AAS 95-417

Globally Stable Feedback Laws for Near-Minimum-Fuel and Near-Minimum-Time Pointing Maneuvers for a Landmark-Tracking Spacecraft

H. Schaub, R. D. Robinett and J. L. Junkins, Texas A&M University, College Station, TX

Abstract

Utilizing unique properties of a recently developed set of attitude parameters, the modified Rodrigues parameters, we develop feedforward/feedback type control laws that globally control a spacecraft undergoing large nonlinear motions using three reaction wheels. The method is suitable for tracking given smooth reference trajectories that spline smoothly into a target state or pure spin motion; these reference trajectories may be exact or approximate solutions of the system equations of motion. An associated nonlinear observer is formulated for state estimation. In particular, we illustrate the ideas using both near-minimum-time and near-minimum fuel rotations about Euler's principal rotation axis, with parameterization of the sharpness of the control switching for each class of reference maneuvers. Lyapunov stability theory is used to prove rigorous global asymptotic stability of the closed loop motion in the end game and during the tracking of the reference motion. The methodology is illustrated by designing control laws for a prototype landmark tracking spacecraft. Simulations indicate practical stability and robustness are achieved in the presence of typical model errors and measurement noise.