Asymptotic Sliding Mode Control Design for Ship Dynamic Positioning System

Ship dynamic positioning control system design is challenging due to its nonlinearity and environmental disturbance. An ideal (conventional) sliding mode utilizes a sign function to make the trajectory reach the sliding surface in finite time but induces the chattering effect. Instead, this paper presents a sliding mode-based ship dynamic positioning control system that utilizes continuous function; therefore, the chattering effect can be avoided. The sliding-mode control system that utilizes the continuous function proposed in this paper has an asymptotic reaching time as well as an asymptotic convergence to the equilibrium point. It is designed using the equivalent control concept. Two scenarios of numerical simulation are presented to show the effectiveness of the proposed control law and to evaluate the performance deterioration if the environmental disturbance-related component is not estimated and is not a part of the control law. Numerical simulation shows that orientation performance is most affected if the control law does not include an estimation of the disturbance-related component. Nevertheless, the designed control law can maintain all state variables near the desired position and orientation.