Active diagnosis of discrete event systems

While the need for accurate and timely diagnosis of system failures and the advantages of automated diagnostic systems are well appreciated, diagnosability considerations are often not explicitly taken into account in system design. In particular, design of the controller and that of the diagnostic subsystem are decoupled and this may significantly affect the diagnosability properties of a system. In this paper we present an integrated approach to control and diagnosis. More specifically, we present an approach for the design of diagnosable systems by appropriate design of the system controller. This problem, which we refer to as the active diagnosis problem, is studied in the framework of discrete event systems (DES). We formulate the active diagnosis problem as a supervisory control problem where the legal language is an "appropriate" sublanguage of the system language. We present an iterative procedure for determining the supremal controllable, observable, and diagnosable sublanguage of the legal language, and for obtaining the supervisor that synthesizes this language. This procedure provides both a controller that ensures diagnosability of the closed-loop system and a diagnoser for online failure diagnosis. We illustrate our approach using a simple pump-valve system.