MPC-based anti-sway control of tethered space robots

Tether sway in debris towing removal perturbs the removal orbit due to the thrust-tether misalignment. Therefore, this paper proposes an anti-sway method using model predictive control (MPC). The method uses platform attitude maneuver to suppress the sway and track the orbit by orienting the platform thrust to the desired direction, which makes the system non-affine and underactuated. This necessitates a dual-loop control structure where the outer loop provides the desired attitude for the inner attitude control loop. For the outer loop, the attitude angular velocity is extracted as a virtual control input. A higher-order hierarchical sliding mode control (HSMC) and a super-twisting-algorithm (STA) based observer are used to obtain the equivalent law for sliding phase. The system states in controller are estimated by an extended kalman filter (EKF). For the inner loop, a MPC-based control torque forces the angular velocity to follow the equivalent law and steers the states to the sliding surface in an optimal manner. Simulation results show that the control objective is fulfilled by the bounded torque and the controller is robust to the estimation errors.