Hoverable structure transformation for multirotor UAVs with laterally actuated frame links

Abstract

This paper presents motion planning strategies that assure stable hovering for a transformable unmanned aerial vehicle (UAV) throughout its transformation. The considered multirotor UAV consists of multiple links equipped with rotors. The UAV can transform its structure by changing the lateral joint angle connecting these links. We first introduce the concept of hoverability, which assures static hovering with a linear state feedback controller. Because the hoverability can be evaluated quantitatively by utilizing a convex hull yielded by the positions of rotors, we propose motion planning methods that leverage the hoverability to achieve transformation while guaranteeing stable hovering with desired margin. As the hoverability can be investigated geometrically without analyzing the dynamics of UAVs, the proposed method can be readily employed for a wide class of transformable UAVs, including the one in which clockwise and counterclockwise rotors are not arranged alternately. The proposed motion planning is designed to secure the specified threshold for hoverability and avoid interferences between links or rotors. We then present two types of costs for motion planning: one minimizes the transformation duration, and the other maximizes the hoverability during the transformation. The proposed motion planning is optimized via Dijkstra's algorithm and demonstrated in simulation.