Overview
Introduction
Market demand and environmental societal pressures require effective manufacturing systems to adapt themselves at an ever-increasing pace. Real life situations can be very unpredictable. For some type of problems, there isn't a fixed solution, so the system needs to be able to take the right decisions in every unique situation.
This creates the need for novel manufacturing control systems that are able to manage production change and disturbances both effectively and efficiently. These systems need to be able to constantly adapt to a lot of different circumstances and evolve according to changing needs.
To meet these new requirements, several new manufacturing paradigms, are being investigated, amongst which is the holonic manufacturing paradigm. This architecture, called PROSA (Product-Resource-Order-Staff Architecture), shall enable easy (self-)configuration, easy extension and modification of the system. In other words, the system allows to add, remove and modify system components during system operation. Also, it allows more flexibility which minimises the need to adapt by maximising their range of 'normal' situations.
After a brief introduction into the holonic manufacturing concept, this page first discusses the structure
of the Holonic Manufacturing System architecture: the concept, the components, their responsibilities and their interactions. The next section motivates this design by comparing it to existing architectures. After this, more detailed aspects of the architecture are discussed: aggregation, specialisation, staff holons and self-similarity. The architecture introduces significant innovations for the way manufacturing systems are driven, which are discussed in the next section. Finally, an overview is given, with links to more detailed pages about the different aspects of the HMS architecture.
Holonic manufacturing concept
The word holon consists of 2 parts: 'holos' = whole and the suffix '-on' which suggests a part.
A holon is a system that is a whole in itself as well as a part of a larger system. It can be conceived as systems nested within each other. Every system can be considered a holon, from a subatomic particle to the universe as a whole. Everything is part of something, and can be viewed as having parts of its own.
The HMS consortium translated 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. It preserves the stability of a hierarchy while providing the dynamic flexibility of a heterarchy.
The HMS consortium developed the following list of definitions to help understand and guide the translation of holonic concepts into a manufacturing setting:
- 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.
Basic holons
There are three types of basic building blocks in a holonic manufacturing system (HMS): product holons, resource holons, and order holons (Figure 1).
Figure 1: Basic Holons
A product holon holds the process and product knowledge to ensure the correct fabrication of the product with sufficient quality. It provides consistent and up-to-date information to the other holons about the product life-cycle, user requirements, design, and process plan and bill of material.
A resource holon consists of a physical part, namely a production resource in the HMS and of an information processing part that controls the resource. It offers production capacity and functionality to the surrounding holons. It holds the methods to allocate the production resources and the knowledge and procedures to organise, use and control these production resources to drive production. Examples of resource holons are machines, furnaces, conveyors, pipelines, pallets, components, raw materials, tools, tool holders, material storage, personnel, energy, floor space, etc.
An order holon represents a manufacturing order. It is an active entity responsible for performing the work correctly and on time. It explicitly captures all information and information processing of a job.
For a minimalistic implementation of a manufacturing system, it suffices to have a holarchy consisting of these three basic holon types. When an order holon arrives in the system, it will first discover what it needs via the respective product holon. The order holon will negotiate with all relevant resource holons to have itself produced by them. As such, the order holon takes care of the logistical aspects (the resource allocation). When an operation starts, the order holon lets the product holon and the resource holons co-operate to perform the technical part of the operation.
The main contribution of this basic control architecture is to get, eventually, everything manufactured in the face of disturbances, uncertainty and change, but with limited guarantees about when and where exactly.
Read more about the interaction between the basic holons...
Aggregation
Interaction between a large number of low-level agents results in a complex system behavior which is difficult to understand, to control and to predict. Structuring the agents in a hierarchy is the appropriate solution to tackle this complexity.
Therefore, aggregated holons are defined as a set of related holons that are clustered together and form on their turn a bigger holon with its own identity. As such, an aggregation hierarchy is formed, which is open-ended at the top and at the bottom. Depending on the study scope of the observer, holons are split up into their sub-holons or treated as a whole.
The aggregation hierarchy is not necessarily a tree-shaped one: holons may belong to multiple aggregations, e.g. a tool can be shared between several workstations. Aggregated holons are no static sets of holons, but can dynamically change their contents depending on needs of the system. Aggregated holons may emerge out of the self-organizing interaction of holons or they may be designed up front. The number of hierarchical levels depends on the specific needs of a certain system, and is not dictated by the architecture.
Specialisation
Specialisation is a concept which is derived from object-oriented methodologies. Already a specialisation took place by separating the basic holons into three kinds: order holons, product holons and resource holons. In a specific architecture, these basic holons may still be too abstract to reason about their place in an architecture. Specialisation can then be used to differentiate between the different kinds of holons. For instance, for the resource holarchy, usually transport holons and production holons like workstations are very different types of resources.
Staff holons
The HMS architecture foresees in the possibility to provide staff holons to assist the basic holons in performing their work. The staff holon is considered as an external expert that gives advice to the basic holon. The basic holon is still responsible for taking the final decision.
When disturbances are absent, the basic holons will follow the advice of the staff holons as well as possible and the system performance may be optimised. However, when disturbances or changes occur in the system and if this will result a bad performance, the basic holons may ignore the advice and take independent actions to do their work.
Example:
The centralised scheduler of a shop, as shown in figure 2, is an example of a staff holon. It has an overview of all resources and all orders, it generates an optimal schedule and it gives this schedule as advice to the individual order and resource holons and to the holons.
