A swarm of more than 1,000 tiny robots can assemble itself into complex shapes such as letters of the alphabet.

Until now, robot swarms that work together as teams had been limited in size to dozens or hundreds of robots. But computer scientist Michael Rubenstein at Harvard University and his colleagues have shown they can program 1,024 tiny robots known as Kilobots, each a little more than three centimeters wide, to assemble themselves into various 2-D shapes, such as a wrench or a five-pointed star, as they reveal today in the journal Science.

"This is a huge accomplishment. No one has demonstrated swarm behavior at that scale before in hardware," says roboticist Hod Lipson at Cornell University, who did not take part in this research.

All Together Now

Robots are expensive and complex, which is a big problem when you're trying to build huge swarms of them. The researchers overcame this challenge by developing relatively cheap and simple bots that are nevertheless each fully autonomous—capable of computation, locomotion, sensing, and communication.

"They only have one sensor, an infrared communications sensor they use to both talk with their neighbors and to measure the distance from their neighbors," Rubenstein says. "And they have a very simple way of moving involving two vibrating motors that allow them to slide over surfaces on three rigid legs—which isn't as good as moving using wheels, but was a compromise to save on cost."

Another challenge: If you have 1,000 robots, some are bound to fail. "It's hard to make a perfect robot, and it's harder to make 1,000 perfect robots, so any algorithm used to get a large swarm of robots to behave properly needs to account for faults in robots," Rubenstein says.

For example, if a robot's motor quits and it does not move, then another robot waiting for it to move also might not move. "And then all the robots don't move and the shape we want them to form doesn't form," Rubenstein says. "That robot may think it is moving, but since it doesn't have wheels and an odometer, it doesn't know that its vibration motors are stuck and that it isn't moving. So we used the infrared sensors on the robots to help them determine their distance from each other and see if they are moving. If they're not, they know to carry out corrective actions to get their motors unstuck."

The researchers use infrared commands to tell the bots what shape to form. The machines then organize autonomously using simple behaviors, such as moving along the edge of the group and communicating with others to maintain a sense of their own location. They're slow, but they get the job done: Moving at one centimeter per second, the robots can form a given shape in a little under 12 hours, Rubenstein says.

Bots, Organize Yourselves

The Kilobots will serve as a test bed for building more advanced robot swarms. "In nature, you see examples of millions of army ants working together, or trillions of cells forming a multicellular creature," Rubenstein says. "We're interested in working toward that goal with robots."

"Just like how single cells can form a multicellular creature, we envision having multi-robot robots," he continues. "You could imagine one changing shape on the fly to adapt to suit its environment—for instance, when walking across flat terrain, it could become a four-legged creature; if it's going across sand, it could become a snake-like creature; and if it's going down a big hill, it could roll to save energy. If an individual robot fails, it can just drop off or get replaced."

"This will push robotics to an entirely new level, especially as swarms of robots are getting affordable," says roboticist Roderich Gross at the University of Sheffield in England, who did not participate in this study.

Robot swarms could also lead to another far-out tech: programmable matter. "You an imagine having a bucket full of a million sand-grain-sized robots, and if you want a wrench, you can reach into the bucket and pull out a wrench, and put it back into the bucket to break the wrench apart for use in another tool," Rubenstein says. "You can adjust the shape of this programmable matter for whatever task or goal you have."

Lipson says that the ability to control swarms of machines could also help us manage fleets of self-driving cars. We're going to need a way to get millions of them to flow smoothly through all the traffic obstacles human drivers must overcome.

"You'd want them to talk with each other, to self-organize," Lipson says. "I'm not saying that this demonstration with Kilobots is the solution to that problem, but the solution will definitely look similar to what they are doing with those robots."

Rubenstein's future plans for the Kilobots include improving their software to make them truly autonomous. "It'd also be useful if, instead of us telling them what shape they should form, they could look at their environment and discover for themselves what shapes they should form that would help them complete their goals," he says.

The Kilobots have been licensed to K-Team, a manufacturer of small mobile robots. But the Kilobot robot design and software also are available open-source for non-commercial use.

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