Relative Depth-Keeping of an Underactuated, Near-Surface Underwater Vehicle in Waves

This paper describes a control method for near-surface depth-keeping by an underactuated autonomous underwater vehicle (AUV). Specifically, we address the problem of maintaining depth relative to the slow variations in free surface elevation for an AUV that is fully submerged, but is operating near the free surface in waves. The nominal depth of the vehicle relative to the undisturbed free surface is five body diameters, which is shallow enough that the added mass and inertia vary with depth, Froude- Krylov forces and diffraction due to passing waves are important, and memory effects cannot be ignored. The slender AUV has independent control over the yaw, pitch, roll, and surge degrees of freedom. A nonlinear control law based on the idea of potential energy shaping is applied to the dynamic model and is shown to be effective at tracking the slow variations in free surface elevation while rejecting faster variations due to waves of shorter wavelength. The control law is applied to a Lumped Parameter Maneuvering (LPM) model of the Virginia Tech (VT) 690 AUV that incorporates the hydrodynamic effects described above.