Satellite Attitude Control Using Magnetic Torquers, a Periodic Time-Varying Control Problem

Abstract

Abstract : Satellite attitude controllers have been designed for a rigid spacecraft whose only actuators are magnetic torque rods. This effort's goals have been to develop a new class of lightweight, moderate accuracy attitude controllers and to evaluate and further develop general methods for the control of time varying systems. Three different classes of controllers have been developed and simulation tested. one based on linear quadratic regulator techniques, one based on sliding mode like concepts, and one based on new H infinity techniques for time varying systems. These H infinity controllers achieve the best performance. In addition to the controller design studies. the issue of attenuation of constant 3 axis disturbances has been addressed. Disturbance attenuation is difficult for this system because it can apply torques only about the 2 axes that are perpendicular to the Earth's magnetic field. It is a challenge to determine how best to counteract a low frequency 3 axis disturbance torque, on average, via judicious use of the fact that the Earth's magnetic field direction varies in time as the spacecraft moves along its orbit. Pointing accuracies on the order of 1 deg or better have been demonstrated in the presence of typical levels of disturbance torque.