Disturbance observer based nonsingular fast terminal sliding mode control of underactuated AUV

In this paper, a disturbance observer based nonsingular fast terminal sliding mode control (NFTSMC) scheme is proposed for the trajectory tracking control of an underactuated autonomous underwater vehicle (AUV). A nonlinear disturbance observer (DO) is designed to estimate complex external disturbances and incorporated into a NFTSMC. In constructing sliding surfaces, the parameter selection in the exponential terms in NFTSMC is expanded. Lyapunov's second method proves the uniformly ultimately bounded stability of the proposed DO based NFTSMC. Simulation is performed to demonstrate the effectiveness of the proposed controller. Compared with the existing NTSMC methods, the convergence rate is improved by the proposed NFTSMC. Moreover, the proposed controller has good robustness to smooth external disturbances. For random disturbances along with an impulse at some time, the proposed controller can also track the disturbance well and achieve good robustness.