Counter-Current and Co-Current Guidance of Underactuated Unmanned Marine Vehicles

A control strategy for counter-current and co-current guidance of underactuated unmanned marine vehicles is presented. The unknown sea current is assumed constant, irrotational, and it may act in any direction of the inertial frame. The control system is based on a pure integral guidance law and two feedback controllers. In particular, the absolute sway velocity is viewed as the error signal of the integral guidance law, and this yields a cascaded pendulum-like closed loop system. It is shown that the chosen guidance law makes the vessel search for the two possible directions having zero absolute sway velocity: the counter-current direction and the co-current direction. The two directions represent a stable equilibrium point and an unstable equilibrium point, respectively. The sign of a gain parameter is seen to define whether the vehicle will converge to the counter-current course or to the co-current course. Local exponential stability is shown for both the counter-current as well as the co-current guidance. The theoretical results are supported by simulations.