Robust 3-D low-altitude airdrop flight control via the sigmoid function-based observer

A robust 3-D trajectory tracking controller for the low-altitude airdrop of the transport aircraft is established in the presence of airflow disturbances, by combining the back-stepping technique and sigmoid function-based disturbance observer. The transport aircraft's nonlinear dynamics during the low-altitude airdrop process is modeled in the affine nonlinear form which consists of the effect of the movement and abrupt drop of the heavy cargo, the ground effect, and the airflow disturbances. The three-dimensional airdrop flight controller design is divided into several cascade subsystems, via the back-stepping technique. In each subsystem dynamic, items caused by the disturbances during the extraction are viewed as part of the "lumped disturbances". They are individually reconstituted and compensated via the sigmoid function-based disturbance observers with high estimation accuracy and nice disturbance attenuation ability. With the estimated the lumped disturbances, an anti-disturbance 3-D back-stepping based controller is proposed for the low-altitude airdrop. Simulations are carried out to verify the proposed control method's effectiveness in improving the robustness and tracking accuracy.