Published in IET Control Theory and Applications Received on 22nd March 2007 Revised on 21st December 2007 doi: 10.1049/iet-cta:20070090 ISSN 1751-8644 Effective fault tolerant control design for nonlinear systems: application to a class of motor control system A. Fekih Electrical and Computer Engineering Department, University of Louisiana at Lafayette, Louisiana, USA E-mail: afef.fekih@louisiana.edu Abstract: An effective fault tolerant control design for nonlinear systems is described. The proposed approach detects the occurrence of a fault in the closed loop system and switches itself between a control strategy designed for nominal operations and a robust control strategy designed for faulty conditions. It achieves a compromise between the need for high performance and robustness by integrating the most advantageous features of each control strategy. The proposed control design is applied to a class of motor control characterised by its highly nonlinear and multivariable dynamic system, induction motor (IM) drives. Simulation experiments in terms of speed and flux responses show the applicability of the proposed approach to IMs subject to faulty conditions caused by a combination of thermal and mechanical stresses for good performance. 1 Introduction Effective control strategies are typically expected to possess the capability of maintaining specific system performance despite faults (changes in system parameters or characteristic properties). According to the SAFEPROCESS technical committee [1], a fault in a dynamical system is defined as: ‘an un-permitted deviation of at least one characteristic property or parameter of the system from acceptable/usual/ standard conditions’. Based on this definition, we call a system ‘healthy’ if nominal plant parameters are in effect and ‘faulty’ when any plant parameter change, that is higher than a set threshold, occurs. Fault tolerant control (FTC) systems are effective control strategies designed with a certain degree of fault tolerant capabilities to guarantee reliable operation and good performance of engineering control systems not only under nominal conditions but also when faults occur [2–4]. The main task to be tackled in achieving fault tolerance is the design of a controller with suitable structure to guarantee stability and satisfactory performance, not only under nominal conditions, but also in the event faults occur. Ideally, in the presence of faults, the closed loop system should be capable of maintaining its safe and reliable operation for a minimum required duration until the next available system repair can be made [5, 6]. In this paper, we design an effective FTC strategy for a class of nonlinear systems subject to faults caused by parameter variations and external disturbances. In the proposed approach, we implement a projection-based method which consists on reconfiguring or restructuring the control law by selecting a particular controller from a pre-designed set of controllers. The proposed approach detects the occurrence of a fault in the closed loop system and switches itself between a control strategy designed for nominal operations and a robust control strategy designed for faulty conditions. That is, healthy controller will operate as long as the system is healthy or the fault is negligible and peak performance will only be sacrificed when a fault is detected and the residual is within certain limits. The idea of engaging the robust controller from the very beginning as it can deal with nominal and faulty conditions is ruled out because it is well recognised in the robust control 762 IET Control Theory Appl., 2008, Vol. 2, No. 9, pp. 762–772 & The Institution of Engineering and Technology 2008 doi: 10.1049/iet-cta:20070090 www.ietdl.org