When you imagine a robot, you might picture R2-D2 in Star Wars, the Omnidroid from The Incredibles or the big-armed machines that build cars on an assembly line. But there’s a new robotic system that doesn’t resemble any of these. Instead, it looks like some kids forgot to pick up their toys.

The robot is a collection of plastic, neon-green disks. Each is about 15 centimeters (6 inches) across. Alone, a single disk can’t do much of anything. It can only expand and contract.

But when a bunch of disks huddle together, things change. Tiny magnets on the disks’ outer rims make them stick together. When one disk expands or shrinks, it pushes or pulls on its neighbors. All of those small pushes and pulls add up. Suddenly the entire blob starts to move — very slowly.

The designers refer to each individual disk as a “particle.” When working as a system, they become what the designers call a “particle robot.” The researchers shared their invention March 20 in Nature. In the new study, they also showed how such a particle robot can accomplish simple tasks, like shuffling toward a light.

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“It’s an innovative mechanism,” says Katia Sycara. She’s a computer scientist at Carnegie Mellon University in Pittsburgh, Pa., who designs multi-robot systems. She did not work on the new invention. But she says it illustrates the wild variety of ways that people can build robotic systems.

At one end of the spectrum of robots you find single-bodied devices. Think R2-D2. These are robots contained in just one body. At the other end of the spectrum are modular robots. These are groups of individual robots that each have their own job but together work on some common task. They include “swarm” robots, which talk to each other and share information about where and how they’re moving.

The new system, says Sycara, is somewhere in between. The disks are individual units, but they bunch together to form a unified team. Their behavior results from their interactions and the laws of physics, not someone telling them what to do.

Natural inspiration

“We wanted to make robots that are very simple and that can respond to changes in the environment,” says Richa Batra. She’s a graduate student at Columbia University in New York City and part of a multi-university team behind the new particle-robotics system.

Scientists behind the project were inspired by nature, Batra explains. In the human body, for example, individual cells work together as muscle tissue. Many other types of cells also move together as a group.

The motion of the robot also reminds Batra of something else in the living world. The blob shuffles along “like a caterpillar moves,” she says. “It bunches up a little, then stretches out.”

Even though the disks don’t communicate directly with each other, they can respond as a group to some signal. The scientists showed this by installing sensors on each disk that could detect light. Then they programmed the disks to expand and contract faster or slower, depending on how intense the light was. When the researchers shone a bright light, their robot crept toward it — the result of all those individual expansions and contractions.





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To make sure the group of particles would not get stuck, the researchers had to consider how friction would affect the disks. Friction is the resistance between two surfaces rubbing together. The disks had to push hard enough to overcome friction. But they couldn’t push each other so far away that their magnets stopped working.

Another challenge the researchers faced was deciding what the disks should look like. For help, they turned to Chuck Hoberman at Harvard University in Cambridge, Mass. He had created what are known as Hoberman Spheres. The clever plastic toys are made of interconnected arms that expand into giant spheres when thrown in the air, and then collapse back into small spheres when caught. The new robotics team recruited Hoberman to design the disks that would become their “particles.” Like his spheres, these, too, get bigger and smaller with minimal effort.

Finally, the scientists had to create a system that could work at different scales. So far, they have built physical robots with more than two dozen disks. But they wanted to show what would happen with groups of hundreds, or even thousands, of particles. That’s where Batra came in. For two years, she wrote computer programs that could predict the behavior of big groups. She showed how a system with 100,000 particles would move. Her software also predicted what would happen if individual disks in the group stopped working.

“That was one of the really beautiful things we were able to look at,” she says. “How many of these particles could be killed off and still have it move?” A lot, as it turns out. Batra ran calculations on robots made from hundreds — even thousands — of particles. Her program predicted that even if one in five disks malfunctioned and stopped moving, the system would still be able to move as a group.

Right now, the robot only moves across a flat surface. Batra says she wants to know what a three-dimensional system might look like. The researchers also don’t know yet how their flat system might be used.

Still, that’s normal for robotics, says Sycara. Each new approach adds to the toolbox that other researchers can later use.

Sycara predicts that future robots, such as this one, will continue to take cues from nature on how they might look and move. “We are going to be seeing more and more biologically inspired designs,” she says.

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.