Observer-based prescribed performance controller independent of initial errors for underwater glider systems under input saturations and state constraints

The motion control of underwater glider (UG) systems holds significant impact on high-precision ocean observation missions. Nevertheless, complex disturbances and substantial uncertainties during the state transitions of periodic V-shaped dives inevitably cause degraded control performance. Herein, a modified prescribed performance control (PPC) strategy is developed in this study for achieving rapid, precise, and robust motion responses under challenging conditions, including unknown initial states, input saturations, state constraints, and complex disturbances. The proposed controller incorporates several innovative features to enhance the control performance. First, both dynamic and steady-state performance for velocity and pitch angle control is considered under a modified PPC, wherein a periodic performance function independent of initial errors is implemented to enhance the system stability against uncertainties during periodic state transitions. Second, the control scheme addresses the inherent challenges of input saturations and state constraints in underactuated UG systems through a novel composite saturation-assisted system. Third, a generalized extended state observer (GESO) is integrated to the PPC for accurate disturbance estimation and compensation. The stability of the integrated system, incorporating the prescribed performance controller, disturbance observer, and saturation-assisted system, is rigorously analyzed using Lyapunov stability theory. Comprehensive comparative simulation studies demonstrate the effectiveness of the proposed controller, confirming its capability to achieve high-dynamic and high-precision robust motion control for UG systems.