Fault tolerant attitude control for a satellite with four reaction wheels

In this paper, a passive fault tolerant control method is proposed for the satellite attitude tracking in the presence of external disturbances, the inertia matrix uncertainties, and reaction wheel faults. To achieve this goal, a modified fast terminal sliding model approach is used due to its robustness against the un-modeled uncertainties and being suitable for the nonlinear system model. The sliding surface variable is chosen to avoid singularity, converge to zero in a finite time, and also reduce the Chatting phenomenon. The stability and finite time convergence of the attitude variables are also demonstrated by the extended Lyapunov condition. In order to increase the accuracy of the controller, the dynamic model of the mentioned actuators is considered. Finally, the simulation is performed on a satellite with four reaction wheels under the mentioned conditions to evaluate the performance of the proposed method. The results show that the proposed method could maintain the stability of the system under actuator faults, and it makes the state variables converge to the desired trajectories in a finite time and also produce chattering-free control signals.