Variable structure attitude control and elastic mode stabilization of flexible spacecraft

The question of attitude control and elastic mode stabilization of a spacecraft (orbiter) with a beam-tip-mass-type payload is considered. It is assumed that bounded but unknown disturbance torques are acting on the spacecraft. Using variable-structure-system theory, a discontinuous three-axis moment control law is derived to control the attitude of the spacecraft. Although this control law accomplishes attitude trajectory tracking, it excites the elastic modes of the beam. A modal velocity feedback design that damps the elastic oscillations using additional actuators at the tip of the beam is presented. Simulation results are presented to show that rotational maneuvers and vibration stabilization can be accomplished in the closed-loop system in spite of disturbance torques and uncertainty in the system