Phase Lag Analysis and Flight Control of a Coaxial Helicopter UAV

This paper identifies the phase lag of a coaxial helicopter UAV to remove the effect of the structural vibration and rotor wake by improper phase lag. This vibration affects the sensor measurement of the flight control computer and deteriorates the attitude control performance of the internal controller. The characteristics that depend on the phase lag in the frequency band are found to identify the phase lag. Then, control commands, body velocity, and angular velocity are visualized and analyzed in a three-dimensional vector field normalized according to the phase lag in a specific motor rotation speed. Additionally, the phase lag is analyzed in the frequency domain using a Fast Fourier Transform using the angular velocity obtained from the gyro. The angular velocity, except for the motor rotation frequency, is filtered according to the cutoff frequency through 2D and 3D power spectrograms to improve the inner-loop control performance. Finally, waypoint flight is performed by applying the selected phase lag for control mixing and inner-loop cutoff frequency.