Sigmoid SuperTwisting Extended State Observer and Sliding Mode Controller for Quadrotor UAV Attitude System With Unknown Disturbance The supertwisting algorithm has been verified its effectiveness in attitude control for quadrotor with unknown disturbance. However, the discontinuous signum function in supertwisting formulation causes the unwanted chattering problem and thus affects the attitude control performance. In this paper, we replace the signum function with sigmoid function and propose the sigmoid supertwisting extended state observer (SSTESO) and sliding mode controller (SSTSMC). Besides, a rigorous convergence analysis of proposed nonlinear error system based on the Lyapunov stability theory is performed, and the parameter selection rule is given. Numerical simulations are implemented in presence of manually designed disturbance in order to verify the advantage of the proposed algorithm compared with original supertwisting algorithm. Results show that the introduction of sigmoid function can not only reduce the chattering effect, but also maintain the advantage of supertwisting algorithm.

Finitetime Attitude Stabilization Control of a Quadrotor with Parametric Uncertainties and Disturbances * In practical applications, some system parameters of the quadrotor UAVs such as inertia moments and thrust coefficients are usually unknown or difficult to measure. Moreover, the quadrotor UAVs are suffering from bounded disturbances and model uncertainties. To address this problem, this paper proposes an adaptive nonsingular terminal sliding mode controller (ANTSMC) is designed to stabilize the quadrotor in the presence of the unknown system parameters and the external disturbances. The adaptive laws are designed to estimate the unknown model parameters to avoid the requirement for the information of model dynamics. In addition, proper adaptive laws are designed to estimate the upper bounds of the external disturbances. Using the estimations, the need for large control gains is avoided. Lyapunov stability analysis have proven that the ANTSMC could guarantees the finitetime convergence of the states of the quadrotor. Experimental results are given to validate the robustness and effectiveness of the proposed controller.