On the Ball: Omnidirectional Mobile Robot has just two moving parts. Good play for simplicity.
Spherical Induction Motor Eliminates Robot’s Mechanical Drive System
By Byron Spice
More than a decade ago, Ralph Hollis invented the ballbot, an elegantly simple robot whose tall, thin body glides atop a sphere slightly smaller than a bowling ball. The latest version, called SIMbot, has an equally elegant motor with just one moving part: the ball. ... "