Distributed Control of Flexible Manufacturing Systems: Implementation of a Specialized Multi-Agent Middleware and Applications of Holonic Concepts
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This PhD thesis reports on the application of the principles of distribution and autonomy from the multi-agent and holonic paradigms to smaller flexible assembly systems. In this specific context, available software technologies were evaluated to be not suitable, and a specialized multi-agent, holonic middleware has been implemented. This thesis presents, therefore, both the application of theoretical concepts, and the implementation and the use of a specific software technology. In order to deal with the constraints of volume, price, production time and product quality, a high degree of automation has become a necessity for many shop-floors. However, compared to manual work, current automation solutions largely reduce the flexibility and reconfigurability of the production systems. Therefore, they increase the cost of product introduction, and changes in the processes or product range. Although it has been reported by many researchers to be the most promising solution, flexible and reconfigurable manufacturing system leads to an increase in the complexity of the equipment and control systems. A strategy to handle the increased complexity is to distribute the control systems. However, distribution adds complexity by itself and throughout the years, several paradigms have been proposed to structure the distribution. Currently the most common paradigms in manufacturing control research are the Multi-Agent System and the Holonic Manufacturing System. Both are based on the principles of autonomy and partial knowledge of the distributed components. Many technologies are available, to implement these physically distributed control systems. Shop-floors have traditionally been dominated by field-buses, but Ethernetbased solutions and TCP/IP are increasing their presence. TCP/IP is a solution for soft real-time communication, which benefits from a highly competitive environment with a flora of software systems available. Available software technologies to develop distributed control systems can be classified in two categories: communication middleware and Multi-Agent Systems. In the context of control systems for smaller flexible manufacturing systems, the available software technologies were evaluated as not been adapted. Communication middlewares are low level and Multi-Agent Systems focus on running a high number of agents in one container while physical distribution and device integration are the important aspects. A specialized holonic middleware was therefore developed. The holonic middleware is a thin holonic layer over a proven communication system. It is different from mainstream Multi-Agent Systems in that it supports multiple programming languages and operating systems, and does not depend on a specific application platform. It is therefore an efficient and practical solution from integration of manufacturing devices to high level control and scheduling. The middleware is open source and available for everyone to use and evaluate. Five applications have been realized to validate the concepts and the software technology. The applications were implemented through separate research projects, with different industrial partners and addressing different aspects of flexible assembly systems. The applications are composed of three semi-industrial assembly systems, a prototype AGV system and an emulation framework. All applications are implemented using the developed holonic middleware. The assembly systems apply the autonomy and partial knowledge concepts to varying degrees, while the AGV system is a typical holonic system exhibiting emergent behaviour. The emulation framework uses the holonic middleware to expose the emulated devices to the network, thus it can be used to run simulations using the exact same control system as the industrial applications. Through the application of distribution and autonomy in the presented applications, this thesis has further demonstrated the feasibility and advantages of the multiagent and holonic principles in the context of flexible manufacturing. In addition, the holonic middleware has been established to be a stable and practical solution to develop semi-industrial manufacturing systems. The variety of the numerous applications presented shows the flexibility and adaptability of the concept and the holonic middleware. Finally, through the development of more complex applications, like the AGV system and the assembly cell for machine tools, the ability of the chosen approach to handle the complexity of flexible manufacturing systems has been demonstrated.