Model analysis and modified control method of ship-mounted Stewart platforms for wave compensation

Offshore installations e.g. marine transportation, oil platforms, etc. are strongly depended on sea conditions. To increase workable time of carrying out these operations, a Stewart platform is installed on a ship to serve as a motion compensation base and equipment on the base can have the same precision with those on the land-fixed base. Since movements of the Stewart platform are influenced by persistent ship motions, they present more complicated dynamical characteristics which make the control issue much more challenges. In the existing results, the established model does not consider the ship motion disturbances or is a linearized model, besides, the actuator velocity is also needed. In this paper, a modified control method is proposed for the ship-mounted Stewart platform. Specifically, the dynamics model considering ship motion disturbances is established and the influence of ship motions on the Stewart platform is analyzed. Through the model analysis, a modified motion controller is proposed with utilizing a multiple degrees of freedom velocity feedforward compensator. Finally, simulations are included to illustrate the effectiveness of the modified control method by contrast.