The tiny 5-inch bot that scientists in Daniel Goldman's lab at Georgia Tech are experimenting with could help lead the way to those kinds of machines. In the journal Science today, Goldman and grad students Chen Li and Tingnan Zhang published their model of how legged animals like lizards move through granular materialsstuff such as sand that comes in loose small particles and can behave like a solid or a liquid. It's a model that could tell robot-builders how certain sizes and shapes of legs will move through shifting materials. "We've now provided our engineering collaborators with tools to aid in design that they didn't have before," he says. And it turns out that ideas developed starting in the 1930s and '40s do an awfully good job of predicting an animal's, or a robot's, performance in sand.
"There is an entire world of animal locomotion that had been more or less unstudied: That is, movement of animals on and within terrestrial materials like sand, which can flow and resolidify around the animal's feet or bodies. We know precious little, and I say that relative to how much we know about how fish swim or birds fly," Goldman says. Although a physicist by training, he tells PM, he took a research side trip into the world of biomechanics. That led him to a field of research that unified the physics of granular materials, the robotic models that have become increasingly popular in animal studies, and imaging techniques such as high-speed X-rays that allow scientists to see a lizard's foot moving beneath the surface of the sand.
Goldman and his colleagues used this approach to study individual life-forms like the sandfish. ("It essentially turns itself into an eel," he says.) But that led to a big question: Could they use what they learned to draw up a set of rules that would tell you how any machine or animal of a particular size and shape moves through these materials? The answer is called terradynamicsit's what the researchers outline in their new study as a potential way to design legged robots (as opposed to terramechanics, the long-standing engineering idea that predicts the behavior of vehicles with wheels or tracks moving over solid, flat surfaces).
To build this model, the lab started with resistive force theory, the '30s idea that scientists used to model how an animal like a nematode moved through a viscous fluid. Basically, it requires that you break down a creature into small parts, calculate the forces on those parts as they move, and then add them up to get a reasonable idea of the total force. When Li and Zhang proposed to use resistive force theory on legged robots, Goldman says, he told them it wouldn't work. But Li tried anyway and found it to be a smashing success. Resistive force theory gave the scientists the total forces on robot legs to within 5 percent of the correct sum. It didn't even matter whether the researchers used small glass balls, large glass particles, poppy seeds, or another granular material.
To refine terradynamics, Chen tested not only different materials, but also different kinds of robot legs. The bot's C-shaped legs moved in what Goldman calls a cockroach gaitthree move, then the other three. But Chen used a 3D printer to craft legs of varying shapes, all of which moved through the sand in the way their theory predicted. Zhang also created a computer model of the team's little robot, allowing the researchers to simulate how a host of different leg shapes would run or swim through the same kind of material.
The upshot, Goldman says, is that robot designers could use this kind of system to find out what method of locomotionbe it wheels, tracks, human-like legs, C-shaped legs, or something elsewould work best in the environment where their robot will need to move. "If you're ever tried to ride a bicycle over a step, that's a hard thing to do with a wheel, but it's easy to step over. Whether you deploy wheels or legs on your device depends upon the environment."
"Animals are sort of an existence proof," he says, in that they show what's possible when it comes to moving through tricky, shifting environments. "If you had a robot that could run across a diversity of substrates at 40 body lengths a second and not go unstable and not dig into the sand, then you'd have a pretty interesting exploratoryand, potentially, search-and-rescue-typedevice."