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Aerobatics, running--skill or simple physics?

Do the difficult aerobatic maneuvers performed by gymnasts and skiers result from a sophisticated series of split-second perceptions and reactions, or are they relatively simple applications of the principles of physics and gravity? This is one of the questions that Leg Lab scientists including postdoctoral associate Rob Playter have tried to answer.

Under the direction of Marc Raibert, professor of electrical engineering and computer science, the Leg Lab (part of the Artificial Intelligence Lab) has been building robots capable of running and jumping in an effort to learn more about locomotion and dynamic balance employed by people and animals. Dr. Playter, who finished his PhD in aeronautics and astronautics last summer, has been concentrating on a particular type of somersault, and he will present his work at the American Association for the Advancement of Science meeting later this month.

"I came into the lab thinking that athletes have these great skills," such as keeping the body straight in maneuvers like a layout somersault, said Dr. Playter, a former gymnast at Ohio State University. He believed that such aerial maneuvers required a person to take continuous readings of his orientation with respect to the ground and make constant compensations. However, he added, "that's all been tempered by my experience in the lab."

A somersault involves rotating the body around an imaginary line entering one side of the rib cage and exiting through the other. In an upright person, this is the intermediate inertia axis, around which rotation is unstable. This means that it's relatively difficult to rotate around this axis without wobbling and twisting. However, Dr. Playter's experiments showed that the rotation can be easily stabilized by passive movements of the arms during the somersaulting movement.

To test his ideas, he created some computer graphic simulations and did mathematical analyses. Later, he built a foot-long wooden doll approximating the dimensions of a human body. To this he attached two arms fitted with springs and pulleys so the arms could move up and down at the shoulder independently of each other. This simple device eliminated all variables except shoulder movement; "I could really isolate what's going on," he said.

Then Dr. Playter built a launcher to fling the doll into a somersault layout. As predicted, the doll twisted several times in the course of its somersault if the arms were fixed in position. However, if they were allowed to move freely with the right springs in the shoulders, the doll could execute several somersaults without twisting at all. He found that if the frequency of the up-and-down oscillation of the arms was equal to the frequency of the doll's rotation, it would somersault with stability.

The work by Dr. Playter and others in the lab has demonstrated that a person doing what seems to be a highly skilled maneuver is only taking advantage, at a very basic level, of forces acting on him or her. Such acrobatic moves "are full of swinging or pendulous behaviors dominated by passive dynamic effects," Dr. Playter said. "The body swings the way it wants to--you can either work with it or against it."

The idea of "working with" the inherent behavior of the body has influenced other Leg Lab robots, including a monopod that moves by bouncing forward like a pogo stick, as well as bipeds and quadrupeds. One of the bipeds is pictured on the cover of this year's student directory (Dr. Playter is the person in the middleof the photo). Another holds the robot land speed record of 13 mph, Professor Raibert said. The researchers have found that the computer controlling each robot doesn't need to tell each joint how to move at every instant; it only needs to adjust the length of the stride and the springiness of the leg. The same principle may apply to the brain and muscles in an animal; much of the work of running could take place almost automatically.

Although robots that can run over uneven ground are not yet feasible (since they would require a visual system to find and select the easiest path), the Leg Lab work has had more immediate applications to video animation. The software that controls robots has been adapted to control computer-animated characters. For instance, Professor Raibert's group produced a computer-animated cartoon called "On the Run" (shown on the cover of the faculty/staff directory) that features bounding one-legged kangaroos and the two-legged robot that exists in real life. His company, Boston Dynamics Inc., is developing the animation software commercially.

By incorporating the basic principles of movement that the robots use, animators can make the creatures run in response to general instructions such as speed and direction without having to specify the details of each individual movement. Although in the early stages, this process greatly eases the labor-intensive aspect of computer animation and makes the behavior more interesting. "Some day, animated characters could animate themselves," Dr. Raibert said. The work has also penetrated the non-animated movie world; the biped robot had a bit part in last year's Rising Sun.

"Existing robots still fall far short of the grace and agility found in animals, but to build such robots is our goal," Drs. Raibert and Playter said.

A version of this article appeared in MIT Tech Talk on February 15, 1995.

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