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This paper is concerned with the analysis of the observability of the discrete event systems (DES) modeled by interpreted Petri nets (IPN). This paper presents three major contributions on the field of the observability of DES. First, an observability definition for IPN is proposed. This definition is more precise than previous ones because it deals with the possibility of determining the system's initial state, using the knowledge of the system's inputs, outputs, and structure. Later, a novel characterization of the IPN exhibiting the observability property that is based on the IPN structure is presented. Finally, a method for designing asymptotic observers is discussed. The main advantage over other methods is that the observer presented herein is given as an IPN, allowing further analysis of the system-observer pair.Index Terms-Discrete event systems (DES), observability, observers, Petri nets (PN).

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Abdmci-This paper deals with model based fault diagnosis of Discrete Event Systems. The model of the system, expressed as an Interpreted Petri Net ( I P N ) describes partially observed events and states, and includes all possible faulty states. Based on a modular modelling methodology, the input-output diagnosability property is introduced and structurally characterized. Then a diagnoser scheme is proposed allowing fault detection and location in polynomial time. K e y w o d s -Model-based fault diagnosis, diagnosability discrete event systems, modelling methods based on intarpreted petri nets. 0-78w-8232-3/04/$17.00 a 2 0 0 4 IEEE 541

This paper presents a method for designing observers of Discrete Event Systems that are modeled with Interpreted Petri Nets. Definitions for observability and structural observability are introduced in order to deal with the computation of initial marking and current marking respectively. The structure of observers is proposed, then an architecture system-observer is defined. Finally, the feedback input to the observer is computed; it leads the observer initial marking to the current system marking.

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