To be able to conduct science investigations on highly sloped and challenging terrains, it is necessary to deploy science payloads to such locations and collect and process in situ samples. A tethered robotic platform has been developed that is capable of exploring very challenging terrain. The Axel rover is a symmetrical rover that is minimally actuated, can traverse arbitrary paths, and operate upside-down or right-side up. It can be deployed from a larger platform (rover, lander, or aerobot) or from a dual Axel configuration. Axel carries and manages its own tether, reducing damage to the tether during operations.

The third-generation Axel Two-Wheeled Rover features a symmetric body and a trailing link, and uses only four primary actuators to control its wheels, tether, and trailing link.
Fundamentally, Axel is a two-wheeled rover with a symmetric body and a trailing link. Because the primary goal is minimal complexity, this version of the Axel rover uses only four primary actuators to control its wheels, tether, and a trailing link. A fifth actuator is used for level winding of tether onto Axel’s spool.

The link serves multiple purposes: it provides a reaction lever arm against wheel thrust, it adjusts the rover’s pitch for pointing its sensors and sampling devices, and it provides redundancy if one of the wheel actuators fails. Turning the trailing link into the ground in lieu of driving the wheels causes the rover body to roll and leads to forward motion of the rover. This tumbling mode of operation has several advantages for operating on slopes and for rolling off rocks if the rover high-centers its body on a rock. Using its tether, Axel is capable of driving down and up steep crater walls and lowering itself down from overhangs or into caves. Running the tether through the trailing link gives Axel greater stability and provides a restoring force for the link, keeping it off the ground during steep slope operations.

Because of its simple design, Axel can readily support different wheel types and sizes ranging from large, foldable wheels to inflatable ones. In this way, it can traverse steep and rocky terrains and tolerate strong impacts during landing or driving. In the case of umbrella foldable wheels, it can change its wheel size depending on the terrain roughness and corresponding rock sizes. Axel wheels have been designed with paddles to enable the rover to traverse rocks that are a wheel radius tall.

This generation of Axel has two science bays, which are large cylinders that fit into and are covered by Axel’s cantilevered wheels. These bays can accommodate up to four small science instruments each. The contact instruments are deployed to the ground via a single-degree-of-freedom, four-bar mechanism. Some optical instruments do not require any deployment and can operate after pointing these sensors using the body actuators. Sampling devices such as a scoop or coring drill may also be deployed by the four-bar mechanism.

Axel co-locates its sensors, actuators, electronics, power, and payload inside the central cylinder and science bays. This configuration provides compactness for launch, and robustness against environmental extremes in planetary missions. The Axel rover is equipped with science instruments, computational and communication modules, stereo cameras, and an inertial sensor for autonomous navigation with obstacle avoidance. Conductors inside the tether allow for the deployed Axel to be charged from and communicate with the parts of the system that remain topside.

A mission can use a single or multiple low-mass Axel rovers to explore and sample high-risk sites. This class of rovers provides new capabilities for steep terrain and cave exploration and sampling beyond what is offered by current state-of-the-art rovers.

This work was done by Issa A. Nesnas, Jaret B. Matthews, Jeffrey A. Edlund, Joel W. Burdick, and Pablo Abad-Manterola of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.


This Brief includes a Technical Support Package (TSP).
Document cover
Axel Robotic Platform for Crater and Extreme Terrain Exploration

(reference NPO-47890) is currently available for download from the TSP library.

Don't have an account? Sign up here.