PMA - Production Engineering, Machine Design and Automation
How to refer to this page:
Bongaerts, 1998, "Integration of Scheduling and Control in Holonic Manufacturing Systems", Ph.D. Thesis PMA/K.U.Leuven, Chapter 3.

Concepts for Holonic Manufacturing

The Roots of Holonics

The term "holonic" is derived from the word "holon", which was introduced by Koestler (1967). The word holon is a combination from the Greek holos = whole, with the suffix -on which, as in proton or neutron, suggests a particle or part. Two observations impelled Koestler to propose this word.

The first observation is from Herbert Simon (1969) who concludes, from his ‘parable of the two watchmakers,’ that complex systems will evolve from simple systems much more rapidly if there are stable intermediate forms than if there are not. Simon’s analysis reveals why every complex adaptive system is hierarchic (in a loose sense).

The second observation, made by Koestler while analysing hierarchies and stable intermediate forms in living organisms and social organisations, is that -- although it is easy to identify subwholes or parts -- "wholes" and "parts" in an absolute sense do not exist. This made Koestler propose the word holon to describe the hybrid nature of subwholes/parts in real-life systems; holons simultaneously are self-contained wholes to their subordinated parts, and dependent parts when seen from the inverse direction.

Koestler also points out that holons are autonomous self-reliant units, which have a degree of independence and handle contingencies without asking higher authorities for instructions. Simultaneously, holons are subject to control from (multiple) higher authorities. The first property ensures that holons are stable forms, which survive disturbances. The latter property signifies that they are intermediate forms, which provide the proper functionality for the bigger whole.

Finally, Koestler defines a holarchy as a hierarchy of self-regulating holons which function (a) as autonomous wholes in supraordination to their parts, (b) as dependent parts in sub-ordination to controls on higher levels, (c) in co-ordination with their local environment.

Holonic Manufacturing Systems

Holonic manufacturing originated in the framework of the Intelligent Manufacturing Systems (IMS) programme. IMS probably is the largest research programme ever launched on manufacturing. Prof. Yoshikawa from Tokyo University originally proposed it with the objective of creating a manufacturing science that can meet the needs of the next century. IMS was conceived as a ten-year precompetitive basic research programme, supported by the governments of the major industrialised countries, for their industries and academics to participate. In 1993-1994, Australia, Canada, the EC, EFTA, Japan, and the US have undertaken a feasibility study for IMS, which comprised six subprojects, or so-called "test cases" (Hayashi, 1993). Since then, international co-operation continued with varying intensity. Since 1997, Europe again fully participates in the IMS projects.

The fifth IMS test case was entitled ‘Holonic Manufacturing Systems: system components of autonomous modules and their distributed control,' also called HMS. After the IMS feasibility study, research on holonic manufacturing has mainly continued in Japanese domestic IMS projects and in some nationally funded projects in the rest of the world. K.U.Leuven (the PMA division) continues its efforts in a nationally funded research project called GOA/HMS -- Concerted Research Action on Holonic Manufacturing Systems. Lasting 4 years, this project combines PMA’s knowledge in flexible shop floor control (Valckenaers, 1993, 1995), non-linear process planning (Detand, 1993), reactive scheduling (Valckenaers, 1994c, 1995, Bongaerts, 1994), and machine controllers (Kruth, 1994b) to develop a holonic architecture for production systems and implement two prototypes or testbeds. In 1997, HMS became one of the first fully endorsed IMS projects. Currently, K.U.Leuven participates in the International HMS Consortium via HANDS, the work package on assembly.

The task of the HMS consortium is to translate the concepts that Koestler developed for social organisations and living organisms into a set of appropriate concepts for manufacturing industries. The goal is to attain in manufacturing the benefits that holonic organisation provides to living organisms and societies, i.e., stability in the face of disturbances, adaptability and flexibility in the face of change, and efficient use of available resources. The HMS concept combines the best features of hierarchical and heterarchical organisation (Dilts, 1991). It preserves the stability of hierarchy while providing the dynamic flexibility of heterarchy.

During the feasibility study, the HMS consortium developed the following list of definitions to help understand and guide the translation of holonic concepts into a manufacturing setting (Valckenaers, 1997):
· Holon: An autonomous and co-operative building block of a manufacturing system for transforming, transporting, storing and/or validating information and physical objects. The holon consists of an information processing part and often a physical processing part. A holon can be part of another holon.
· Autonomy: The capability of an entity to create and control the execution of its own plans and/or strategies.
· Co-operation: A process whereby a set of entities develops mutually acceptable plans and executes these plans.
· Holarchy: A system of holons that can co-operate to achieve a goal or objective. The holarchy defines the basic rules for co-operation of the holons and thereby limits their autonomy.
· Holonic manufacturing system: a holarchy that integrates the entire range of manufacturing activities from order booking through design, production, and marketing to realise the agile manufacturing enterprise.
· Holonic attributes: attributes of an entity that make it a holon. The minimum set is autonomy and co-operativeness.

