 This paper was not presented at any IFAC meeting. This paper was recommended for publication in revised form by Associated Editor R.K. Boel under the direction of Editor T. Basar. * Corresponding author. Tel.: #44-116-223-1303; fax: #44-116- 252-2619. E-mail address: ce@sun.engg.le.ac.uk (C. Edwards) Automatica 36 (2000) 541}553 Sliding mode observers for fault detection and isolation  Christopher Edwards*, Sarah K. Spurgeon, Ron J. Patton Control Systems Research, Department of Engineering, University of Leicester, Leicester LE1 7RH, UK Control Systems Research, Department of Engineering, University of Leicester, Leicester LE1 7RH, UK Department of Electronic Engineering, University of Hull, Hull HU6 7RX, UK Received 8 July 1998; revised 15 March 1999; received in "nal form 17 August 1999 Abstract This paper considers the application of a particular sliding mode observer to the problem of fault detection and isolation. The novelty lies in the application of the equivalent output injection concept to explicitly reconstruct fault signals. Previous work in the area of fault detection using sliding mode observers has used disruption of the sliding motion to detect faults. A design procedure is described and nonlinear simulation results are presented to demonstrate the approach.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Fault detection; Fault isolation; Observers; Sliding mode; Output injection; Equivalent control 1. Introduction It is well known that the core element of model-based fault detection in control systems is the generation of residual signals which act as indicators of faults. The residual signals are generated using estimates of and a comparison with real measured quantities. For the design of residual generators, various approaches have been discussed in the literature. Fault detection using the parity space approach is considered in (Gertler, 1991; Patton, 1988; Patton & Chen, 1991; Patton, Frank & Clark, 1989) whilst (Ge & Fang, 1988; Massoumnia, 1986) employ fault detection "lters. Besides these ap- proaches, the class of observer-based approaches have been the most widely considered (Chen, Patton & Zhang, 1996; Ding & Frank, 1990; Ding, Guo & Frank, 1994; Duan, Patton, Chen & Chen, 1997; Frank, 1990, 1994; Ge & Fang, 1988; Gertler, 1988, 1991; Hou & Mu K ller, 1992, 1994; Patton & Chen, 1991, 1997). Patton and Chen (1994) have shown that the observer-based and parity space residual generators are related mathemat- ically and under certain conditions are identical. An additional concept in the fault diagnosis literature is the failure detection "lter in which the structure of the ob- server ("lter) is specially designed to provide powerful fault isolation properties (Wilsky, 1976). The basic idea behind the use of the observer for fault detection is to estimate the outputs of the system from the measurements by using some type of observer, and then construct the residual by a properly weighted output estimate error. The residual is then examined for the likelihood of faults by using a "xed or adaptive threshold. Certain decision rules can then be applied to determine if a fault has occurred. A decision process may be based on a simple threshold test on the instantaneous values or moving averages of the residuals. When the system under consideration is subject to unknown disturbance or un- known inputs, to achieve e!ective fault detection, the e!ect of the disturbance has to be de-coupled from the residual signal to avoid &false alarms' in detection. This problem is known in the literature as robust fault detec- tion or fault detection using unknown input observers (Patton, 1997) and has been considered by many authors, for example, Chen et al. (1996), Dalton, Patton and Chen (1996), Hou and Mu K ller (1992, 1994), Patton and Chen (1991, 1992), Patton et al. (1989), Patton and Willcox (1987). Chen et al. (1996) discuss extensions to the failure detection "lter theory and provide an important link between this theory and the use of unknown input observers as a robustness problem. When, in the 0005-1098/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 5 - 1 0 9 8 ( 9 9 ) 0 0 1 7 7 - 6