Jean-Claude Gentina

A New Cyclic Scheduling Algorithm for Flexible Manufacturing Systems

Flexible manufacturing system control is an NP-hard problem. A cyclic approach has been demonstrated to be adequate for an infinite scheduling problem because of maximal throughput reachability. However, it is not the only optimization criterion in general. In this article we consider the minimization of the work in process (WIP) as an economical and productivity factor. We propose a new cyclic scheduling algorithm giving the maximal throughput (a hard constraint) while minimizing WIP. This algorithm is based on progressive operations placing. A controlled beam search approach has been developed to determine at each step the schedule of the next operations. After presenting the main principles of the algorithm, we compare our approach to several most known cyclic scheduling algorithms using a significant existing example from the literature.

A method for hierarchical modeling of the command of flexible manufacturing systems

The present paper focuses on the modeling of the command and proposes a hierarchical and modular approach which is oriented on the physical structure of FMS. The requirements issuing from monitoring of FMS are discussed and integrated in the proposed model. Its modularity makes the approach open for extensions concerning as well the production resources as the products. As modeling tool, we have chosen object Petri nets. The first part of the paper describes desirable features of an FMS command such as safety, robustness, and adaptability. As it is shown, these features result from the flexibility of the installation. The modeling method presented in the second part of the paper begins with a structural analysis of FMS and defines a natural command hierarchy, where the coordination of the production process, the synchronization of production resources on products, and the internal coordination are treated separately. The method is rigorous and leads to a structured and modular Petri net model which can be used for FMS simulation or translated into the final command code. >

A framework to design a distributed diagnosis in FMS

When a failure occurs in a flexible manufacturing system (FMS), it is crucial to react quickly in order to maintain the productivity of the system. This requirement leads us to introduce the concept of on-line monitoring. The basic idea is the automation of the procedures of detection, diagnosis and error recovery. In this paper, reasoning mechanisms are proposed for the implementation of an on-line diagnostic system. These mechanisms are based on a distributed processing of symptoms that enables the problem of real-time constraint to be solved. They implement two kinds of reasoning: temporal and an hypothetical.

Formal approach of fms cyclic scheduling

This correspondence is related to the determination of both control and scheduling of flexible manufacturing systems under cyclic command. Different approaches can be found in the literature, but we focus on those which respect the optimal throughput while minimizing the work in process. So, we recall methods of performance evaluation developed during the last 20 years. The last part is devoted to a new approach of cyclic scheduling using a Petri net. This method uses algebraic tools (dioids) developed for the study of marked graphs. In this way, the problem of the scheduling is progressively transformed into a problem of the search of solution(s) on a system of equations

Petri net modeling of ratio-driven flexible manufacturing systems and implications on the WIP for cyclic schedules

In this paper we present a systematic approach of modelling ratio-driven flexible manufacturing systems with Petri nets. We focus on the impact of universal or dedicated transport resources and the way the operation sequences are modeled as closed loops. For a particular problem setting, minimizing the necessary work in process (WIP) while saturating the critical machines, we show that the modeling approach based on timed event graphs, in the way it is commonly used, may induce unnecessary constraints on the WIP. In order to overcome this problem we propose a new technique for the modeling of production ratios and establish tight lower bounds for the WIP, giving thereby a measure for the performance increase obtained.

Hierarchical and dynamic production control in flexible manufacturing systems

Abstract Job shop control involves directing the flow of products through the manufacturing system. Centralized and decentralized approaches have been proposed. This paper introduces first a mix approach in the organization of real time control level in flexible manufacturing systems: a hierarchical organization permits coordination of distributed decision centres and heterarchical consideration confers a partial autonomy to sub-level decision centres. Second, corrective actions against unexpected events are introduced to maintain as far as possible the forecasted scheduling.

A heuristic algorithm for the computation of cyclic schedules and the necessary WIP to obtain optimal cycle time

Presents a heuristic algorithm to determine a cyclic schedule with optimal cycle time while minimizing the work in process (WIP). The respect of the optimal cycle time is a hard constraint during the computation of the schedule, while the minimization of the WIP is considered as an objective. The algorithm builds the schedule progressively and its complexity allows the use under real-time constraints.<<ETX>>

Transformation of the cyclic scheduling problem of a large class of FMS into the search of an optimized initial marking of a linearizable weighted T-system

We show that Petri net is a pertinent model to compute analytically the cyclic scheduling of flexible manufacturing system (FMS) with shared non-preemptive resources and assembly or disassembly of multiple components. The proposed method is based on two techniques: 1) the linearization of Petri net conflicts on resources; and 2) the linearization of weighted T-system will give a deterministic marked graph if a specific condition of normalization of the transitions is satisfied. We show that it is possible to compute the best cyclic scheduling in p steps with respect to the cycle time by means of an optimal (min,+) set of linear equations. By using (max,+) algebra it is possible to discriminate among the candidate solutions, the solutions giving the minimum time cycle.

Synchrony theory applied to control problems in flexible manufacturing systems

Within the synchrony theory, the fairness concept plays an important role in the analysis of conflict resolution in Petri net models. We apply this concept to a class of conflicts in flexible manufacturing systems and develop a simple regulation scheme that ables to integrate constraints on the mean outcome of the conflicts.<<ETX>>

State dependent release control in flexible manufacturing systems

In this paper we address the problem of release control in a flexible manufacturing system subjected to a ratio-respect constraint. We show how this constraint can be included into a product-oriented stochastic Petri-net model and evaluate on an example the effect on the system throughput of the relaxation of an instantaneous ratio-respect constraint. Conflicts are solved according to decision rules and the evaluation is carried our through the calculation of the underlying Markov chain.<<ETX>>