Frequency-Based Robust Control for Micro Quadrotors Flying in Narrow Vertical Spaces

Unmanned aerial vehicles (UAVs) are ideal for tasks such as inspections in confined spaces, owing to their maneuverability and independence from terrain constraints, unlike terrestrial robots. However, the aerodynamic effects in confined spaces are greater than that in open areas, because of the propeller-induced flow (PIF). This research addresses this issue as a robust-control challenge and develops a frequency-based robust-control solution. Preliminary experiments reveal that robust control is more suitable than feed-forward control, because of the complex relationships between flight and environmental conditions. To counteract the PIF-based aerodynamic effects in confined spaces, we propose a frequency-based robust controller employing a nonlinear disturbance observer (NDO). The NDO gain design is based on wind-gust frequencies. Comparative experiments, including static and dynamic scenarios, are conducted between the conventional controllers (proportional-integral–derivative and geometric controllers) and proposed robust solution. The results demonstrate improved quadrotor performance compared to that of nonrobust controllers, exhibiting enhanced hovering and navigation within a 0.4 m narrow space under 3 m/s wind when using a Crazyflie. This research provides valuable insights for improved safety and control in confined environments and serves as a guide for developing advanced control strategies and safety protocols for potential real-world applications.