CNC Dictionary

[BobWarfield]

NC, it is uncanny that you have chosen to attach somthing labeled "Stepper vs Servo" in your note, although I realize it contains a lot more wisdom than just that. The reason I mention that is that while driving home, I decided it would be fun to try to write a quick and dirty simulation of the effects of open loop vs closed loop and backlash while drawing a circle. I know the latter is one of your pet concerns, and it is indeed an excellent test, as well as being easy for these crude simulations. I won't dwell on it too much here as I want to start a separate thread on the simulation so people can comment as they see fit.

I will say that I have added a section to the bottom of the CNC Dictionary page called "CNC Tutorials", and it is there that I have placed the simulator page. I plan to use that section for things that are too lengthy to stick in the Dictionary.

I do think I have the explanation for your issue of needed to run your cam machine more slowly than expected even though it is a servo system, and discuss that at the bottom of the page.

The simulator page is:

http://www.thewarfields.com/MTStepperServoBacklash.htm

I must say, I am spending entirely too much time on this, but it is a fascinating way to organize my own research and learning, so it's worth it. I have also been able to find some time to make actual chips in the shop as well as progress on my CNC lathe conversion, so perhaps all is not lost.

Best,

BW

[NC Cams]

The attachment started out merely being Marris' "stepper vs servo" dissertation at the bottom of the submission.
However, things kept getting added as time went on and I never went back and renamed it.

Knowledge isn't knowing everthing, rather knowledge is knowing where to find the answer to your or someone else's question.

The circle cutting thing is something we stumbled across during our cam profile cutting project. We jumped in to trying to cut a cam profile. The code would puke as we used parts of canned code to get a combo of arcs in concert with point to point milling to cut the shape.

Once we got that issue resolved, we uncovered massive hysterisis in the system and it showed up at direction changes and also at 45 deg from the direction changes. On a hunch, we tried milling a circle using ONLY canned code and POOF, the problems stuck out like a sore thumb. At that point, it was either lame code (unlikely as it was proven commercial code) or hardware anomalies.

Using the circle cutting test, we slowly but surely uncovered and fixed the various mechanical vagaries that simultaneous sin wave velocity profiles cause (motors see these when circle cutting). Besides, we were able to simply cut the same part a bit smaller and not waste master blanks while testing.

ALthough we verified via our tests and with a Haas that the circle cut test does effectively challenge a servo control system, we never gave steppers a though - we don't have any on our milling equipment.

Recently, the same problem areas showed up in a thread were steppers were having problems at the 45 deg points - points where both motors are concurrently encountering max velocity and varying accelerations.

Turned out this was the same point where steppers were being induced into resonance. From what we learned and what we saw with the stepper problem another member had, it became obvious that there is a complex interaction that takes place for a machine to cut a perfectly round circle - regardless of the type of controller.

This simple test readily shows how well your system can rub its head and pat its stomach. Once we got our system to cut round circles, we then worked on speed. Only by doing iterative work with feed rates did we learn that slower IPM's gave even less error potential than faster ones.

When we looked into the velocity feed back loop via iterative servo tuning did we learn that higher speeds resulted in higher errors in f/b loop and vice versa for lower errors .

Some things you are taught, some things you're born with and other things you simply stumble across by dumb luck and persistance.

[BobWarfield]

I have added some brief tutorials to the CNC Dictionary (as well as a bunch of additional definitions over time) that may be useful for beginners:

- Breakout Board I/O: Round up of the different kinds of boards, some of their I/O capabilities, and some other considerations.

- CNC Control Panels: A survey of commercial and hobbyist panels and pendants.

- CNC Software: Just a high level view of what the different software does and how it works together.

- Stepper/Servo Backlash Simulator

The tutorials are at the bottom of the main page:

http://www.thewarfields.com/MT/MTCNCDictionary.htm

Best,

BW