Second order sliding mode controllers for altitude control of a quadrotor UAS: Real-time implementation in outdoor environments

This article deals with the design and real-time implementation of three second order sliding mode controllers for the altitude tracking of a quadrotor aircraft. A comparative study based on the analysis of the tracking error was performed in order to determine the controller presenting the best performance in a real-time application at outdoors environments. The compared strategies were the Classical First Order Sliding Mode Controller, the Super Twisting Sliding Mode Controller, the Modified Super Twisting Sliding Mode Controller and the Nonsingular Terminal Super Twisting Sliding Mode Controller. The last three controllers are based on the second order sliding mode technique, and they ensure robustness with respect to modeling errors even under external disturbances while reducing the chattering phenomenon in comparison with first order sliding mode controllers. Lyapunov stability theory is used to prove convergence in finite time of the altitude tracking error in the different proposed control laws. In order to demonstrate the effectiveness of the proposed solutions, an extensive set of simulation and real-time experimental results are presented.