Relative Velocity Control and Integral LOS for Path Following of Underactuated Surface Vessels

A control technique for path following applications of underactuated surface vessels in the presence of constant irrotational ocean currents is revisited and improved. It is shown that in steady state the presented guidance law paired with measurements of the absolute speed and the relative speed of the vessel yields to an estimation of the ocean current. The control strategy is based on a modified two-dimensional Line-of-Sight (LOS) guidance law with integral action and two feedback controllers. The integral effect in the LOS guidance is introduced to counteract slowly varying disturbances like wave drift, wind load and current. In particular the effect of sea current is studied and the chosen integration law is defined to reduce the risk of wind-up effects. Moreover, due to the irrotational nature of the ocean current, only relative velocities are used in the feedback loop. Compared to the approach based on absolute velocities, redefining the vessel model with relative velocities significantly simplifies the control system. Closed loop uniform local exponential stability is achieved for path following of straight-line paths. Finally, simulations are presented to support the theoretical results.