Partial attitude stabilization underactuated control with disturbance observers for post-captured space robot

In this study, a takeover controller is proposed for a post-captured space robot with the underactuated property under the effect of unknown environmental disturbances and the gravitational gradient moment. The (w, z) parameter is introduced to describe the attitude, and an extended kinematics is derived by the additional consideration of the orbital motion of the space robot. To eliminate the effect of disturbances on the underactuated axis, two disturbance observers is proposed for the actuated axes and underactuated axis. Based on the extended kinematics and the observers, a kinematic controller is applied to stabilize the attitude. Moreover, a dynamic controller is derived to achieve the partial stabilization of the post-captured space robot around the specific axis. The convergences of the controllers are proven. The simulation results show that the state variables converge to demand values, and the angular velocity of the underactuated axis maintains at a constant value.