Fuzzy adaptive optimal fault-tolerant trajectory tracking control for underactuated AUVs with input saturation.

Control of underactuated autonomous underwater vehicles (AUVs) not only needs to consider fault-tolerance, but also pursues optimal performance. To this end, a fuzzy adaptive optimal fault-tolerant trajectory tracking control scheme is proposed, which realizes three-dimensional optimal trajectory tracking control of underactuated AUVs with actuator faults and constraints, dynamic uncertainties, and environmental disturbances. The features of the scheme are: (1) System outputs are redefined by a coordinate transformation to handle the underactuation problem, and an asymmetric smooth saturation model is designed to constrain control inputs. (2) An adaptive disturbance observer is constructed to estimate compound disturbances consisting of actuator faults, uncertain dynamics, disturbances, and saturation compensation values. (3) A backstepping controller is designed to transform the original system into an equivalent tracking error dynamic system with an affine nonlinear form while compensating for compound disturbances. (4) A generalized fuzzy hyperbolic model and adaptive dynamic programming are utilized to establish an optimal controller to optimize control energy and tracking errors. Through analysis, all signals of the closed-loop system are uniformly ultimately bounded, and tracking errors can converge to arbitrarily small intervals. Pure software simulation and hardware-in-the-loop test results demonstrate the advantages and feasibility of the proposed scheme. • A novel fuzzy adaptive optimal fault-tolerant control scheme is proposed for three-dimensional trajectory tracking control of underactuated AUVs. • An adaptive disturbance observer and an asymmetric smooth saturation model are designed for handling actuator faults, uncertain dynamics, disturbances and input saturation. • An adaptive dynamic programming-based optimal controller is established by a generalized fuzzy hyperbolic model and an improved parameter updating law is designed for reducing conservativeness of system stability and requirements for initial parameter adjustments, respectively. [ABSTRACT FROM AUTHOR]