Field Deployment of the Tethered Robotic eXplorer to Map Extremely Steep Terrain

Abstract

Mobile robots outfitted with a supportive tether are ideal for gaining access to extreme environments for mapping when human or remote observation is not possible. This paper details a field deployment with the (TReX) to map a steep, tree-covered rock outcrop in an open-pit gravel mine. TReX is a mobile robot designed for the purpose of mapping extremely steep and cluttered environments for geologic and infrastructure inspection. Mapping is accomplished with a 2D lidar fixed to an actuated tether spool, which rotates to produce a 3D scan only when the robot drives and manages its tether. In order to handle motion distortion, we evaluate two existing, real-time approaches to estimate the trajectory of the robot and rectify individual scans before alignment into the map: (i) a continuous-time, lidar-only approach that handles asynchronous measurements using a physically motivated, constant-velocity motion prior, and (ii) a method that computes visual odometry from streaming stereo images to use as a motion estimate during scan collection.Once rectified, individual scans are matched to the global map by an efficient variant of the ICP algorithm. Our results include a comparison of estimated maps and trajectories to ground truth (measured by a remote survey station), an example of mapping in highly cluttered terrain, and lessons learned from the deployment and continued development of TReX.