The Mind-Spinning Potential of the Spherical Motor


Imagine a computer mouse that can push back against a user's fingers to simulate braille dots. Or a robotic arm for fine-tuning circuit boards on orbiting satellites with greater accuracy than any human hand could ever manage. These represent just two of the myriad possibilities for the world's first truly spherical motor.

Unveiled earlier this month by researchers at Johns Hopkins University, the circular-shaped motor laced with magnets can spin to apply force in three dimensions. "We can rotate it in any direction we want," explains mechanical engineering professor Gregory S. Chirikjian, who created the spherical motor.

UP, DOWN, ALL AROUND. That flexibility is remarkable when compared with traditional mechanical motors, which have moving parts that rotate around a single axis and thus can apply force in only one dimension. By contrast, the new motor's wide range of motions could make future models extremely useful to everyone from NASA engineers to manufacturers of medical equipment.

While today's technology uses combinations of conventional motors to create three-dimensional motion, the process is cumbersome and inefficient. So the idea of a spherical motor has long intrigued scientists, who had never quite figured out how to build one. The key problem was a simple matter of physics: Determining the precise and equidistant placements of magnetic force for a large number of points on a sphere presents a mind-boggling problem. Researchers at Georgia Tech took a big step forward recently when they constructed a motor that can spin in a cone-shaped range of motions. But the Johns Hopkins model can spin throughout the full range of a sphere. The equidistant placement was key, says Chirikjian.

Stein holds the "saddle" of electromagnets, Chirikjian points to the permanent magnets on the sphere's interior

CIRCULAR LOGIC. Chirikjian first enlisted Hopkins math professor Edward Scheinerman to solve the placement riddle. Then he and graduate student David Stein glued 80 magnets in a precise pattern inside a plastic sphere about the size of a typical globe of the world you might find in any library. Chirikjian and Stein mounted the sphere atop a hollowed-out hemispherical saddle lined with 16 circular electromagnets. They marked each electromagnet with a number to allow them to write the necessary computer program that would control the motion of the spherical motor. By activating two or more of these electromagnets, an operator can create a force that pulls on certain permanent magnets within the sphere and spins it in the desired direction.

Scientists see tremendous potential in spherical motors. The robotic arms used widely today require at least six conventional motors to replicate the three-dimensional motion of a human limb. But because the spherical motor more accurately simulates the motion and force projection of a human shoulder, robotic arms using spherical motors would only need three of Chirikjian's inventions to achieve a far greater range of motion than their one-dimensional counterparts.

The magnet-filled globe rests atop the computer-activated saddle

Fewer moving parts would mean less friction and reduced energy consumption. And fewer joints would allow robotic arms to be more accurate. "Each joint adds a little bit of play, a little bit of wiggle to the arm. When you have six joints, that adds up," says Chirikjian.

Spherical motors could also be used to create omni-directional gears, even tactile sensations. For example, a computer trackball that uses a spherical motor could actually provide resistance against pushing fingers to create the impression of raised braille dots. "You could create the sensation of bumping into a wall in a maze game, or even the feeling of a ball hitting a racket in a game of computer Pong," says Chirikjian, whose basic science research was funded by the National Science Foundation.

Chirikjian, Stein, and Scheinerman have applied for two U.S. patents on their work. Commercial applications likely remain years away, depending on when scientists can shrink the control mechanism and eliminate the need for a PC to guide the motor. Commercial spherical motors will also need to be more robust in construction. All the same, the motors should prove both economical to construct and easy to either scale up or scale down, says Chirikjian. That's good news for engineers developing more efficient machines, for users of assistive technology, and for computer gamers looking to get a better feel for their desktops. By Alex Salkever in New York


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