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Shape-shifting robots take form
[April 25, 2008]

Shape-shifting robots take form

(New Scientist Via Acquire Media NewsEdge) HOW would you like to have your very own shape-shifter? Perhaps a liquid metal T-1000 Terminator to help around the house. Or a universal tool kit that could reshape itself into any implement at the press of a button. For an astronaut in orbit, an army mechanic in remote terrain or even a homeowner trying to fix a furnace on a cold winter night, it could be just the thing.

Well, one day maybe. The traditional approach to building shape-shifting devices has been to use materials based on shape memory alloys, polymer sheets or nanoparticles. But these have proved difficult to control and have other limitations, so researchers have begun taking a different and less exotic tack.

Their approach is known as self-reconfigurable robotics, and it takes advantage of recent advances in robot hardware, communications and control algorithms. Last year a team of US researchers from different universities put together a blueprint for shape-shifting based on cell-like robotic "modules" that can rearrange themselves to create different shapes. They made their pitch to the Defense Advanced Research Projects Agency. As a result, DARPA set aside $4 million for a set of six-month studies to design modules that could be mass-produced for demonstrations. The agency is prepared to fund 18-month contracts to pursue the most promising designs.

Instead of trying to control individual molecules or create nanoparticle fluids that morph from puddles to silver-skinned cyborgs, DARPA programme manager Mitch Zakin is pursuing what he calls "programmable matter". These are so-called "mesoscale" mini-machines, a millimetre to a centimetre in size, that can arrange themselves to form whatever shape is desired. Initially, Zakin expects the outcome to be devices the size of small Lego pieces, but as the technology improves the modules and the machines assembled from them should scale down further. Ultimately you could tell a sack of "smart sand" what to do, and the grains would assemble themselves into a hammer, a wrench or even a morphing robotic aircraft. "It's making machines more like materials, and materials more like machines," says Daniela Rus, a robotics researcher at the Massachusetts Institute of Technology.

The current era of reconfigurable robots began in 1988, when Toshio Fukuda of Nagoya University in Japan designed a "cellular robot" that could change its form. In the 1990s, roboticist Mark Yim, then at the Palo Alto Research Center in California, built a snake-like robot that could be put together in different configurations to move around in different ways. Each part contained a microprocessor, sensors and a motor to allow it to move relative to its neighbours. Then in 2002, computer scientists Seth Goldstein and Todd Mowry of Carnegie Mellon University in Pittsburgh, Pennsylvania, began using on-board electromagnets to move modules around and get them to stick together in an approach they called "claytronics" .

Generations of graduate students and researchers have cobbled together similar demonstrations, but few have progressed beyond slow-moving modules the size of toy blocks. "We haven't made much progress in two decades," admits roboticist Hod Lipson of Cornell University in Ithaca, New York. For his part, Lipson has demonstrated robots that assemble themselves from simple components, and he thinks this is where the future lies. He envisions millions of mobile modules that can join together like living cells to make finely granulated structures.

Last year Zakin decided, after talking with researchers including Lipson, that DARPA should try to help move things in that direction. At a systems and technology symposium last August, Zakin spoke about modules that would couple, uncouple and rearrange themselves into new shapes and structures. They would transfer information and energy among themselves, and have the internal smarts to respond almost organically to their environment. To do this, each module would require its own motor to move it, an electromagnet or other means to hold on to other modules, and a microprocessor to coordinate its motion .

The first challenge is to provide the modules with an efficient way to align themselves relative to each other. Yim, who is now at the University of Pennsylvania in Philadelphia, has developed a promising method. He is working with modules made up of a pair of metal pieces linked by a motorised hinge. Plates on the side and bottom of these modules have magnets that can be used to join them to adjacent modules in any of four orientations. Some, but not all, modules also contain a digital camera and processor. The advantage of using cameras as sensors is that they use very little power, he says, and they don't interfere with each other.

Yim gives a dramatic demonstration, which he calls the first case of "self-assembly after explosion". It starts with a 15-module robot walking on two legs. A student kicks it apart and it falls into three pieces, each made up of four "ordinary" modules and one camera module. The pieces wiggle across the floor towards one another, guided by images from the camera and computer-vision algorithms. They then reconnect, re-forming the original robot, which slowly rises and walks on.

A device that can put itself back together like this is all well and good, but an important question remains: how should shape-shifting modules be arranged in the first place? The simplest design is a snake or chain - which has been demonstrated in many labs - with each module attached to one or two others. By twisting the segments back and forth, an assembled snake can slither across the floor. Some variants also have arms and legs: Yim recently assembled a record 56 modules into a 14-legged robot that can walk. He claims that the control system would allow even more modules to be added, but in general a snake design doesn't scale up well to thousands of modules. That's partly because it would be hard for an on-board computer to keep track of the many directions of movement, but the main difficulty is that the control signals have to pass from module to module, along the length of the entire structure.

