Double-loop chattering-free adaptive integral sliding mode control for underwater vehicles

Abstract

To solve the trajectory tracking problem of underwater vehicles in the presence of unknown dynamics and external disturbances, a double-loop adaptive integral sliding mode controller that yields chattering-free performance is proposed in this paper. The designed double-loop controller consists of an outer-loop position controller (the kinematic controller) and an inner-loop velocity controller (the dynamic controller). The outer-loop controller is designed based on the desired position and current position and it serves as the virtual control input to achieve accurate position tracking. A saturation function is adopted to replace the conventional signum function to overcome the chattering problem of the outer-loop sliding mode. And then, the outer-loop controller is considered as the reference velocity for the inner loop, and the inner-loop controller (the real control input) is developed to make the actual velocity of the vehicle to precisely track the reference velocity. A continuous adaptive term is designed instead of the discontinuous switching term in the conventional sliding mode to estimate and compensate the unknown dynamics and external disturbances and to eliminate the chattering effects of the inner-loop sliding mode. The global asymptotical stability of the double-loop tracking system is analytically proven using Lyapunov stability theory and Barbalat's lemma. Numerical simulation performed on the dynamic model of ODIN AUV demonstrates the effectiveness of the proposed approach.