Robust and Adaptive Path-Following Control of an Underactuated Ship

The design of a vessel path-following control system based on a full, realistic, nonlinear model is considered. The control objective is to force a surface, course-unstable vessel to track a predefined geometric path. We study an underactuated ship characterized only by a surge control force and yaw control moment, typical of many supply vessels. The assumption is made that the ship's model parameters are unknown, while significant external disturbances and unmodeled dynamics exist. Therefore, the design procedures make use of robust and adaptive control techniques. The controller synthesis uses adaptive output feedback linearization and H ∞ optimal control techniques. In this way, the proposed control scheme assures position tracking despite various uncertainties. Because the considered design method leads to a nonminimum phase system, the problem of how to stabilize unstable zero dynamics arises. The presented simulations are based on a realistic ship model in terms of the structure and experimentally identified parameters. The simulations illustrate the effectiveness of the proposed algorithms.