MARINE LOCOMOTION: A TETHERED UAV−BUOY WITH AN INTEGRATED CONTROL SYSTEM

Abstract

Unmanned aerial vehicles (UAVs) are reaching offshore. In this thesis, the novel problem of a marine locomotive quadrotor UAV, which manipulates the surge velocity of a floating buoy by means of a cable, is formulated. The proposed robotic system can have a variety of novel applications for UAVs where their high speed and maneuverability, as well as their ease of deployment and wide field of vision, give them a superior advantage. In addition, the major limitation of limited flight time of quadrotor UAVs is typically addressed through an umbilical power cable, which naturally integrates with the proposed system. A detailed high-fidelity dynamic model is presented for the buoy, UAV, and water environment, in a simplified two-dimensional planar case and in the full scale three-dimensional case. Furthermore, a Directional Surge Velocity Control System (DSVCS) is proposed to allow both the free movement of the UAV around the buoy when the cable is slack, and the manipulation of the buoy's surge velocity when the cable is taut. Using a spherical coordinates system centered at the buoy, the control system commands the UAV to apply forces on the buoy at specific azimuth and elevation angles via the tether, which yields a more appropriate realization of the control problem as compared to the Cartesian coordinates, where the traditional x-, y-, and z-coordinates do not intuitively describe the tether's tension and orientation. The proposed robotic system and controller offer a new method of interaction and collaboration between UAVs and marine systems from a locomotion perspective. The system is validated in virtual high-fidelity simulation environments (MATLAB/Simulink, and ROS-Gazebo), which were specifically developed for this work, while considering various settings, operating conditions, and wave scenarios. On the control systems side, a practical guideline is proposed for designing and tuning adaptive backstepping control systems by leveraging the similarity with PID control laws for a class of second-order nonlinear systems. A complete set of mathematical formulations, visual aids, and a well-structured algorithm are provided to exploit the benefits of the established link. This aims at facilitating the adoption and dissemination of advanced nonlinear control laws, namely adaptive backstepping and its variants, in more real-life and industrial applications while benefiting from the legacy of PID tuning rules. Furthermore, the proposed guideline allows for upgrading primitive PID controllers to more advanced nonlinear control system, and assessing their stability margins using the provided algorithm without much added complexity. The adaptive backstepping control law is formulated as a two degrees-of-freedom control law that combines the sum of a feedback PID control component and a feedforward model compensation component. The relationship between backstepping and PID gains is provided in the form of a third-order polynomial, and a simplified second-order one, with practical tuning guidelines. The paper culminates with devising an algorithm to design and tune backstepping gains based on the established PID similarity. The proposed control law and tuning methodology are validated on a quadrotor unmanned aerial vehicle (UAV) system in both numerical simulations as well as experimentally on a physical quadrotor UAV platform.