A Balancing Act: Ballbot from Carnegie Mellon Rolls out on One Wheel

by David Templeton | Submitted Wednesday Feb 04, 2009 [03:23 PM]

For people who aren't circus performers it's a ridiculous notion: Balancing atop a soccer ball as an efficient means of traveling across a room. The floor is flat and the ball is round, so it's obviously unstable and goes against all common sense. Unless you are Ralph Hollis.

The Carnegie Mellon University research professor of robotics has created a 5-foot, 95-pound robot that travels atop a single aluminum sphere coated in urethane.

Introducing "Ballbot" -- a self-contained, battery-operated robot that goes in any direction without the need for extra maneuvering room.

Watching it move without falling requires unwilling suspension of disbelief. One could swear it violates major laws of physics.

"We're at the point where it works, but there are many issues remaining," Dr. Hollis said. "But it proves that one wheel is sufficient for a mobile robot."

Although dynamically stable, humanoid robots with human-styled legs are expensive and complicated, based on current technology, making them less practical for use in houses or offices, he said.

Throughout his career in robotics, he said, he's learned that taller robots with three or four wheels can tip over when they accelerate too rapidly or travel on ramps. Such robots require wider wheel bases, making them awkward and unstable.

Believing "simple is best," he sat down about four years ago in his home workshop to figure out how best to make robots roll, and creativity ruled the day. He concluded that a ball would be ideal.

The only challenge -- and it was a formidable one -- was making the tall, thin and rather heavy robot balance atop an orb smaller than a soccer ball and use it as a source of mobility.

Using such curved logic and robotics know-how, Dr. Hollis and his team of graduate students eventually made the human-sized Ballbot twist, turn and travel in any direction without the need for room to maneuver.

It almost can turn on a dime: The part touching the floor is no larger than a silver dollar.

Ballbot relies on an internal sense of balance thanks to fiber-optic gyroscopes that measure inertia, pitch and roll angles from vertical and send hundreds of signals per second to a computer. The computer, in turn, controls four rollers that turn the ball to make sure Ballbot is in position to stand or roll without tipping over.

"It's more stable than the typical robot," Dr. Hollis said. "It doesn't like to tip over."

Before test runs, the researchers used to place a black inner-tube around Ballbot to prevent damage if it does fall, but no longer.

Dr. Hollis and his research team discussed Ballbot in October during the International Symposium for Robotics Research in San Francisco. In May, he delivered a paper on Ballbot during the International Conference on Robotics and Automation in Orlando, Fla.

Dr. Hollis' Ballbot is getting strong reaction from colleagues.

"It's very interesting and very unique," said Ruzena Bajcsy, a professor of electrical engineering and computer science at University of California, Berkeley. "The big issue is how to keep it stable, and I think he's figured it out.

"He's figured out that a ball can be an actuation or mobility device in small spaces," Dr. Bajcsy said. "In my mind that's the biggest contribution."

Louis Whitcomb, professor of Dynamic Systems and Control Laboratory at Johns Hopkins University's Department of Mechanical Engineering, described Dr. Hollis as a pioneer in robotics.

"I think it's really cool," he said. "There are the Lewis and Clark types in science and he's definitely in that category."

Dr. Hollis is writing an article about Ballbot for an upcoming edition of Scientific American magazine.

With a degree in physics, Dr. Hollis worked 16 years for IBM, the last eight as manager of advanced robotics in IBM's Thomas J. Watson Research Center. As a student at the University of Colorado, he created the first mobile robot with a computer on board in 1977.

In 1993, he took a position at CMU's Robotics Institute, where he's working on several robotics projects, including precision-assembly robotics that can assemble small precision systems automatically.

Another project involving "haptics" uses magnetic levitation, among other techniques, to give the computer operator an interactive sense of touch. The technology makes it seem as though the person at a computer is touching things in a three-dimensional image on the screen.

Making robots move efficiently has long been a fascination of Dr. Hollis.

Key to success, he said, is keeping the hardware mechanically simple so the software does the hard work. At home he developed the one-wheel concept and designed and built most of the 200 precision parts.

Working with National Science Foundation grants the past two years, he and two graduate students, Anish Mampetta and Eric Schearer, and former graduate student Tom Lauwers, produced Ballbot with assistance from George Kantor, a CMU robotics project scientist.

Dr. Hollis said a tall, thin robot that can move in any direction will allow it to have meaningful relationships with people.

Ballbot seems to defy logic in other ways.

The computer and other heavier equipment is placed near the top of the robot. It would seem to make it unstable. But weight near the top makes it fall more slowly, much the way it's easier to balance a broomstick on one's hand when the heavier bristled end is highest, Dr. Hollis said.

Ballbot's ball works in reverse of the ball operating a computer mouse. The ball in the mouse controls the computer, but the computer in Ballbot controls the ball.

Tugged or pushed, Ballbot moves away from the force to remain vertical, proving its stability and reluctance to tip over.

Ballbot can stand indefinitely subject to battery power, but also can be programmed to lower three legs, or kickstands, to provide stability while resting. Much like humans, it's stable only when the power is on.

"Now we have to see how it will perform around people and furniture," Dr. Hollis said.

Next, the team will provide the robot with a drive so its cylindrical trunk can rotate to face any direction without the ball needing to turn.

The team also hopes to add a head and arms that would swing to help it keep its balance. But those additions will require Ballbot's computer to be reprogrammed to adjust to fluctuations in weight distribution as the arms move. The arms are necessary to make it functional in homes and offices, Dr. Hollis said.

Although it needs some tweaking to make it faster and more graceful, Ballbot's method of mobility marks a major step in robotic evolution.

Creating a robot that moves well in any direction "is the holy grail of mobile robots," Dr. Hollis said, as he repeats the idea that led to its creation.

"One is enough."

People who read this, also read...

Tags:navigation  robotic+platform  balance  Carnegie+Mellon 

back to top

blog comments powered by Disqus
Topic Suggestion?
Send it to our editors using our simple submission form