If you choose to use pneumatics to do this, there are a few good discussion threads from January and February 2004.
Also, some other sources -
www.cyberblue234.com / go to the multimedia and there is a video of pneumatic staging (ie starting and stopping mid-point on the travel).
also check the white papers, technical section, for a slide on the pneumatic circuit layout to do this staging. the title is "Controlling Pneumatics - Sketch".
I am the author of the file upload if that helps the search. It was uploaded in January or February 2004.
Basically, you control the movement of the cylinder by controlling the exhaust of the solenoid valve. I believe the slide shows doing that with two switches - one to control the direction and one to start and stop the cylinder. We ended up finding a way to link the solenoids and control it with one 3 way switch (extend - hold - retract).
It is difficult to repeatedly go to the exact same stop point. Air is very compressible, so the cylinder might move a little. Most cylinders were more accurate in one direction - depending on how they were loaded.
Good Luck. Take some time to experiment.