Adaptive Observer-Based Super-Twisting Sliding Mode Control for Low Altitude Quadcopter Grasping

This article offers an improved robust altitude control solution for an unmanned aerial vehicle (UAV) load grasping system at low altitude under ground-effect and varying loads. We propose a novel technique for adaptive gain selection of the higher-order sliding mode observer (HOSMO). The adaption rate is proportional to the absolute value of the errors computed between the real noisy position measurements and their estimation provided by the observer. In addition, it can adjust to bidirectional disturbance bounds found in UAV grasping applications to avoid overestimation of the gains. The disturbance observer is integrated with a super-twisting sliding mode controller to achieve robust altitude control, effectively attenuating the chattering phenomenon. Moreover, disturbance-based gain conditions are avoided because of the adaptive law for the HOSMO. System stability and finite-time convergence of the adaptive HOSMO are investigated using Lyapunov theory, even in the presence of Lebesgue-measurable noise. Validation is performed through multi-stage simulations using a PX4-powered Clover drone Gazebo simulator and real-time experiments involving low-altitude pick-and-place scenarios with a COEX Clover drone equipped with a rigid gripper mechanism.