Skater: A Novel Bi-modal Bi-copter Robot for Adaptive Locomotion in Air and Diverse Terrain

TL;DR

This work tackles the challenge of robust, efficient locomotion for aerial-ground robots by introducing Skater, a longitudinal bi-copter with two passive wheels and a unified actuation system. A unified dynamic model and differential-flatness analysis are developed for both aerial and ground modes, enabling a single NMPC framework to track trajectories and switch modalities smoothly. Real-world experiments and benchmarks demonstrate strong terrain traversability, enhanced steering via vectored thrust, and substantial ground-energy savings, validating the approach and its practical potential. The findings offer a compact, versatile platform and a control paradigm that can guide future multimodal robot designs for diverse terrains.

Abstract

In this letter, we present a novel bi-modal bi-copter robot called Skater, which is adaptable to air and various ground surfaces. Skater consists of a bi-copter moving along its longitudinal direction with two passive wheels on both sides. Using a longitudinally arranged bi-copter as the unified actuation system for both aerial and ground modes, this robot not only keeps a concise and lightweight mechanism but also possesses exceptional terrain traversing capability and strong steering capacity. Moreover, leveraging the vectored thrust characteristic of bi-copters, the Skater can actively generate the centripetal force needed for steering, enabling it to achieve stable movement even on slippery surfaces. Furthermore, we model the comprehensive dynamics of the Skater, analyze its differential flatness, and introduce a controller using nonlinear model predictive control for trajectory tracking. The outstanding performance of the system is verified by extensive real-world experiments and benchmark comparisons.

Figures (12)

• Figure 1: Real-world trajectory tracking experiments. (a) Snapshot of aerial-ground hybrid trajectory tracking test. (b-c) Snapshots of trajectory tracking tests on slippery and rough ground surfaces.
• Figure 2: Prototype of the proposed bi-modal bi-copter robot.
• Figure 3: Theoretical minimum traversing width of common multicopter configurations, which is typically designed to utilize the minimum number of actuators necessary for a specified number of rotors. The width of single-rotor aircrafts is used as the baseline.
• Figure 4: Illustration of the hardware details for the proposed system.
• Figure 5: Coordinate definitions and force analysis. $f_r$, $f_l$, and $F_n$ represent the ground rolling friction force, lateral friction force, and normal force on the wheels, with 'left' and 'right' subscripts indicating their application to the left and right wheels.