  In this paper, the incremental nonlinear dynamic inversion (INDI) method is
applied to the control system design of coaxial rotor UAVs. The aerodynamic
uncertainty and anti-disturbance problems are solved in the control system
design. The designed controller gives the UAV excellent flight performance and
control robustness. An incremental gain method (IGM) is proposed for the delay
problem of the state derivative. This method has the advantages of less
calculation load and simple parameter adjustment, which provides excellent
convenience for applying INDI controllers to coaxial rotor UAVs. The principle
of IGM is demonstrated by the stability analysis method of the discrete system,
and the parameter selection strategy of the IGM is analyzed in detail by
simulation. The advantages of the controller design method are verified by
comparative flight tests of nonlinear dynamic inversion (NDI) and INDI. The
experimental results show that the average trajectory tracking error of INDI is
only 58.3% of that of NDI under the same wind speed. When the angular
acceleration delay is less than 0.06 s, IGM can easily keep the system stable.
In addition, INDI has better robustness under obvious model errors and strong
input disturbance.
