Quote Originally Posted by Neale View Post
I ask this question from a position of ignorance, but if you use step/dir with a modern AC servo plus appropriate driver, does that mean that there is a PID tuning issue but it is hidden from the user and is pre-programmed into the driver somewhere?
It's not simple to be honest because what you are actually dealing with is 3 PID controllers in what's called a Cascade control setup. Position, Current and Velocity PIDS.

The Controller just deals with the Position PID and the Amplifiers deal with Position, Current and/or Velocity PIDS. When setting up an Amplifier you choose if running in Velocity or Current Mode. (Some will let you change between modes using inputs but won't get into that.)

So before you can properly set up position in the controller the Motor and amplifier first have to be tuned to each other regards Velocity or Current. This is done in the Amplifier.
Now with an Analog controller, you have to tune the Position PID in the controller so the voltage can react quickly to Position error Feed back and make any corrections needed in the position loop to try and reach Zero Error. However, In reality, the error never actually reaches Zero and there is always some slight error that is constantly trying to be corrected by the controller's Position and Amp's Position PID's. So you can say servo never actually stops moving, this means the Position PID has to correct and settle quickly so the motor doesn't twitch n jiggle around while trying to constantly reach Zero Error.

Now with Step n Dir then I'm not quite sure how Full Closed-loop back to the controller is done because never used Step n Dir Controller that is a Fully closed loop but I would imagine the principle is very similar but except instead of outputting a Voltage it sends out pulses to try and correct position error.

This is a very simple explanation to what is quite a complex system and I'm no expert with servos either so please excuse if any of what I said not fully correct.