Robust attitude control and failure analysis for small satellites.

In the last years, the technological improvement and the resulting miniaturization of processors and electronics gave small satellites new capabilities and performance, previously possible only with larger satellites. As the development made possible to inexpensive and high-quality imagery as well, more satellites have been used for Earth imagery purpose and, in the next few years, this trend will not change easily. The smaller mass also allows to achieve lower launch cost, therefore giving access to space to universities and non-governmental companies. This played as a breakthrough for the entire space economy. Since the beginning of XXI century, this increased usage led to bigger attention by customers for any kind of failure, that can affect the spacecraft at any moment. In detail, the reduced mass turns the spacecraft more sensitive to the external perturbations and the MEDs (Momentum Exchange Devices) have more limitations due to their smaller dimensions. For this reason, the key feature of the proposed research is on the design of a robust control system, able to withstand parametric uncertainties (within the plant and bounded) and matched failure of the actuation system. The main objective is the design of an H∞ controller, starting from Linear Matrix Inequality (LMI) formulation. This method allows achieving the required robustness, with the uncertainty derived from the unknown angular momentum of the reaction wheels and including uncertainties in the spacecraft system. The obtained controller, suitably designed for attitude control maneuver, is a “unique” state-feedback controller for both uncertainties. The closed-loop system is evaluated for different initial conditions, including attitude positions far from the desired conditions. The effectiveness of the proposed approach is demonstrated by extensive simulations considering a pyramidal baseline configuration. Moreover, a fault detection method based on the theory of parity equation is proposed. The control law can ensure mission accomplishment in case of one wheel failed, considering a proper Fault Detection, Isolation and Recovery (FDIR) procedure. A mission scenario based on an optical Earth Observation mission is tested, in which different spacecraft configurations and failures are described.