Redundant Observer-Based Tracking Control for Object Extraction Using a Cable Connected UAV

A novel disturbance observer (DOB) based controller is proposed for quadrotors tethered via an elastic cable to improve performance in object extraction tasks. Unlike a typical rigid connection, the elastic cable offers compliance but introduces forces proportional to the quadrotor’s position, creating challenges in accurately estimating and compensating for external disturbances. The proposed method jointly estimates the vertical disturbance and the stiffness coefficient of the cable by leveraging the redundant measurements across all three translational channels. A pseudoinverse technique is used to determine the observer gain functions, such that the estimation of the two quantities is decoupled and stable. Compared to standard DOB, which assume nearly constant disturbances, the proposed approach can quickly adjust its total force estimate as the tethered quadrotor changes its position or tautness of the tether. Two experimental scenarios are conducted: tracking performance under a constant tether strain, and an object extraction task, where the quadrotor manipulates a nonlinear mechanism mimicking the extraction of a wedged object. In both cases, the proposed controller significantly outperforms standard DOB and extended state observer methods in repeated tests.