A Fractional Nonlinear PI-Structure Control for Robust Attitude Tracking of Quadrotors

A robust attitude quadrotor control based on a novel fractional order PI nonlinear structure is proposed and experimentally validated. The proportional action depends on a nonlinear transformation of the sliding variable, while the integral action of fractional order rejects nondifferentiable disturbances, such as turbulent effects and gust winds, enforcing a sliding motion in finite time. The exponential convergence of a quaternion-based error is guaranteed for well-posed attitude tracking, and the regularity of the control signal can be adjusted with respect to the fractional order. Notably, the proposal includes, as particular cases, first-order and second-order sliding mode control schemes. Experiments illustrate the viability of the proposal, and a comparative study versus PD, PI-like, first-order sliding mode and second-order sliding mode supertwisting controllers is presented and discussed.