[jimwick]

Yes, intermediate positioning is definitely possible. We built a fairly successful prototype this summer, although we didn't really have the necessary equipment to do the job right.

The basic idea is that you use a two position valve to control direction, and then use simple open/close valves on each exhaust to control motion. Nothing moves until ONE of the two exhaust valves is opened. Speed can be controlled by modulating air flow in the exhausts. We did not have solenoid operated open/close valves, so we just used manual ones from the hardware store. The system worked fairly well using the two manual valves.

I believe this can all be done using a three position valve. They are pricey so we haven't tried it yet.