Multi-robot Mapping of Lava Tubes.

Terrestrial planetary bodies such as Mars and the Moon are known to harbor volcanic terrain with enclosed lava tube conduits and caves. The shielding from cosmic radiation that they provide makes them a potentially hospitable habitat for life. This motivates the need to explore such lava tubes and assess their potential as locations for future human outposts. Such exploration will likely be conducted by autonomous mobile robots before humans, and this paper proposes a novel mechanism for constructing maps of lava tubes using a multi-robot platform. A key issue in mapping lava tubes is the presence of fine sand that can be found at the bottom of most tubes, as observed on earth. This fine sand makes robot odometry measurements highly prone to errors. To address this issue, this work leverages the ability of a multi-robot system to measure the relative motion of robots using laser range finders. Mounted on each robot is a 2D laser range finder attached to a servo to enable 3D scanning. The lead robot has an easily recognized target panel that allows the follower robot to measure both the relative distance and orientation between robots. First, these measurements are used to enable 2D (SLAM) of a lava tube. Second, the 3D range measurements are fused with the 2D maps via ICP algorithms to construct full 3D representations. Thismethod of 3D mapping does not require odometry measurements or fine-scale environment features. It was validated in a building hallway system, demonstrating successful loop closure and mapping errors on the order of 0.63m over a 79.64m long loop. Error growth models were determined experimentally that indicate the robot localization errors grow at a rate of 20mm per meter travelled, although this is also dependent on the relative orientation of robots localizing each other. Finally, the system was deployed in a lava tube located at Pisgah Crater in the Mojave Desert, CA. Data was collected to generate a full 3D map of the lava tube. Comparison with known measurements taken between two ends of the lava tube indicates the mapping errors were on the order of 1.03m after the robot travelled 32m.