Sheer cliff faces present a traversal challenge for most wheeled robots on the market, but researchers at the University of Tokyo say they’ve developed a two-robot framework that works pretty reliably in their testing. In a newly published paper on the preprint server Arxiv.org (“UAV/UGV Autonomous Cooperation: UAV assists UGV to climb a cliff by attaching a tether“), they describe an unmanned aerial vehicle (UAV) — in this case a quadcopter — that can tether a ground-bound drone to an anchor at the top of walls and steep inclines, allowing it to climb up unimpeded.

“[We] propose a novel cooperative system for an Unmanned Aerial Vehicle (UAV) and an Unmanned Ground Vehicle (UGV) which utilizes the UAV not only as a flying sensor but also as a tether attachment device,” the authors of the paper explain. “[It enhances] the poor traversability of the UGV by not only providing a wider range of scanning and mapping from the air, but also by allowing the UGV to climb steep terrains with the winding of the tether.”

So how’s it work? The UGV is permanently attached via mechanized winch and cable to the UAV, a custom-made quadcopter with an Nvidia Jetson TX2 chipset, a flight controller, and a raft of sensors including a modular fisheye camera, time-of-flight sensor, inertial measurement unit (IMU), and laser sensor. To estimate its orientation and chart out an obstacle-free path to the anchor point, the UAV taps the IMU to perform a technique called visual inertial odometry (VIO) and a predictive control model that communicates roll, pitch, yaw rate, and more to the flight controller.

Robot UAV UGV

Above: The UAV attaches a tether to an anchor, allowing the UGV to pull itself up with a winch.

The two robots communicate over wireless and funnel information through Robot Operating System (ROS), an open source robotics middleware. ROS runs on the UGV’s onboard computer and translates voxel data collected by the UAV to a grid map, which it uses to find a suitable anchor and perform basic path planning and cliff detection.

The researchers considered a range of tether attachment methods initially, among them a grappling hook, a tether-winding technique that had the UAV detect and wrap around a pole-like structure, a gripper-like grasping device, a peg inserted into the ground, and a hybrid of the grappling hook and winding technique. But they eventually settled on the last in the list — the hybrid technique — because of “its simple mechanism” and “high chance of successful attachment.”

In experiments, the researchers report that the UAV’s localization and position control was “good enough” to complete a fully autonomous mission, and that the UGV managed to successfully climb both vertical walls and stairs. Moreover, in a field test, they say the two robots together “successfully” navigated through a field, detected an anchor, attached the tether to said anchor, and climbed a cliff — despite a few failure cases when the UGV became entangled in the cable.

In future work, they hope to build a “smarter” tether tension control system and tether-aware planning, power supply, tether unhooking, and communication systems.