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.

Mobile robots supported by an electromechanical tether can safely explore extremely rugged terrain in resource-limited environments. While a tether provides power, wired communication, and support on steep surfaces, it also reduces maneuverability; in cluttered environments the tether will contact obstacles, forming intermediate anchor points. In order for the robot to avoid tether entanglement, it must localize itself with respect to any added anchor points. Accordingly, we present a first approach towards nonvisual localization and mapping that utilizes tether measurements and wheel odometry to jointly estimate vehicle trajectory and tether-to-obstacle contact points. The proposed method is inspired by FastSLAM, where instead of updating a map of landmarks, tether length and bearing measurements are used to update sequential lists of anchor points for every particle representing a belief of the robot's trajectory. Results from both simulation and experiment using our Tethered Robotic eXplorer (TReX) demonstrate that (i) our method is more accurate than odometry alone, and (ii) we are able to map intermediate anchor points nonvisually.