Int. J. Critical Computer-Based Systems, Vol. 8, No. 2, 2018 141 Copyright © 2018 Inderscience Enterprises Ltd. Fault diagnosis of discrete-event systems based on the symbolic observation graph Abderraouf Boussif* and Mohamed Ghazel Univ Lille Nord de France, IFSTTAR, COSYS, ESTAS, F-59650 Villeneuve d’Ascq, France Email: abderraouf.boussif@ifsttar.fr Email: mohamed.ghazel@ifsttar.fr *Corresponding author Kais Klai LIPN, CNRS UMR 7030, Univ. Paris 13, Sorbonne Paris Cité, France Email: kais.klai@lipn.univ-paris13.fr Abstract: Fault diagnosis of discrete-event systems (DESs) has received a lot of attention in industry and academia during the last two decades. In DES based diagnosis, the two main discussed topics are offline diagnosability analysis and online diagnosis. A pioneering approach that led to the development of various techniques is based on the so-called diagnose. However, this approach suffers from the combinatorial explosion problem due to the exponential complexity of construction. To partially overcome this problem, an efficient approach to construct a symbolic diagnoser is proposed in this paper. The proposed approach consists in constructing a diagnoser based on the symbolic observation graph (SOG), which combines symbolic and enumarative representations. The construction of the diagnoser as well as the verification of diagnosability are performed simultaneously on the fly, which can considerably reduce the state space of the diagnoser and thus the overall running time. To evaluate the efficiency and the scalability of the approach, some experimental results are presented and discussed based on a DES benchmark. Keywords: fault diagnosis; discrete-event systems; diagnosability analysis; diagnoser; symbolic observation graph; on-the-fly verification. Reference to this paper should be made as follows: Boussif, A., Ghazel, M. and Klai, K. (2018) ‘Fault diagnosis of discrete-event systems based on the symbolic observation graph’, Int. J. Critical Computer-Based Systems, Vol. 8, No. 2, pp.141–168. Biographical notes: Abderraouf Boussif is a Post-doc Researcher with the Evaluation and Safety of Automated Transport Systems (ESTAS) Research Team, French Institute of Science and Technology for Transport, Development Networks (IFSTTAR), University of Lille (France). His research interests are mainly in formal verification and fault diagnosis of discrete event systems.