Tracking Rigid Body Motion Using Thrusters and Momentum Wheels

Abstract

Tracking control laws are developed for a rigid spacecraft using both thrusters and momentum wheels. The model studied comprises a rigid body with external thrusters and with rigid axisymmetric wheels controlled by axial torques. Modified Rodrigues parameters (MRPs) are used to describe the kinematics. The thruster torques and the axial motor torques are computed to track given attitude motions, using the angular velocity error and MRP error to develop linear and nonlinear control laws. Three different controllers are developed. The first controller uses thruster torques based on a bang-bang control law, while momentum wheels are used to correct tracking errors. The second controller is similar, but is designed to use the thruster torques in such a way that the momentum wheels are not used at all unless there are initial condition errors. The third controller uses linear feedback for the wheels and nonlinear feedback for the thrusters. In all three cases, the controllers are shown rigorously to result in globally asymptotically stable closed-loop systems.