Energy-efficient exact-time chattering-free recursive sliding mode control of UUVs based on disturbance observer

High-precision and fast-response trajectory tracking control with high energy efficiency for unmanned underwater vehicles (UUVs) plays an increasingly important role in numerous marine tasks. Although the predefined-time control offers fast convergence of the system trajectories, it tends to overestimate the actual settling time and produce excessive control effort at the initial stage which may result in actuator saturation. This paper presents an energy-efficient trajectory tracking control strategy with exact-time convergence for UUVs under disturbances. Firstly, by employing the trigonometric-type function, an exact-time recursive sliding mode control (ETRSMC) scheme is presented, where the tracking errors reach zero at exact reference time for arbitrary initial conditions and the initial excessive control inputs are avoided. Then, an exact-time disturbance observer (ETDO) is developed for accurately estimating the external disturbance at the exact time independent of the initial estimation error, and then integrated to attenuate external disturbances while keeping the small discontinuous control gain of the designed ETRSMC scheme. Comparative numerical simulations are performed for two trajectory tracking scenarios. Compared with the predefined-time counterpart, the proposed ETDO-ETRSMC scheme guarantees exact-time convergence at any user-defined settling time, reduces peak control effort by 88.2%-98.7%, and lowers average energy consumption by 35.8%-96.9% across extensive simulation scenarios, while maintaining comparable steady-state accuracy and effectively suppressing chattering.