pic: 6 speed?? Great in theory - lets see how it works!

[Biff]

There are threads about multi positioning with air cylinders. If I was going to do it that way I would use a control valve with a center off, no power all ports closed. A slider pot attached to the piston, and a really good programer.
You can stop a piston in any point of it's travel with the first, you can tell where it is with the voltage generated from the slider on a analoge input to the controler. The bounce and drag may cause occilation problems. And if your coding is not up to it, all bets are off. Other wise I would use shaft with very coarse thread, almost like that of a Yankee screwdriver, and a multi turn pot; driven as suggested by a globe motor. Good luck