The consensus control problem under a leader-follower formation scenario is investigated in this paper, where the interactive link is considered as directed topology. A fixed-time distributed observer (FDO) is proposed to estimate the states of the leader node, whose states are supposed to be accessible for only partial followers. Furthermore, a weighted consensus protocol (WCP) is deployed on unmanned aerial vehicles (UAV) to attain formation tracking. The fixed-time convergence of FDO is proved through Lyapunov theories. A comparative simulation is conducted with FDO against a typical linear observer and WCP against a conventional consensus protocol. The results depict that the faster convergence speed and lower formation tracking error can be reached by FDO and WCP. An outdoor experiment with an unmanned ground vehicle (UGV) as leader and three quadrotor UAVs as followers is performed to enhance the application of the proposed approach.

To address the formation control issue of multiple unmanned aerial vehicles (UAVs), a finite-time control scheme based on terminal sliding mode (TSM) is investigated in this paper. A quadcopter UAV with the vertical takeoff property is considered, with cascaded kinematics composed of rotational and translational loops. To strengthen the application in the low-cost UAV system, the applied torque is synthesized with an auxiliary rotational system, which can avoid utilizing direct attitude measurement. Furthermore, a terminal sliding mode surface is established and employed in the finite-time formation control protocol (FTFCP) as the driven thrust of multiple UAVs over an undirected topology in the translational system. To maintain the safe flight of the UAV clusters in an environment to avoid collision with obstacles or with other UAV neighbors, a pigeon-hierarchy-inspired obstacle avoidance protocol (PHOAP) is proposed. By imitating the interactive hierarchy that exists among the homing pigeon flocks, the collision avoidance scheme is separately enhanced to generate the repulsive potential field for the leader maneuver target and the follower UAV cluster. Subsequently, the collision avoidance laws based on pigeon homing behavior are combined with the finite-time sliding mode formation protocol, and the applied torque is attached as a cascaded structure in the attitude loop to synthesize an obstacle avoidance cooperative control framework. Finally, simulation scenarios of multiple UAVs to reach a desired formation among obstacles is investigated, and the effectiveness of the proposed approach is validated.

This paper presents a fixed-time command-filtered backstepping control method for high-order nonlinear networked systems subject to input saturation. A fixed-time command filter is introduced to cope with the complexity explosion caused by the backstepping method. On this basis, the error compensation mechanism is used to eliminate the effect of filter error. Furthermore, input saturation and prescribed performance are considered to meet the actual engineering requirements. Subsequently, a fixed-time state-feedback controller is designed by combining the backstepping method and command filter. This designed controller not only has the advantages of conventional command-filtered backstepping control but also ensures fixed-time convergence characteristics. Finally, two examples are provided to illustrate its effectiveness.