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Author Topic: CNC drive AXBB-E  (Read 614 times)

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Re: CNC drive AXBB-E
« Reply #10 on: March 09, 2019, 03:47:02 PM »
Hi Russ,
PMDX has a great reputation and a loyal following.

Steve has stated in other threads that the PMDX Mach4 lathe threading feature is mature. Further I understand that
it will accommodate either spindle index signals OR spindle encoder signals. What is not clear, at least to me, is
whether PMDX supports PID, however the tone of Steve's replies suggests not.

As you are no doubt aware Mach3 assumed a near constant spindle speed during threading operations to ensure accurate
threading and reliable thread synchronization. Mach4 is very similar.

This has traditionally has meant that the spindle motor requires much greater torque than that required by the cutting action
on the workpiece, usually with significant rotational momentum. If your spindle is not sufficiently powerful or you are taking
deep and aggressive threading passes the quality of the thread degrades.

Warp9 TD elected to pair their lathe threading feature with spindle PID. This means that with suitable feedback, either
index or encoder, the spindle control analogue output will vary in a manner to restore the spindle speed. Potentially
allowing the use of a smaller less capable spindle than had hitherto been required. I have attached a screen shot of the
ESS plugin Spindle page to give you a sense of how Warp9 have done it.

Craig
My wife left with my best friend...
     and I miss him!
Re: CNC drive AXBB-E
« Reply #11 on: March 09, 2019, 06:51:52 PM »
 
Quote
Warp9 TD elected to pair their lathe threading feature with spindle PID. This means that with suitable feedback, either
index or encoder, the spindle control analogue output will vary in a manner to restore the spindle speed. Potentially
allowing the use of a smaller less capable spindle than had hitherto been required

Interesting application. "Potentially" being the key word.
Would like to hear from lathe users regarding the actualities of the technique.  :)

Thanks again Craig,
Russ
 
Re: CNC drive AXBB-E
« Reply #12 on: March 09, 2019, 07:54:24 PM »
Hi,
yes 'potentially' is indeed the keyword.

Take an AC Step/Direction servo. A stream of pulses, let say 10,000 pulses per second are applied to a servo with a
10,000 count per revolution encoder. The servo will turn one revolution per second. Should the servo be loaded
it will slow somewhat, but the servo drive recognizes that its slipping behind and drives the servo to catch up.

Modern servos can maintain angular position and/or angular speed to within very small fractions of 1%, in the case of the
servo I described above to within 2 parts in 10,000 or so, or 0.02%. More modern servos with 20 bit encoders are very
much better again.

There are limits of course, if the load exceeds the torque the servo can produce it will slip behind and cannot catch up,
a servo would fault 'following error'.

Excluding the situation where the servo is overloaded the ability of the servo to accurately follow its commanded position
(or speed) is determined by the frequency at which it can correct itself. Modern servos (and VFDs) have typically 'refresh
rates' of 5-10kHz. This results in a closed loop servo bandwidth of 500Hz to 1kHz. Such a servo can respond and correct
a fluctuation in position (or speed) with 1/500th-1/1000th of a second, pretty damn good.

The ESS PID refresh rate is approximately 1kHz for a servo bandwidth of 100Hz. About 10th thats available by
an AC servo but still INFINITELY better than open loop. That is what ESS spindle PID does, it makes a feedback servo
of an ordinary spindle motor, albeit with somewhat reduced bandwidth.

You will certainly recall conversations on the forum where the concept of Mach enacting feedback has been discussed.
The communication delays preclude Mach itself from doing a good job. External motion controllers like the Hicon and CSMIO/A
can do so by adding a PID controlled analogue voltage to an analogue servo drive. They add that ability at considerable
cost. The ESS has enacted a similar feature but with one motor only ( the spindle) at no extra cost.

Thus the 'potential' is in fact 'ACTUAL' within the exception of overload, for which PID feedback cant fix, nor any other means either.

Craig
My wife left with my best friend...
     and I miss him!
Re: CNC drive AXBB-E
« Reply #13 on: March 09, 2019, 09:11:58 PM »
Interesting ... and very well explained.

Thank you Craig,
Russ