Adaptive steering control for an azimuth thrusters-based autonomous vessel

The proposed paper presents the design and development of the combined guidance and control strategies for the autonomous navigation of unmanned vessels characterised by azimuth-based thrust architecture. Autonomous marine vehicles (AMVs) are consolidated technological tools commonly employed for different tasks such as exploration, sampling and intervention. With the final aim of autonomous shipping, the AMVs capabilities have to be migrated and adapted towards the reliable and safe control of commercial-like unmanned vessels. These last are spreading thanks to a number of technological research projects. The employment of unconventional hull shapes combined with propulsive layout based on azimuth thrusters requires robust guidance techniques to provide precise and reliable motion control during navigation. The paper introduces a dual-loop guidance and control scheme able to provide advanced navigation capabilities. An inner control loop, devoted to the actuation of the azimuth thrusters, allows the tracking of reference course angle (namely the autopilot). Such a control loop is characterised by a modified PD regulation scheme, where a novel adaptive derivative component is inserted in order to improve the convergence curve towards the required course reference. The outer guidance loop, based on Lyapunov and virtual-target approach, allows the vessel to track generic desired paths, thus enhancing the autonomous navigation capabilities. The paper will provide a deep design and analysis approach for the developed techniques, as well as simulation results of the combined guidance and control scheme, proving the reliability of the proposed approach in different operative conditions.