Modelling and Control of Anti-Roll Tanks
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A ship out at sea is exposed to environmental forces in addition to those generated by the ship's actuators. It is these forces that make the ship move. However, in some cases the ship moves in an undesirable way. Because the moment of inertia in roll is smaller than the other moments of inertia, motion in roll becomes the least dampened of all the unwanted motions of a ship. Therefore, nowadays ships are often equipped with roll reducing devices. A common device is the anti-roll tank, which delivers its stabilizing moment by moving the tank fluid from side to side. The topic of this thesis is to study and analyze how the ship's dynamics in sway, roll and yaw is influenced by an u-shaped anti-roll tank and how to control the tank fluid to make a stabilizing moment in roll. In order achieve this, a 3 DOF lateral model of a ship equipped with a u-tank will be derived using Lagrangian mechanics. The model's validity will be analyzed by comparing it to a well known and proven model. In order to increase the u-tank's performance, several controllers, both passive and active, will be designed. Furthermore, the model with its controllers will be implemented in Matlab/Simulink and exposed to regular and irregular beam seas. The wave loads will be calculated from the Froude-Krylov forces and moments.