[ahecht]

Very interesting. Another method I've seen for coupling two motors whose speed needs to be independent of each other is the modified planetary used in the Toyota Prius. In the Prius, either the gas or the electric motor can run at any speed, and both will provide power to the wheels. "Gearing" is done by adjusting how much power comes from the lower RPM electric motor or the higher RPM gas motor.

The gearbox itself is almost identical to a standard planetary gearbox, except that the Sun and the Ring are both driven, one by the electric motor, and one by the gas motor. The Planet Carrier is then connected to the drive shaft. The advantage is that, if needed, both motors, each going at their optimum speed, could power the wheels at the same time.

Another idea that I came up with over the summer (while playing with mindstorms, of all things), is to use a standard differential, except that one wheel output is replaced by a second motor input. Mechanically, this would function identically to the Prius' system, with all the same advantages. However, building a dual power planetary gearbox or a differential (I'm not sure if they are available cheaply off the shelf) may be just as complex as building a CVT.