zProbe - drift in small measurements



  • Hello!

    I am currently calibrating a new Delta and although I have now built 4 of those I get a calibration problem I cannot remember ever having before.
    First I am moving along the trusted guide in this respect: https://duet3d.dozuki.com/Wiki/Calibrating_a_delta_printer

    The Piezo-Precision Endstop works reliably enough to have the mesh bed leveling with more than 130 points without an error.

    In preparation to verify the sensor is working reliably I wrote a script for Homing and than moving all the way down, probing, than back up again.
    Then agan the same movement with fast travel down to shave of time. I get a standard deviation of 7 micron and 4 micron and a total range of 20 micron and 10 micron respectively. So far so good, but now the weirdness starts...

    When I try to check the repeatability of the probing from Z5 to the bed, I usually do 20 runs, the results drift gradually.

    I had first results of 742 micron and a last of 672 micron or 782 micron. But the result is not moving to the other extreme in one step. From one measurement to the next there usually is not a bigger step than 10 micron, while 50% of the time it also stays the same (48 micron for 4 cycles for example) and rarely it jumps back. This leaves me with an awful ... in StDev and ... of overall range.

    But the exciting thing about this is that this only happens when I only go up Z5. When I conduct the same test run with Z25 or Z250 the results improve. For Z25 the StDev is 7 micron and the range 20 micron. For Z250 11 micron StDev and 30 micron range.

    Of course some of that effect can be due to changing temperatures. The printer is made from aluminium and the distance between the probing point and the endstops is a meter, the total travel app. 700mm. That would grow the frame by 23micron per C. However this effect should be most visible when Homing and probing again and again. But there is no such effect visible.

    Last but not least drift appears to be growing the finer my approach macros are. I wrote the small scrip G91 -> G1 Zn -> G90 for n=-0.01/-0.02/-0.03 and it appears the smaller the movements I use to approach the further I get (when approaching manually).

    So how can it be that the probing point drifts into one direction when I use small movements? Any suggestions? My thought is, that if there is such a drift in repeated measurements on the same spot it might as well be the case during the mesh bed leveling, which defeats the purpose of the same and explain a sort of bent mesh bed visualisation.



  • On top of that, I noticed that the speed of the probing changes sometimes. So sometimes it probes at appr. F500 and ignores the F300 and sometimes it goes with the regular F300.

    M208 S1 Z-3
    G28 Z
    M561
    M564 S0;	Allow movement below software endstop
    G1 Z2 F8000
    M558 P1 I1 R0.8 F300; Adjust probing characteristics: Standard Speed F500
    M291 P"Adjust height until nozzle grips paper" R"Please adjust..." S2 T0 Z1
    G92 Z0;		define new height
    G1 Z25;		nozzle up 5mm
    G30 S-1;	probe the bed without resetting
    G1 Z25;		nozzle up 5mm
    G30 S-1;	probe the bed without resetting
    G1 Z25;		nozzle up 5mm
    G30 S-1;	probe the bed without resetting
    G1 Z25;		nozzle up 5mm
    G30 S-1;	probe the bed without resetting
    G1 Z25;		nozzle up 5mm
    G30 S-1;	probe the bed without resetting
    G1 Z25;		nozzle up 5mm
    G30 S-1;	probe the bed without resetting
    G1 Z25;		nozzle up 5mm
    G30 S-1;	probe the bed without resetting
    G1 Z25;		nozzle up 5mm
    G30 S-1;	probe the bed without resetting
    G1 Z25;		nozzle up 5mm
    G30 S-1;	probe the bed without resetting
    G1 Z25;		nozzle up 5mm
    G30 S-1;	probe the bed without resetting
    G1 Z25;		nozzle up 5mm
    

    In this case the probing speed and corresponding measurements were as follows:

    slow 0.112
    fast 0.082
    fast 0.082
    slow 0.112
    slow 0.112
    slow 0.112
    slow 0.102
    slow 0.112
    slow 0.102
    fast 0.072

    It is obvious that the measurements vary according to the probing speed, however I set the probing speed at the start, to make sure, but did not adjust the probing speed in between the cycles, and the config.g probing speed is also at the F300.



  • The vaying-probing-speed-problem also appears when doing a multi-probe Delta Calibration, so much so that the default tolerance is being triggered.

    And another problem also appears randomly. I don't know whether it is the same underlying issue, but I'll just throw it against this wall and see whether someone can make sense of it: Sometimes when I home axis W, Axis A is also moving albeit into the wrong direction. Sometimes when I approach the heatbed (sometimes when I type G1 Z1, xyz towers move down) A also moves along, usually the xyz movement is slower then but accelerating slowly as well.

    ... I am Using a Duet WiFi V1.02 with a Duex5 V0.9

    Firmware Name: RepRapFirmware for Duet 2 WiFi/Ethernet
    Firmware Electronics: Duet WiFi 1.02 or later + DueX5
    Firmware Version: 2.03 (2019-06-13b2)
    WiFi Server Version: 1.22
    Web Interface Version: 1.22.6



  • Please try upgrading to the latest 2.04 RC. There were some delta bugs in 2.03.


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