Terradynamics and design of tip-extending robotic anchors

TL;DR

The paper tackles the challenge of anchoring in hard-to-reach or low-gravity environments by studying tip-extension terradynamics and contrasting it with traditional rigid intrusion. It derives four design insights—extending beyond a critical depth, adding hairs, staying near-vertical, and using multiple small roots—that together enable high extraction strength with low insertion effort. Building on these principles, the authors design a lightweight soft robotic anchor that self-anchors without external reaction forces, deploying to 45 cm in Martian regolith simulant and achieving about 120 N of anchoring force for a 300 g device (anchoring-to-weight ratio > 40:1). These findings offer practical, low-mass anchoring strategies for subsurface sensor deployment and other extraterrestrial operations, with broad implications for robotic anchoring in granular media.

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.

Figures (15)

• Figure 1: Overview of the motivation and mechanisms of soft robotic anchor. (A) Rigid insertion and (B) plant root growth are fundamentally different mechanisms of anchoring in the ground. (C) Growth offers advantages in remote and low-gravity environments for robotic anchoring. (D) Rigid insertion requires high force because it must overcome both tip and side resistances. (E) Conversely, tip-extension enables "zero-reaction-force self-anchoring" because anchoring forces along the sides of the device counter tip resistance beyond a critical depth.
• Figure 2: Insertion and extraction forces measured at the base of rigid intruder (blue), hairless (red), and hairy tip extender (orange) in sand. (A) Insertion force as a function of normalized insertion time to a depth of 15 cm, where time = 0 represents no insertion and time = 1 indicates full insertion. Depth during insertion was not directly measured. The average times for full insertion were 13.5 s for the rigid intruder, 10.3 s for the hairless, and 12.5 s for the hairy tip extender. The mean standard deviations of the shaded regions are 1.1 N, 0.85 N, and 1.08 N, respectively. Inset: Zoom of hairless tip extender. (B) Extraction force as a function of normalized time. The average extraction time for all the devices was 13.5 s. The mean standard deviations of the shaded regions are 0.05 N, 0.09 N, and 0.17 N, respectively. (C) The devices used for insertion and extraction experiments: rigid intruder, hairless tip extender, and hairy tip extender. (D) Ratio of maximum extraction to maximum insertion force. The location of maximum forces is marked by an "x" in (A) and (B). The error bars represent the lower and upper bounds of the peak extraction-to-insertion force ratios.
• Figure 3: Particle Image Velocimetry (PIV) reveals granular flow dynamics during insertion and extraction of an intruder (top row), hairless tip extender (bottom row). Heatmaps are constructed by sequentially concatenating vertical slices captured at the midpoint over the entire duration of insertion and extraction. The experiments were conducted in a confined sand testbed. The color maps represent the magnitude and direction of particle velocities, and the dashed diagonal lines represent the approximate tip position of both devices. The dashed rectangles mark the onset of extraction, which is shown in detail in the enlarged subset figures at the bottom right. (A) During intruder insertion, the nearby particles are continuously displaced. (B) In contrast, during tip extension, the particles around the tip are displaced, while the sand above the growing region remains nearly stationary. (C-D) At the onset of extraction (inset), the tip extender (D) exhibits less particle movement than the rigid intruder (C).
• Figure 4: The effects of diameter and angle on insertion and extraction. (A) Resistive forces measured for tip extenders with varying diameter during insertion (top) and extraction (middle), and the corresponding extraction-to-insertion ratio (bottom). Solid curves represent the quadratic and linear fits to the insertion and extraction forces, respectively, and the resulting linear fit to the ratio. (B) Resistive forces measured for a pair of tip extenders with varying insertion angles during insertion (top), extraction forces (middle), and the corresponding extraction-to-insertion ratio (bottom).
• Figure 5: Self-anchoring experiments. Insertion and extraction force of a tip extender in a force-controlled setup in loose sand demonstrates the "zero-reaction-force self-anchoring" phenomenon. The insertion force is measured as the minimum weight on a tip extender to prevent it from backing out of the sand as it grows deeper. The maximum insertion force was recorded at 3 cm intervals. The RFT model is shown as a dashed line. Tip force is not directly measured and equals net force minus extraction force.