The spherical induction motor (SIM) invented by Prof Hollis, and Masaaki Kumagai, a professor of engineering at Tohoku Gakuin University in Tagajo, Japan, eliminates the mechanical drive systems that each used on previous ballbots. Because of this extreme mechanical simplicity, SIMbot is said to requires less routine maintenance and is less likely to suffer mechanical failures.
Induction motors aren’t new, but the SIMbot’s rotor is spherical enabling it to move omnidirectionally at up to 1.9m/s.
Though Prof Hollis said it is too soon to compare the cost of the experimental motor with conventional motors, he said long-range trends favour the technologies at its heart.
“This motor relies on a lot of electronics and software,” he explained. “Electronics and software are getting cheaper. Mechanical systems are not getting cheaper, or at least not as fast as electronics and software are.”
SIMbot's mechanical simplicity is a significant advance for ballbots, a type of robot that Prof Hollis maintains is ideally suited for working with people in human environments. Because the robot's body dynamically balances atop the motor's ball, a ballbot can be as tall as a person, but remain thin enough to move through doorways and in between furniture. This type of robot is inherently compliant, so people can simply push it out of the way when necessary. Ballbots can also perform tasks such as helping a person out of a chair, helping to carry parcels and physically guiding a person.
Until now, moving the ball to maintain the robot's balance has relied on mechanical means, using an ‘inverse mouse ball’ method, in which four motors actuate rollers that press against the ball so that it can move in any direction across a floor, while a fifth motor controls the yaw motion of the robot itself. However, the belts that drive these rollers wear out and need to be replaced, and the system then needs to be recalibrated. The researchers said that the new motor's solid-state system would eliminate that time-consuming process.
The rotor of the spherical induction motor is a precisely machined hollow iron ball with a copper shell. Current is induced in the ball with six laminated steel stators, each with three-phase wire windings. The stators are positioned just next to the ball and are oriented slightly off vertical.
The six stators generate travelling magnetic waves in the ball, causing the ball to move in the direction of the wave. The direction of the magnetic waves can be steered by altering the currents in the stators.
Getting rid of the mechanical drive eliminates a lot of the friction of previous ballbot models, but virtually all friction could be eliminated by eventually installing an air bearing, Prof Hollis said. The robot body would then be separated from the motor ball with a cushion of air, rather than passive rollers.