Nonlinear Observer Design For Ship Dynamic Positioning System

The dynamic positioning (DP) system resists against the disturbance by wind, wave and current by the use of thrust system on the ship, and then maintains the desired position and heading.The total ship motion is the sum of the low frequency motion due to the wind, current, second-order wave drift and thrusters, and the high frequency motion due to first-order wave disturbances. The performance of the high frequency motion is periodic oscillation and will not lead to the change of the average positions. In order to avoid unnecessary fuel cost and tear on the thrusters, the high frequency should not enter the feedback loop, and only the low frequency motions are counteracted by the thrust system. There is needed to use wave filtering techniques filtering out the high frequency component from the position and heading measurements. However, in most case measurements of the ship velocities are not available. Hence, estimates of the velocities must be computed from noisy position and heading measurements through a state observer. Filtering and state estimation are important features of a DP system.In the traditional DP systems the wave filtering and state estimation problems are solved by using Kalman filters. It must linearize the kinematic equations about a set of predefined constant yaw angles, typically 36 operating points in steps of 10°, to cover the whole heading envelope. Global exponential stability (GES) cannot be guaranteed if Kalman filter is used. However, the numerous covariance tuning parameters may be difficult.In order to solve the problem when using the Kalman filter, this thesis design a nonlinear observe. The observer is proven to be passive and GES. The number of tuning parameters is reduced to a minimum. The proposed observer includes features like estimation of both the low-frequency position and velocity of the ship from noisy position measurements and filter out the WF motion. Then, considering that the sea state is constantly changing, and the observer should therefore be able to automatically adjust the estimation of the LF motion to the slowly-varying wave model parameters. So a gain-scheduled observer is derived by measuring the wave model parameters on-line. Also, the gain-scheduled observer is proven to be GES. The performances of the two observers are simulated on a computer model of a supply vessel.