The Robustness of Tether Friction in Non-idealized Terrains

TL;DR

This paper investigates the use of tether wrapping around natural objects to exploit the capstan effect, significantly increasing traction and load resistance for mobile robots in outdoor terrains, demonstrated through various control applications.

Contribution

It introduces a practical, robust method leveraging natural objects for force amplification via tether wrapping, validated through experiments and multiple robotic control scenarios.

Findings

Force amplification up to 774x using tether around stones

Capstan model explains force amplification on irregular objects

Method effective under variable environmental conditions

Abstract

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.