Hydraulic Linear Motion Precision Controll

I think for this you would actually be commanding the stepper motor with position commands, but those position commands would correlate with the velocity command of the hydraulic servo. So you'd command the stepper to move to X number of steps to move the control arm to Y angle resulting in opening the valve to move the hydraulics at N velocity. So in essence the position of the steeper would be the velocity command.

But in then end like Andy said, it's all just numbers, and the control doesn't know what the numbers really mean, It is up to you to scale and plug them in where they need to go to work. And I see no reason this couldn't work the same as any ordinary analog servo system. Where in a servo you have your +/-10v analog signal will be connected to your step generators position command (and scaled appropriately) instead of to a drive's analog velocity command input.

Start with an ordinary analog servo config, Then where the pid's output would be connected to the analog output command. You would connect that instead to your stepgen's position command. If you are using a Mesa hardware stepgen I wouldn't bother with using the usual PID to stepgen velocity command setup. I think i'd just connect directly to the stepgen's position command input.

You might find that the existing screws and other intermediate hardware between the stepper and the control valve are nessisary to have the required resolution to adequately control the valve with the stepper motor. (This is just a guess not having seen how it is really set up.) If that isn't the case simpler is almost always better.