Adaptive continuous Quasi-Fixed-Time integral terminal sliding mode attitude control for BiFlying-Wings tail-sitter unmanned aerial vehicles during flight mode transition

To address the attitude control problem of biplane tail-sitter unmanned aerial vehicles (BFWTSUAV) during flight mode transitions, which are susceptible to various uncertainties such as unmodeled nonlinear dynamics and external disturbances, this paper proposes an adaptive continuous Quasi-Fixed-Time integral terminal sliding mode controller (ACQFITSMC). The proposed ACQFITSMC combines the advantages of Quasi-Fixed-Time terminal sliding mode, integral sliding mode, and adaptive estimation techniques, ensuring fixed-time convergence of attitude errors, strong robustness, and effective suppression of chattering. Unlike traditional methods, the proposed controller incorporates smooth reference attitude and angular velocity planning, which significantly enhances the system’s dynamic performance, suppressing overshoot and actuator saturation. Theoretical analysis demonstrates that the proposed method guarantees practical stability with fixed-time convergence of the closed-loop system. Comparative simulations with the incremental nonlinear dynamic inversion (INDI) controllers, which are widely used for tail-sitter UAVs and existing fixed-time terminal sliding mode controllers, verify the superior performance of the proposed controller. Finally, outdoor flight experiments validate the practicality and superiority of the proposed controller in real-world applications.¹