Path following for formations of underactuated marine vessels under influence of constant ocean currents

In this paper the straight line path following problem is considered for formations of underactuated marine vessels. The vessels are affected by a constant ocean current that is bounded, and irrotational with respect to the inertial frame. A Line-of-Sight (LOS) guidance law with two feedback linearising controllers is used to achieve path following of each individual vessel. Integral action is added to the LOS guidance law to compensate the effects of the ocean current acting on each vessel. In addition to the individual geometric task of path convergence, the vessels must also achieve the formation control task. More specifically, the vessels have to move along the desired path with a specified relative inter-vessel distance and with a constant desired velocity. This task is accomplished using a nonlinear formation control law. The closed-loop dynamics are analysed using theory for feedback-interconnected cascaded systems. It is shown that the origin of the closed-loop error dynamics of the combined path following dynamics and formation dynamics is uniformly globally asymptotically stable. Simulation results are presented in a case study.