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Scoot, scramble and roll.

Invertebrate biologists have good reason to think their data could inspire even the best engineer. Real-life walking, sliding, inching and somersaulting creatures showcase quite a few possibilities for robot locomotion.

"Nowhere is there greater diversity in locomotor designs than in arthropods," says Robert J. Full, a comparative physiologist at the University of California, Berkeley. These critters -- crabs that scoot sideways just out of reach of waves, centipedes that scramble up tall walls, stomatopods that curl up and roll away backwards -- make their way through environments that would immobilize the most sophisticated machines.

"[Conventionally designed] robots never move in a way that's similar to animals," says Full.

To understand how arthropods scoot, scramble and roll, he and others have begun breaking these movements down into their simplest mechanical components. They find, however, that deriving general principles for use in robot design requires a bit of ingenuity.

To get a better handle on crab-scooting and cockroach-running, for example, Full constructed a force plate -- a platform with sensors underneath that connect to a computer. The sensors detect when a leg pushes down. As the crab or cockroach runs across the plate, Full films it with a high-speed video camera and then analyzes leg positions frame by frame, correlating the movements with force plate measurements.

These and other studies show that no animal, not even a tumbling stomatopod, moves as a wheel does. Instead, animals' legs act like pendulums during slow walks and like springs when the animal speeds up. Full has found that animals with two, four, six and even eight legs all produce similar patterns on force plates, indicating that "the body is being propelled alternately by two sets of legs," he says. "The differences come when you look at individual legs." Thus, three legs of an insect, two legs of a poodle and four legs of a crab act as units equivalent to one leg of a human. But in each animal, the role of each leg varies. "I believe the legs are positioned and develop forces to minimize the torque at all the joints," Full explains.

He found another parallel between animals with different numbers of legs when he examined the relationship between speed and stride. Trotting fourlegged animals, for example, speed up by switching to a gallop: They take longer strides rather than move their legs faster to accelerate. Full discovered that a crab and a mouse of equal weight will switch gaits as the same speed -- about 1 meter per second -- and the same stride frequency, or number of times all the legs cycle per second.

"That suggests that there are general principles that can be applied to animals with tremendous differences in body form," he says. "Those same concepts can be transferred to nonbiological systems."

Such close examination has yielded other surprises about the versatility of moving arthropods. Scientists once thought that cockroaches always kept three legs on the ground and so remained stable all the time as they moved. This static stability contrasts with the dynamic stability of running humans, who would fall over if stopped in midstride but who stay upright because all the forces balance out as they move through each stride cycle.

Full's videos, however, show that cockroaches can reach speeds of 1.5 meters per second -- more than 3 miles per hour. As the roach accelerates, it leans farther back, first depending on the back four legs and then, at top speeds, zooming along on just two. "This rejects the notion that arthropods require static stability," says Full. It also shows that an animal or robot can be capable of both dynamic and static stability, he says.

"The key is not to exactly mimic the biological system but to take the concepts and see if they can be transferred to a design to make it better," says Full, who has provided MIT engineers with data about leg length and position for possible use in the design of new robots. He also sees himself as benefiting from such collaboration: "We can provide biological inspiration for them, and the questions they ask us will help us define how we need to quantify these systems in the animal world."
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Title Annotation:arthropods and robotics
Author:Pennisi, Elizabeth
Publication:Science News
Date:Nov 30, 1991
Words:694
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