Jean-Marie Proth

F. Dicesare

George Harhalakis

Manuel Silva-Suarez

F. Vernadat

M. Silva Significant changes have been occurring in industrialized countries since the Second World War. Production is moving towards sophisticated high qUality products, economy of scale has been replaced by economy of scope, jerky demands are progressively replacing steady demands, and competi tiveness is becoming a worldwide phenomenon. These trends require highly automated manufacturing systems with small set-up times and high flex ibility. As a consequence, implementation and running costs of modem manufacturing systems are drastically increasing, whereas their fields of application remain limited, and every day become even narrower, which increases the risk of early obsolescence. This is the reason why designers are trying to improve the preliminary design phase, also known as the 'paper study phase'. The preliminary design phase includes, but is not limited to, the func tional specification, and the evaluation of the system. Many tools exist to support the functional specification of manufactur ing systems. IDEFO is one of these tools. It leads, using a top-down ap proach, to a precise functional description of the required system. However, its use cannot be extended further. In general, the evaluation starts with a modeling step, which depends on the evaluation tool used, and ends by applying the model to find out its main dynamic characteristics. Two main approaches can be used to perform this task, namely simulation and math ematical approach. Using simulation, the modeling tool is either a classical computer language, or a simulation language.

Practice of Petri Nets in Manufacturing

MengChu Zhou

Petri Net Synthesis for Discrete Event Control of Manufacturing Systems develops two essential resource-sharing concepts: parallel and sequential mutual exclusions and theoretical results in Petri synthesis. A parallel mutual exclusion (PME) is defined to model a resource shared by independent distributed processes, and a sequential mutual exclusion is a sequential composition of PMEs, modeling a resource shared by sequentially-related processes. A hybrid synthesis methodology for Petri net models and controllers is proposed using top-down, modular, and bottom-up design ideas and the mutual exclusion theory. An aggregate Petri net model is refined by replacing places and /or transitions with basic design modules which are mathematically and graphically described. Petri net design methods are presented for such buffers as automatic storage and retrieval systems. Using the proposed method synthesizes both Petri net structure and feasible initial markings, guaranteeing that resulting Petri nets have desirable system properties such as freedom from deadlock and cyclic behavior. A Petri net controller is extended to error recovery for automated manufacturing systems. The theory can guarantee that the desired system properties achieved by the original design will be preserved when the controller is augmented to deal with an error in the prescribed methods. Control code has been directly generated from Petri net definitions. The algorithm and implementation details are given for a flexible manufacturing system. Using the approach presented in Petri Net Synthesis for Discrete Event Control of Manufacturing Systems, engineers and research workers can develop their own discrete event control applications and experiments.

F. Dicesare

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