Path Following of Underactuated Surface Vessels in Presence of Unknown Constant Environmental Forces: Preliminary Results

A disturbance rejection technique for path following applications of underactuated surface vessels, originally designed to compensate for ocean currents, is shown to successfully counteract for wind and wind waves as well. The overall effect of wind and wind waves is modeled as an unknown combination of constant forces and moments acting on the vessel. Disturbances in surge and yaw are canceled through feed-forward and adaptive techniques in the autopilot. However, due to the underactuation, the drifting effect of the environmental forces in the sway direction can not be canceled in this way. In order to eliminate the drifting effect of wind and wave forces in the sway direction, a modified two-dimensional Line-of-Sight (LOS) guidance law with integral action is proposed. The integral effect of the LOS guidance is introduced to counteract slowly varying disturbances like wave drift, wind load and currents. The closed-loop stability analysis reveals global κ-exponential stability properties for the guidance subsystem, where the perturbing effect of the autopilot on the guidance law is neglected. The theoretical results are supported by simulations.