The Johns Hopkins Gazette: January 16, 2001
January 16, 2001
VOL. 30, NO. 17


Spherical Motor Allows Ball-Based 3-D Movement

Magnet-guided device could lead to cheaper and more precise robotic joints

By Phil Sneiderman

Johns Hopkins Gazette Online Edition

Engineers at Johns Hopkins have invented a globe-shaped motor that is capable of rotating in any direction. The device, which uses electromagnets controlled by a computer, could give robotic arms greater flexibility and precision and might even allow the lowly computer mouse to guide the hand of the computer user, instead of the reverse.

The spherical motor

These advances could come about because the new spherical motor permits a wide range of unhindered mechanical motion. "A conventional motor turns on an axis, moving in one direction," Gregory S. Chirikjian, an associate professor in the Whiting School's Department of Mechanical Engineering, explains. "What we've developed is a new type of spherical motor. Basically, there's a ball inside, and we can rotate it in any direction we want."

David Stein and Gregory Chirikjian with the spherical motor that could be used in robotic joints or an interactive computer mouse.

Magnetic forces and complex computer software make the device work. For their prototype, Chirikjian and doctoral student David Stein mounted 80 permanent magnets inside a hollow sphere, arranging them in a precise pattern. Like a scoop of ice cream nestled into a cone, the magnet-laden sphere was then placed into a tapered base atop a "saddle" made of 16 circular electromagnets, each marked with a number. By activating two or more of these electromagnets, the operator causes them to attract certain permanent magnets inside the sphere. This attraction pulls the ball into a new position.

The idea of a spherical motor is not new. But the Hopkins engineers believe their model is superior to previous efforts because it can achieve a much greater range of motion. Chirikjian and Stein described their device in a paper presented at a conference sponsored by the American Society of Mechanical Engineers. Also, Chirikjian and Stein, along with a third collaborator, Edward Scheinerman, a professor of mathematical sciences at Hopkins, recently applied for two U.S. patents covering components they developed for the prototype.

With further refinement, the inventors say, the spherical motor could replace the conventional motors that are now used to move robotic arms in three dimensions. Currently, a robotic arm needs six or more conventional motors to position and orient objects in three dimensions. But the spherical motor would behave like a human shoulder joint, rather than an elbow joint. As a result, Chirikjian says, three spherical motors could give a robotic arm a greater range of motion than arms that have six traditional motors. "You'd be able to use far fewer joints because each spherical motor would have more freedom of motion," he says. "This would also enable the robotic arm to be more accurate because every time you have a joint, you introduce a little bit of play, a little bit of wiggle to the arm. When you have six or more traditional motors, that little bit of wiggle adds up. If instead you could use only three spherical motors, you'd have much less jiggling."

The inventors envision other applications for the spherical motor. "You could also turn these motors upside down and use each one as a three-dimensional wheel," Chirikjian suggests. "It would not only turn around an axis like a conventional wheel, it would have omnidimensional characteristics. For example, if you put a ball in a socket, you can roll it any way you want, unlike a regular wheel, which can only go in one direction without slipping.

"You could also use the spherical motor technology to create a computer mouse that pulls you around, if you wanted to interact with your computer. You'd be able to argue with your computer--it wants you to go one way, and you want to go another. This technology could be used in games or as a way to have intelligent agents in a computer interact with the physical world. Right now, most everything on a computer is visual in nature. But one can imagine that in the future this interaction will involve more of the sense of touch."

The National Science Foundation provided funding for this research.

Related Web Sites

Gregory Chirikjian's Web page
Robotics research at Johns Hopkins