Topics on Current Compensation for Path Following Applications of Underactuated Underwater Vehicles

Ocean currents compensation techniques for underactuated underwater vehicles based on a three-dimensional Line-of-Sight (LOS) guidance law with integral action are discussed and applied in order to achieve precise path following. The current is considered as constant, unknown and acting in all directions, including directions in which there are no actuator forces. An integral term introduced in the guidance law counteracts the current by allowing the vehicle to side-slip and pitch while keeping the desired course. It is shown that the integral action in the LOS guidance law performs a vectorial sum between the vehicle's relative velocity and the unknown current velocity to compensate for the drift. Path following of straight lines is considered and the problem is addressed at the kinematic level only since the LOS guidance law can be seen as a decoupling tool between the actuated surge dynamics and the unactuated sway and heave dynamics of the vehicle. Stability of the closed loop kinematic system is analyzed and simulation results show path convergence.