New Adaptive Sliding Mode Controller for Depth Control of Autonomous Underwater Robot

Sliding mode control is a robust controller against modeling imprecisions and external disturbances, successfully employed to the dynamic positioning of autonomous underwater robot. In order to improve the performance of the whole system, the discontinuity in the control law must be smoothed out to avoid the undesirable chattering and unwanted ripples. One of the disadvantages of conventional sliding mode is great vulnerability in the presence of noise. However, noise and some initial condition causing undesirable chattering phenomenon and unwanted ripples in the control input. This paper describes the development of a depth control system for autonomous underwater robot. In this paper we used the sliding surface term and its derivation with adaptive gains in control law instead of the sign function with fixed gain. The proposed controller has been designed to solve great vulnerability of sliding mode control at the presence of noise. For contrast one of factors causing chattering, big controller gain, the gain of controller adapted according to state condition and uncertainties. Due to in the proposed controller there is no sign function, so our controller is not vulnerability to noise. Using this controller, ripples and unexpected sharp peak of the input control signal were canceled and control signal was smoother than conventional sliding mode controller with boundary layer. The stability and convergence properties of the closed-loop system are analytically proved using Lyapunov stability theorem. Simulation results are presented in order to demonstrate the control system performance.