A New Method of Thruster Control in Positioning of Ships Based on Power Control

Positioning systems include different control functions for automatic positioning and guidance of marine vessels by means of proper action of the thruster and propeller devices. These may be controllable pitch propeller (CPP) with fixed speed, controllable speed with fixed pitch propeller (FPP), or controllable pitch and speed in combination. The positioning controller computes commanded forces in surge and sway and moment in yaw. The thruster allocation algorithm determines the corresponding force and direction each thruster and propeller device must produce in order to fulfil the positioning controller commands. Conventionally, the final pitch or speed set-point signals are determined from stationary propeller force to speed/pitch relations based on data from the propeller manufacturer. However, these relations are strongly influenced by the local water flow around the propeller blades, hull design, operational philosophy, vessel motion, waves and water currents. In this paper, a new method based on torque control and power control of the propeller and thruster devices is introduced. Instead of calculating the propeller speed and pitch set-point signals based on stationary functions, the propeller force to moment and power relations are used. This method gives a significant improvement in the performance and the stability of the electrical power plant network, while the positioning accuracy and bandwidth are improved. Simulations demonstrate the performance of the proposed controller schemes.