Crystal morphTo get around this problem, researchers are experimenting with an architecture in which modules are arranged in a lattice-like 3D pattern, a bit like atoms in a crystal. The lattice offers parallel paths for control signals, so different elements in the lattice can move simultaneously. The regularity of a lattice also makes it easier to rearrange or model on a computer than a chain, says Mark Moll, a computer scientist at Rice University in Houston, Texas. All in all, he says, the lattice design should scale up to large numbers of modules more easily.

Take the "Miche" system that Rus has developed at MIT. It is a self-assembling robot with a twist: instead of building a robot structure by adding modules, Miche starts with a large lattice and removes modules, like a sculptor chipping into a block of marble. Rus begins by stacking together 26 cubic modules, each measuring less than 5 centimetres to a side. When stacked, the robotic modules communicate with their neighbours through infrared sensors on their faces. Switchable magnets hold the lattice together.

Rus assembles the lattice on a table and lifts it into the air. While the electromagnets are switched on, the pieces hold together, but when control signals turn off the magnets holding a particular block it drops to the table. As more blocks drop away, a dog-shaped robot emerges. The experiment makes the point that to get a shape you want, it may be simpler to remove modules from a lattice structure than to add them or move them around.

Although researchers have not yet assembled more than a few dozen modules, they are making plans to build more complex structures. Assembling one module at a time into a predetermined spot, autonomously or by hand, simply won't work for millions of modules. It would take too long and modules would wind up in the wrong places. "You have to give up the idea of deterministic manipulation," Lipson says. Instead he suggests a probabilistic approach, in which large numbers of modules are shaken up so they end up filling a desired volume. Individual modules wouldn't have to find their way to assigned slots; instead, they could adapt to the niche they find themselves in, based on information from other modules and an overall control system.

Electrical engineer Eric Klavins of the University of Washington in Seattle has demonstrated this idea by shuffling "programmable parts" on an air-hockey table. Each module is an equilateral triangle 12 centimetres to a side, with a magnetic latch on each face. When two triangles collide, they latch together while their on-board processors communicate with each other via an infrared link to decide whether or not to remain attached. In this way Klavins can pre-program the triangles to build up complex structures such as parallelepipeds and hexagons. His physical demonstrations have been limited to a small number of modules, but his computer models have extended the idea to larger scales.

Ultimately his goal is not to specify the behaviours precisely, but rather to make the structures assemble themselves. Klavins is inspired by the way molecules form living cells and living cells form tissues, and he aims to glean lessons from nature to apply to robotic modules. He is now trying to build complex structures using simple, cheap materials by designing them from the bottom up. "The fundamental ideas are very similar for programming proteins and programming structures," he says. "But I don't know what kind of technology we'll be able to use."

No matter how it is achieved, any big change in scale will raise problems that so far have been addressed only in theory. Control and communications with millions of modules is a huge challenge, and special algorithms will be needed to decide how much information individual modules need. "How do the modules talk to each other to come to a consensus and agree on a series of actions?" Moll asks. Will all modules be identical, or will different modules be needed for different tasks? How will energy be distributed among millions of modules?

Then of course there is the enormous engineering challenge of shrinking the size of each module. Even stepping down to Zakin's mesoscale poses serious hurdles. Shrinking motors is very difficult, because they have to be both strong and energy-efficient. One approach, Yim suggests, is to use external energy - shake the system to get modules to settle into place - but that would shift the problem to control and sensing, which will still need to be worked out on the mesoscale.

So what lies ahead for programmable matter? DARPA issued its request for proposals last October, and in the next two years developers aim to demonstrate a system with at least 1000 modules - an ambitious 20-fold increase in complexity over the biggest system to date. A universal tool kit is further off, but at least it won't have to compete with everyday hardware in terms of costs. The Pentagon would gladly pay a premium for a single item that saves soldiers from hauling a truckload of special-purpose tools into combat, just as NASA would pay for one that saves weight on space missions.

For practical uses, the overall strength and robustness of any robotic tool will be crucial. DARPA asks only that the first round of implements have the strength of plastic, but that won't do for a universal tool kit. And how will robotic modules deal with the inevitable problems faced by field equipment such as dirt and misalignment? Questions like these will have to be answered.

If the project succeeds, a universal tool kit will be just the start. Robotic modules that can rearrange themselves could also repair themselves by moving modules to replace damaged sections. Imagine a vehicle that could repair its own dents and scratches, or clothing and other gear that could recover from wear and tear. Further down the road, reconfigurable robots might learn to make new modules from raw materials, thereby becoming self-replicating. Perhaps shape-shifting robots will become the new explorers, plumbing the depths of the ocean and exploring planets too hostile for humans.

We have a long way to go, but even a child can grasp the universal appeal of shape-shifting. Rus says that when her daughter was 4 years old, she said she wanted to be able to "pull toys out of the wall". The girl wanted her own universal toy kit. If she doesn't grow up too fast, maybe she'll get one.

Copyright ? 2008 Reed Business Information - UK. All Rights Reserved.

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