How do you control the length a piston will extention/retraction?

[MrForbes]

Originally Posted by 1075guy

While the trapped air in the cylinder can effectively turn it into a spring, I would argue that it CAN be controlled to a reasonable level of accuracy, as evidenced by 1075's 2007 arm. We did however, use, in addition to the ram driving the arm, another ram which clamped down on a piece of Al angle attached to the arm that slid through a piece of Al square tube. This basically functioned as a physical brake, reducing the "air spring" effect.

Adding a mechanism to limit travel can be a big help, you could also have a movable physical stop (perhaps a small servo could actuate a pin or something, as long as the load is removed before you try to move it)