Appointed Fixed Time Observer-Based Sliding Mode Control for a Quadrotor UAV Under External Disturbances

In this article, a study of the appointed fixed time control problem is presented for the quadrotor unmanned aerial vehicle attitude control system subject to external disturbances. Based on the appointed fixed time sliding mode variable and the adaptive technique, a novel sliding mode observer is first established to estimate the external disturbances within the appointed settling time. Subsequently, an appointed fixed time controller is proposed to track the desired attitude regardless of the initial states. To circumvent the singularity problem of the above controller, a novel improved adaptive sliding mode control law is designed by employing a nonlinear function and a modified sliding mode observer. Moreover, the chattering phenomenon is attenuated by replacing the sign function with hyperbolic tangent function. The appointed practical fixed time stability of the closed-loop attitude control system is proved and analyzed rigorously utilizing the Lyapunov theory. Finally, the effectiveness and fine performance of the proposed schemes are illustrated by numerical simulation results.

I. Introduction

In Recent Years, quadrotor unmanned aerial vehicles (UAVs) have been wildly investigated due to the potential applications in military and civil fields [1], [2], such as reconnaissance and surveillance, logistics delivery, city management, power-lines inspection, etc. Compared with the other types of UAVs, the quadrotor UAV possesses the advantages of simple structure, strong maneuverability and controllability, and so on. Four fixed blades and motors are mounted on the quadrotor, using the variation in motor speed for the flight control [3]. However, it is still a challenging issue to design the robust control method for a quadrotor UAV because it is a typical nonlinear and underactuated system [4]. What’s worse, there always exist uncertainties such as unknown external disturbances during the flying missions. It inevitably raises the difficulty to achieve the high-accuracy and high-stability flight control of the quadrotor UAV control system.