Gain scheduled depth control of an underactuated AUV near the free surface

This study employed hydrodynamic coefficients obtained from two-dimensional simulations of a circular cylinder near the free surface to model the vertical-plane dynamics of an underactuated AUV using a slice-based approximation approach. The resulting three-degree-of-freedom model incorporates depth-dependent nonlinear quadratic damping terms parameterized by the dimensionless submergence gap ratio. Based on depth-dependent scheduling parameter identification, a PID-based depth controller is designed to determine the AUV’s stern plane command, which is subsequently embedded into a feedforward trim scheduling control framework. The proposed approach is validated through depth-keeping trial conducted in a water tank. Overall, experimental results demonstrate the effectiveness and energy saving of embedding depth-dependent characteristics into the control model for underactuated AUVs near the free surface.