Setting up Servos in step/dir position mode

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06 Jun 2021 09:14 - 06 Jun 2021 10:08 #211282 by RoberCNC
Hey dudes.
I'm trying to migrate a setup with steppers to a machine with servos. First of all, I have tried to work without touching anything and the servos move, the directions are correct, etc. but they move at a very low speed. I understand that I have to modify parameters in the INI and HAL file, I thought it would only be to adjust the new values ​​of stepscale and little else but reading around here I see that many more things interact like PID, FF'S... and before touching anything I would like to know well that, the truth is I'm quite lost, this servo thing is completely new to me. The servos are DMM DYN2 in position mode step/dir with Mesa 5i25 + 7i76. I attach basic INI and HAL files to work with.I don´t know if itis a good idea to work from a configuration from steppers or if would be better to make a totally new config with thePncconf Wizard.

File Attachment:

File Name: CNC_2021-06-06.hal
File Size:14 KB

File Attachment:

File Name: CNC_2021-06-06.ini
File Size:6 KB
Attachments:
Last edit: 06 Jun 2021 10:08 by RoberCNC.

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06 Jun 2021 13:08 #211295 by PCW
Step/dir servos are setup exactly like stepmotors/step drives,
You do not need to change any PID setting unless you are using
encoder feedback to LinuxCNC,

Is stepscale correct? (do the axis move the correct distances?)

160 steps per mm seems small for something like a mill or lathe.
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06 Jun 2021 14:22 #211302 by RoberCNC
So it's just like I thought, only modify step scale according to servo's parameters.
No, stepscale is not correct, it must me something like 2000 (10000pulse per rev/ 5 mm ballscrew pitch).
Being the encoder integrated in the driver I could have feedback to LinuxCNC?

Thanks.

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06 Jun 2021 14:49 #211303 by PCW
You can have encoder feedback but its probably more
trouble than its worth unless you have feedback from
linear scales

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06 Jun 2021 18:51 #211316 by RoberCNC
Understood Peter, thank you very much for the info.
I must be missing something in math, if I apply gear number (electronic gear reduction) = 2500 * 4 = 10000 pulses needed to rotate 1 revolution, if the ball screw pitch is 5 mms, then Stepscale should be 10000 / 5 = 2000, right?
Putting this value in the INI file, I get the following message:
"hm2 / hm2_5i25.0: Stephen.00.maxvel is too big for current step timings & position-scale, clipping to max possible"

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06 Jun 2021 19:20 #211326 by tommylight

"hm2 / hm2_5i25.0: Stephen.00.maxvel is too big for current step timings & position-scale, clipping to max possible"

That is just a warning, given the step time and step space and scale with the requested velocity, the stepgen can not reach the maximum step rate so it clipped to what it can do, meaning it lowered the maximum speed the machine can move at.
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06 Jun 2021 20:12 #211343 by RoberCNC
Hey Tommy, how are you doing? then the math is ok. What exactly does that imply? I guess I can cut that number in half.

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06 Jun 2021 20:54 #211350 by PCW
Yes, you would have to reduce the step time and step space to reach your requested velocities with the current step-scale. With a steptime and stepspace of 5.050 usec, the maximum steprate would be about 99 KHz but the requested stepgen velocity of 50 mm/sec at 2000 steps per mm needs a 100KHz step rate.

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06 Jun 2021 20:54 #211351 by tommylight
Doing fine, thank you.
No idea about the math, most probably it is useless as most position servo drives will have another setting for the number of pulses per revolution that has nothing to do with the encoder.
But if that is the used resolution, then yes the math is correct.
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06 Jun 2021 22:00 #211358 by RoberCNC

Yes, you would have to reduce the step time and step space to reach your requested velocities with the current step-scale. With a steptime and stepspace of 5.050 usec, the maximum steprate would be about 99 KHz but the requested stepgen velocity of 50 mm/sec at 2000 steps per mm needs a 100KHz step rate.

Are they reduced following some reasoning, I don't know if they maintain a numerical relationship, or am I lowering the values in trial and error mode?

And on the other hand, using the feedback of the encoder without scales, I understand that it would not solve in any way the possible mechanical backlash that the machine may have.

Thank you guys very much, reading you is always learning.

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