Development and experimental testing of nanosatellites attitude control using mixed magnetic/mechanical actuation

The Attitude Control System is one of the most important subsystems yet one of the least developed for what concerns the CubeSats. Magnetorquers are often employed as main actuators, thanks to their reduced dimensions. However, a spacecraft actuated by magnetorquers alone suffer from an instantaneous underactuation. Reaction wheels offer better performance but are prone to failure and must be desaturated periodically. It's possible to overcome the limits of both types of actuators by employing an ACS equipped with both types of actuators. In this work, an attitude control law integrating three orthogonally placed mangnetorquers and one reaction wheel is studied and experimentally tested on a dynamic simulator. The first part of the thesis regards the design of the control law. The required control is distributed between actuators using a geometric approach. The control approach is studied through numerical simulations and tuned for a spacecraft of a nanosatellite class. Nonlinear spacecraft dynamical model is considered, as well as worst-case external disturbances, magnetic field and attitude estimation error. The stability of the control law with respect to parameters variations is studied by means of Monte-Carlo approach. In the second part, the designed control law is validated through Hardware-in-the-loop testing on an attitude simulator testbed developed at the University of Bologna. Before-use facility calibration is described. In particular, to provide almost disturbance-free rotational dynamics, the gravity torque acting on the platform have to be compensated. This is done using a novel approach employing jointly shifting masses and magnetorquers (responsible for angular velocity damping). Then, the proposed control law is implemented on-board of the simulator and experimentally assessed. The results show high accuracy and robustness with respect disturbances.