IEEE Robotics & Automation Magazine
44
1070-9932/09/$26.00
ª
2009 IEEE
DECEMBER
2009
Axel
A Minimalist Tethered Rover
for Exploration of Extreme
Planetary Terrains
BY PABLO ABAD-MANTEROLA,
JEFFREY A. EDLUND, JOEL W. BURDICK,
ALBERT WU, THOMAS OLIVER,
ISSA A.D. NESNAS, AND JOHANNA CECAVA
R
ecent scientific findings suggest that some of the most
interesting sites for future exploration of planetary
surfaces lie in terrains that are currently inaccessible
to conventional robotic rovers. To provide robust
and flexible access to these terrains, we have been
developing Axel, the robotic rover. Axel is a lightweight two-
wheeled vehicle that can access steep terrains and negotiate
relatively large obstacles because of its actively managed tether
and novel wheel design. This article reviews the Axel system
and focuses on those system components that affect Axel’s
steep terrain mobility. Experimental demonstrations of Axel
on sloped and rocky terrains are presented.
Motivation
Despite the great successes of the Mars Exploration Rovers
(MERs) [1], some of the richest potential science targets for future
exploration missions lie in terrains that are inaccessible to state-of-
the-art Martian rovers, thereby limiting our ability to carry out in
situ analysis of these rich opportunities. For example, bright new
deposits, which may be ice flows, have been discovered hundreds
of meters below the rims of steep craters in the Centauri Montes
region on Mars (Figure 1). While the Opportunity rover has
imaged layers of bedrock in the vertical promontories of Cape St.
Vincent in Victoria crater, these g
eological features are currently
inaccessible to conventional sampling methods. The high-reso-
lution orbiter images of stratified deposits of ice and dust reveal a
very challenging terrain, which, if it could be navigated, would
provide important clues to the geological and hydrological past
of Mars [2]. The recently reported Martian methane plumes [3]
rise over heavily cratered terrains in the Arabia Terra and Syrtis
Major regions. Without new mobility platforms, it will be diffi-
cult to directly access the surface of this region to assess if the
methane comes from a biological or geological origin. Similarly,
Titan, Europa, Enceladus, and the Earth’s moon also offer chal-
lenging surface features with associated scientific targets. A new
generation of planetary exploration robots is needed to access
the challenging terrains to probe, sample, and measure. New
inquiries of this sort could lead to significant scientific rewards.
Robotic Mobility for Extreme Terrain
Mechanisms and algorithms for robotic mobility in steep and
complex terrains have been investigated for several decades. Pro-
posed approaches include multilegged quasistatic walkers [4],
bipedal walkers [5], hopping machines [6], snakelike mechanisms
[7], and wheeled vehicles with complex wheel designs [8] or
chassis [9]. Critical issues in evaluating the viability of these
approaches for space applications include the robustness and
mechanical complexity inherent with the approach, the total sys-
tem mass, the energy required per traverse distance, the ability to
carry out in situ scientific studies and sample gathering, and the
ability to recover from faults. Most previously proposed methods
have one or more shortcomings with respect to these criteria.
For exploring challenging topographies, an actively con-
trolled tether combined with a conventional mobility platform
Digital Object Identifier 10.1109/MRA.2009.934821
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Authorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on January 8, 2010 at 14:50 from IEEE Xplore. Restrictions apply.
(using, e.g., wheels, legs, or tracks) may provide a useful means
to enable very steep terrain access. One such example was the
Dante tethered robot [10] that descended into the Mt. Spurr
volcano in 1994 using its tether and an eight-legged walking
frame. In the 1990s, following orbital imagery of Mars stratig-
raphy, a number of different mission concepts were proposed
for in situ science investigations that included legged and
wheeled robots. The Cliff-bot [11] was an example of a
wheeled robot that used a dual tether system to help manage
its traverse across a cliff face, which has been demonstrated on
cliff faces in Svalbard, Norway. In addition to the legged and
multiple-wheeled robot approaches, some of the earlier con-
cepts also advocated the potential advantages of using light-
weight tethered platforms, although none of these efforts led
to an implementation.
The Axel System: Overview
To provide access and in situ sampling in areas of extreme ter-
rain, the Jet Propulsion Laboratory (JPL) and the California
Institute of Technology (Caltech) have been collaborating to
develop the Axel rover. Axel is a minimalistic robot consisting
of two wheels connected by a central cylindrical body, a caster
arm, and an actively controlled tether passing through the caster
arm (Figure 2). The caster arm, in addition to controlling the
tether, also provides a reaction force against the terrain necessary
to generate forward motion when traveling on flat ground.
Axel’s minimalist design overcomes some of the limitations
found in prior tethered robots. Dante’s operation on Mt. Spurr
was cut short when it tipped over; it had no built-in mecha-
nism to recover an upright posture. Because of its symmetry,
Axel has no upside
–
down posture and thus does not suffer
from this failure mode. Cliffbot similarly has no tip-over
recovery and uses two tethers. Like Dante, Axel’s tether is paid
out by an onboard motor, an advantage compared with Cliff-
bot’s more complicated offboard tether-management system.
Because of Axel’s low mass, onboard battery, and wireless com-
munication link, the tether is a simple, high-strength cord as
opposed to Dante’s heavy tether with embedded power and
signal conductors.
Axel’s minimalist design satisfies many of the severe con-
straints imposed by space mission design. Because the rover
uses only three actuators to control its wheels, caster arm, and
tether, its total mass is low (the current prototype weighs
approximately 22 kg, and we expect a smaller flight-qualified
version with a mass of approximately 8
–
10 kg). Its simplistic
design improves mechanical robustness. All of its electronic
components can be centralized in the body, simplifying
thermal control design for operation in extreme cold.
Mission Concept
We expect Axel to operate in hazardous terrain via the use of a
host platform as an anchor. Since Axel’s body acts as a winch,
the host platform requirements are reduced to a simple mount.
The host platform could be a lander, a larger rover, a habitat,
or even an astronaut. Once the anchor point has been secured,
Axel can descend overstep promontories, navigate through
rocky terrain, take images, collect soil samples, and then return
by reeling in its tether. Figure 3 portrays a hypothetical
scenario in which Axel is deployed from the Mars Science
Laboratory (MSL) [12], an example of a host platform that
could potentially carry an Axel as a method of sampling in
extreme terrain.
There are some key advantages to this tethered approach for
planetary exploration missions. The risk to overall mission suc-
cess of descending into craters or similar topographies is mini-
mized, as the host can detach Axel’s tether should it fail and
then continue with other mission objectives. Axel is also small
and light enough for more than one copy to be hosted from an
600 m
New Deposit
September 2005
August 1999
Figure 1.
Photos from the Mars Global Surveyor orbiter
camera showing recent flows in a crater of the Centauri
Montes region. (Courtesy of NASA’s Mars Global Surveyor.)
Cameras
Paddle Wheels
Caster Arm
Sampling Device
Tether
Figure 2.
Photograph of Axel with key features labeled.
Axel is a lightweight two-wheeled
vehicle that can access steep
terrains and negotiate relatively
large obstacles.
IEEE Robotics & Automation Magazine
DECEMBER
2009
45
Authorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on January 8, 2010 at 14:50 from IEEE Xplore. Restrictions apply.