This study aimed to design a linear matrix inequality (LMI)-based robust control system and a failure identification method for small spacecraft. The key feature of the proposed method is its capability to withstand failure in the actuation system by means of the observer and controller gain definition. An effective approach for withstanding failure is required due to the small capabilities of the momentum exchange device (MED) and the system’s sensitivity to external perturbations. Passive fault-tolerant approaches can be included in the design process of the control algorithm. In particular, the main objective of this study was the design of an $H_{\infty }$ controller that started from an LMI formulation and considered the parametric uncertainties and matched failure of the actuation system. In addition, a fault detection method based on sliding mode observers was proposed to include an active disturbance correction algorithm and improve the system robustness and performance during spacecraft stabilization. The closed-loop system was evaluated for different initial conditions, including attitude positions that are far from the desired condition. The effectiveness of the proposed approach was demonstrated using extensive simulations that considered a pyramidal actuation configuration and hardware constraints.

Free-flying robots have been recently developed to operate on-board the International Space Station (ISS) as semi-autonomous robotic assistants. NASA Astrobee robots are the new free-flyers proposed by NASA, and equipped with a 3 degree of freedom manipulator. This research aims to realize an accurate mathematical model of the robot attitude dynamics, and an attitude controller for the NASA Astrobee system. The robot has been modeled coupling the attitude dynamics with the manipulator motion, and robust attitude controllers are proposed to stabilize disturbances and configuration changes induced by the manipulator usage. First, a second-order twisting sliding mode controller is designed to achieve a trade-off among control law flexibility, robustness and accuracy. Among robust control strategies, sliding mode controllers are characterized as low complexity, and low computational cost control methods, making the system able to achieve reactivity and stability also when parameters are changed. The performance of the proposed control method are compared with a backstepping controller that include an iterative algorithm to compute an adaptive control, as function of disturbances and orientation error. The performance are evaluated and analyzed considering several scenarios in Matlab Simulink simulations, and combining Simulink and ROS Gazebo to run co-simulations with the NASA Astrobee simulator. Simulations with variable body mass, links masses, and different gripped objects have been run to test the controllers performance in off-design conditions.