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BowGo! CMU robotics researchers develop a pogo stick that aims high

Monday, May 28, 2001

By Byron Spice, Science Editor, Post-Gazette

Robotics researchers have turned a pogo stick into a pogo-Go-GO stick.

By replacing the metal coil spring of a conventional pogo stick with a fiberglass bow, Carnegie Mellon University researchers have turned the bouncing toy into a jumping machine. The device, which they call a BowGo, thus far has been able to clear obstacles as high as 42 inches.

Co-inventor Ben Brown aboard the BowGo on the Carnegie Mellon University campus. Brown has personally cleared 38 inches with the high-tech pogo stick. (Lake Fong, Post-Gazette)

"It's pretty scary when you think about it," said Ben Brown, a co-inventor who personally has cleared 38 inches with the BowGo. "So we try not to think about it."

The user's feet are 15 inches above the bottom of the BowGo's footpad, so the heights that users reach is considerable. Brown, a project scientist at CMU's Robotics Institute, and Illah Nourbakhsh, an assistant professor of robotics, are careful to wear helmets when using the contraption and, after outfitting it with an extra wide footpad, have decided it's safer to hop on grass rather than concrete.

Yet even now they are working on a lighter, more energetic version that could launch them to even greater heights. And they, along with post-doctoral fellow Garth Zeglin, have applied for a patent in hopes that they can find a company to make and market the thing. (Pogo sticks were originally patented in 1919 by George Hansburg, an Illinois furniture maker.)

It would seem that CMU robotics experts might have more important things to do than play with a pogo stick on steroids. But if you think of the BowGo as the leg of a robot, not a toy, you begin to grasp the project's true nature.

For Brown, 55, it began in the early 1980s when he and Marc Raibert, then a CMU robotics researcher, began working with hopping robots. These robots were, in essence, computer-controlled pogo sticks. They had hydraulic legs and were attached by cables to a stationary computer and to electrical power.

A one-legged hopping robot hardly sounds practical, but it enabled reseachers accustomed to slow-moving robots to explore the potential of speedy, maneuverable robots.

The high-tech BowGo, center, contrasted with two regular, spring-powered pogo sticks. The BowGo uses a fiberglass bow that can store three to five times as much energy as a steel spring of the same weight. (Lake Fong, Post-Gazette)

"It's the simplest running machine you can build," Brown explained. "We don't see one-legged things in nature, but we do see kangaroos, which sort of act like one-legged things."

Brown still has videos showing the washtub-shaped robot hopping madly around the lab. But the hydraulic leg required so much power that, even with miniaturized computer controls, the robot never could be untethered. So Brown moved on to other projects; Raibert leaped to the Massachusetts Institute of Technology, where he established its Leg Lab before joining private industry.

About five years ago, however, Brown begin thinking about running robots again. NASA was interested in extraterrestrial robots that could leap or hop while exploring Mars and other planets, moons and asteroids.

"When you're hopping, you're really flying and you can go over large obstacles," he explained.

Walking or rolling robots, by contrast, are limited in the size of boulders and crevasses they can negotiate by the length of their legs or the diameter of their wheels.

Extraterrestrial hopping is not a new idea. The Soviet space program reportedly studied a hopping machine for exploring the moon, Brown noted.

In the late '60s, Stanford University aeronautical engineers also worked on a lunar pogo stick; equipped with gyroscopes for stability, rockets for steering and a seat for the astronaut, it was intended to jump 30 feet straight up in the low lunar gravity, or to make leaps 50 feet long.

Brown and Zeglin began their experiments on an inclined air table, which simulated low gravity. Rather than use an expensive, power-hungry hydraulic leg to make it hop, they opted for a cheap, low-tech bouncing leg fashioned out of piano wire that was bent into a bow.

They found that the wire bow was quite efficient in storing and releasing energy; 80 percent of the energy used to bend the bow as the leg fell against the surface could be released to send the leg upward again.

Related story

In their quest for high-powered hopping, inventors in the U.S. and elsewhere have also turned to piston power.


That sort of efficiency suggested that a running robot might be feasible after all, if the leg could be scaled up.

"The pogo stick, or BowGo, was a way to test that," Brown said. "Of course, it also sounded like fun."

It also sounded like fun to a group of mechanical engineering students at the Oregon Institute of Technology a couple years ago, when producers of the TV version of the Guinness Book of World Records offered to award them the height record for pogo sticks if they could design a pogo stick that could jump 60 inches.

Guinness keeps track of pogo stick records for consecutive hops (177,737), for distance covered (23.11 miles in 12 hours, 27 minutes) and even for jumping the 1,899 steps of the CN Tower in Toronto (57 minutes, 51 seconds), but not for height.

"The two senior mechanical engineering students did a great job of designing and machining the device, which is about 6 feet high and a half foot around," said their faculty adviser, Robert Rogers. They used a custom-wound spring and installed bearings to reduce friction along the main shaft.

But the highest they ever got was 18 inches. "What we have learned so far is that, with this conventional spring design, there is a point of diminishing returns as far as weight versus energy storage capacity of the spring," they wrote in their project report. "The higher the target jump, the heavier and stronger the components need to become. Basically, this design is so heavy and cumbersome that it is hard to control."

  Online Graphic:
What makes the BowGo go



A lighter, better-lubricated version, they concluded, might reach 36 inches, but no more.

The beauty of the fiberglass bow, Brown said, is that it can store three to five times as much energy as a steel spring of the same weight. Also, when compressed, coil springs tend to buckle and press up against the pogo stick's plunger, increasing friction.

The latest version of the BowGo, built early last year, is 45 inches high, weighs nine pounds and has an aluminum body. Its plunger can travel 15 inches. The fiberglass bow has a spring force in excess of 600 pounds. The bow is offset from the main body in such a way as to soften the impact when the device hits the ground.

To reduce friction, the BowGo uses rollers to guide the plunger, rather than the plastic bushings common in toy pogo sticks.

A new version will be lighter and will have a plunger that can travel 50 percent farther. The length of travel is a limiting factor in how much energy can be stored, Brown explained.

Brown and Nourbakhsh have talked with several companies interested in manufacturing the BowGo, including one that envisions it as a specialty item for circus performers and other entertainers. Brown guessed a BowGo might sell for about $200 if mass produced.

Bow-legged robots are only one of the other possible applications for the technology. "I seem to be alone on this," Brown said, "but I believe it could lead to some viable means of locomotion." By adding a small power source and computer controls to maintain stability, he suggested, the BowGo might be transformed into personal transportation device.

"It would be like riding on the back of a kangaroo," he said. "Its [potential] is unexplored territory and I'd like to explore it."

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