Trajectory tracking of small helicopters using explicit nonlinear MPC and DOBC

Small-scale helicopters are very attractive for a wide range of applications due to their unique features. However, autonomous flight of a small helicopter is quite challenging because it is naturally unstable, has strong nonlinearities and couplings, and are very susceptible to wind and small variation of helicopter structures. A robust nonlinear control scheme is proposed to address these issues. It consists of a nonlinear model predictive controller and a nonlinear disturbance observer. First, an analytic solution for the nonlinear MPC is developed based on the nominal model and under the assumption that all disturbances are measurable. Then a nonlinear disturbance observer is designed to estimate the influence of external force/torque inducted by wind turbulence, unmodelled dynamics and variations of the helicopter structure. Simulations and flight tests including hovering under wind gust have been carried out to demonstrate the performance of the proposed control scheme.