Digital robust nonlinear controller for position and attitude stabilization of an autonomous quadrotor unmanned aerial vehicle

Unmanned aerial vehicles, which are aircraft without human pilots or passengers on board, expanded to many applications as digital control technologies improved and costs fell. However, the highly nonlinear dynamics in these systems cause traditional methods to be inadequate, so the importance of nonlinear control methods for these systems has increased. This study focuses on nonlinear stabilizing control of an autonomous quadrotor UAV based on an approximate dynamical model. Position and attitude digital controllers are designed using a backstepping controller with integral action directly in the discrete-time domain. Firstly, the discretized dynamics of the quadrotor are introduced. Due to the underactuated structure of autonomous quadrotor UAVs, the digital controllers are designed to track the altitude positions and yaw angle of the quadrotor to their reference trajectories, while also stabilizing the pitch and roll angles. In the design procedure, position controllers automatically generate the desired trajectories of pitch and roll angles. Robustness against parametric deviations, unmodeled dynamics, and external disturbances is achieved with integral action, thus ensuring an offset-less steady-state response. The asymptotic stability of the closed-loop system under the proposed controllers is demonstrated according to Lyapunov theory. Detailed simulation results, along with comparative studies, are presented to illustrate the effectiveness and feasibility of the proposed controller.