Progress in Development of the Axel Rovers

NASA’s Jet Propulsion Laboratory, Pasadena, California

Progress has been made in the development of a family of robotic land vehicles having modular and minimalist design features chosen to impart a combination of robustness, reliability, and versatility. These vehicles at earlier stages of development were described in two previous NASA Tech Briefs articles: “Recon figurable Exploratory Robotic Vehicles” (NPO-20944), Vol. 25, No. 7 (July 2001), page 56; and “More About Reconfigurable Exploratory Robotic Vehicles” (NPO-30890), Vol. 33, No. 8 (August 2009), page 40. Conceived for use in exploration of the surfaces of Mars and other remote planets, these vehicles could also be adapted to terrestrial applications, including exploration of volcanic craters or other hostile terrain, military re connaissance, in spection of hazardous sites, and searching for victims of earthquakes, landslides, avalanches, or mining accidents. In addition, simplified versions of these vehicles might be marketable as toys.

Figure 1. A Prototype Axel Rover can operate in a free or tethered configuration. Testing of this version is complicated by a need to properly wrap the power cable around the axle housing. It has been proposed to eliminate the power cable and install rechargeable batteries in a future version.

The most basic module in this family of reconfigurable robots is the Axel rover, which has a cylindrical body with two main wheels and a trailing link. Inside its body are three motors and associated mechanisms for driving the two wheels and for rotating the link 360º around its symmetrical body. The actuated link serves several purposes:

It is used as a lever arm to react to the wheels thrust to move Axel in multiple directions.
It is used to rotate the Axel housing in order to tilt, to the desired angle, any sensors and instruments mounted on or in the Axel housing.
It provides an alternative mobility mode, which is primarily used in its tethered configuration. Turning the link into the ground in lieu of driving the wheels causes the Axel housing and wheels to roll as a unit and thereby leads to a tumbling motion along the ground. With a tether mounted around Axel’s cylindrical body, the link serves as a winch mechanism to reel and unreel the tether raising and lowering Axel over steep and vertical surfaces (Figure 1).

Figure 2. Extended Axel Configurations show design versatility.

Sensors, computation, and communication modules are also housed inside Axel’s body. A pair of stereo vision cameras provides three-dimensional view for autonomous navigation and avoiding obstacles. Inertial sensors determine the tilt of the robot and are used for estimating its motion. In a fully developed version, power would be supplied by rechargeable batteries aboard Axel; at the time of reporting the information for this article, power was supplied from an external source via a cable.

In and of itself, the Axel rover is fully capable of traversing and sampling terrains on planetary surfaces. By use of only the two main wheel actuators and the caster link actuator, Axel can be made to follow an arbitrary path, turn in place, and operate upside-down or right-side-up. If operated in a tethered configuration, as shown in Figure 1, it can be made to move down and up a steep crater wall, descend from an overhang to a cave, and ascend from the cave back to the overhang, all by use of the same three actuators. Such tethered operation could be useful in searching for accident victims or missing persons in mines, caves, and rubble piles. Running the tether through the caster link enhances the stability of Axel and provides a restoring force that keeps the link off the ground for the most part during operation on a steep slope.

In its extended configuration, two Axel modules can dock to either side of a payload module to form the four wheeled Axel2 rover (Figure 2). Additional payload and Axel modules can dock to either side of the Axel2 to form the Axel3 rover, extending its payload capacity and its mobility capabilities.

This work was done by Issa A. Nesnas, Daniel M. Helmick, Richard A. Volpe of JPL, Pablo Abad-Manterola, and Jeffrey A. Edlund of Caltech; Raymond Cipra and Damon Sisk of Purdue University; and Raymond H. Christian and Murray R. Clark of Arkansas Tech University for NASA’s Jet Propulsion Laboratory. NPO-45553