Sliding-mode control of ship-mounted Stewart platforms for wave compensation using velocity feedforward

Offshore installations e.g., marine transportation, oil platforms, etc., are strongly dependent on sea conditions, which means low working efficiency and economic loss due to unworkable sea conditions. To increase the workable time of carrying out these operations, a Stewart platform is installed on a ship deck to serve as a motion compensation base, and equipment such as cranes and drilling platforms can be placed on the base to eliminate the effect of wave-induced ship motions. Herein, ship-mounted Stewart platforms work in the non-inertial frame, and their movements are influenced by persistent and unpredictable ship motions. As a result, they present much more complicated dynamical characteristics, which bring much more challenges for the controller design. To deal with the aforementioned problem, this paper presents a sliding-mode control scheme for the ship-mounted Stewart platform. Specifically, by employing Kane's method, the dynamics model is established with the effect of ship's motion on the top platform considered. Furthermore, a novel velocity feedforward compensator is proposed to improve control performance. A command-filtered based sliding-mode backstepping controller is subsequently developed. Finally, it is rigorously proven that the control errors are bounded employing Lyapunov-based analysis, and simulations are included to illustrate the effectiveness of the proposed control scheme.