[Image above] Shivam Chopra holds the robot he helped to develop as a Ph.D. student in the research group of Nicholas Gravish at the University of California, San Diego. This robot, inspired by turtle hatchlings, can move untethered through sand. Credit: UC San Diego
While many people associate sand with fun at the beach, sand is also a critical material resource that serves as a key component in nearly all aspects of our infrastructure, including concrete, asphalt, brick, and glass.
Yet the increasing demand for sand, similar to the demand for other critical material resources, is raising concerns about a global shortage. And though numerous studies have called for the development of a global sand governance strategy to address this imminent threat, little headway has been made.
This holdup is due in part to a lack of reliable methods to gauge, monitor, and compare the scale of sand extraction and consumption activities. Unlike water or air, robots cannot be easily deployed to gather data in a sandy environment. That is because movement through granular media, such as sand, presents numerous challenges. For example,
- Very large resistive forces due to frictional resistance between sand grains,
- Nonzero yield stresses that cause unpredictable solid/fluid transitions, and
- Extremely limited opportunities for sensing obstacles.
Additionally, the robot body must be tightly sealed so that sand grains cannot penetrate interfaces, which would cause rapid degradation of mechanical components and subsequent failure.
Research in the last 15 years on moving across or within granular environments has mostly focused on two basic approaches: 1) robots that use peristaltic body expansion and elongation, like earthworms; and 2) undulatory robots that use body bending to effectively “swim” in sand, like the sandfish lizard.
Robots based on the first approach face challenges to obtaining autonomous, untethered operation. Robots based on the second approach have successfully navigated shallow, low-density plastic beads, but the resistive forces are comparatively small compared to movement in sand; current actuators may not be able to generate motion in that case.
Appendage-driven robots would have several advantages over these other approaches, including the ability to detect obstacles and generate large propulsive forces through a wide stroke. But they face a fundamental problem: due to the frictional resistance of sand (i.e., nonzero yield stress), an elastic appendage can get stuck in a deformed configuration if the elastic restoring stress is below the yield stress.
Fortunately, nature can serve as prime inspiration for many challenges that researchers face, and this situation is no exception. In a recent open-access paper, researchers from the University of California, San Diego, described how they developed an easily deployable, untethered, appendage-driven robot by observing the motion of sea turtle hatchlings.
As explained in a UC San Diego press release, scientists still do not fully understand how robots with flipper-like appendages move within sand. So, the UC San Diego team conducted extensive simulations and testing, which led them to select a tapered body design and shovel-shaped nose. They also added two foil-like surfaces on the sides of the nose to keep the robot at level depth in the sand.
The robot, which was tested in both a lab tank and La Jolla Shores Beach, traveled at a speed of 1.2 millimeters per second—roughly 4 meters, or 13 feet, per hour. It was able to detect obstacles above its body by monitoring changes in the torque generated by the movement of its flippers, though not obstacles below or directly in front of it.
Next steps include exploring ways to increase the robot’s speed, such as by increasing the gait cycle frequency or making the appendages longer. The researchers also plan to modify the design so that the robot can burrow into and out of the sand, rather than being buried at its maximum depth of 5 inches.
The open-access paper, published in Advanced Intelligent Systems, is “Toward robotic sensing and swimming in granular environments using underactuated appendages” (DOI: 10.1002/aisy.202200404).