Hydraulic Ram control - S7-1200

Hi,

I am about to start a project controlling a hydraulic ram using a Siemens S7-1200 controller.

It will need to be quite steady control, by this I mean maintaining a load to within +/-2% with a dynamic attachment on it. I.e. the attachment being pulled by the ram will elongate when underload, so the hydraulics needs to account for the mechanical movement and not just let the load release to zero.

Can anyone point in the direction of reading material for valve selection, assuming I will need a PI or PID loop.

Thanks,

So very much like this post from a strangely familiar sounding name
http://www.plctalk.net/qanda/showthread.php?t=125732