Hybrid iteration sliding mode control for ultra-low altitude airdrop level off

For the ultra-low altitude airdrop level off stage, many uncertain factors such as ground effect, sensor measurement errors and low altitude airflow interfere the precision of trajectory tracking, which exert serious threats on the aircraft safety and mission performance, a hybrid iteration sliding mode flight controller was designed. In the first order sliding mode, a global dynamic switching function was adopted, which eliminated the reaching stage of sliding mode and ensured the whole response robustness. In the second order sliding mode, a nonlinear integral switching function was adopted and the overshoot caused by the integral term was brought to the first order sliding mode, so not only the tracking accuracy was guaranteed but also the dynamic performance was improved. Lyapunov stability theory and Barbalat lemma analysis show that the flight control law can completely reject constant model perturbation and disturbances, and can control the upper bounds of the steady errors with dynamic model perturbation and disturbances. Simulation results confirm the robustness and ascendant tracking performance of the proposed approach.