Fast terminal sliding-mode control based on unknown input observer for the tracking control of underwater vehicles.

The control of underwater vehicles is always challenging due to nonlinear dynamics, unmodeled dynamics, system uncertainty, and environmental disturbances. In this paper, a robust control scheme is designed for the tracking control of underwater vehicles with system uncertainty and time-varying environmental disturbances. First, the dynamic model of underwater vehicles is simplified to an explicit expression for the inertia matrix without the need for complex hydrodynamic coefficients. Then, according to the invariant manifold theorem, a single-parameter unknown input observer (UIO) is designed to estimate uncertainty. Based on the simplified model and UIO, a fast terminal sliding mode control with an inverse function (IFTSMC) is designed. The finite-time stability of the closed-loop system is proved. Finally, to evaluate the performance of the proposed control scheme, comparative numerical simulations are conducted with 20% dynamic uncertainty and time-varying interference. The results indicate that the proposed control scheme is effective and robust under dynamic uncertainty and time-varying disturbances. • Based on invariant manifold theorem, an unknown input observer is designed. • A reaching law with arctangent function and inverse hyperbolic function is designed. • A closed-loop controller considering measurement noises is designed. [ABSTRACT FROM AUTHOR]