Motion Planning for the ATHLETE Rover with Reinforcement Learning

Legged locomotion is attractive because it can enable a robot to traverse far more varied terrain than a wheeled rover is capable of. In the context of planetary exploration, this is especially attractive as the sites of greatest scientific interest tend to be characterized by difficult terrain. For example, it would be extremely difficult for a wheeled rover to make its way into a lunar crater in search of water. Planning legged locomotion is, however, a more difficult problem the wheeled lomotion. Compared to wheeled robots, legged robots tend to have a much larger number of degrees of freedom. A planner for a legged robot therefore has to plan in a high-dimensional configuration space, placing considerable demands on the planner’s efficiency. The problem is further complicated when uneven terrain is considered. Classical motion planning techniques applied to legged locomotion generally assume a flat workspace with obstacles to be avoided. In the case of uneven terrain, there are no explicit obstacles, but a motion planner must take care to maintain the stability of the robot at all times while moving over sloped and uneven surfaces. For articulated legs, contact with the ground creates a closed-loop kinematic chain. Algorithms such as probabilistic roadmaps must be adapted to efficiently handle these closedloop constraints, as a randomly sampled point in the configuration space has zero probability of exactly satisfying them. Especially when the robot is intended for cooperative tasks with humans, an additional problem is planning trajectories which appear natural to a human observer. A planner for a humanoid bipedal robot, for example, might generate bizarre-looking arm motions which aid in balance, even on relatively flat terrain where such motions are unnecessary. A planner for legged locomotion should ideally encode constraints that encourage natural-looking motion. The specific robot considered in this paper is the ATHLETE (All-Terrain Hex-Limbed Extra-Terrestrial Explorer) robot developed by the Jet Propulsion Laboratory (JPL). ATHLETE is intended to be a lunar rover, and is especially designed for movement over broken and uneven terrain. Its hexagonal frame is designed for carrying large Fig. 1. The ATHLETE rover