Nonlinear Robust Controller for Miniature Helicopters Without Singularity

A singularity-free nonlinear robust controller is proposed for miniature helicopters to accomplish trajectory tracking missions. The helicopter model is decomposed into a cascaded structure with some unmodeled dynamics. The controller development applies the hierarchical strategy. First, a position loop controller is designed based on a saturation control scheme using differential hyperbolic tangent functions, and criteria for choosing controller parameters are established to ensure the finite-time position tracking and to obviate singularity in the command attitude extraction. Singularity may also occur during the attitude tracking when the pitch of the helicopter reaches $\pm \frac{\pi }{2}$ . Based on a barrier Lyapunov function, an attitude loop controller is put forward with a backstepping technique, and an initial condition constraint for the nonsingular attitude tracking is deduced. Simulations illustrate the applicability of the proposed controller.