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Leader-follower formation control of multiple vertical takeoff and landing UAVs: Distributed estimator design and accurate trajectory tracking

In this paper, we study the leader-follower formation control problem of multiple vertical takeoff and landing (VTOL) unmanned aerial vehicles (UAVs) with limited communication. In particular, the leader's trajectory is only accessible to a subset of the followers and the followers only have access to their neighbours' information. Distributed estimators are developed for each VTOL UAV to obtain accurately the desired information asymptotically. Given the outputs of estimators, the solution of the leader-follower formation control problem of multiple VTOL UAVs is simplified into separately solving the tracking problem of each VTOL UAV with a converging input. Since the dynamics of VTOL UAV system is under-actuated, a hierarchical strategy is introduced such that a command force and an applied torque are synthesized in sequence to achieve the position tracking to the desired position and the attitude tracking to the command attitude extracted from the command force. In addition, an auxiliary system with appropriate parameters is introduced to guarantee the nonsingular command attitude extraction and to avoid the use of the unavailable desired information. It is shown that, the differences between the positions of the followers and the one of the leader converge to desired offsets. Simulations are provided to validate the proposed theoretical results.