US Patent 7997209 Propulsion mechanism

There is provided a differential propulsion mechanism including two or more concentric and mutually counter-rotating first wheels (4, 6, 8, 10), mutually reacting and balancing the torque of a motor drive (20) interacting with the wheels. The motor drive has a stator attached to one of the first wheels to power a first wheel over a first track (12, 14, 16, 18), a rotor coupled to a mechanical link, at least indirectly connecting the rotor with a second of the two or more first wheels to power the second wheel over a second track, and a concentric connecting device affixed for coupling a payload thereto or for coupling the mechanism itself to another device.