Figure 2: A centralised scheduler is a staff holon to the order holons and resource holons.
Other examples of staff holons are on-line shop floor control holons, (central) process sequence planners, CAD-systems, and even MRP-systems.
The Holonic Manufacturing Architecture
Holonic architectures were introduced to provide an answer to some shortcomings in existing control system architectures. Two architectural styles are frequently used: the hierarchical and the heterarchical architectures.
- Hierarchical architectures introduce levels of control and have a pyramidal structure. There are strict master-slave relationships between the elements. Control decisions are operated top-down and status is reported bottom-up.
- Advantages: High and predictable performance.
- Disadvantages: Local controllers are helpless when cut off from their directing supervisors. This makes realising robustness very difficult and increases the coupling between the different elements. Also, decision making is often based on obsolete information.
- Heterarchical architectures allow for full autonomy between the elements in the architecture. The cooperation between elements is arranged via an explicit negotiation procedure (e.g. contract-net protocol) or via indirect coordination. Full local autonomy is maintained during the cooperative process.
- Advantages: Increase of robustness. Because elements function autonomously, they should not fail when other elements malfunction.
- Disadvantages: Reduced predictability of the control system, possible incompatibility issues and no opportunities for optimisation.
The holonic architectural style contributes to the responsiveness of the control system. This architectural style combines the advantages of both the hierarchical and the heterarchical architectural style and is able to handle the disadvantages.
- High and predictable performance (hierarchical)
- robustness against disturbances and the agility (heterarchical)
- A holon functions as part of a bigger whole, forming a hierarchy with other holons for a period of time. Higher-level holons advise the lower-level holons, which is an advantage in handling disturbances.
Self-similarity
All holons inherit a common interface and behavior from the basic resource, product or order holons. Holons of the same type have similar interfaces and behavior. Horizontal self-similarity relates to holons with different specialisations on one level of aggregation. Vertical self-similarity means that resource holons on a higher level in the hierarchy work similar to lower level resource holons.
Innovations
The HMS architecture introduces significant innovations.
Structure is decoupled from control algorithms and technical aspects are decoupled from logistical aspects. This means that different holons don't need to know all details of orders when they communicate to each other. When an order holon has to perform an operation on a certain part, it doesn't need to know how to do this operation. Instead it only needs to know which resource holon can perform the operation, then it passes the order to the resource holon which will know how to execute the order. All logistical matters are handled by the order holon, so the resource holon doesn't need to know details of the order.
The decoupling in the PROSA reference architecture should enable an intensive reuse of sub-systems in a wide range of manufacturing systems. This will lead to higher-quality implementations of these sub-systems, resulting in increased reliability, increased performance, reduced installation and maintenance costs, and a higher development pace.
The decoupling of structure from control allows to reuse structural modules in a different logistic chain, just by only replacing the control algorithms; the decoupling of technical from logistical aspects allows to reuse control algorithms in logistically similar factories just be replacing the product holons; default functions, such as the deadlock prevention mechanism, can be reused in all systems, but can eventually be overwritten by system-specific solutions if required.
Conclusion
Future manufacturing systems need to cope with frequent changes and disturbances. As such, their control requires constant adaptation and high flexibility. Holonic manufacturing is a highly distributed control paradigm that promises to handle these problems successfully. It is based on the concept of autonomous co-operating agents, called ‘holons'. These holons are particles of the holonic reference architecture for manufacturing systems, which is being developed at PMA-KULeuven.
The HMS architecture PROSA (Product-Resource-Order-Staff Architecture) consists of three types of basic holons: resource holons, product holons and order holons. Every basic holon type focuses on different responsibilities of the manufacturing system. The holons exchange process knowledge, production knowledge, and process execution knowledge respectively.
The basic holons are structured using the object-oriented concepts of aggregation and specialisation. Aggregation is used to focus on different levels of holons. Specialisation is used to focus on different functionalities of holons.
Staff holons can be added to assist the basic holons with expert knowledge. Staff holons are optional elements which may assist the basic holons with expert knowledge to help them perform their task. Staff holons permit the incorporation of centralised solutions. This is useful for problems for which there does not exist a distributed solution, and it allows easy migration from current hierarchical systems to holonic solutions. Since staff holons are only giving advice to the basic holons, they do not introduce hierarchical rigidity into the system. A descriptive model of the data, functions, and behaviors of the holons, shows more detailed aspects of the architecture.
After comparing PROSA with existing manufacturing control approaches, it is concluded that PROSA is covering all aspects of both hierarchical and heterarchical control architectures. As such, PROSA can be regarded as a generalisation of the two former approaches.
The resulting architecture has a high degree of self-similarity, which reduces the complexity to integrate new components and enables easy reconfiguration of the system. Due to the horizontal self-similarity, special cases of orders, products, and resources can be handled similar to nominal cases. The vertical self-similarity avoids the need for strict hierarchical levels. It allows for individual resource holons to belong to several holarchies at several hierarchical levels.
More importantly, PROSA also introduces significant innovations: the system structure is decoupled from the control algorithm, logistical aspects can be decoupled from technical ones, and, and PROSA opens opportunities to achieve more advanced hybrid-control algorithms.
See also
- Read the original article or download the pdf.
