Terradynamics and design of tip-extending robotic anchors

TL;DR

This paper investigates the mechanics of tip-extending robotic anchors inspired by tree roots, providing design principles and demonstrating a lightweight device capable of anchoring in loose Martian soil with high efficiency.

Contribution

It offers a rigorous understanding of granular mechanics for tip extension and applies these insights to develop a deployable, lightweight robotic anchor.

Findings

The robotic anchor can deploy sensors 45 cm deep in Martian regolith simulant.

The device achieves an anchoring force of 120 N with a weight of 300 g.

Design insights improve anchoring efficiency and force-to-weight ratio.

Abstract

Most engineered pilings require substantially more force to be driven into the ground than they can resist during extraction. This requires relatively heavy equipment for insertion, which is problematic for anchoring in hard-to-access sites, including in extraterrestrial locations. In contrast, for tree roots, the external reaction force required to extract is much greater than required to insert--little more than the weight of the seed initiates insertion. This is partly due to the mechanism by which roots insert into the ground: tip extension. Proof-of-concept robotic prototypes have shown the benefits of using this mechanism, but a rigorous understanding of the underlying granular mechanics and how they inform the design of a robotic anchor is lacking. Here, we study the terradynamics of tip-extending anchors compared to traditional piling-like intruders, develop a set of design insights, and apply these to create a deployable robotic anchor. Specifically, we identify that to increase an anchor's ratio of extraction force to insertion force, it should: (i) extend beyond a critical depth; (ii) include hair-like protrusions; (iii) extend near-vertically, and (iv) incorporate multiple smaller anchors rather than a single large anchor. Synthesizing these insights, we developed a lightweight, soft robotic, root-inspired anchoring device that inserts into the ground with a reaction force less than its weight. We demonstrate that the 300 g device can deploy a series of temperature sensors 45 cm deep into loose Martian regolith simulant while anchoring with an average of 120 N, resulting in an anchoring-to-weight ratio of 40:1.