This paper deals with the robust tracking problem for a group of Quad-Rotors forming a time-varying geometric pattern, i.e. formation control, in the presence of external disturbances. Such a robust control strategy is able to deal with any formation scheme such that there exists a directed spanning tree from the leader agent to all other agents. The proposed output-control strategy is based on a Continuous Sliding-Mode Control (Continuous-SMC) approach which is composed of a Finite-Time Sliding-Mode Observer (FT-SMO) and hierarchical cascade Continuous-SMCs. It is assumed that only the positions and angles of each Quad-Rotor are available to measure. Despite the disturbances acting on all the Quad-Rotor dynamics, this control strategy provides uniform finite-time stability for the altitude and yaw angle tracking and formation errors, and uniform exponential stability for the forward and lateral tracking and formation errors. Moreover, four different types of Continuous-SMCs can be used in the proposed strategy. Numerical simulations illustrate the performance of the proposed strategy.
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