Analysis on Attitude Fluctuation in Vertical Descent of Multi-rotor UAVs

In continuous vertical descent, multi-rotor UAVs' attitude will meet the problem of attitude fluctuation and tends to divergence. The problem comes from the flight controller instead of vortex-ring state. Based on an eight-rotor UAV's nonlinear model in hovering mode, the control law is analysed by using transfer function. The first reason is that the inner ring and outer ring bandwidths of the PID cascade controller do not match well. And the other one is that the lower linear operating point and lower average rotor speed during descent lead to less thrust and torque per propeller, which make it more difficult to resist wind disturbances. By adjusting the position loop's parameters in descent, the eight-rotor UAV reduced the requirement of horizontal position accuracy, and the stability of horizontal attitude control increased, which finally obtained a better performance.