Abstract
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.
Highlights
THERE is an increasing development for offshore industry such as oil platforms, wind turbines and offshore cranes, etc [1], [2]
To present the influence of ship motions on the Stewart platform, we investigate the distribution of the influence factors Kas, Kvs, Kam, Kvm and Kg, which are nonlinear functions of qb
In this paper, a modified control method has been proposed for the ship-mounted Stewart platform for wave compensation
Summary
THERE is an increasing development for offshore industry such as oil platforms, wind turbines and offshore cranes, etc [1], [2]. Unlike traditional land-fixed Stewart platform systems, their bases are equipped on the ship deck, whose motions are influenced by sea waves and ocean currents, and they work in a non-inertial frame. This means that, to accurately carry out marine transportation, it is required to address the ship motions, which make the system dynamics more complex and stronger nonlinearity. We still need to describe the velocity vectors in the inertial reference frame to obtain dynamics equation based on Kane method, and they are written as ωs = [RbmJωsm, Jωm] q = Jωsq (22).
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