Fig. 1: The Axel Rover

The Axel rover system is a family of platforms aimed at providing versatile mobility for scientific access and human-oriented exploration of planetary surfaces in the solar system. The basic component of this system is the Axel rover shown in Figure 1.

A primary goal of the Axel system design is minimal complexity. Therefore, the basic Axel rover uses a symmetrical design, with only three actuators to control its wheels and a trailing link. The link serves several purposes: it provides a reaction lever arm against wheel thrust, it adjusts the rover's pitch for pointing its stereo cameras, and it provides redundancy if one of the wheel actuators fails. Using only three actuators, this rover is capable of following arbitrary paths, turning-in-place, and operating upside-down or right-side-up. Axel can readily support different wheel types and sizes ranging from large foldable wheels (as shown in Figure 3) to inflatable ones. In this way, it can traverse steep and rocky terrains, and tolerate strong impacts during landing or driving. Additionally, Axel is designed with co-location of its sensors, actuators, electronics, power, and payload inside the central cylinder. This configuration provides compactness for launch, and robustness against environmental extremes in planetary missions. The Axel prototype in Figure 1 is equipped with computational and communication modules, stereo cameras, and an inertial sensor for autonomous navigation with obstacle avoidance.

Fig. 2: Axel concept as a tethered marsupial rover for steep terrain access.

Another major advantage of the single Axel design is its potential use as a part of a larger system. One concept employs a single Axel deployed by tether from a larger rover for access to steep terrain, as shown in Figure 2. Alternatively, Axel rovers can be arranged in a family of configurations to carry larger payload modules, as shown in Figure 3. With planned in-field docking capability, Axels can be used for a variety of exploration purposes before and after payloads are deployed.

JPL led the team which developed the Axel rover in collaboration with Prof. Raymond Cipra at Purdue University and Prof. Murray Clark at Arkansas Tech University. Current work on Axel is done in collaboration with Prof. Joel Burdick of Caltech.

JPL's conceptual development of the Axel system began in 1999, and has been independently paralleled by the University of Minnesota Scout rovers (first published in April 2000). Due the minimalist design of these concepts, Axel and Scout share many features. Some differences of the Axel system are: fully symmetrical design, 360 degree actuated link, and front and back payload module interfaces.

Fig. 3: The Axel Rover Family. Top left and clockwise: (a) Axel in a stowed configuration for flight, (b) an Axel in its surface mobility configuration, (c) an Axel2 with two Axels carrying a single payload module, and (d) and Axel3 with two payload modules.

POC: Issa A.D. Nesnas