Lyapunov Redesign Approach for Robust Output Regulation of a Class of Autonomous Underwater Vehicles using Conditional Servocompensator

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This paper investigates the output regulation problem for a class of Autonomous Underwater Vehicles (AUVs) having non-minimum phase nonlinear dynamics, via conditional servocompensators in steering plane. The 6DOF dynamic model of AUV when decoupled in steering plane and transformed into strict normal form results in unstable zero dynamics. A robust output feedback Sliding Mode Control (SMC) is then designed employing Extended High Gain Observer to achieve semi-global asymptotic stabilization of the internal state. The regulator design is inspired from the pioneering work of Isidori [1] and Khalil [2] where controller is constructed using internal model dynamics and a robust globally bounded output feedback controller. With the inherent leverage of incorporating any stabilizing controller in the design framework, the servocompensator is introduced in the Lyapunov redesign framework for the closed loop system retaining the asymptotic robustness properties of the overall feedback design. Furthermore the controller ensures that servocompensation is only provided in a neighborhood of the zero-error manifold, while forcing the trajectory to the manifold outside it, which tends to effectively improve the transient response, while achieving zero steady-state tracking error and rejecting matched uncertainties. Analytical results and simulations are included to shown that the proposed controller ensures zero error convergence with tight stability margins and paves the way for further advancements to achieve exponential stability in output regulation problems.