Anti-Sway Control of Suspended Loads on Shipboard Robotic Cranes

Currently, the speed at which constructing materials can be transferred from a transport ship to an offshore construction site is limited by sea conditions. Rough sea conditions cause the payload to sway making load transfer difficult and time-consuming. The objective of this research is to develop a real-time, command compensating control for reducing sea state induced payload sway for shipboard robotic cranes. The future use of this control strategy will be to facilitate faster “ship-to-offshore construction site” payload transfer in rough sea conditions. In this study, only the sea-induced rotational motion of the ship is considered, since it is assumed that a station-keeping control maintains a constant position of the ship. This rotational motion is modelled using pitch-yaw-roll Euler angles. The shipboard robotic crane is modelled as a spherical pendulum attached to a threedegree- of-freedom manipulator. The three degrees-of-freedom are azimuth (rotation about an axis normal to the ship’s deck), elevation (rotation about an axis parallel with the ship’s deck, also referred to as luffing), and liftline length. An inverse kinematics based approach and a sliding mode control strategy are explored. Both approaches use the azimuth and the elevation capability of the crane manipulator to maintain a horizontal position of the suspended load to reduce sea-induced payload sway.