A novel approach to 6-DOF adaptive trajectory tracking control of an AUV in the presence of parameter uncertainties

• An adaptive back-stepping controller was presented for AUV with parameter uncertainty.
• Projection algorithm was used to update the estimate of the unknown parameters.
• Another adaptive controller is designed using saturation functions.
• Simulation results verified convergence of tracking errors asymptotically.
• Stability of both controllers is proved using Lyapunov theory and Barbalat׳s lemma.

In this paper, the trajectory tracking control of an autonomous underwater vehicle (AUVs) in six-degrees-of-freedom (6-DOFs) is addressed. It is assumed that the system parameters are unknown and the vehicle is underactuated. An adaptive controller is proposed, based on Lyapunov׳s direct method and the back-stepping technique, which interestingly guarantees robustness against parameter uncertainties. The desired trajectory can be any sufficiently smooth bounded curve parameterized by time even if consist of straight line. In contrast with the majority of research in this field, the likelihood of actuators׳ saturation is considered and another adaptive controller is designed to overcome this problem, in which control signals are bounded using saturation functions. The nonlinear adaptive control scheme yields asymptotic convergence of the vehicle to the reference trajectory, in the presence of parametric uncertainties. The stability of the presented control laws is proved in the sense of Lyapunov theory and Barbalat׳s lemma. Efficiency of presented controller using saturation functions is verified through comparing numerical simulations of both controllers.