UAV precise ATOL techniques using UWB technology

Autonomous landing of an UAV on a mobile platform is currently one of the most explored research areas. It emerges as a powerful solution in many sophisticated civil and military applications where human intervention is not always available or sufficiently responsive. Examples of this are continuous flight tasks or long distances to be covered, where mobile charging stations are needed. It is therefore clear that in all these operations the position accuracy of both UAV and mobile platform is of vital importance. This thesis examines the development of an autonomous landing system based on ultrawide-band ranging sensors. A pose estimation filter and a robust control algorithm are proposed, enabling precise tracking and autonomous landing both on the stationary or moving platform. Firstly, they are tested in a large number of simulated scenarios, where it is possible to model both UAV and UGV dynamics and all sensors with their realistic noise. Finally, the algorithms are implemented on the real embedded system, allowing a landing accuracy from 5 to 10 cm. Ultrawide-band is a suitable technology for service robotics because of its high ranging precision, obstacle penetration capabilities and robustness against interference. It is in fact possible to accurately compute the relative position of both UAV and UGV by installing UWB ranging sensors on the two systems. Positioning is achieved through a least squares multilateration algorithm, which takes as input the distances given by the UWB devices and returns the relative position of the two systems in the rover reference frame. This information is very noisy and needs to be rotated in the drone reference frame for control purposes. Therefore, it is then fused with UAV sensors and UGV compass data in a loosely coupled Kalman filter, allowing up to 5cm accuracy when the drone is within 1m from the rover. The filtered relative position estimate is then passed to a gain scheduling PID speed controller, which ensures fast tracking and acceptable overshoot in both chase and landing phases. The first step is handled by a proportional control algorithm, while the second by a proportional-integrative-derivative one. Since the main problem of the proposed landing system is the misalignment between the drone and rover compasses, this control algorithm is designed to be robust with respect to orientation errors, allowing successful landings with errors up to 40 degrees. Finally, a predictive control variant is proposed, in which the indirect UGV speed estimate is used to compute at a 10Hz rate the optimal control setpoint and allow autonomous landings at speeds above 3km/h.