Falling in line: Visual route following on extreme terrain for a tethered mobile robot

This paper describes visual route following for a cliff-climbing, tethered mobile robot for the purpose of autonomously traversing extreme terrain in the presence of obstacles. When the robot's tether contacts an obstacle, an intermediate anchor is formed. In order to detach from intermediate anchors and avoid entanglement, the robot must backtrack along its outgoing trajectory. We use the Visual Teach & Repeat (VT&R) algorithm to autonomously repeat a manually taught path. However, our problem is complicated by the fact that the robot's tether must (i) remain taut regardless of inclination, (ii) allow the robot to drive freely, and (iii) provide motion assistance when wheel traction is reduced on steep slopes. To enable visual route following over varied terrain, we have developed a novel tether controller that selects a safe, steady-state tension based on the robot's inclination while also accounting for vehicle motion. Experiments are performed on our Tethered Robotic Explorer (TReX), which autonomously repeats paths while tethered in both flat-indoor and steep-outdoor environments in the presence of obstacles.