Engineers at MIT have created the world’s first robotic transformer caterpillar—the prototype for a machine that will metamorphose not just into a butterfly, but into a plethora of different shapes. It’s the latest technology to borrow one of the natural world’s most common, efficient and ingenious manufacturing processes—the origami-like folding of one- or two-dimensional forms into three-dimensional shapes.
The prototype is called the milli-motein—“milli” because of the small scale at which the device is built, “motein” because the robot was inspired by the folding of proteins. Proteins are made up of amino acids, in long strands linked together by ribosomes, that fold themselves into the complex shapes that determine their functions in the body. The milli-motein was built to be able to curl and lock itself into shape in a similar way.
So far the prototype is only that, a prototype: it’s a chain four links long. Just building that, though, required some rather nifty engineering. The terms of the DARPA grant that funded the work required that each module—each motorized link in the chain—be smaller than one cubic centimeter. At that scale the designers had to rethink how to make the parts of their robot move. Because they wanted something where the modules could be cheaply mass-produced, they required their design to have as few moving parts as possible—they eschewed gears, for example. And because of the laws of physics, the amount of heat generated by the tiny motors was proportionally greater than it would be at a larger scale. According to one of the engineers, Ara Knaian, as they started experimenting with different motor types, they were frustrated by the fact that generating enough torque to actually lift and twist the segments of the milli-motein inevitably burned out the motors. “I had all this smoldering, charred wreckage on my desk,” he says.
The solution they hit on was a tiny motor that paired electromagnets and permanent rare-earth magnets. The concept was the same one that runs the enormous crane magnets used to move scrap metal and cars at junkyards—the switchable polarity of the electromagnet can either augment or negate the more powerful permanent magnet, turning the apparatus on (pick up the car) or off (drop it). The great thing about these so-called electropermanent magnets is that they only require power to turn on or off. Once in the on position, they don’t require additional energy, since the permanent magnet is doing the work.
The MIT team arranged their paired magnets in a cross shape with a steel ring around them—the whole thing is a centimeter in diameter and two millimeters thick. Turning on and off the electromagnets in paired sequence caused the steel ring to rotate around the magnets, creating the torque of the motor. And like the junkyard magnets, the motor only requires power when it moves, not to hold its position—allowing the milli-motein to powerlessly lock when it’s in the right configuration and not burn out its mechanical muscles.
Knaian and his co-designers, Neil Gershenfeld, head of MIT’s Center for Bits and Atoms, and Kenneth Cheung, call their invention the “electropermanent motor,” and it’s already getting attention from industry. They’re working with the precision device maker Moog (MOG/A) to design airplane flap controls that use the motors—the advantage being that if they burn out in mid-flight they’ll simply lock into place, rather than flapping around like traditional motors, a much more dangerous problem—as well as improved versions of the rotating ion thrusters that steer satellites.
As for the milli-motein itself, commercial applications, Knaian concedes, are further in the future. So far, transformers remain the province of toys (sometimes very, very high-end ones). The milli-motein could eventually do reconnaissance missions in hostile environments, be sent in to diagnose mechanical failures in hard-to-reach places, or even repair machinery in outer space. But, Knaian says, “It’s not something you’re going to find in Best Buy (BBY) in five or 10 years.” That being said, he argues that it would be rash to write off shape-changing robots because they seem fanciful or frivolous today. He points to the famous (probably apocryphal) quote attributed to Thomas Watson, IBM’s mid-century chairman, who said that he thought that the global market for computers was “about five.” The smaller, more reliable, and cheaper a technology gets, he argues, the more uses we tend to find for it.
“Even in the near-term I have the feeling that there must be a killer app even in its current state,” Knaian says. “I’m just not sure what it is.”
