J. Wang, X. Liao, and Z. Yi (Eds.): ISNN 2005, LNCS 3498, pp. 584590, 2005. ' Springer-Verlag Berlin Heidelberg 2005 Fault Detection in Reaction Wheel of a Satellite Using Observer-Based Dynamic Neural Networks Zhongqi Li, Liying Ma, and Khashayar Khorasani Department of Electrical and Computer Engineering Concordia University, Montreal, Quebec H3G 1M8 Canada Abstract. This paper presents a methodology for the actuator fault detection in the satellites attitude control system (ACS) by using a dynamic neural network based observer. In this methodology, a neural network is used to model a nonlinear dynamical system. After training, the neural network, it can give very accurate estimation of the attitude positions of the satellite. The difference be- tween the actual and the estimated outputs is used as a residual error for fault detection. The simulation results show advantages of this method as compared to the method based on a generalized Luenberger linear observer. 1 Introduction Rigid attitude control of spacecrafts has been widely investigated in the past several years [1-3]. There are a variety of control techniques that can be used in attitude con- trol system (ACS). In this paper, we apply the classical PID controller in the system. Through the PID controller, the satellite will meet the ACS accuracy requirements (in our case, the satellite should be maintained within 0.25 o of a Earth-pointing attitude in all the three axes for any attitude set point change in a specified range) in several minutes. For attitude change of satellite, we use a three-axis stabilized system with on-axis reaction wheels for control actuation. Normally, the actuator subsystem contains 4 reaction wheels (3 active and 1 redundant) and three magnetorquers used for momen- tum dumping. In this sense, reaction wheel is an essential element of attitude control system. Reaction wheels are momentum exchange devices which provide reaction torque to a spacecraft and store angular momentum. An accurate mathematical model is presented in [4]. Reaction wheels working condition and performance heavily affect the attitude control of satellite. For better attitude control, any fault in the reac- tion wheels should be detected as early as possible. Mainly, there are three types of faults that can happen in reaction wheels that are of interest to us. They are bus volt- age fault, current loss (power loss) and temperature change fault in the wheel. 2 Analysis Three separate control loops will be used to command the three reaction wheels con- trol of each satellite axis. The single-axis control block diagram is shown below.