Three-Dimensional Position and Velocity Regulation of a Quad-Rotorcraft Using Optical Flow

We address the problem of hover flight and translational velocity regulation of a quad-rotorcraft unmanned aerial vehicle (UAV) with the main objective of allowing the vehicle to navigate autonomously. This paper complements and improves previous researches considering multiple-camera systems and nonconventional sensors, which deal only with stabilizing the aerial vehicle in hover or in take-off and landing tasks. A vision system has been implemented to estimate the vehicle's altitude, lateral position, and forward velocity during flights. It is shown that, using visual information, it is possible to develop control strategies for different kinds of flying modes, such as hover flight and forward flight at constant velocity. The stability of the closed-loop system is ensured by implementing a hierarchical control strategy. In the first stage, the performance of the proposed methodologies was validated under a simulation environment, showing satisfactory and promising results. Next, real-time experimental applications, consisting of autonomous hover and forward flight at constant velocity, were successfully achieved, validating the effectiveness of the proposed imaging algorithm and vision-based controller.