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how is duet auto leveling calculated?

Posted by shadowphile 
how is duet auto leveling calculated?
July 21, 2018 07:02PM
I've just finished V3 of my new large Delta with heavier 20x40 towers and massive water-jet cut metal bed, 10mm diameter rod arms with mag-ball joints, etc. Lots of other cool refinements for ease of use and precision.
It's working fantastic, zero skew and spot-on dimensions with no adjustments required! Hard to believe actuallly....
I'm still using my old spring-loaded optical-switch z-probe and finished my effector tilt H parameters. I get a pre-autocal error of .171mm and post-cal of .017 so I am VERY pleased. (especially since I'm using v-wheels on the towers for my carriages).
However, when I manually touch down at all my probe points I don't get anything that looks as good as .017mm bed error (I assume RMS).
Here are my offsets going CW around the the circumference of my bed (350mm):
.13,.16,.16,.01,-.16 (these are deviations from the measured center offset which always zero out anyway before each print)
This is a peak-to-peak variation of .32mm and and an RMS of .113 (.05 if I subtract the mean but I'm not sure if I'm calculating this right).aro
My H offsets range are in the 0 to .1 range, worse than I hoped but not too bad for such a large machine,
Do these numbers look reasonable? I still don't see how the reported .017mm bed error is calculated.
It may be impractical but I would like to see .1 pk to pk across the bed for large thin objects. The carriage mounts and effector are CNC and the 1/4" thick bed is super flat.

Edited 2 time(s). Last edit at 07/21/2018 07:06PM by shadowphile.
Re: how is duet auto leveling calculated?
July 22, 2018 03:02AM
The figure it gives for deviation after calibration is a predicted figure, because it doesn't probe the points a second time to measure the actual deviation. You can check the actual deviation after calibration by running auto calibration again and looking at the "deviation before" figure. If your delta geometry is good and the only errors are things that auto calibration can correct for, then the actual deviation will be close to the predicted figure. If the actual deviation is a lot higher, then your printer has geometrical errors that auto calibration (with whatever number of factors you specified) doesn't correct for.

Edited 1 time(s). Last edit at 07/22/2018 03:03AM by dc42.

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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