Autonomous Heavy Object Pushing Using a Coaxial Tiltrotor

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. An experimental video can be found at https://youtu.be/Gdmcmoz_UjU
Note to Practitioners
—The motivation of this work is to enable heavy object manipulation, especially pushing operation, using a multirotor platform. To push a heavy object, which has not been treated in existing works, is challenging due to the presence of a large unknown interaction wrench and rotor saturation issue. To resolve this problem, we first build an
$H$
-shaped coaxial tiltrotor that can effectively utilize rotor thrusts to generate horizontal interaction force. Then, a robust controller is designed to address uncertainty in the interaction wrench for the tiltrotor. Finally, we propose an optimization-based control allocation to avoid rotor saturation. Real-world experiments including heavy object pushing and ablation study with respect to the control allocation law are conducted to show the effectiveness of the proposed framework. We validate the use of the coaxial tiltrotor in additional comparative experiments with a conventional multirotor which fails in pushing a
$60$
kg cart. The proposed framework can be utilized in various applications such as disaster relief operations, emergency rescue and aerial delivery.