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Like a circus performer, SIMbot balances on a ball, and it only has 1 moving part

Ralph Hollis, a research professor at Carnegie Mellon University’s Robotics Institute, has been building “ballbots” — tall, thin robots that glide around on a sphere a bit smaller than a bowling ball — for the past 12 years. His latest iteration, however, is his most impressive yet — a deceptively simple robot whose only moving parts are its body and the ball.

“It’s one of the world’s simplest mobile robots that’s still capable of moving rapidly and navigating from place to place,” he told Digital Trends.

Named SIMbot (standing for “spherical induction motor”), the new robot was a joint project led by Hollis and Masaaki Kumagai, a professor of engineering at Tohoku Gakuin University in Tagajo, Japan.

“With SIMbot, we wanted to eliminate all the mechanical components,” Hollis continued. “Instead of rollers, motors, belts, gears and other components associated with ballbots that had been built before, we had the idea to have the ball wheel be a specially designed rotor. This is a hollow iron sphere with a copper layer on top of it. We then use an induction principle to directly generate torques on that sphere, thereby making it a closed-loop spherical induction motor.”

One of the big advantages of a ballbot, compared to a multiwheeled robot, is that they can more easily move in areas like corridors, doorways. and — most importantly — around people.

“The problem with multiwheeled robots is that, if you make them tall enough to be able to interact with people, they become very top-heavy,” Hollis said. “To counter that, you then need to make their base very wide to stop the robot from tipping over, which makes it hard to maneuver in many environments.”


A robot like SIMbot, on the other hand, is not only slender, but can easily move in any direction thanks to its ball base. It can move at speeds comparable with a fast walk.

“It’s definitely just a research project at this point,” Hollis said. “However, one of the outstanding characteristics of SIMbot is that it’s omnidirectional compliant — meaning that if you push on them, they’ll move away from you. It also means they can be easily guided around by a person.”

This compliant behavior, he said, offers some interesting possibilities when it comes to human users. “We’ve already started doing experiments where subjects are led by it around our lab,” he said. “The intention is that, in the future, it might be possible for visually-impaired or elderly people to be guided around by a SIMbot-type robot.”

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