Etienne Craye

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.

Optimal Control of a Nested-Multiple-Product Assemble-to-Order System

In this paper, we study an assemble-to-order system consisting of n products assembled from a subset of m distinct components where the products have a modular nested design, i.e. product i has only one additional component more than product i − 1. In particular, we study the optimal production and inventory allocation policies of such systems. Components are produced on independent production facilities one unit at a time, each with a finite production rate and exponentially distributed production times. The components are stocked ahead of demand and therefore incur a holding cost per unit per unit of time. Demand from each product occurs continuously over time according to a Poisson process. The demand for a particular product can be either satisfied (provided all its components are available in stock) or rejected. In the latter case, a product-dependent lost sale cost is incurred. In this situation, a manager is confronted with two decisions: when to produce a component and whether or not to satisfy an incoming product order from on-hand inventory. We show that, for the production of a component, the optimal policy is a base-stock type where the base-stock level depends on all other components’ inventory. We also show that, for inventory allocation, the optimal policy is a multi-level rationing policy where the rationing levels depend on all other components’ inventory. We propose a simple heuristic that we numerically compare against the optimal policy and show that, when carefully designed, it can be very effective.

Toward an Implementation of Recovery Procedures for FMS Supervision

Abstract This paper first specifies Recovery and gives some trends about Recovery strategies. Then, it focuses on Reconfiguration. Several reactivity levels are defined according to the failure impact, flexibilities reserved during the exploitation phase, and flexibilities corresponding to the whole potentialities of the architecture. These enable to know if the FMS can react with its current configuration, or if its configuration has to be changed to go on with the production, or if the production has to be changed to maintain FMS availability. The Recovery procedures are implemented through a model named Operational Accessibility Graph, that has been emphasized by the definition of new attributes and methods. Some algorithms, based on the graph theory, are presented.

Robust control of multi-product job-shops in repetitive functioning mode

This work deals with the robustness of dynamic discrete event systems including time interval constraints. It presents a method to compute the passive robustness by the decomposition of the P-time Petri net model into four sets. A computing algorithm of a lower bound of the maximal time disturbances allowed on a given point is provided. Also, we focus on the active robustness by giving some definitions and lemmas. Finally, we present a theorem containing a local sufficient condition for the active robust control use.

A functional graph approach for alarm filtering and fault recovery for automated production systems

Alarm processing is one of the functions of the monitoring and the supervision of automated productions systems. However, these systems become more and more complex. Actually, this induces that standard alarm processors do not manage efficiently all the alarms that occur. This paper deals with intelligent alarm processing system (IAPS). The goal of this type of system is to reduce the number of alarms that are presented to the operators and to assist them to select corrective actions in case of fault occurrence. In this paper, we propose a prototype developed under PROLOG. Different concepts such as validation, causal inhibition or synthesis alarms characterize this prototype features. The filtering process is independent of the nature of the process (continuous or discrete) and of the way the raw alarms are produced.

Estimation of failure probability of milk manufacturing unit by fuzzy fault tree analysis

In conventional fault tree analysis, probabilities of the basic events were treated as precise values; sometimes this probability does not correspond to real situations of systems because of ambiguity and the imprecision of some basic events. In order to overcome this disadvantage, a new fuzzy probabilistic method, based on the fuzzy set theory and the Buckley approach, is used in the fault tree analysis for quantifying the basic event uncertainty. According to failure modes, a fault tree of a milk manufacturing unit is constructed. Failure in the milk manufacturing workshop is analysed using the proposed method.

Toward an implementation of recovery procedures for flexible manufacturing systems supervision

Abstract This paper first specifies Recovery and gives some trends about Recovery strategies. Then, it focuses on Reconfiguration. Several reactivity levels are defined according to the failure impact, flexibilities reserved during the exploitation phase, and flexibilities corresponding to the whole potentialities of the architecture. These enable to know if the FMS can react with its current configuration, or if its configuration has to be changed to go on with the production, or if the production has to be changed to maintain FMS availability. The Recovery procedures are implemented through a model named Operational Accessibility Graph, which has been emphasized by the definition of new attributes and methods. Some algorithms, based on the graph theory, are presented.

Study of the diagnosability of automated production systems based on functional graphs

Functional graphs are a convenient representation that we have introduced to model automated production systems. They are useful for the monitoring and the supervision of manufacturing processes or other industrial processes, such as chemical processes. An approach based on relational theory and graph theory is presented in this paper. This approach allows to characterize formally structural properties of a functional graph and to map it into a set of relations translating all the complete paths existing in the initial graph. Two kinds of functional graphs are analyzed and algorithms exploiting their structures are presented. We introduce the concept of diagnosability as a system property that reflects the possibility to observe the behavior of a system with respect to faults. The diagnosability is defined and analyzed by means of computable states and mathematical relations. Propositions explaining causality relations between functions of a functional graph are given.

Tolerance evaluation of flexible manufacturing architectures

Within the framework of FMS dependability, this article aims to apply the concept of tolerance to flexible manufacturing architectures. This enables flexible architectures to be studied and the one that could best react if failure occurs to be chosen. The methods proposed here are based on a process representation. They could be used on a designed architecture in a feedback process: The analysis is performed off line in order to check whether the workshop is tolerant or the addition of some flexibilities can make the system more tolerant.

A Process Modelling for Supervision in FMS : Operational Accessibility Graph

Abstract Control supervision implies specific functions such as Monitoring, Working Modes Management and Piloting. To manage the complexity of FMS, the process has to be modelled. This paper deals with a process modelling in order to integrate works about supervision. The integration is based on the definition of a library that contains reference elements based on the concept of operation. These elements are used with the notion of accessibility in graph to get the final model. Some algorithms are proposed to exploit the model and examples are given for Recovery integration.