Identification and control of a direction unstable ship

Leszek Morawski, Janusz Pomirski,

e-mail: lmoraw@wsm.gdynia.pl e-mail: jpomir@wsm.gdynia.pl

Precision in ship movements’ control is of great consequence to a ship safety, but control of a ship with a wide zone of direction unstability causes many problems to precision control. These problems particularly appear during course-change manoeuvre, when a ship should be stabilised at the new course and the real course oscillates for a long time, often with high amplitude.

The paper presents identification results of ship dynamics and a design of nonlinear controllers, which were used in a ship’s autopilot for controlling a direction unstable ship. A dynamics of the reduced isomorphic VLCC tanker model in the scale of 1:24 was identified.

Two controllers were applied for steering of ship. One of these controllers was a turn controller, while the other stabilised the course of the ship. Synthesis of both algorithms is based on nonlinear Norrbin model of a ship with direction unstability. Both algorithms contain nonlinear feedback, which cause that the closed loop course-keeping system behaviors similar to the linear dynamics of 2^nd order, and dynamics of the closed loop turn rate stabilization system can be described by the inertial linear dynamics of 1^st order.

The switching between both algorithms depends on some kind of “a competition” of the signals, which are consecutively generated by both controllers. During small course changes the control does not come to the turn rate stabilization controller. During big course changes the commanded rudder angle should be big, therefore the turn rate stabilization controller switches on. The course-keeping controller takes the control back when course error becomes small enough. The chosen method results in a continuity of the commanded rudder signal. The performance specifications for the control system (transient signals decaying, time of regulations, dumping ratio and overshoots) are met.

Simulation tests of the control were performed using an identified physical model of a direction unstable VLCC tanker.