Reduced traction limits the ability of mobile robotic systems to resist or apply large external loads, such as tugging a massive payload. One simple and versatile solution is to wrap a tether around naturally occurring objects to leverage the capstan effect and create exponentially-amplified holding forces. Experiments show that an idealized capstan model explains force amplification experienced on common irregular outdoor objects - trees, rocks, posts. Robust to variable environmental conditions, this exponential amplification method can harness single or multiple capstan objects, either in series or in parallel with a team of robots. This adaptability allows for a range of potential configurations especially useful for when objects cannot be fully encircled or gripped. These principles are demonstrated with mobile platforms to (1) control the lowering and arrest of a payload, (2) to achieve planar control of a payload, and (3) to act as an anchor point for a more massive platform to winch towards. We show the simple addition of a tether, wrapped around shallow stones in sand, amplifies holding force of a low-traction platform by up to 774x.

RHex (pronounced "rex") is a unique hexapedal robot that uses hybrid wheel-legs (whegs) to get around. It's surprisingly adaptable, able to adjust its gait to conquer a variety of obstacles and terrains, and it can even do some impressive parkour. RHex has been around for nearly two decades, which is practically forever in robot years, but because of how versatile it is you still see it doing cool new stuff from time to time. Carnegie Mellon University's Robomechanics Lab uses a fancy US $20,000 version of RHex called X-RHex Lite "to explore the connection between dynamic locomotion and perception," but they've only got one robot since it's wicked expensive, which limits the amount of research and outreach they can do. To fix this, they've designed a much smaller version of RHex called MiniRHex that you can build yourself for about $200.