Abstract

The general objective of this Ph.D. thesis is to study the dynamics and control of rigid and flexible spacecraft supported by a high-fidelity numerical simulation environment. The attitude control system is one of the critical technology elements, a significant cost driver on most spacecraft programs and hence a key area of study and research. The thesis will focus on the different aspects of the problem including the modeling of flexible dynamics and the design and implementation of a suitable subclass of an attitude control system. We have named this the Attitude Recovery System which would be initiated autonomously in an emergency situation such as a thruster misfire, a micro-meteorite collision, a mechanical wheel malfunction, or any other large disturbing torques. The system proposed could equally well be used in the normal attitude acquisition phase beginning right after the launcher tip-off, which could typically impart unwanted angular body rates to the spacecraft. The demand for greater attitude pointing precision, attitude maneuvering or recovery with the increased use of lightweight and flexible materials necessitates the consideration of flexible dynamics in the control strategy. These highly nonlinear dynamics which increase the order of the system are extremely difficult to model with high degree of accuracy. A general model for attitude and flexible dynamics for a class of spacecraft is hence derived in detail based on the so-called hybrid coordinates approach. The spacecraft considered has a star topology with a rigid central bus and flexible plate-type appendages. Given that the flexible spacecraft is under-actuated, the input-output feedback linearization technique is specifically used to partition the system into two distinct parts, namely an external linear system and an internal unobservable nonlinear system. A general internal/zero dynamics theorem for a class of nonlinear systems is proved and then applied to a flexible spacecraft which results in a linear asymptotically stable zero dynamics. The overall closed-loop stability of the flexible spacecraft is also analyzed rigorously and shown to be locally asymptotically stable using the Lyapunov theory. The robustness of the controller against modeling and parametric uncertainties is examined through extensive numerical simulations