Robust self tuning of fuzzy sliding mode control

In this article, an adaptive fuzzy sliding mode with Proportional + Integral + Derivative (PID) sliding surface is employed for depth control of an autonomous underwater vehicle (AUV). The sliding surface defined the dynamic behavior of a system in sliding mode. Usually, PD-type sliding surface is selected as a hyperplane in the system state space. An integral term included in the sliding surface expression that resulted in a type of PID sliding surface as hyperbolic function for eliminating chattering effect. At first, we address the design of proposed control scheme for nonlinear trajectory control and then we provide two types of self tuning techniques for estimating gain factors in control algorithm. In first tuning technique, coefficients of PID sliding surface are determined from fuzzy logic tuning scheme. While, second tuning technique based on non-fuzzy self tuning for updating hitting gain of fuzzy sliding mode control (FSMC), which depends on the normalized change in sliding surface and its number of fuzzy partitions. The Lyapunov stability analysis is also given to ensure the closed loop dynamic stability of underwater vehicle in vertical plane. The simulation results are presented to make a quantitative comparison with the traditional PID based sliding mode control. It is demonstrated that the proposed control law improves the tracking performance of system dynamic model in case of external disturbances and parametric uncertainties.