Adaptive second order sliding mode for robust motion control of underwater vehicles

To improve the tracking control performance, in the presence of parametric uncertainties, for an autonomous underwater vehicle (AUV), the sensitive parameters are first identified, using the direct sensitivity analysis method. Then, an adaptive second order sliding mode controller is proposed to ensure the robust stability and performance, without any a priori knowledge on the upper bound of perturbations. To this end, a proportional integral derivative (PID) structure is adopted, as the sliding surface. The unknown sensitive parameters are estimated by some adaptive mechanisms. On the other hand, the second order sliding mode control (2-SMC) is developed to neutralize the effects of unstructured uncertainties and disturbances. The closed loop stability is shown, using the Lyapunov stability theorem, and verified by various numerical simulations.