stepper motor closed loop question

Hello!

I'm Ahn, currently designing my first small CNC router. I've seen many articles recommending ac servo motor for a better quality of the result. However, because of the budget issue, I decided to use servo stepper motors.

My question is about the concept of closed-loop control. I thought CNC(or PLC level controller) receives encoder signal from each motor and modify pulses for each driver (with PID or similar algorithm). However, what I just figured is that the motor driver does PID based servo control!!

(1) So, for normal closed-loop control, does CNC fully trust(?) each driver and send incent pulse signal???

(2) If it does, then is there any reason to send encoder signal to CNC controller? Is it just for monitoring? (encoder -> driver ->(?) CNC)

Thank you.

For closed loop step motors I would not bother returning encoder information to LinuxCNC unless you have one of these situations:

1. You have linear encoders and wish to setup a second feedback loop

2. You want to disable the drives without losing position

3. You want to monitor the actual motion performance with LinuxCNC

4. You want to use LinuxCNCs smarter following error monitoring rather than a fixed error limit

Also for low cost drives that do not output encoder signals, you would have to tap into
the drives encoder signals which is not a great idea because the encoder ground
will be common with the (rather noisy) motor ground and low cost drives do not use
differential encoder signals