How do I model a piston?

Hi,

I want to model an assembly that uses a piston to raise an arm. But for control purposes, I'm going to use the hinge on the arm to get to the correct angle. I would like to model the piston that is supposed to be doing the control as a passive object that will just extend and retract as the arm moves. The piston is joined to the arm and a support by hinges as well.

My question is how do I set up the piston? I tried creating a spring joint between the two piston objects, but I had a hard time making it work.