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Reconfigurable robots make strides as milli-moteins

Nov. 30, 2012
The device doesn’t look like much — a caterpillar-sized assembly of metal rings and strips resembling something you might find buried in a home-workshop drawer.

The device doesn’t look like much — a caterpillar-sized assembly of metal rings and strips resembling something you might find buried in a home-workshop drawer. But the technology behind it, and the long-range possibilities it represents, are remarkable. The petite contraption is called a milli-motein, a name that blends its millimeter-sized components with a motorized design inspired by proteins, which naturally fold themselves into incredibly complex shapes.

This tiny robot may be a harbinger of future devices that could fold themselves into almost any shape imaginable. The device was conceived by Neil Gershenfeld, head of MIT’s Center for Bits and Atoms, and graduate students Ara Knaian and Kenneth Cheung, and is described in a paper recently presented at the 2012 Intelligent Robots and Systems conference.

“It’s effectively a one-dimensional robot that can be made in a continuous strip, without conventionally moving parts, and then folded into arbitrary shapes,” explains Gershenfeld.

To build what is reportedly the world’s smallest chain robot, the research team had to invent an entirely new kind of motor — not only small and strong, but also able to firmly hold its position even with power switched off. The team met these needs with a new system called an electropermanent motor.

The motor is similar in principle to the giant electromagnets used in scrapyards to lift cars, in which a powerful permanent magnet is paired with a weaker magnet whose magnetic field direction can be flipped by an electric current in a coil. The two magnets are designed so that their fields either add or cancel, depending on which way the switchable field points. Using this method, the force of the powerful magnet can be turned off at will — for example, to release a suspended car — without having to power a huge electromagnet the entire time.

In the new miniature version, a series of permanent magnets paired with electromagnets are arranged in a circle and drive a steel ring situated around them. The key innovation, Knaian explains, is that the magnets do not take power in either the on or the off state, but only use power in the changing state, thereby minimizing overall energy usage.

The milli-motein concept follows up on a paper, published last year, which examined the theoretical possibility of assembling any desired 3-D shape simply by folding a long string of identical subunits. That paper, co-authored by Cheung, MIT professor Erik Demaine, alumnus Saul Griffith, and research scientist Jonathan Bachrach, proved mathematically that it was possible for any 3-D shape to be reproduced by folding a sufficiently long string. What's more, the research concluded that it is possible to figure out how to fold such a string, as well as determine the exact steps needed to successfully reach the desired endpoint.

“We showed that you could make such a universal system that’s very simple,” says Cheung.

While he and his colleagues have not yet proved a way of always finding the optimal path to a given folded shape, they did find several useful strategies for arriving at practical folding sequences. Demaine points out that the folding of the shape doesn’t have to be sequential, moving along the string one joint at a time.

“Ideally, you’d like to do it all at once, with each of the joints folding themselves into the desired configuration simultaneously so that the loads are distributed," says Demaine.

Other researchers, including some at MIT, have explored the idea of fashioning reconfigurable robots from a batch of separate pieces that could self-assemble into different configurations, an approach sometimes called “programmable pebbles.” But Gershenfeld’s team found that a string of subunits capable of folding itself into any shape could be simpler in terms of control, power, and communications than using separate pieces that must find each other and assemble in the right order.

It’s part of an overall approach, Gershenfeld explains, to turning data into things. In an article in the current issue of the magazine Foreign Affairs, he describes a technology roadmap for accomplishing that, and its policy implications. He and his colleagues have established a global network of more than 100 “fab labs” that provide community access to computer-controlled fabrication tools. Today, the design information is contained in an external computer rather than in the materials being manufactured, but the research goal is to digitize the materials themselves so that they can ultimately change their own shape, as the milli-motein does.

The milli-motein is part of a family of such devices being explored at size scales ranging from protein-based “nanoassemblers” to a version where the chain is as big as a person, Gershenfeld says. Ultimately, a reconfigurable robot should be small, cheap, durable, and strong, says Knaian. The MIT research could lead to robotic systems that can be dynamically reconfigured to do many different jobs rather than repeating a fixed function, and that can be produced much more cheaply than conventional robotics.

The development of the milli-motein was supported by the U.S. Defense Advanced Research Projects Agency’s Maximum Mobility and Manipulation and Programmable Matter projects. For more information, visit

Image courtesy of Neil Gershenfeld, MIT.

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