DISTURBANCE OBSERVER-BASED NONLINEAR MOTION CONTROL OF THE PARALLEL PLATFORM FOR WAVE COMPENSATION

Offshore installations, such as marine transportation and shipboard cranes, are highly vulnerable to sea conditions, resulting in reduced efficiency and economic losses. To reduce the impact of waves on offshore equipment operations, this paper presents a nonlinear controller for a parallel compensation platform to isolate the load motion from the ship motion. The parallel platform is equipped with multiple hydraulic actuators so that this platform can obtain the advantages of the hydraulic system such as large-power density and large output power. Considering the complication of the platform dynamics model under non-inertia and the nonlinear characteristics of the hydraulic system, this paper proposes a nonlinear cascade controller based on inverse kinematic model, which contains a disturbance observer and a sliding model surface. The disturbance observer accounts for the complicated non-inertial forces and other nonlinearities. In addition, the outer inverse kinematic model tracking loop uses sliding mode surface to guarantee the tracking transient performance, with desired hydraulic driving force as control output. The inner force control loop is designed through the backstepping method. Finally, Lyapunov-based analysis demonstrates the stability of the closed-loop system, and simulation results verify the effectiveness of the compensation performance in the proposed scheme.