Cincinnati Millacron X axis slide

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05 Nov 2012 22:16 #26280 by andypugh

the machine moves well, although I have not managed to move 10000mm/min. I've only come to 7000mm/min. when 7000mm/min move faster than I get a warning in the "servodrivers".

This almost certainly has nothing to do with the PID tuning. It may be that you need a lower max accelleration. Looking at a Youtube Video it looks like the machine has a very heavy X-axis slide.

I do not understand that the input and output units of the PID?

The PID has two inputs (position-cmd and postion-fb) and one output.
Depending on the source of those inputs, and where the output goes to (in HAL) their units can be very different, and the optimum Pgain (etc) can vary by many orders of magnitude.

You may well want some Dgain, but that will need to be very small relative to Pgain. If Pgain is 5 then I would expect the correct Dgain to be 0.0005 (maybe 10x smaller or 100x bigger)

However, your f-error doesn't seem all that bad, if the units are mm. 20um error at the acceleration peaks?

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05 Nov 2012 22:37 #26282 by jlviloria
Andy.

thanks for your support. This is leaving me bald. but worth it.

when you say 20um. mean. 20 micrometros?

This also confuses me. the idea is to produce parts with good accuracy and repeatability. 0.02mm settings. f-error is perfect or need minus?

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05 Nov 2012 22:58 #26283 by andypugh

when you say 20um. mean. 20 micrometros?

Yes, I couldn't work out how to type 20µm at the time. (I did manage to find a way to turn my screen upside-down though...)

This also confuses me. the idea is to produce parts with good accuracy and repeatability. 0.02mm settings. f-error is perfect or need minus?

0.02mm is not very good for lathe precision, but your steady-state error seems to be very much lower. (0.002mm) and that is the state the machine will be in during slow machining operations. (in fact, in the vast majority of lathe machining operations the critical axis is completely stationary).

I suggest that you leave the tuning for the moment, and see how the parts turn out.
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06 Nov 2012 01:01 #26294 by PCW
The error spike is going to be proportional to acceleration and
you are very unlikely to use this much acceleration when actually
cutting, especially on a fine cut.

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06 Nov 2012 14:38 #26299 by jmelson
I can tell you need some FF2 to reduce those acceleration spikes.
Because they are so pointy, you may also be calling for more acceleration
than the drives can provide. Start with lower MAX_ACCEL settings
in the ini file, and use FF2 to reduce them as much as possible.
Then, turn up the acceleration until the error spikes again, and back off
to leave the drives some margin.

FF2 is used in VERY small amounts, 0.001 is a good place to start.
It is easy to give too much, and the error flips over to the other
side. The easy way to check is reduce FF2 a little, if the error
gets smaller, you are over correcting.

Jon
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