Fixed-wing Unmanned Aerial Vehicle (UAV) has typical characteristics such as underactuation and nonlinearity, which poses challenges in autonomous waypoint tracking design. Additionally, during close formation flight, there is significant motion coupling and aerodynamic interference among unmanned aerial vehicles, further complicating high-performance control design. To address the autonomous waypoint tracking problem in close formation flight of fixed-wing unmanned aerial vehicles, considering requirements for trajectory smoothness, motion coordination, and tracking robustness, a multi-level robust cooperative tracking control architecture is proposed based on the fully-actuated system modeling method. This architecture consists of three core components: motion planning at upper-level, cooperative filtering at middle-level, and robust cooperative tracking control at lower-level. The upper-level generates feasible and smooth motion trajectories in real-time by integrating the virtual structure method with iterative linear quadratic regulator based on discrete waypoint. To improve the transient performance of tracking control, distributed cooperative filtering is introduced to filter the planned trajectories, generating reference signals for each individual unmanned aerial vehicle. Finally, considering challenges such as aerodynamic coupling and system uncertainties in close formation, a robust cooperative tracking control method based on uncertainty and disturbance observers is designed, achieving precise trajectory tracking and reliable formation maintenance. The proposed robust cooperative tracking control architecture addresses various constraints and challenges in close formation flight, realizing the comprehensive design of formation motion planning and tracking control, which effectively enhances system autonomy, coordination, and robustness. Finally, test of the proposed method is conducted through simulation of close formation flight with five fixed-wing unmanned aerial vehicles, validating the effectiveness of the approach.

An active disturbance rejection control strategy based on Uncertainty and Disturbance Estimator (UDE) is designed, analyzed, and validated for underactuated 3-Degree-of-Freedom (3-DOF) helicopter affected by significant model uncertainties and external disturbances. In detail, an attitude trajectory tracking strategy of the elevation and travel angles is developed using feedback linearization based on the idea of full-drive theory for the helicopter experimental setup, which exhibits severe dynamic coupling and obvious underactuated characteristics, while the stabilization of the internal state (pitch angle) is guaranteed. Then, the first-order UDE is further proposed and combined with nominal control law to construct a robust control scheme for the design of the virtual control algorithm of the three channels with inner-outer loop structure, which can effectively compensate for uncertainty and external disturbance of the helicopter. The control scheme addresses the underactuated characteristics of the plant with continuous and smooth control signals, while the disturbance compensator has fewer parameters and the adjustment of system performance is concise and clear. Based on the singular perturbation theory, the convergence of tracking error, the input-state stability of the closed-loop system, and the influence of UDE parameters on the ultimate bound of tracking error are analyzed. In addition, the potential multiple time scale attributes of the system are revealed. The necessity of disturbance compensation, the effectiveness of UDE, and the convenience of control parameters adjustment are verified by both simulation and experimental results.