Autonomous Heavy Object Pushing Using a Coaxial Tiltrotor

* Status: Accepted for publication in IEEE Transactions on Automation Science and Engineering (T-ASE)
* Authors: Sunwoo Hwang¹, Dongjae Lee¹, Changhyeon Kim, and H. Jin Kim (The first two authors contributed equally to this work)
* Abstract: Aerial physical interaction (APhI) with a multirotor-based platform such as pushing a heavy object demands generation of a sufficiently large interaction force while maintaining the stability. Such requirement can cause rotor saturation, because the rotor thrust enlarged for interaction force may leave a reduced margin for attitude stabilization. We first design an H-shaped coaxial tiltrotor that can generate a sufficiently large interaction force than a conventional multirotor. We then propose an overall framework composed of high-level robust controller and low-level control allocation for the coaxial tiltrotor to ensure robustness against uncertain motion of the unknown interacting object and to overcome the saturation issue. To guarantee the robustness at all time, we design a controller based on a nonlinear disturbance observer (DOB). Then, we formulate a problem of computing low-level actuator inputs avoiding rotor saturation as a tractable nonlinear optimization problem, which can be solved real-time. The proposed framework is validated in extensive real-world experiments where the 3.3 kg tiltrotor successfully pushes a cart weighing up to 60 kg. An ablation study with the tiltrotor shows effectiveness of the proposed control allocation law in avoiding rotor saturation. Furthermore, a comparative experiment with a conventional multirotor shows failure in the same setting, which validates the use of the coaxial tiltrotor.
* Contact: [e-mail] (Sunwoo Hwang), [e-mail] (Dongjae Lee)