Towards Articulated Mobility and Efficient Docking for the DuAxel Tethered Robot System

Sites of increasing interest for planetary science, such as craters, cold traps, and vents lie in terrains that are inaccessible to state-of-the-art rovers. The Jet Propulsion Laboratory, in collaboration with Caltech, is actively developing a tethered mobile robot, Axel, for traversing and exploring extremely steep terrain, such as Recurring Slope Lineae on Mars and vertical pits on the Moon. However, on Mars, where landing-site ellipses are large due to the presence of an atmosphere, Axel may need to traverse several kilometers from its lander untethered given its finite tether carrying capacity (~300 m). This paper proposes a novel design for a hybrid mobility system that allows a pair of Axel rovers to dock, lock, and drive long distances as a four-wheeled, articulated steering vehicle. The design improves upon prior efforts to achieve DuAxel mobility by leveraging two actuated docking mechanisms attached on opposite ends of a central module to enable `sit/stand' functionality; the prior DuAxel system was limited to skid steering, which was inefficient due to Axel's grouser-style, high-friction wheels. In the proposed system, the `sit' configuration is achieved by aligning each dock parallel to the surface, allowing one Axel to detach and explore while the other remains docked and serves as a backup. While `sitting', the central module rests on the ground and is outfitted with shovel-style wedges for passive anchoring to sandy terrain (an optional drill can be integrated for anchoring to rock). In order to `stand', the exploring Axel reattaches, locks, and both docks are rotated until Axel's tether caster arm is upright and the central module is lifted off the ground. Once upright, each Axel rotates about a pivot point for articulated, all-wheel steering, which is accomplished by applying differential wheel torques. The main contributions of this paper are i) a detailed systems design of the docking mechanism and central module, ii) kinematic modeling of articulated mobility and `sit/stand' docking functionality, and iii) initial testing in a relevant environment to characterize the mobility of the proposed system.