An autonomous flight control (AFC) law is designed for a small-scale unmanned helicopter with the hierarchical dynamic inversion (HDI). HDI is based on the hierarchical structural properties of a nonlinear system to partition the nonlinear system into a number of subsystems so that the dynamic inversion based methods are easily applicable to control design of these subsystems. The recursive control (RC) technique is applied in the AFC design because the attitude subsystem is non-affine nonlinear in the control variables. RC is one of the dynamic inversion based methods and is applicable to control design of a class of non-affine nonlinear systems so that there is no iterative numerical computation needed. There is no gain scheduling based technique needed in the AFC design with HDI. The designed AFC laws for the small-scale unmanned helicopter are verified in simulation on the basic 6-DOF nonlinear model of the helicopter. The simulation results demonstrate that the resulting closed-loop system from the AFC law designed with HDI can achieve better flight performances than the closed-loop system resulting from that designed with the linear quadratic regulator in a wide range of flight conditions. The simulation results also show that the closed-loop system resulting from the AFC law designed with HDI is capable of carrying out autonomous flight.Keywords: unmanned aerial vehicle, nonlinear system design method, helicopter flight control design, dynamic inversion, autonomous flight control design.
NomenclatureB gb Transformation matrix from the NED frame to the body frame B ψsr Transformation matrix form the NED frame to the half NED frame B φ Transformation matrix on roll angle B θ Transformation matrix on pitch angle B ψ Transformation matrix on yaw angle B β System matrix in model of main blade flapping motion B δ Control matrix in model of main blade flapping motion C C = cos( ), represents one of φ, θ, ψ, ψ sr C d0 Drag coefficient of main blades C lmr Lift coefficient of main blades C ltr Lift coefficient of tail bladesgains, denotes subscripts F Feedback gain matrices, denotes subscripts except b F b Aerodynamic force in the body frame, N I Identity matrix with appropriate dimensions J Inertial matrix J xMoment of inertia about x-axis of the body frame, kg·m 2 J y Moment of inertia about y-axis of the body frame, kg·m 2 J z Moment of inertia about z-axis of the body frame, kg·m 2 J xz Product of inertia about x-axis and z-axis of the body frame, kg·m 2 M b Aerodynamic moment in the body frame, N·m R mr Span of main blades, m R tr Span of tail blades, m S S = sin( ), represents one of φ, θ, ψ, ψ sr T mr Thrust generated by the main blades, N T tr Thrust generated by tail blades, N T s Sampling period, s V ab Velocity with respect to air flow in the body frame, m/s V gb Velocity with respect to ground in the body frame, m/s V gg Velocity with respect to ground in the NED frame, m/s V gh Horizontal ground speed, m/s V mr An expression of a velocity on main blades, m/s V n Horizontal velocity with respect to grou...