Flying Calligrapher: Contact-Aware Motion and Force Planning and Control for Aerial Manipulation

TL;DR

To enable aerial manipulation with time-varying contact forces, the authors develop a contact-aware trajectory planner and a hybrid motion-force controller for a fully actuated hexarotor. The system accepts target strokes via touchscreen or manual waypoints, plans dynamically feasible motion-force trajectories that respect friction and surface constraints, and robustly tracks them while visualizing stroke linewidth through a sponge-pen end-effector. A novel end-effector and an integrated touchscreen interface enable intuitive, high-precision aerial calligraphy. Experimental results on letters demonstrate motion and force tracking with an IoU of $0.59$, end-effector RMSE of $2.9\text{ cm}$, and force RMSE of $0.7\text{ N}$, validating the approach for general aerial manipulation tasks involving variable contact forces.

Abstract

Aerial manipulation has gained interest in completing high-altitude tasks that are challenging for human workers, such as contact inspection and defect detection, etc. Previous research has focused on maintaining static contact points or forces. This letter addresses a more general and dynamic task: simultaneously tracking time-varying contact force in the surface normal direction and motion trajectories on tangential surfaces. We propose a pipeline that includes a contact-aware trajectory planner to generate dynamically feasible trajectories, and a hybrid motion-force controller to track such trajectories. We demonstrate the approach in an aerial calligraphy task using a novel sponge pen design as the end-effector, whose stroke width is positively related to the contact force. Additionally, we develop a touchscreen interface for flexible user input. Experiments show our method can effectively draw diverse letters, achieving an IoU of 0.59 and an end-effector position (force) tracking RMSE of 2.9 cm (0.7 N). Website: https://xiaofeng-guo.github.io/flying-calligrapher/

Figures (9)

• Figure 1: (a) Our aerial manipulator is writing a Chinese calligraphy artwork on the wall. (b) "AIR" and (c) "2024" written by our system.
• Figure 2: Pipeline of the proposed system. Users either manually define the target strokes or draw the target letter through a touchscreen interface. Then, the trajectory planning module generates a dynamic-feasible motion-force trajectory. A hybrid motion and force controller is designed for the UAM to track the reference trajectory. Finally, a novel end-effector pen is designed for the UAM to draw the target letters.
• Figure 3: The coordinate frames used in this paper. Specifically, $\mathcal{W}$, $\mathcal{B}$, $\mathcal{E}$, $\mathcal{S}$, and $\mathcal{C}$ stand for the world, vehicle body, end-effector, arm, and contact frame, respectively.
• Figure 4: System overview. (a). Side-view of our UAM platform. (b). Design of the sponge pen as the end-effector. (c). Calibrated stroke linewidth and contact force relation.
• Figure 5: Results of three repeated trials for the letter 'A'. Top row: Final written image. Middle two rows: Time history of end-effector position and contact force tracking performance. Bottom row: visual comparison between the written letter and ground truth image, evaluating the overall writing performance.