ATHLETE: A limbed vehicle for solar system exploration

As part of the Human-Robot Systems project funded by NASA, the Jet Propulsion Laboratory has developed a vehicle called ATHLETE: the All-Terrain Hex-Limbed Extra-Terrestrial Explorer. Each vehicle is based on six wheels at the ends of six multi-degree-of-freedom limbs. Because each limb has enough degrees of freedom for use as a general-purpose leg, the wheels can be locked and used as feet to walk out of excessively soft or other extreme terrain. Since the vehicle has this alternative mode of traversing through or at least out of extreme terrain, the wheels and wheel actuators can be sized for nominal terrain. There are substantial mass savings in the wheel and wheel actuators associated with designing for nominal instead of extreme terrain. These mass savings are comparable-to or larger-than the extra mass associated with the articulated limbs. As a result, the entire mobility system, including wheels and limbs, can be about 25% lighter than a conventional mobility chassis. A side benefit of this approach is that each limb has sufficient degrees-of-freedom to use as a general-purpose manipulator (hence the name "limb" instead of "leg"). Our prototype ATHLETE vehicles have quick-disconnect tool adapters on the limbs that allow tools to be drawn out of a "tool belt" and maneuvered by the limb. A power-take-off from the wheel actuates the tools, so that they can take advantage of the 1+ horsepower motor in each wheel to enable drilling, gripping or other power-tool functions. This paper describes the applicability of the ATHLETE concept to exploration of the moon, Mars and Near-Earth Asteroids (NEAs). Recently, the focus of human exploration beyond LEO has been on NEAs. One scenario for exploration of a NEA has been likened to a submarine exploring a wrecked ship - humans would sit in a "bubble" and approach the asteroid for up-close examination and robotic manipulation. What is important is to ensure that the bubble doesn't collide with the asteroid surface, nor float away. Multiple limbs, such as available on ATHLETE, allow for precise positioning and anchoring so as to enable the human bubble to maximize its exploration potential. A microgravity testbed has been constructed in the ATHLETE lab, with six computer-controlled winches able to lift ATHLETE and payloads so as to simulate the motion of the system in the vicinity of a NEA. Accurate 6-axis force-torque sensors will measure the applied forces and moments wherever the vehicle touches a simulated asteroid surface. These measured forces can be used to compute the resultant motion of the vehicle in the microgravity environment, and the winches then move the vehicle along the computed trajectory. Preliminary test results from this system are described.