Safety flight envelope calculation and protection control of UAV based on disturbance observer

Abstract

In this paper, as for the unmanned air vehicle (UAV) under external disturbance, an attainable-equilibrium-set-based safety fight envelope (SFE) calculation method is proposed, based on which a prescribed performance protection control scheme is presented. Firstly, the existing definition of the SFE based on attainable equilibrium set (AES) is extended to make it consistent and suitable for the UAV system under disturbance. Secondly, a higher-order disturbance observer (HODO) is developed to estimate the disturbances and the disturbance estimation is applied in the computation of the SFE. Thirdly, by using the calculated SFE, a desired safety trajectory based on the time-varying safety margin function and first-order filter is developed to prevent the states of the UAV system from exceeding the SFE. Moreover, an SFE protection controller is proposed by combining the desired safety trajectory, backstepping method, HODO design, and prescribed performance (PP) control technique. In particular, the closed-loop system is established on the basis of disturbance estimation error, filter error, and tracking error. Finally, the stability of the closed-loop system is verified by the Lyapunov stability theory, and the simulations are presented to illustrate the effectiveness of the proposed control scheme.