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Plant growth and burrowing animals inspire breakthrough technology.
by Janet Preus
Soft robotics have received deserved attention lately, and ongoing research is demonstrating real progress in this science, particularly when used in exoskeletons. Mobile robots that can climb, fly or even swim have also had quite a lot of attention, but robots that can burrow in the ground are studied far less and have not been so well understood.
That is changing, as researchers at the University of California Santa Barbara (UCSB) and the Georgia Institute of Technology are breaking new ground in the study of burrowing robots. The researchers took their cue from plants and animals accustomed to navigating underground to develop a fast, controllable soft robot that can burrow through sand, the UCSB news service reports.
Strategies that worked
It may sound like a simple project, but it is anything but, which may explain why no one has overcome the challenges previously. “The biggest challenges with moving through the ground are simply the forces involved,” says Nicholas Naclerio, a graduate student researcher in the lab of UCSB mechanical engineering professor Elliot Hawkes and the lead author of a paper published in the journal Science Robotics.
“Whereas air and water offer little resistance to objects moving through them, the subterranean world is another story,” Naclerio says. “If you’re trying to move through the ground, you have to push the soil, sand or other medium out of the way.”
To do this, the team started with a vine-like soft robot designed in the Hawkes Lab that mimics plants and the way they navigate by growing from their tips, while the rest of the body remains stationary, UCSB reports. The researchers learned that tip extension keeps resisting forces low and localized only to the growing end; if the whole body moved as it grew, friction over the entire surface would increase as more of the robot entered the sand until the robot could no longer move.
Burrowing animals also inspired their work with a strategy called “granular fluidization,” which suspends the particles in a fluid-like state and allows the animal to overcome the high level of resistance presented by sand or loose soil. This was accomplished with a device that shoots air into the region just ahead of the growing tip, enabling it to move into that area, something like a southern sand octopus.
“The biggest challenge we found and what took the longest to solve was when we switched to horizontal burrowing, our robots would always surface,” Naclerio says. “It’s much easier to push the sand up and out of the way than it is to compact it down.”
NASA’s interest : a Q & A
NASA is among the interested parties in this breakthrough research. Advanced Textiles Source (ATS) asked Naclerio to answer a few questions about NASA’S interest, the use of textiles in this type of robotic technology and the other possible applications for a robot with burrowing capability.
ATS: It appears that this sort of soft robotic device would be impossible without the proper fabric. What sort of performance characteristics did you find that you needed in the fabric? What fabric did you use, in the end, to make the tunneling robot?
NACLERIO: We iterated through several types of airtight. ripstop nylon fabric constructions when making our robot. The characteristics we required for our pneumatic everting robot were strength, flexibility, suppleness, airtightness, abrasion resistance and low friction.
The initial prototypes were made of two layers of 50 micrometer-thick silicone and urethane impregnated ripstop nylon from Seattle Fabrics, adhered together with temperature vulcanizing silicone adhesive (Sil-Poxy, Smooth-On Inc.). One layer was oriented with its threads along the axial and circumferential axis of the robot and the other at a 45° angle for axial and torsional stiffness, respectively.
Our later versions of the robot were made of XPac TX07 fabric (Rockywoods) and sealed with 0.51DCF Dyneema composite fabric (Ripstop by the Roll) and a pressure sensitive adhesive (3M 9482PC). The advantage of the XPac fabric is that it is a coated ripstop nylon with Kevlar threads running at a 22.5° angle, which give the robot torsional stiffness.
The discontinued TX07 version of the fabric had a film coating on one side, which made it easy to seal with the Dyneema and PSA tape. Most of the other XPac fabrics have a taffeta coating, making them more difficult to seal.
ATS: Can you tell us more about NASA’s interest? What would they be using it for, exactly? How could the device accomplish this for them?
NACLERIO: We are excited to explore the potential uses of our work with NASA. Our burrowing method is well suited for dry, low-gravity, extraterrestrial environments, where reactive forces may be difficult to produce. Example applications include thermal sensor placement on Mars, volcanic tunnel exploration on the moon, asteroid sampling or anchoring and granular ice exploration on Enceladus, a moon of Saturn.
ATS: What do you know about the sort of material NASA would need, if they were to use this on the moon, for example?
NACLERIO: We have not specifically looked into materials for extraterrestrial use yet, but it will be important to consider things such as extreme temperature changes, gas permeability, abrasion resistance, and vulnerability to environmental contaminants. Another challenge with soft robots in space is providing a source of pneumatic power. Potential methods include compressed gas tanks, gas generators or collecting gas from the environment.
ATS: How else might this device be used? Can you suggest certain industries or specific applications?
NACLERIO: Our work presents a terradynamic understanding of burrowing in granular media and applies the key results to design a soft robot that controls subterranean interaction forces to achieve fast, steerable burrowing in three dimensions. Although our method of burrowing may not be ideal for soil penetration beyond a few meters, it offers improved performance in long, shallow, directional burrowing.
The mechanisms for controlling subterranean forces investigated are useful beyond the scope of a small burrowing robot. Future work could examine integrating tip extension with conventional drilling and exploration technologies.Potential applications on Earth include soil sampling; minimally invasive irrigation, wire, or geothermal loop installation without trenches; erosion control; search and rescue; and granary inspection.
Beyond burrowing
The UCSB report says the technology not only enables new applications for fast, precise and minimally invasive movement underground, but also lays mechanical foundations for new types of robots. Understanding the mechanics of how plants and animals have mastered subterranean navigation opens up many possibilities for science and technology, says Daniel Goldman, Dunn Family Professor of Physics at Georgia Tech.
“Discovery of principles by which diverse organisms successfully swim and dig within granular media can lead to development of new kinds of mechanisms and robots that can take advantage of such principles,” he says. “And reciprocally, development of a robot with such capabilities can inspire new animal studies as well as point to new phenomena in the physics of granular substrates.”
Research for this paper was conducted also by Mason Murray-Cooper, Yasemin Ozkan-Aydin and Enes Aydin at the Georgia Institute of Technology.
Janet Preus is senior editor of Advanced Textiles Source. She can be reached at jlpreus@ifai.com.