Time-varying dynamic modeling and trajectory tracking control for variable-load unmanned aerial vehicle

The dynamic variation issues of variable-load unmanned aerial vehicle (UAV) used in agricultural plant protection activities was addressed by a disturbance-resistant control system based on PD (proportional-derivative) sliding mode control. First, a time-varying dynamic model was developed by analyzing the variations in mass, center of gravity and moment of inertia across time. Then a trajectory tracking control approach based on PD sliding mode control was designed to develop an inner-loop attitude controller and an outer-loop trajectory controller to accomplish precise and closely coupled trajectory tracking. Numerical simulations were conducted to verify the trajectory tracking performance, demonstrating accurate tracking of the desired trajectory with standard deviations of 0.0507, 0.1613 and 0.0002 m in the horizontal, lateral and vertical directions, respectively. In terms of attitude control, the system exhibited favorable performance on the roll, pitch and yaw axes, with small transient errors and rapid convergence. Flight experiments further demonstrated that the UAV accurately followed the specified path, and errors in both straight and twisting segments satisfied control criteria. This control system ensured efficient and steady trajectory tracking, offering theoretical and application references for intelligent and precise agricultural plant protection activities.