Author Topic: accuracy issue  (Read 2427 times)

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Offline fristot

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accuracy issue
« on: May 12, 2009, 10:23:46 AM »
Hello,
I use A 4th axis to make windings. The motor driver uses 8 pulses per step. Motor has 200 step/rev. And for this axis, 1 unit is 1 turn. So I configured 1600 steps/units (at 1200 units/min)  This is quite fast but OK with 35kHz Kernel.
My problem is that after many turns, the motor 0 position is no longer correct. It shifts about 0.15 turn per 100 turns. (i.e. missing 0.15 * 1600 steps to actually reach the correct position). There is no torque on the motor, so no risk to lose steps - and acceleration is very slow.
So I tried replacin 1600 by 1602.4 (+0.15%). It helps but does not seems to be perfect.
A few month ago, I did not have this problem with the same settings. So I tried with an old copy of the .xml setting file, and still the problem occurs. So probably a hardware issue ?
Also the other axis do not seem to have this problem (with 200 steps/unit)
I use a laptop (X31). And it could also be a cause ? Pulse freq is stable (36137Hz and time in interrupt is around 4usec)

I'm lost. Thanks in advance. Pierre.


Offline Jeff_Birt

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Re: accuracy issue
« Reply #1 on: May 12, 2009, 11:21:06 AM »
The first thing to try is cutting your velocity and acceleration in half. If this works well it would be a clue that perhaps your initial settings were on the borderline of being too fast. Also check that mechanically things on that axis are in good shape. Nothing loose or binding etc. If you use the same type of driver for each axis you can swap drivers to to see if the problem follows the driver or axis.

Laptops can cause problems, but I would expect you to see the problem on every axis.
Happy machining , Jeff Birt
 

Offline fristot

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Re: accuracy issue -solved
« Reply #2 on: May 12, 2009, 11:28:28 AM »

I can't believe it ! It was actually simply a loose coupling between the motor and axis.  I tested it though. It was very hard to resist by hand.
But I think that with the vibrations, it could make some angle shift...  Now it works.

Thanks a lot Jeff !

Pierre.

Offline Jeff_Birt

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Re: accuracy issue
« Reply #3 on: May 12, 2009, 11:55:08 AM »
Glad you got it sorted.  ;D
Happy machining , Jeff Birt