A robust attitude controller for a spacecraft equipped with a robotic manipulator

It is essential to have the ability to control the attitude of a spacecraft while an onboard robotic manipulator is operating. The paper presents a comprehensive dynamic model for a reaction wheel actuated spacecraft and a robust controller that can maintain the attitude of the spacecraft as required while the manipulator follows the desired trajectory. Unlike previous work, this paper reformulates the dynamic equation of a Free-Flying Space Robot (FFSR) with actuating Reaction Wheels (RWs) by taking into account the contribution of RWs to the angular momentum of the entire system. Given strong nonlinearities and multiple inputs of the system, diagonalization is first used to transform the strongly coupled problem into multiple single-input problems by introducing virtual torques. When involving system uncertainties, the defined virtual torque is accurately associated with the actual torque using nominal value of the uncertain inertia matrix in order to guarantee stability of original system. Smoothed Sliding Mode (SMC) controllers are designed for each single-input system provided that the bounds of uncertainties can be estimated. A spacecraft mounted with a 3-DOF manipulator is used in simulation to demonstrate the robustness of the control law when applied to space manipulators.