Sliding mode control based on passive nonlinear observer for dynamic positioning vessels

An observer sliding mode controller is presented for dynamic positioning vessels. The proposed controller is robust to variations in environmental conditions and unmodeled components. The motions of the ship in real world contain the low-frequency and high-frequency parts. The high-frequency motions are harmful or deathful for the controller and may cause the divergence of the system. To overcome the problem, the passive nonlinear observer is adopted to eliminate the high-frequency data in the position and heading. Moreover, the observer can also estimate the low-frequency motions of the ship. Since the model of the motion is strongly nonlinear, the sliding mode technique is appropriate and simple because it can use the nonlinear terms directly when designing the controller. In order to reduce the chattering of the controller, the sign function is replaced by the saturation function. The simulation results indicate the validity of the controller.