Laurent Piétrac

Multi-model approach to discrete events systems: application to operating mode management

In this paper, we propose a framework for designing suitable switching control decisions for discrete event systems (DES) whose structures change as they develop in different operating modes. Control decisions consist of either an event in a sequence to occur enabling an event or preventing the event from taking place disabling an event. Our contribution enables to adopt different modeling approaches and ensures switching between all designed process models when there is commutation between the operating modes. Thus, in the context of supervisory control theory (SCT), we propose that each model automaton represents process functionning in a specific operating mode. Specifications imposed on any operating mode could be conflicting. An attractive alternative is switching control, in which a different controller is applied to each operating mode [P. Charbonnaud, F. Rotella, S. Medar. Process Operating mode Monitoring Process: Switching Online the Right Controller, IEEE transactions on systems, Man and Cybernetics, Part C 31(1) pp 77-86. 2002; M. Zefran, J. Burdick, Design of switching controllers for systems with changing dynamics, in: Proceedings of the 37th conference on Decision and control, 1998, pp. 2113-2118]. Control of process functionning means that both process and specification models must be associated with one specific operating mode. Based on supervisory control theory, our work focuses on operating mode management in particular when the process is subject to failure. The adopted approach (multi-model) assumes that only one attempted operating mode is activated at any one time, while the others are considered desactivated. The problem of commutation and tracking between all designed models (process and specification) is formalised by the proposed framework. In this context, several questions are raised. Is the process engaged in a state which is compatible with the atteined mode ? Are the specifications consistant with each starting state ?. Are the specification conflicting ? Can all defined states be reachable ? To answer correctly these questions, a mode switching mechanism must be formalised.

A supervisor implementation approach in Discrete Controller Synthesis

We investigate the implementation of supervisors generated by symbolic BDD-based Discrete Controller Synthesis (DCS). The implementation technique proposed is able to solve both control non-determinism and the structural incompatibility introduced by symbolic DCS. We highlight and illustrate interesting structural properties of the supervisor implementation. Our technique is illustrated on a reallife example modeling a System-on-chip component: a serial to parallel converter.

Identification of incompatible states in mode switching

Mode management is one of the problems in discrete events systems control design. Even based on a simple specification, it is very difficult to prove that models of each mode and mutual interaction are correct. This paper demonstrates that supervisory control theory is an effective tool for detecting specification incompatibilities because it clearly separates process, models and specifications. We use simple cases to present a method that introduces flexibility into mode specification. This method can be used to adjust or to modify incompatibilities between specifications and thereby promotes correct mode switching.

Supervisory control based on multi-face modelling of discrete event systems

This paper reports a supervisory control design methodology based on the multi-face modelling of discrete-event systems in order to allow rapid prototyping and flexible implementation of controllers for reactive systems. Although Supervisory Control Theory assures that the closed loop system meets the prescribed requirements, it uses ordinary finite state machines as process models, which results in complicated and large-scale controllers. A new modelling methodology simplifies modelling by introducing functional models based on tasks, which allow the reduction of component models. The paper presents a multi-level supervisory control architecture for systems modelled in the framework and derives the properties of the overall control system. Propositions for the implementation of such supervisory architectures are also made.

Advantages and Drawbacks of the Logic Programme Synthesis using Supervisory Control Theory

Abstract Concurrent engineering, re-engineering and reactivity become more and more real in the design concept. Particularly in the design of automatic control efficiency is attempted in terms of quality, quickness, validation and dependability at the different development steps. Nevertheless more an industrial process is automatically controlled and more it is important to manage the performance of the control design. Several investigations are conducted today in the use of assistance tools in the design of proper control laws which will be applied to Programmable Logic Controllers (PLC). In another way, attentions emerged from the synthesis concept using language formalisms. The proposed idea here is to combine these two approaches, i.e. to persuade the designer that synthesis is able to assist the design of logic program. In that way, advantages and drawbacks of the logic program synthesis using supervisory control theory are here discussed. This paper includes two main parts, the first one is devoted to a brief description of the supervisory control particularly described in terms of ability in determining some important properties, the second one based on an applicative example leads to a discussion on the transfer ability of the theory.

A Design Method for Synthesizing Control-Command Systems out of Reusable Components

This paper investigates an industrial design issue related to code reusability: building control-command systems out of Commercial off the shelf (COTS) components. The design method proposed uses in synergy the formal verification (FV) and the discrete controller synthesis (DCS) techniques. COTS are formally specified using temporal logic and/or executable observers, and coded according to their formal specification. New functions are built by assembling COTS together. The COTS assembly operation is not error free: the resulting assembly may not achieve the desired function it is supposed to. For these reasons, COTS assemblies need to be formally verified and if errors are found, they must be corrected using DCS. The resulting system is ready for hardware (e.g. FPGA) implementation.

Forbidden and Preforbidden States in the Multi-model Approach

This paper deals with operating mode management of discrete event systems (DES) and this contribution is based on supervisory control theory (SCT). Our aim is to extend SCT by introducing a mechanism for managing different operating modes for the controlled system. An operating mode corresponds to a specific system structure (engagement or disengagement of different system components) and specified tasks. Mode management will consist in controlling switching between modes with a view to handling models of reasonable size. Our approach is a multi-model one and involves representing a complex system by a set of simple automata models, each of which describes the system in a given operating mode. The adopted approach assumes that only one attempted operating mode is activated at a time, whilst other modes must be deactivated. The switching problem may be defined as finding compatible states, when controlled system behavior switches from one operating mode to another. The major contribution of this paper is the avoidance of switching from states (called forbidden states) with ghost compatible states in the selected operating mode. These states are called ghost because their existence would potentially violate a defined selected mode specification

Design of switching supercisors for reactive class discrete event systems

Abstract Based on the decentralized supervisory control, Lin and Wonham (1988), Lafortune et al. (2001), the present paper proposes a contribution to the supervisory control of systems (with different production objectives) that react to exceptional events (failure events for instance). Existing work on decentralized control of Discrete Event Systems (DES) focuses on problems where each decentralized control supervisor controls and observes some events in a system and must together achieve some prescribed goals Rudie and Wonham (1992), Yoo and Lafortune (2002). In this model the supervisors act simultaneously on the plant (process) and are sometimes conflicting. Our approach is based on the concept of decentralized control and we propose a procedure which allows in one hand to avoid the conflict problem and on the other hand to manage the commutation (switching) between two operating modes: nominal and degraded modes labelled respectively N and D. The developed control strategy for tackle the conflict problem and for manage the switching between two operating modes will be also implemented by the automata.

C-3C: A structure for high reliability with minimal redundancy for batteries

This paper deals with the structures involving the storage cells of electrical energy and on the reconfiguration of these storage systems. It places the existing structures in a matrix configuration to compare their performances in terms of reliability. Then, a different solution: the C-3C structure, for limiting the redundancy for fault tolerance of the cells is proposed and compared with existing patterns. This new configuration, besides it minimizes redundancy, improves battery reliability and increase reliability of a matrix structure of cells.

Equivalence of behaviors between centralized and multi-model approaches

This paper presents a comparison between centralized approach and multi-model approach based on Supervisory Control Theory (SCT). The centralized approach uses both the whole process and specification to compute the controlled process. The multi-model approach, on the other hand, is used basing on various modal perspectives. This approach allows to build smaller models, which lead to smaller scale and better understanding of the latter. The comparison is made basing on a number of conditions, which all ensure the identical behavior between the controlled processes of each discussed approach. An example of a manufacturing system illustrating the comparison is also presented.