ATHLETE (All-Terrain Hex-Limbed Extra-Terrestrial Explorer) is a multipurpose mobility platform for planetary surfaces. It is a cross between a wheeled rover and a walking robot, and travels using powered wheels mounted on the end of each of six robotic limbs. Each limb is a fully articulated robotic manipulator with six or seven degrees of freedom.
ATHLETE’s wheel-on-limb mobility design provides great flexibility for mobility over different types of surface features. Using the wheels to drive is highly efficient over benign terrain. However, to support the limbed design, ATHLETE uses wheels with lower torque and smaller diameter relative to the robot's size and weight, as compared to wheeled-only rovers like the Spirit and the Opportunity. This lowers the overall surface pressure on the wheels, making ATHLETE vulnerable to slipping or embedding in soft terrain, and wheel stalling on steep terrain when traction is good.
ATHLETE’s limbed design makes it possible for the robot to extract itself from such situations by locking the wheels and employing a walking gait. While these two mobility modes, driving and walking, provide ATHLETE with mobility solutions over any type of terrain, ATHLETE’ s complexity and flexibility make possible the development of hybrid mobility modes that combine aspects of both driving and walking. A mobility mode that enables faster forward progress than walking over terrain unsuitable for driving is particularly desirable.
A new concept was developed for a walking-driving hybrid in which wheels are repositioned by sliding them along the ground, rolling the wheels in coordination with the motion of the limb. This mobility mode is referred to as the Sliding Gait, or SGait. For soft, loose, or steep terrain, in which wheeled roving fails, SGait enables mobility by allowing some wheels to be repositioned while other wheels act as anchors. Because all six wheels maintain contact with the ground at all times, SGait motions easily maintain a conservative polygon of support, allowing efficient multi-limb gaits like the alternating tripod gait to be used for a significant time savings over free walking. Using force feedback while rolling wheels extends the usefulness of SGait to terrain that is rutted, bumpy, or strewn with small obstacles, as force control can be employed to maintain favorable wheel loading as the limb complies to terrain features.
The SGait algorithm uses the kinematic capability of ATHLETE’s limbs to reposition wheels independently of the robot’s payload deck while keeping them in contact with the ground. The wheel is rolled along the ground in coordination with limb repositioning. This can be done for individual wheels or groups of wheels, allowing the approximation of any six-limbed walking gait. Between or during wheel repositioning motions, the payload deck is shifted to maintain a desired stable position above the wheels.
The SGait concept is similar to the inching concept, but is made much more capable by the maneuverability of the wheel-on-limb ATHLETE mobility system. With inching, each expansion and contraction of the suspension system carries the rover center of gravity (CG) forward. In the SGait implementation, however, the kinematic capability of each limb allows the wheels to be repositioned with minimal effect on the CG position, which further reduces slipping in soft terrain. In addition, SGait can conform to a variety of terrain types, making the algorithm effective over a wider range of surface conditions.
This work was done by Julie A. Townsend, Curtis L. Collins, and Jeffrey J. Biesiadecki of Caltech for NASA’s Jet Propulsion Laboratory.