麻豆淫院

Imagine that you have a big box of sand in which you bury a tiny model of a footstool. A few seconds later, you reach into the box and pull out a full-size footstool: The sand has assembled itself into a large-scale replica of the model.

That may sound like a scene from a Harry Potter novel, but it鈥檚 the vision animating a research project at the Distributed Robotics Laboratory (DRL) at MIT鈥檚 Computer Science and Artificial Intelligence Laboratory. At the IEEE International Conference on Robotics and Automation in May 鈥� the world鈥檚 premier robotics conference 鈥� DRL researchers will present a paper describing algorithms that could enable such 鈥渟mart sand.鈥� They also describe experiments in which they tested the algorithms on somewhat larger particles 鈥� cubes about 10 millimeters to an edge, with rudimentary microprocessors inside and very unusual magnets on four of their sides.

Unlike many other approaches to reconfigurable robots, smart sand uses a subtractive method, akin to stone carving, rather than an additive method, akin to snapping LEGO blocks together. A heap of smart sand would be analogous to the rough block of stone that a sculptor begins with. The individual grains would pass messages back and forth and selectively attach to each other to form a three-dimensional object; the grains not necessary to build that object would simply fall away. When the object had served its purpose, it would be returned to the heap. Its constituent grains would detach from each other, becoming free to participate in the formation of a new shape.

Distributed intelligence

Algorithmically, the main challenge in developing smart sand is that the individual grains would have very few computational resources. 鈥淗ow do you develop efficient algorithms that do not waste any information at the level of communication and at the level of storage?鈥� asks Daniela Rus, a professor of computer science and engineering at MIT and a co-author on the new paper, together with her student Kyle Gilpin. If every grain could simply store a digital map of the object to be assembled, 鈥渢hen I can come up with an algorithm in a very easy way,鈥� Rus says. 鈥淏ut we would like to solve the problem without that requirement, because that requirement is simply unrealistic when you鈥檙e talking about modules at this scale.鈥� Furthermore, Rus says, from one run to the next, the grains in the heap will be jumbled together in a completely different way. 鈥淲e鈥檇 like to not have to know ahead of time what our block looks like,鈥� Rus says.

Conveying shape information to the heap with a simple physical model 鈥� such as the tiny footstool 鈥� helps address both of these problems. To get a sense of how the researchers鈥� algorithm works, it鈥檚 probably easiest to consider the two-dimensional case. Picture each grain of sand as a square in a two-dimensional grid. Now imagine that some of the squares 鈥� say, in the shape of a footstool鈥� are missing. That鈥檚 where the physical model is embedded.

According to Gilpin-author on the new paper, the grains first pass messages to each other to determine which have missing neighbors. (In the grid model, each square could have eight neighbors.) Grains with missing neighbors are in one of two places: the perimeter of the heap or the perimeter of the embedded shape.

Once the grains surrounding the embedded shape identify themselves, they simply pass messages to other grains a fixed distance away, which in turn identify themselves as defining the perimeter of the duplicate. If the duplicate is supposed to be 10 times the size of the original, each square surrounding the embedded shape will map to 10 squares of the duplicate鈥檚 perimeter. Once the perimeter of the duplicate is established, the grains outside it can disconnect from their neighbors.

Rapid prototyping

The same algorithm can be varied to produce multiple, similarly sized copies of a sample shape, or to produce a single, large copy of a large object. 鈥淪ay the tire rod in your car has sheared,鈥� Gilpin says. 鈥淵ou could duct tape it back together, put it into your system and get a new one.鈥�

The cubes 鈥� or 鈥渟mart pebbles鈥� 鈥� that Gilpin and Rus built to test their algorithm enact the simplified, two-dimensional version of the system. Four faces of each cube are studded with so-called electropermanent magnets, materials that can be magnetized or demagnetized with a single electric pulse. Unlike permanent magnets, they can be turned on and off; unlike electromagnets, they don鈥檛 require a constant current to maintain their magnetism. The pebbles use the magnets not only to connect to each other but also to communicate and to share power. Each pebble also has a tiny microprocessor, which can store just 32 kilobytes of program code and has only two kilobytes of working memory.

The pebbles have magnets on only four faces, Gilpin explains, because, with the addition of the microprocessor and circuitry to regulate power, 鈥渢here just wasn鈥檛 room for two more magnets.鈥� But Gilpin and Rus performed computer simulations showing that their algorithm would work with a three-dimensional block of cubes, too, by treating each layer of the block as its own two-dimensional grid. The cubes discarded from the final shape would simply disconnect from the cubes above and below them as well as those next to them.

True smart sand, of course, would require grains much smaller than 10-millimeter cubes. But according to Robert Wood, an associate professor of electrical engineering at Harvard University, that鈥檚 not an insurmountable obstacle. 鈥淭ake the core functionalities of their pebbles,鈥� says Wood, who directs Harvard鈥檚 Microrobotics Laboratory. 鈥淭hey have the ability to latch onto their neighbors; they have the ability to talk to their neighbors; they have the ability to do some computation. Those are all things that are certainly feasible to think about doing in smaller packages.鈥�

鈥淚t would take quite a lot of engineering to do that, of course,鈥� Wood cautions. 鈥淭hat鈥檚 a well-posed but very difficult set of engineering challenges that they could continue to address in the future.鈥�

This story is republished courtesy of MIT News (), a popular site that covers news about MIT research, innovation and teaching.