Design of Robust Finite-Time Nonlinear Controllers for a 6-DOF Autonomous Underwater Vehicle for Path Tracking Objective

In this paper, kinematic and dynamic equations of a 6-DOF (Degree Of Freedom) autonomous underwater vehicle (6-DOF AUV) are introduced and described completely. By developing the nonsingular terminal sliding mode control method, three separate groups of control inputs are proposed for the autonomous underwater vehicle subjected to uncertainties including parametric uncertainties, unmodeled dynamics, and unknown disturbances from ocean. All classes of suggested inputs are able to steer the mentioned underwater vehicle to the desired path within finite times. For all of them, innovative nonlinear sliding surfaces are defined possessing several optional parameters. The global finite-time stability is proven for the closed-loop system of the aforementioned underwater vehicle injected by each class of proposed inputs. More, three applicable inequalities are derived to determine the convergence finite times related to suggested inputs. Obtained inequalities reveal that the mentioned finite times are dependent on initial conditions and optional parameters of control inputs. Finally, three suggested inputs are separately simulated on the Naval Postgraduate School Autonomous Underwater Vehicle II (NPS AUV II). Simulation results illustrate that all proposed inputs can fulfill the trajectory tracking objective for the NPS AUV II properly.