Erik Demaine - The robotic equivalent of a Swiss army knife
Reconfigurable robot a step toward something that can become almost anything.
David L. Chandler, MIT News Office
November 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. But the
technology behind it, and the long-range possibilities it represents, are quite
The little device is called a milli-motein — a name melding its millimeter-sized
components and a motorized design inspired by proteins, which naturally fold
themselves into incredibly complex shapes. This minuscule robot may be a harbinger of
future devices that could fold themselves up into almost any shape imaginable.
The device was conceived by Neil Gershenfeld, head of MIT’s Center for Bits and
Atoms, visiting scientist Ara Knaian and postdoctoral associate Kenneth Cheung, and is described in a paper
presented recently at the 2012 Intelligent Robots and Systems conference. Its key feature, Gershenfeld says:
“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.”
To build the world’s smallest chain robot, the team had to invent an entirely new kind of motor: not only small and
strong, but also able to hold its position firmly even with power switched off. The researchers 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 (one that, like an ordinary bar magnet, requires no power) is paired with a weaker magnet (one
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. Thus, the force of the
powerful magnet can be turned off at will — such as to release a suspended car — without having to power an
enormous electromagnet the whole time.
In this new miniature version, a series of
permanent magnets paired with electromagnets are
arranged in a circle; they drive a steel ring that’s
situated around them. The key innovation, Knaian
explains, is that “they do not take power in either
the on or the off state, but only use power in the
changing state,” using minimal energy overall.
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 former
Computer Science and Artificial Intelligence
Laboratory research scientist Jonathan Bachrach,
proved mathematically that it was possible for any
3-D shape to be reproduced by folding a sufficiently
long string — and that it’s possible to figure out how
to fold such a string, and the exact steps needed to
successfully reach the desired endpoint.
“We showed that you could make such a universal
system that’s very simple,” Cheung says. 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,” he says, with each of the joints folding
themselves to the desired configuration simultaneously so that the loads are distributed.
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. “You can just pass signals down the chain,” Knaian says.
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.
Hod Lipson, an associate professor of mechanical and aerospace engineering and computing and information
science at Cornell University, says, "This result brings us closer to the idea of programmable matter — where
computer programs and materials merge to form a new kind of matter whose shape and function can be
programmed — not unlike biology. Many people are excited today to learn about 3-D printing and its ability to
fabricate any shape; Gershenfeld’s group is already thinking about the next episode, where we don’t just control
the shape of objects, but also their behavior."
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,” Knaian says, adding that right now, “it’s not
possible to get all of those.” Still, he points out, “Biology is the existence proof that it is possible.”
The MIT researchers’ work 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
The development of the milli-motein included recent graduate Maxim Lobovsky SM '11 and undergraduate
students Asa Oines and Peter Schmidt-Neilsen (who worked on the project as visiting high-school students). The
work was supported by the U.S. Defense Advanced Research Projects Agency’s Maximum Mobility and
Manipulation and Programmable Matter projects.