This paper describes the robust control system design for a ship dynamic positioning system. The control design is based on an approximate linear model derived from the nonlinear hydrodynamic equations governing the horizontal motions of the ship. The nonlinear models of the ship, seawaves, current, wind and thrusters are derived and simulated for control design verification. The H control design technique is employed to design the controller. The control problem is formulated in state-space form and the design specifications are translated into requirements on the weighting functions of the error signal and the thrusters input. A tuning procedure is proposed based on the wind and wave disturbances. The controller is initially tested on the nonlinear ship model and simulation results are presented to demonstrate the robustness of the H controller. Tank tests results are then presented to assess the controller performance
The design of predictive PID controllers with similar features to the model-based predictive controllers (MPC) is considered. Corresponding to a prediction horizon of size M, a bank of M parallel conventional PID controllers is defined. All the controllers have the same three terms gains, but the ith controller operates on the future error signal at time (t+i). A known future set point and dead time can be easily incorporated and the input constraints can be handled via the solution of constrained MPC problem or integral windup techniques. The stability of the closed loop system is studied and the optimal values of PID gains are derived. The controller reduces to a conventional PI or PID controller for the first or second-order system, respectively. The controller can be implemented on the existing process control hardware and the conventional PID tuning knobs can be used to adjust the controller performance. Simulation studies for a number of different systems show that the controller performance is close to a MPC type control algorith
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