[MrForbes]

The magnet sensor is an electrical device that can sense a magnet on the piston in the cylinder. You need to order the "M" option when you order the cylinders, so they wil put the magnet inside. You can move the clamp on sensor to wherever you want along the cylinder. And you can probably put a few sensors on one cylinder if you need to figure out just where the piston is.

However, you may have fun trying to control how far you move the piston, since air is compressible, and you have to play some games with the valve setup to make it be able to stop partly extended.