Setting up lathe speed with 7i76 digital pot

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01 Jan 2019 18:17 - 01 Jan 2019 18:17 #123278 by PCW
_Maybe_

Do you have any documentation on the drive?
Are you sure potentiometer+ from the drive is DC? (relative to potentiometer-)

The 7I76 (voltage output) potentiometer replacement is meant for DC voltage controlled drives,
not all inexpensive SCR drives work this way however
Last edit: 01 Jan 2019 18:17 by PCW.

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01 Jan 2019 18:35 #123279 by Stigoe
No, but it is the same clone of a KBMM-225 (or something like that) as used on most of these Chinese benchtop mills and lathes.

I measured 5.5v DC across the original potmeter, so I'm fairly certain it is DC controlled. I can test if there are any AC there as well.

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01 Jan 2019 18:47 #123280 by Stigoe
I just measured across spindle- and spindle+ and there was no AC there, only DC.

I've also managed to get .spinout to track commanded RPM in both low and high range by editing the scale.

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01 Jan 2019 19:44 #123285 by PCW
One thing you might try first is to run the drive with a isolated power source supplying the potentiometer. If this works, the 7I76E Digital potentiometer should as well.

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01 Jan 2019 21:00 #123290 by Stigoe
I don't think I'll have time to try this tomorrow before I leave for work, but I'll try when I get back sometime early February.

Anyway, thanks for the help so far.

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16 Jan 2019 06:46 - 16 Jan 2019 06:49 #124282 by Contract_Pilot
You will need to tine it in the VFD and the Belt gearing of the machine.. See my unfinished build!! www.stevenrhine.com/?p=2432
Last edit: 16 Jan 2019 06:49 by Contract_Pilot.

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04 May 2019 19:32 #132765 by Stigoe
It took a bit longer longer than February before I managed to test this (it was barely above freezing in my workshop when I came back from work in February...), but I've tested now. I connected a separate 12V DC PSU with negative from PSU to both digital pot negative and motor controller and positive to digital pot positive.. This helped and I could adjust the speed of the spindle from PP, but now I need some help setting up the sale properly.

I did a few quick tests. The PSU is 12V and when I commanded S200, I had 1.32V and a speed of 180RPM. With S400, I had 2.64V and a speed of 600RPM.

Since PP uses a bit different configuration for spindle speed than vanilla LinuxCNC, how do I calculate the scale and which parameters do I use?

Br,
Stig

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06 May 2019 18:07 #132982 by andypugh
It sounds like the scale might not be far wrong, but the linearity is not all that you might wish.

The 7i76 sounds to be outputting exactly he right voltage for a 1500 rpm spindle. But the actual spindle seems to not be responding linearly.

You could consider setting up a PID for closed-loop spindle speed control, or maybe tweak the commanded rpm with the "lincurve" HAL component.

Or you could not bother, as all that really matters if if the spindle speed is "too fast" or "too slow" and the actual numbers are not 100% relevant.

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09 May 2019 18:33 #133269 by Stigoe
Thanks!

I guess I can try to use lincurve, but I guess I need to use 2 since I have a two-speed lathe and switch between the lincurves somehow based on whether I've selected low- or high-speed?

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09 May 2019 18:46 #133271 by andypugh
If the speed ranges overlap then yes, you would use two lincurves.
Just choose the required output with a "mux" component.

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