Three-axis Motion Compensated Crane Head Control*

:Offshore operations can be harsh and demanding and set personnel and equipment at risk. Ships will be exposed to the elemental forces of wind, waves and current, which will influence offshore crane operations considerably. This paper addresses the use of a crane head, constructed as a Delta parallel robot, to compensate for the motions of the ship in three axes. This type of robot has a rigid and accurate structure, but because of its highly nonlinear nature, advanced control algorithms must be derived. This paper includes both forward and inverse kinematics for the robot, as well as velocity kinematics and workspace analysis. The kinematics of a full crane system, with the robot as its head, has been modelled, and a simulator which includes a model of a supply vessel is created. The disturbances on the system from the elements are translated and rotated to the crane head frame of reference for use in the compensation procedure. PID controllers are used to control the crane head, and simulations are conducted to verify that the crane head is able to compensate for the motions created by waves.