Dynamic Compensation System for Landing Objects on a Moving Ship
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The focus of this thesis is directed towards the difficulties that may occur during a ship-to-ship load transfer operation at sea conditions. Today few ships have the ability to exchange motion-data about the motion dynamics, making referenced compensation problematic. It exists crane solutions where the free-hanging load can be compensated so that it holds relatively still referred to the world frame, but not to the receiving ship frame. In this thesis two concept are developed coping with these problems. First, a laser measurement system consisting of laser distance sensors, measuring the distance down to the surface underneath the load. The measurements is used to describe the motion of the surface related to roll, pitch and heave; effectively collecting motion-data about the receiving ship, independent of ship-to-ship communication. An extended Kalman filter is implemented to filter the signals from the laser sensors. The second concept is a load tilt mechanism for tilting the free-hanging load in parallel synchronization with the surface underneath the load. This is achieved by tilting the load based on the comparison between angle measurements from the laser system, with data from the on-board IMU. The main part of the project constitutes of a practical approach, this including prototyping of the concepts, and construction of a small scale laboratory for experimentation consisting of several components. Firstly a 3 DOF parallel manipulator platform, imitating the load-receiving ship motion was constructed. Inverse kinematics was used in order to actuate the platform, with a wave spectrum analysis implemented as the motion reference. Secondly, a crane including the laser measurement system, and a heave compensation system handling the heave motion by feeding the hoist-wire accordingly. Finally, the load tilt mechanism is constructed, regulated by a step-response-modelled feed-forward PI-controller. Experiments was performed in the laboratory setup as follows: By running the motion platform, data from the measurement system were used in an algorithm for estimating the platform movement in terms of roll, pitch and heave. The heave compensation system then moves the load relative to the platform from estimated heave motion. The load tilt mechanism angles the load to imitate the estimated roll and pitch motion, effectively moving the load in parallel synchronization with the platform. The load can now be landed in a more controlled manner. The results shows that the measurement unit is able to provide accurate estimates of the platform dynamics, and that the tilting mechanism is able to adequately tilt the load. As a final experiment, the impact during compensated and manual load landing operations are compared, which revealed a significant reduction of the impact force.