disclaimer:
I'm not picking on the Gecko drives, they were just a good example (and I was looking at using them)

----background:

I'm working on converting a manual Series 1 Bridgeport to CNC. I plan to use a PC running EMC/Linux for control. I already have the ballscrews installed and I have the mechanical aspect mostly under control. I'm still undecided on Z the axis setup, but I've got a good idea for what I'm doing with the X and Y axes.

I like to think that I have a basic understanding of circuits, electronics and control systems, but I'm still somewhat unsure of the electronics aspect of the conversion.

----

I had decided to use a servo system for my conversion.
I was looking at doing a PC -> breakout board -> Gecko drive -> servo motor setup.

----confusion:

In my research, I've found that a lot of motor drives take "step/direction" input. This makes sense to me for use with stepper motors, which being open loop do not give feedback to the controller.
(and step/direction is an obvious input format)


However, I see that the Gecko servo drives, (and others) take step/direction" input as well. With these servo drives, the encoder feedback is sent straight to the motor drive. From my quick read of the Gecko manual, the only input to the PC is the err/res channel.
(if connected? I got the impression that this was optional)


I'm confused by this.


My thinking is that, despite using servos, a PC/Gecko/servo system won't be any different than a stepper system from the PC (controller) point of view. The computer will output a step/direction, which the Gecko will send to the motor. The PC won't receive any F/B and will be running as a open loop. It seems that having the encoder feedback to the drive will eliminate a(the?) major advantage of servo motors being feedback systems.

Yes, it looks like the servo drives will be implementing localized closed loop control over their respective axes (for their individual steps). Since the axes are not linked I believe they can come out of sync, leading to elliptical error when cutting circles. I'd think that the way to correct this would be to collect all the F/B in the controller and compare the two axes doing the circle to each other and the circle vector... or go really slow, in tiny steps to insure that there is no overshoot/fall-behind... which is how you'd run a stepper motor system. (?)

From the controller's point of view, isn't the only difference between a stepper and servo system using this style of servo drive that the servo drive can notify the controller (through the err/res channel) when the servo misses a "step" (or several...) and is no longer on course, allowing for the controller to kill the program?

----question:

Am I thinking this through correctly?

I will be doing more research, but are there hobby level/DIY servo drive setups that combine the F/B at the controller level?
("3 axis closed loop", vs "open loop over top of 3 independent closed loop axes")

I want to say that a PC running 3 Gecko servo drives is still an open loop system, though with localized closed loop control on the individual axes. Is that correct?
How much difference will having a localized closed loop controller on each axis make vs just using a stepper motor system?

Thanks for your assistance.