Position tracking control of a helicopter in ground effect using nonlinear disturbance observer-based incremental backstepping approach

Helicopters are highly nonlinear and internally unstable air vehicles, the dynamic response of which can be strongly influenced by flight conditions (wind gust, ground effect, etc.). Therefore, it is a challenging task to design a reliable flight control system (FCS) with all safety and performance requirements satisfied. This paper investigates the robust position tracking problem of a helicopter in ground effect (IGE). Based on a highly reliable and computational efficient finite state representation of rotor flow field IGE, a novel nonlinear disturbance observer-based incremental backstepping (NDOIBS) controller is designed to track position commands (all derivatives of the reference trajectory are known) under the influence of system uncertainties and external disturbances. Without requiring the exact knowledge of helicopter dynamics, the NDOIBS approach guarantees that instant control increments are derived in terms of Lyapunov theory and ensures robustness in the presence of mismatched disturbances whose first derivatives are bounded. It is shown that all state variables of the closed-loop system are semi-globally uniformly ultimately bounded (SGUUB). In addition, to further improve the horizontal position tracking performance, rotor state feedback (RSF) technique and a disturbance feedback strategy are applied to develop a pitch stability augmentation system (SAS). Finally, controller performance is demonstrated through numerical simulations using the Bo-105 utility helicopter. With the efficiency and robustness properties verified, the suggested NDOIBS control scheme shows great potential for implementing advanced FCS designs in existing helicopters.