Additional sources of inspiration

While the roots of the HMS concept are insights provided by Koestler (1967) in the nature of real-life complex adaptive systems, PMA has been looking for additional sources of inspiration. Waldrop (1992) presents a number of fundamental properties of complex adaptive systems, like autocatalytic sets, positive feed-back and the coupling between system development and the future environment in which such system will reside. Van Brussel et al. (1995) investigate how the understanding of these mechanisms may contribute to the design and development of complex adaptive systems in manufacturing.

The emergence of complex adaptive systems requires the existence of an autocatalytic set: a set of elements that catalyses the creation of its own elements. In biology, this property refers to the generation of offspring. In manufacturing and information technology, it means that software that has gained an early and wide market share, gets early and plenty support for the further development and testing of that software, and for that reason alone becomes better than the competitive products.

The autocatalytic set is a system characterised by positive feedback. The presence of its members increases the rate at which new set elements are created, which in turn increases this rate even further. An undesirable consequence of positive feedback, which cannot be ignored, since it has significant impact, is lock-in: the system becomes locked in the solution that it selects first; a huge effort is required to switch at a later instant. The system becomes history-dependent or path-dependent. Developers of complex artefacts must be aware of lock-in. However, they are facing a dilemma: early solutions that are "marginally-good-enough" and manage to become member of an autocatalytic set, ordinarily become winners in a competitive world. Developers are well aware that this results in sub-optimal solutions, but do not have an established methodology to avoid it.

The coupling between the development of a system and its future environment is a third factor that influences the development of complex adaptive systems. The future environment of a complex system may depend on the success that system will have. While (ab)using this principle may lead to success (lock-in) of that system, optimising the goals of individuals to the detriment of global goals, understanding the mechanisms behind it may protect the customers or competitors of the locked-in solution against the drawbacks of lock-in. This reasoning lead to design principles that preserve flexibility, like 'low and late commitment' (Valckenaers, 1993).

Practical requirements for holonic manufacturing

As reasoning on autonomy, co-operation and complex adaptive systems may seem quite abstract, work on holonic manufacturing has considered from the beginning how these concepts lead to specific requirements for manufacturing systems.

From the outset, the prevalent software technology to implement the concepts of holonic manufacturing appeared to be intelligent co-operating agents, also called multi-agent systems (Wooldridge, 1995, Stirling, 1993, Rao, 1991). Multi-agent systems are also used in heterarchical control, and provide the software with opportunities for taking the initiative to take autonomous decisions.

Secondly, the hope to combine the best of hierarchical and heterarchical control formed a practical inspiration source for the research on holonic manufacturing. Holonic manufacturing systems shall combine the high and predictable performance promised by hierarchical systems with the robustness against disturbances and the agility of heterarchical systems.

Thirdly, holonic manufacturing shall address the problem of rising costs for the development and maintenance of complex software. It shall avoid the rigidity of hierarchical systems and shall fully support the system evolution to comply with changing requirements (e.g. new products, new or evolving technologies, unpredictable demands). Reuse of components shall reduce development costs, improve software quality and ease the migration towards a new paradigm like holonic manufacturing. Consequently, reconfigurability of the HMS is an important aspect.

Fourthly, to ease the operation of a holonic manufacturing system, this latter needs to be self-configuring, learning and self-organising. The increased flexibility resulting from an agile and reconfigurable manufacturing system may put a high load on the system operators, such that the holons in a HMS shall assist the operator to control the system: holons shall autonomously select appropriate parameters settings, find their own strategies and build their own structure. Fifthly, holonic manufacturing shall preserve a place for the human in the system, since he/she is the most flexible, and intelligent component in the system.

Sixthly, holonic manufacturing shall consider an evolutionary approach to implement all the above requirements. Since the requirements are quite ambitious, it is more pragmatic to plan intermediate steps towards the fully intelligent manufacturing system. This provides a smoother migration path towards holonic manufacturing and ensure the ability of the system to support continuous adaptation, migration and evolution.

Copyright 1996, Katholieke Universiteit Leuven
Page design: Jo Wyns
Information provider: PMA Division KULeuven
Comments for the authors: Jo Wyns
http://www.mech.kuleuven.ac.be/pma/project/goa/concepts.htm