+/-10V or closed loop step/dir?

[JAZZCNC]

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.

[Voicecoil]

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.

I would guess that the loop moves the motor to within at the minimum +/-1 increment of the encoder resolution (e.g. on the usual closed loop steppers +/-1/1000 revolution). There will have to be a bit of hysteresis (i.e. a window of accuracy) otherwise if you tried to take it to a certain position it would tend to hunt back and forth 1 encoder step.

[JAZZCNC]

I would guess that the loop moves the motor to within at the minimum +/-1 increment of the encoder resolution (e.g. on the usual closed loop steppers +/-1/1000 revolution). There will have to be a bit of hysteresis (i.e. a window of accuracy) otherwise if you tried to take it to a certain position it would tend to hunt back and forth 1 encoder step.

It's a fair bit more complicated than just that and can't really compare Closed-loop stepper system to real Servo drives as they simple in comparison.

Like Moray says Full closed-loop servo system is a lot of black magic and I while I personally understand PID and what all it's terms mean and handle in the system. I tried to keep my explanation simple and kept it mostly just to position but the reality is that it's a bit more complicated than just position when getting a Servo driven machine running nice.
So when I said don't understand how Step n Dir Closed-loop is done what I probably should have said was don't understand how the PID for Step n Dir output is handled by the Controller.

Regards the hunting back n forth then that's exactly what does happen. The Closed-loop system is constantly making corrections to maintain position, Torque or speed and this is where the black magic and mathematics come into play. It's also why setting a Servo machine up to be accurate with smooth movement, fast accelerations and directional changes with quick settle times can be a challenge. Because changing any one of the PID settings in either drive or controller can affect the others and how the system works. Real hair pulling stuff.!!

That said Modern Servo drives are getting much better and come with built-in tuning aids to help tune motor to drive. The Old AC and DC Amplifiers like what's on some of my machines are what real nightmares are made from. They are also very unforgiving and blow up for Fun which is an area where modern servo drives are much much more forgiving.