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On decentralized on-line scheduling of FMS
Our work is concerned with the scheduling problem for medium sized (5-10 machines or workplaces) flexible manufacturing systems which are part of a larger production process. A two-layer control structure for the overall control problem is considered. The general philosophy is to divide the scheduling problem into a number of local decentralized problems: the optimal sequencing and routing of the jobs within flexible manufacturing system or small workshops. The subsystems are coordinated by an upper level scheduler. It assignes the due dates for the completion of operations in the subsystems and determines the earliest possible starting times from the dependencies among the operations and the externally determined starting conditions. The predominant goal of the local on-line scheduling algorithms must be the minimization of the lateness of the local jobs, despite the inevitable disturbing factors. The better this goal is achieved (under the restriction of efficient use of theavailab le ressources), the easier is the coordination of the subsystems. We therefore investigate in this paper the behaviour of local on-line scheduling algorithms. The local control laws are based on priority rules augmented by look-ahead strategies. Their effectivity is compared for a real-world example.