Gantrykins with HM2 Servo in closed Loop

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28 Aug 2012 11:22 #23707 by juwi
Hi

I have a homebuild CNC actually driven by 2 Granite devices and 2 Dugong Drives in step/dir mode by emc softstepper. I am planning to rebuild it using AC Servos with Stoeber Posidyn SDS AC Servo Controllers, with 5i25/7i77.

I have two things I don't understand quite good for now. First, I have two independent Y-Axis Motors. If the are getting out of synchronous running, it would probably screw the machine. The Portal weighs ~180kg, so even one Motor just doing nothing would be harmful to the geometry.

If I understood correctly, the Gantrykins are just virtually bind the joints to an axis? I read I cannot jog in Intervals (1mm, 10mm) anymore after that, is a MPG with switchable stepwidth affected by this problem?

Second, the SDS AC Drive has a velocity controller that can be configured using integral and proportional term plus a lowpass (set to zero as stated here in the forum)
Both P- and I-Term can be set in percent, what would make sense or do I have to tune both of them?
Actually they are set to 60% proportional and 30% integral. Wuold it be wise to set the drive only to integral and leave the rest to linuxcnc?

Thanks in advance!
julian

PS: early Video of the machine:

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28 Aug 2012 12:18 #23709 by andypugh
juwi wrote:

I have two things I don't understand quite good for now. First, I have two independent Y-Axis Motors. If the are getting out of synchronous running, it would probably screw the machine. The Portal weighs ~180kg, so even one Motor just doing nothing would be harmful to the geometry.

Can you take the X-axis off and decouple the Y sides during the tuning process (perhaps add some dummy mass). Servo runaway/oscillation could be as unpleasant as you fear.

If I understood correctly, the Gantrykins are just virtually bind the joints to an axis? I read I cannot jog in Intervals (1mm, 10mm) anymore after that, is a MPG with switchable stepwidth affected by this problem?

I am not sure. You could try modifying the sample gantry config to test it.

Second, the SDS AC Drive has a velocity controller that can be configured using integral and proportional term plus a lowpass (set to zero as stated here in the forum)
Both P- and I-Term can be set in percent, what would make sense or do I have to tune both of them?

I think you would need to tune both to give the best velocity command following behaviour. This would be best done in a configuration where a tuning error won't wreck the machine.

Actually they are set to 60% proportional and 30% integral. Wuold it be wise to set the drive only to integral and leave the rest to linuxcnc?

I don't think so. LinuxCNC will be issuing velocity commands to the drive, and the velocity-to-current loop in the drive needs to be working properly.

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28 Aug 2012 13:56 - 28 Aug 2012 13:58 #23712 by juwi

[...]me wimpering about shearing machine to death...[...]

Can you take the X-axis off and decouple the Y sides during the tuning process (perhaps add some dummy mass). Servo runaway/oscillation could be as unpleasant as you fear.

That is doable, as I am moving to my now hobbycellar soon the machine will be "parted"... Is there a possibility (a real, not a theoretical) that combining the axes after tuning leads into new harmonic interactions between the two drives?

If I understood correctly, the Gantrykins are just virtually bind the joints to an axis? I read I cannot jog in Intervals (1mm, 10mm) anymore after that, is a MPG with switchable stepwidth affected by this problem?

I am not sure. You could try modifying the sample gantry config to test it.

Ok. Ill try this and perhaps our wisdom will increase ;)

Anyway I am wondering how in such beautiful an mighty software such a "on top" Kinematic is needed for a gantry config ;) This doesnt look "right"...
I would offer my humble programming abilities if this will be helpful?

Second, the SDS AC Drive has a velocity controller that can be configured using integral and proportional term plus a lowpass (set to zero as stated here in the forum)
Both P- and I-Term can be set in percent, what would make sense or do I have to tune both of them?

I think you would need to tune both to give the best velocity command following behaviour. This would be best done in a configuration where a tuning error won't wreck the machine.

Actually they are set to 60% proportional and 30% integral. Wuold it be wise to set the drive only to integral and leave the rest to linuxcnc?

I don't think so. LinuxCNC will be issuing velocity commands to the drive, and the velocity-to-current loop in the drive needs to be working properly.


I feared that. Ok, this WILL take time, but I'll try.

Thanks a lot,
Julian
Last edit: 28 Aug 2012 13:58 by juwi.

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28 Aug 2012 14:30 #23714 by andypugh
juwi wrote:

Anyway I am wondering how in such beautiful an mighty software such a "on top" Kinematic is needed for a gantry config ;) This doesnt look "right"...


it isn't completely necessary. You can simply connect axis.1.motor-pos-cmd to both PID modules in the HAL.
The only reason to run Gantrykins is to allow the two sides to home independently to individual home switches for auto-squaring. If you never want the two sides to move independently then you can just use trivkins.

Note that gantrykins isn't "on top", it replaces trivkins.

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28 Aug 2012 14:36 #23715 by juwi
andypugh wrote:

juwi wrote:

Anyway I am wondering how in such beautiful an mighty software such a "on top" Kinematic is needed for a gantry config ;) This doesnt look "right"...


it isn't completely necessary. You can simply connect axis.1.motor-pos-cmd to both PID modules in the HAL.
The only reason to run Gantrykins is to allow the two sides to home independently to individual home switches for auto-squaring. If you never want the two sides to move independently then you can just use trivkins.

Note that gantrykins isn't "on top", it replaces trivkins.


Ahh, I understand. Ok, using brakes and absolute encoders on the servos there is no need for Gantrykins, except as a "second machine" for time-to-time rehoming...
Wonderful :) That made my day and sounds like a perfeect match :)

Thanks a lot!
yours,
Julian

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21 Oct 2012 20:20 #25567 by juwi
andypugh wrote:

[...]
it isn't completely necessary. You can simply connect axis.1.motor-pos-cmd to both PID modules in the HAL.
[...]


Is that still the way with SSI absolute position feedback devices?

Best regards,
julian

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21 Oct 2012 20:22 #25568 by andypugh
juwi wrote:

[Is that still the way with SSI absolute position feedback devices?


It ought to be, yes, but I can't claim to have tried it.

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