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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:

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

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

Thanks a lot,
Julian

juwi wrote:
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.

andypugh wrote:
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

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

Best regards,
julian

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