An Unmanned Aerial Vehicle (UAV) is subjected during its flight to external forces effects (mainly produced by wind) which might induce vibrations to its system. Thus, an automatic control strategy is required for providing flight stability, and to ensure a proper system response against bounded external disturbances. In a quadrotor vehicle, the variation in its rotors angular speed allows controlling roll, pitch and yaw angles, and at the same time the motion in the 3D space. Therefore, in this paper, a Proportional Integral Derivative (PID) approach is adopted for the control of a four-rotor helicopter. Moreover, for providing robustness to the closed-loop response, a Generalized Extended State Observer (GESO) is used. Then, an equivalent disturbance gain is computed, and the matched and mismatched disturbances are estimated; Parametric uncertainties, as well as unmodeled dynamics, are also compensated by the designed control scheme. Finally, several numerical assessments are presented, for which the yielded results show that the proposed controller is capable to properly compensate injected disturbances to the quad-rotor as well as to perform trajectory tracking tasks. Also, in order to specify quantitatively the system performance with and without considering disturbance compensation, the Integral of Time multiply by Absolute Error (ITAE) index is calculated in both these cases.
