Robust Output Feedback Depth Control Design for a Class of AUVs in Diving Plane

This paper presents a robust depth control design for a class of Autonomous Underwater Vehicles (AUVs). We consider a SISO system by extracting 3 DOF decoupled equations of motion from a nonlinear mathematical model. This system is then bifurcated into internal and external subsystems by transforming it into a normal form. For state-feedback design, where all external states are considered to be measurable, a robust control scheme based on Sliding Mode Control (SMC) is proposed. The proposed controller achieves semi-global asymptotic stability in the presence of AUV model uncertainties and/or parametric variations. The work is then extended to output feedback design by using observer design approach of Boker & Khalil of combining an Extended High-Gain Observer and an Extended Kalman Filter so as to estimate the whole state vector. Mathematical analysis shows that, under output feedback controller, trajectories of the closed-loop system approach the invariant manifold in finite time and stay there then after. A simulation example is included to show the efficacy of the proposed controller.