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I want to check how stable the output of the bedleveling protocol G29 was by running it multiple times. When running it four times, I got this as subsequent output

Recv: Bilinear Leveling Grid:
Recv:       0      1      2
Recv:  0 -0.056 -0.475 -0.605
Recv:  1 +0.124 -0.349 -0.595
Recv:  2 -0.023 -0.344 -0.673
Recv:  

Recv:       0      1      2
Recv:  0 +0.218 -0.183 -0.325
Recv:  1 +0.399 -0.073 -0.328
Recv:  2 +0.251 -0.072 -0.399
Recv: 

Recv:       0      1      2
Recv:  0 +0.498 +0.090 -0.049
Recv:  1 +0.677 +0.211 -0.038
Recv:  2 +0.531 +0.217 -0.117

Recv: Bilinear Leveling Grid:
Recv:       0      1      2
Recv:  0 +0.772 +0.368 +0.222
Recv:  1 +0.949 +0.482 +0.231
Recv:  2 +0.806 +0.495 +0.154

Which are quite unstable, and strange results. It seems as if there is an increment of around +0.27/0.28 for each probe point in the next iteration. How can this happen? What are the right bed level settings right now?

  • Printer: Anet A6
  • Probe: BLTouch
  • Software: Marlin 2.0
  • Bed level mode: Bilinear
  • Amount of probe points: 3x3
  • Probe surface: glass bed

For reference, here are the Configuration.h and the Configuration_adv.h.

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Actually the sensor reads the bed correctly in terms of shape, not in actual reproducible distance.

Plotting the grids:

enter image description hereenter image description here enter image description hereenter image description here

Results in pretty much the same shapes, so, when correcting for the maximum displacement and plotting all results in a single graph gives:

enter image description here

So the sensor does seem to produce reproducible bed geometry, but not the actual values.

Misinterpreting your initial question I assumed a default Anet sensor, this official ROKO SN04-N sensor is reported to not be very accurate and has a maximum hysteresis of 10 % error of the detecting distance, which is 10 % of 5 mm (so ± 0.5 mm). The overall maximum measurement is in grid 4 with a value of 0.949 mm and in grid 1 with a value of 0.124 mm; 0.949 mm - 0.124 mm = 0.825 mm is well within the ± 0.5 mm. This seems odd as the hysteresis should be in play on all measurements as the probe goes up and down. I've used this sensor myself, but replaced it for a more reliable sensor like the LJ18A3-8-Z/BX.

Now that it is clear that an accurate sensor is being used, there might be some play/backlash present in your printer. You should check the probe mount and the Z-axis.

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  • $\begingroup$ Oh wow @0scar, how sweet of you to take this effort in the plotting! I agree with you that it seems to get the overal 'shape' of the bed. Note however that I am using the BLTouch, which seems pretty accurate seen from the M48 results (I believe 0.0009 standard deviation). I will see if I can do this test again, but then add a G28 in between the results.. or maybe an M500 command. I can imagine that the routine somehow accumulates all the measurements $\endgroup$ – zwep Nov 16 '20 at 14:35
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    $\begingroup$ @zwep I read Anet A6 probe, now I understand that you mean printer Anet A6 and a BLTouch probe and BLTouch software! Sorry for the miscommunication, I'll alter the answer, but the main message will be the same, you need to check the bracket and the Z-axis for play or hysteresis/backlash. $\endgroup$ – 0scar Nov 16 '20 at 14:43
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    $\begingroup$ Hi, just did an additional test with MULTIPLE_PROBING set to 3 and this gave consistent results! $\endgroup$ – zwep Nov 16 '20 at 18:44

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