product development, to reduce risk, and to penetrate local markets.
c. Effective and efficient coordination becomes critical to reap the benefits of collaboration.
d. Centralized control of various entities with different objectives, locations, and cultures is almost out of question. Effective coordination will be a key challenge for entities who want to thrive in the new paradigm.
A "holonic manufacturing system" (HMS) is a way of organizing a manufacturing system to meet the above challenges. In an HMS, key elements such as machines, work centers, plants, parts, products, persons, departments, or divisions have autonomous and cooperative properties. These elements are called "holons," a word coined by combining "holos" (the whole) and "on" (a particle) following Koestler [4]. In an HMS, each holon's activities are determined through the cooperation with other holons, as opposed to being determined by a centralized mechanism. An HMS could therefore enjoy high agility, which is an important characteristic for future manufacturing systems. How to define holons for a given problem context, what should be the appropriate holonic architecture, how to design effective communication and coordination mechanisms, and how to migrtate from current systems to HMSs, however, are critical issues to be investigated before HMS can be realized in practice.
THE HOLONIC MANUFACTURING SYSTEMS PROJECT
The HMS project was one of the six test cases of the Intelligent Manufacturing System (IMS) Feasibility Study Program. The objective was to explore mechanisms for cooperation among industrial companies, universities, and research institutes to promote research in and adoption of "holonic" technologies for manufacturing systems. Within the HMS project, partners on planning, scheduling, and control included: United Technologies Corporation (UTC) -- users' requirements and technology needs; Toshiba -- assembly planning; UConn -- optimization-based scheduling; Hitachi -- dynamic task planning and data driven control; Katholieke Univ. Leuven and Kobe Univ. -- real-time scheduling and control; and Keele University -- intelligent and distributed knowledgebase and database.
COOPERATION METHOD FOR HOLONIC SCHEDULING
The highlights of UConn's research are:
a. Many critical issues in manufacturing planning and scheduling were identified through survey of and direct discussion with UTC engineers and scientists and other HMS partners.
b. A holonic planning and scheduling architecture was designed. The optimization-based scheduling methodologies developed by UConn [5] were successfully mapped onto the holonic architecture, and provides a theoretical foundation for the effective cooperation among holons to achieve globally near-optimal system performance.
c. The results obtained in (b) were implemented on the Intellectual Distributed Processing System, Toshiba's distributed object-oriented operating system installed at UConn, for a simulated simple robotic assembly test bed. This object-oriented implementation laid a foundation for the future development of scheduling systems [6,7].
d. Building on the relationship developed during the HMS project, further collaborative research and development efforts with both Toshiba and UTC are currently under way.
COOPERATION METHOD FOR HOLONIC SCHEDULING
For the simulated simple robotic assembly test bed considered, three classes of holons were defined: product holons, machine type holons, and the coordination holon. Individual holons decide their actions through cooperating with other holons. Effective cooperation is therefore critical to achieve high system performance. This, however, is mostly an open issue.
Lagrangian relaxation is a mathematical optimization technique to optimize an objective function subject to various constraints. It is inherently a decomposition and coordination method, where a complicated problem is decomposed into many simple subproblems by relaxing coupling constraints using Lagrange multipliers. Through iteratively solving the subproblems and updating the multipliers, globally near-optimal solutions can be obtained. We have developed Lagrangian relaxation methods for the scheduling of several generic manufacturing environments.
The decomposition and coordination nature of the Lagrangian relaxation technique makes it an ideal candidate (as opposed to other methods such as branch-and-bound) to be mapped onto the holonic architecture. Subproblem solving is performed by individual product and machine type holons, and is coordinated through multipliers which are updated by the coordinator holon. The Lagrangian relaxation technique thus provides a theoretical foundation for guiding the cooperation among holons, leading to globally near-optimal performance.
CONCLUDING REMARKS
It is very easy to under-estimate the importance of the holonic concept. Is it a new jargon that will be fashionable for one or two years and then gradually dies out? A precious lesson can be learned by comparing conventional programming languages (e.g., Basic, FORTRAN, Pascal, and C) versus object-oriented programming languages (e.g., C++ and Smalltalk). The encapsulation of data and methods within individual objects (autonomy) and the clear delineation of responsibilities and relationships among them (cooperation) make objective-oriented languages the overwhelmingly preferred languages for new software development in recent years. The lack of proper coordination within a shop or within an enterprise could be a nightmare for all who are involved. Holonic concept points a very promising direction to meet this challenge. It is therefore conceivable that the holonic will become the dominating trend for manufacturing and beyond, whatever name it is called. There are, however, major issues to be resolved. We will continue the efforts towards the theoretical understanding and practical implementation of HMS.
ACKNOWLEDGMENTS
The research was supported in part by a joint grant from National Science Foundation (DDM 9311994), United Technology Corporation (UTC), and UConn's Advanced Technology Center for Precision Manufacturing. The authors would like to thank Mr. R. S. Lopatka and Dr. J. M. Oblak of UTC, and Dr. S. Tamura and Mr. T. Hasegawa of Toshiba.
REFERENCES
2. Suda, H., "Future Factory System Formulated in Japan," Parts 1 and 2, Techno Japan, Part 1: Vol. 22, No. 10, October 1989, pp. 15-25; Part 2: Vol. 23, No. 3, March 1990, pp. 51-61.
3. Nagel, R. N., and R. D. Dove, 21st Century Manuf. Enterprise Strategy, PA: Iacocca Institute, Lehigh Univ., 1991.
4. Koestler, A., The Ghost in the Machine, The Macmillan Company, 1968.
5. Luh, P. B., and D. J. Hoitomt, Scheduling of Manufacturing Systems Using the Lagrangian Relaxation Technique, IEEE Transactions on Automatic Control, Vol. 38, No. 7, July, 1993, pp. 1066-1080.
6. Hasegawa, T., L. Gou, S. Tamura, P. B. Luh, and J. M. Oblak, "Holonic Planning and Scheduling Architecture for Manufacturing," Intnl. Working Conf. on Cooperating Knowledge Based Systems, Keele, U.K., June 1994, pp. 125-139.
7. Gou, L., T. Hasegawa, P. B. Luh, S. Tamura, and J. M. Oblak, "Holonic Planning and Scheduling for a Robotic Assembly Testbed," Proceedings of the Fourth International Conference on Computer Integrated Manufacturing and Automation Technology, Troy, NY, October 1994, pp. 142-149.