INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL Int. J. Robust Nonlinear Control 2010; 20:673–693 Published online 18 May 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rnc.1461 Application of passivity and cascade structure to robust control against loss of actuator effectiveness M. Benosman ∗, † and K.-Y. Lum Temasek Laboratories, National University of Singapore, 5A Engineering Drive 1, #09-02, Singapore 117411, Singapore SUMMARY In this paper the problem studied is passive fault tolerant control with respect to loss of actuator effectiveness, for nonlinear models. The main idea of the work is the formulation of the faulty models as cascade systems of the healthy models with the fault models. Based on this simple formulation, passivity theory is used to ensure the stability of the faulty system. We also study the problem of input saturation. The type of faults studied here are constant as well as time-varying faults. The validity of the proposed controllers is shown on several numerical examples. Copyright 2009 John Wiley & Sons, Ltd. Received 22 June 2008; Revised 25 February 2009; Accepted 26 February 2009 KEY WORDS: passivity; cascade systems; robust fault tolerant control; loss of actuator effectiveness; nonlinear model; saturated control 1. INTRODUCTION Fault tolerant control (FTC) aims at achieving accept- able performance and stability for the safe fault-free system as well as for the faulty system. Many methods have been proposed to deal with this problem. For survey papers on the field of FTC, the reader may refer to [1, 2]. While the available schemes can be classified into two types: passive and active FTC [2], the work presented here falls into the first category of passive FTC. Indeed, active FTC aims at ensuring the stability and some performances, possibly degraded, ∗ Correspondence to: M. Benosman, Temasek Laboratories, National University of Singapore, 5A Engineering Drive 1, #09-02, Singapore 117411, Singapore. † E-mail: tslmb@nus.edu.sg Contract/grant sponsor: Defence Science and Technology Agency; contract/grant number: POD51324 for the post-fault model, and this by reconfiguring on-line the controller, based on the fault detection and diagnosis (FDD) block that detects isolates and estimates the current fault [2]. Contrary to this active solution, another solution consists on using a unique robust controller that will deal with all the expected faults. This solution has the drawback to be reliable only for the class of faults expected and taken into account in the design of the passive FTC. Furthermore, the performances of the closed-loop are not optimized for each fault scenario. However, it has the advantage to avoid the time delay due to online diagnosis of the faults and reconfiguration of the controller, required in active FTC [3, 4], which is very important in practical situations where the time windows during which the system stay stabilizable is very short, e.g. the unstable double inverted pendulum example [5]. In practical applications passive FTCs complement active FTC schemes. Indeed, passive FTCs are necessary during the fault detection and estimation phases [6], where Copyright 2009 John Wiley & Sons, Ltd.