Discrete Variable Structure Control of Ship Autopilots Based on Bech's Equation

In ship autopilot design, the non linear behaviour of ship motion and the influences of uncertain factors can be more exactly characterized if the Bech's equation with uncertain terms is used. This paper presents a new approach for the ship steering autopilot design based on the Bech's equation and the discrete variable structure control theory. Firstly, a new discrete reaching law, which is stable at the origin and can eliminate the chattering caused by conventional discrete reaching law parameters, is proposed. Subsequently, an algorithm for uncertainty estimation is developed. The estimator can adaptively compensate the influence of system uncertainties and has high precision, especially for slowly varying uncertainties. Robust stability of closed loop system can be ensured if the changing rate of system uncertainties is bounded. Simulation results show that the dicrete variable structure controller designed here can track a desired course well and exhibits good performance robustness.