[kirtar]

One thing you can do is take a scale (that is to say a reference such as a meter stick) and use a video camera. This may require a high speed camera that you don't have though depending on how fast this is. What you can do is tape it with the scale and go frame by frame (remember, frames have a constant rate).

If you're using a piston and not a rotary pneumatic, you can actually just focus on the piston and put the scale there so you won't reallly need a high speed camera. Basically, start at the first frame in which it starts to move (record measurement), go to the frame in which it is fully extended and then divide the distance traveled by (Numer of frames/Frame rate), and then carry through the calculation through any linkage you have it hooked up to.

Hope that made sense and was helpful.