Observer-Based Robust Attitude Control of Liquid-Filled Flexible Spacecraft in a Fully Actuated System Framework

An observer-based robust attitude controller is proposed for a complex spacecraft. The spacecraft consists of a rigid carrier, a flexible appendage, and a partially liquid-filled tank. The appendage is modeled as a Bernoulli-Euler beam, and the liquid sloshing is equivalent to a lumped mechanical multimode model. The spacecraft dynamical equation is derived from the theorem of angular momentum. On the basis of the fully actuated system approach, a second-order attitude system in terms of error quaternions is deduced. To estimate the lumped disturbance consisting of inertial uncertainties, liquid sloshing, appendage vibration, and external disturbances, an extended state observer is constructed. A robust controller is designed based on the fully actuated system approach and the observer. The stability of the controlled system is proved via Lyapunov’s direct method. The present work finds a meaningful discovery that the proposed controller can be free from the unwinding phenomenon, and a brief analysis of the unwinding-free performance is provided based on the stability proof. The simulation results demonstrate the effectiveness of the proposed controller.