Robust and adaptive reconfigurable control for satellite attitude control subject to under-actuated control condition of reaction wheel assembly
Satellite mission life are primarily dictated by the state of health of its Reaction Wheel Assembly (RWA), especially for commercial GEO satellites. Propulsion based actuating devices can't replace or accommodate for the loss of RWA because thrusters based pointing control is not as accurate as the RWA. The RWA is the primary set of actuators (as compared to thrusters for orbit maintenance and maneuvering) mainly responsible for the satellite mission for accurately and precisely pointing its payloads to the right targets to conduct its mission operations. The RWA, consisting of either a set of four individual wheels mounted in a pyramid or three in an orthogonal configuration, is mainly the primary control actuators of the satellite during its normal operations. Future GEO satellites will be required to achieve much longer lives than their typical 15 years normal life expectancy. Driven by customers' demands/goals and competitive market have challenged Attitude Control Subsystems (ACS) engineers to develop better ACS algorithms to address such a stringent requirement. There are two main directions to design satellite's under-actuated control subsystem: (1) Attitude Feedback with Zero Momentum
Principle and (2) Attitude Control by Angular Velocity Tracking via Small Time Local Controllability concept. Successful applications of these control laws have been largely demonstrated via simulation for the rest to rest case. Limited accuracy and oscillatory behaviors are observed in three axes for non-zeros wheel momentum while realistic loss of a wheel scenario (i.e., fully actuated to under-actuated) has not been closely examined! This study revisits the under-actuated control design with detailed set ups of multiple scenarios reflecting real life operating conditions which have put current under-actuated control laws mentioned earlier into a re-evaluation mode since rest to rest case is not adequate to truly represent an on orbit failure of a single wheel. The study is intended to facilitate the ACS community to further develop a more practical under-actuated control law and present a path to extend these current thinking to address a more realistic reconfigurable ACS subject to a dynamic transition from a 3 RWs mode to 2 RWs mode.