In short, Full’s cockroach work revealed a natural sweet spot: an ideal, easy-to-calculate springiness for any manner of limb. From that realization, only a small conceptual leap was needed to design the first efficient walking robot. Working with scientists at the University of Michigan and McGill University, Full went on to build Rhex (for robot hexapod), a giant mechanical roach that acts like a bread box bouncing on six pogo sticks. Rhex has no touch sensors, no feedback mechanisms, no programmed navigational ability, no grasp whatsoever of the physical world—just six bouncing legs. Yet it works marvelously, traversing rough terrain without a stumble. Most incredibly, when given a sideways poke, it quickly regains its composure. Self-stabilization, Full realized, doesn’t require neural wiring; it’s a free by-product of bouncing. “It’s built into the mechanical form,” he says.
Insects may be stable for the same reason, Full suggests. On his laptop, he has a video of a cockroach racing over a field of uneven blocks. “How do they run over this terrain?” he muses. “They don’t even slow down! Are they sensing everything, feeding it back through their brains? It can’t be. It’s too fast.”
To test his theory, Full sent his live roach back to the treadmill. This time, his team attached a miniature sideways-pointing jet pack on its back. Partway through its run, the jet pack would fire, knocking the roach off balance with a known force. To Full’s astonishment, the roach recovered its balance in less than 10 milliseconds—faster than any neural reflex. The brain wasn’t wasting energy on the body’s stability. The roach was staying upright without thought.
Because the brain doesn’t need to monitor stability, the insect is free to perform higher-order activities, like sensing its whereabouts and plotting a course. “The antennae need only give a general command,” Full says. “They don’t have to speak to the legs.” With that in mind, Full teamed up with Stanford University’s Center for Design Research to build Sprawl, a small six-legged robot with a sensor antenna and a simple course-correcting algorithm. Then came Ariel, a crablike robot built in collaboration with the Massachusetts firm iRobot. Ariel is six-legged, amphibious, and the first robot capable of remaining stable in shallow, rough surf. If it flips over, the legs reorient so that the top of the body becomes the bottom, allowing Ariel to move upside down. Full, eagerly funded by the Office of Naval Research, is now engineering Ariel to perform such tasks as finding underwater mines.
Full insists that his designs don’t mimic nature. They’re just inspired by it. Natural selection does not produce optimal systems, he says. Real limbs and their functions are constrained by the developmental process—they must grow and change over time—and by the fact that they perform multiple tasks. “Natural selection is not engineering,” Full says. “It’s tinkering with what you have.” The key in designing Ariel was realizing that its joints required just two degrees of motion, not the countless possibilities found in living crabs. “Crabs fight with each other, mate, jump on each other,” Full says. “We don’t need to put that in. Robots aren’t reproducing—yet.”
Meanwhile, Full is thinking extraterrestrially. For years he has been trying to get the rover builders at NASA to see the error of their ways: Wheels are slow, unstable, and marginally maneuverable on rocky terrain, he says. Rhex would have a field day on Mars. (When Full showed NASA a video of Rhex scrambling down a rocky hill, he
tinted the hill orange, for a Mars-like effect.) Back in January, when the Mars rover was stuck on its landing
|The world’s most efficient climbing robot may one day be modeled on the gecko. Geckos stick to walls with five-toed feet that are covered by half a million microscopic hairs per foot. The hairs are so tiny that they’re attracted to surfaces by van der Waals forces, which govern objects of molecular size.|
platform, Full couldn’t help but chuckle: The wheels of progress move slowly. “We’re still trying to convince them that legs are the way to go,” he says.
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