Influence of Hydrostatic Stability on Orientation Control of Autonomous Underwater Vehicles

Currently, most Autonomous Underwater Vehicles (AUVs) are designed to be hydrostatically stable, i.e.\ their Center of Gravity (CG) is placed under their Center of Buoyancy (CB). While such a design provides the vehicle with passive stability against disturbances in pitch and roll, the restoring forces also constrain its maneuverability.
An alternative is marginally stable vehicle design (CG and CB placed preferably close to each other), offering high maneuverability at the cost of continuous need for disturbance compensation. Hydrostatic orientation control (active shifting of CG and/or CB) enables for high maneuverability with passively stable equilibrium, but for large hydrostatic stability also large masses and/or volumes have to be shifted.

If high maneuverability is demanded, choosing the optimal of these concepts is often not trivial. Therefore, the goal of this thesis was to examine the effects of hydrostatic stability on overall system stability, control performance and energy consumption.

The impact of hydrostatic stability on overall system stability was investigated in 1 DOF (pitch) applying nonlinear stability theory. A simulation study was conducted to examine the influence on control performance and energy consumption. For simulation purposes, a simple empirical thruster model was derived, based on experimental data. Thruster experiments were conducted using a thruster test stand designed and manufactured within this thesis. For experimental validation of the obtained results, a 1 DOF seesaw-like test stand was built, but not put into operation, yet.

The thesis's results indicate that global asymptotic stability can be achieved for feedback controlled vehicles independent of hydrostatics. Concerning control performance and energy consumption, simulation results suggest that the influence of hydrostatic stability is of high importance, if thrusters with slow dynamics are used, but only of minor relevance, if the thrusters offer a high dynamic range. Therefore, operation of a marginally stable AUV appears reasonable, if thrusters with such fast dynamics are used. If the thruster have relatively slow dynamics, increasing hydrostatic stability seems to considerably improve control performance, however, this effect saturated for large metacentric heights.