Li, Bo

Ocean is human’s last frontier on Earth with most of its space inaccessible to human and remains
largely unexplored. For the protection of our ocean and its sound development, unmanned autonomous
underwater vehicle AUV, plays an increasingly important role. However, today’s AUV can’t function in a
strong current environment. Propeller-driven AUVs typically move at speeds of up to 1.5-2.0 m/s, and
thus strong ocean currents could push AUVs way from the planned paths. And their control surfaces
may not work properly, especially when AUVs are maneuvering. Extra thrusters may be added to
improve the maneuverability, yet the endurances of the vehicles will be shortened since extra thrusters
consume more power. On the other hand, buoyancy-driven underwater gliders, using internal actuators,
are characterized by long endurance. However, gliders typically move at horizontal speeds of about 0.3
m/s, which make gliders unsuitable for the missions in strong ocean currents. In the present research, a
hybrid AUV design will be studied which combines the capabilities of both AUVs and underwater
gliders. The proposed AUV will be propeller-driven yet the maneuverability of the vehicle in both
horizontal and vertical planes will be achieved by using internal actuators instead of control surfaces
and extra thrusters. The research will mainly focus on the control strategy of an AUV in a horizontal
plane by using internal actuators to exploit the vehicle’s coupling effect of the roll motion on horizontal
motions to maneuver AUV in a strong current environment.

This dissertation concerns the dynamics and control of an autonomous underwater
vehicle (AUV) which uses internal actuators to stabilize its horizontalplane
motion. The demand for high-performance AUVs are growing in the field of
ocean engineering due to increasing activities in ocean exploration and research.
New generations of AUVs are expected to operate in harsh and complex ocean environments.
We propose a hybrid design of an underwater vehicle which uses internal
actuators instead of control surfaces to steer. When operating at low speeds or in
relatively strong ocean currents, the performances of control surfaces will degrade.
Internal actuators work independent of the relative
ows, thus improving the maneuvering
performance of the vehicle.
We develop the mathematical model which describes the motion of an underwater
vehicle in ocean currents from first principles. The equations of motion of a
body-fluid dynamical system in an ideal fluid are derived using both Newton-Euler
and Lagrangian formulations. The viscous effects of a real fluid are considered separately.
We use a REMUS 100 AUV as the research model, and conduct CFD simulations to compute the viscous hydrodynamic coe cients with ANSYS Fluent. The
simulation results show that the horizontal-plane motion of the vehicle is inherently
unstable. The yaw moment exerted by the relative flow is destabilizing.
The open-loop stabilities of the horizontal-plane motion of the vehicle in
both ideal and real fluid are analyzed. In particular, the effects of a roll torque and
a moving mass on the horizontal-plane motion are studied. The results illustrate
that both the position and number of equilibrium points of the dynamical system
are prone to the magnitude of the roll torque and the lateral position of the moving
mass.
We propose the design of using an internal moving mass to stabilize the
horizontal-plane motion of the REMUS 100 AUV. A linear quadratic regulator
(LQR) is designed to take advantage of both the linear momentum and lateral position
of the internal moving mass to stabilize the heading angle of the vehicle. Alternatively,
we introduce a tunnel thruster to the design, and use backstepping
and Lyapunov redesign techniques to derive a nonlinear feedback control law to
achieve autopilot. The coupling e ects between the closed-loop horizontal-plane
and vertical-plane motions are also analyzed.