Nonlinear Time-invariant Feedback Control of an Underactuated marine Vehicle along a Straight Course

A nonlinear, closed-loop, time-invariant controller that globally stabilizes an underactuated marine vehicle on a straight course is proposed- Traditional surface vessel linear course tracking autopilots are designed applying linear control methods on the linearized model, thus yielding only local results. Indeed due to the underactuated nature of the system, the model cannot be feedback linearized thus the most common and perhaps powerful tools of nonlinear control theory are ruled out and an alternative design strategy must be considered. At present no other smooth, time-invariant controller globally achieving the control objective is known to the authors. The overall convergence, stability and robustness to environmental disturbances is addressed and simulations are provided to show the controllers behaviour.