IR probe on Kapton Tape?



  • Hello all! 🙂

    I recently purchased a roll of polyimide tape from Conrad Electronic (https://www.conrad.de/de/p/polyimid-folie-50-mm-hitzebestaendig-1550728.html) and now I've put it on my aluminium printbed. It was previously covered with blue painters tape which worked great with the IR sensor board. But now with the Kapton Tape it does trigger but only from a very close distance (< 1 mm). This wouldn't be a problem in itself if it could detect the surface from a distance (~20 mm) and slow down until it triggers like it did with blue tape.
    Now I'm wondering why it doesn't work like it should since the website about the IR probe and various other people on the internet state that it worked with Kapton Tape.
    I also tried putting the Kapton Tape on the blue tape but that also doesn't work.
    My guess is that the Kapton scatters the IR light so much that the probe only detects the surface when directly above it. Is that possible? And what could I try to improve it?

    Thanks in advance! 🙂

    Edit: I just noticed that the LED on the IR probe blinks when the sun shines on it and consequently won't trigger on anything. On the website it says that the probe isn't affected by sunlight... so maybe something's wrong with my probe?

    Edit2: I just read the website completely and at the bottom it says that direct sunlight interferes with the probe... my bad. But the original problem still persists.



  • Try angling the sensor slightly. Or try using something other than an IR sensor... or just a different IR sensor.
    If you really really really want to go for impractical, put some squares of blue tape where the sensor is reading. Or maybe try some sharpie.

    Its possible that the kapton is acting like a prism, or its just absorbing the IR (which seems more likely).

    IR sensors are only slightly less flaky than capacitive sensors in the testing I've seen, any change in the reflectivity of the target will significantly change the sensing distance.



  • @Eumldeuml Is the probe in analog mode or digital mode?

    In digital mode, I get about a 3.5mm trigger distance against both bare aluminum and aluminum with Kapton applied. I can't test analog mode right now because I'm on a Duet3 which doesn't support it.

    Try digital mode and see of it makes a difference.



  • Thanks for your replies!
    @theruttmeister: Angling the probe could be a solution, I'll try that later!
    Squares of blue tape is kinda impractical since I'd like to use mesh leveling and I use about 40 points... Also I can't live with the fact that the mesh then won't represent the exact surface. The curse of a perfectionist I guess... 🙄

    @gtj0: I use it on a Duet WiFi which uses analog mode according to the website. Is there a way to switch to digital mode?

    Yesterday I also found some weirdness in the readings of the probe which are displayed in the DWC as numbers from 0 to 1000:

    Kapton on Blue Tape:
    Z0.6-: 537 (triggered)
    Z0.65+: 466
    Z1.6+: 0
    Z5.95+: 466 (possibly because of the surrounding blue tape, since I only used one strip of Kapton for testing purposes)
    Z20.6+: 0

    Kapton on Blue Tape after scuffing it up slightly:
    Z0.65-: 537 (triggered)
    Z0.7+: 466 (surface detected but not triggered)
    Z2.2+: 0 (no surface detected)

    Kapton on bare aluminium:
    Z0.4-: 537 (triggered)
    Z0.45+: 1000 (error)
    Z0.8+: 466 (surface detected but not triggered)
    Z1.5+: 0

    ("+" after a Z-value means "this Z-value and bigger")

    So what this means is that when probing, the print head goes down (or the bed up in my case), then it senses the approaching surface (represented by a value of 466) but then it goes to 0 again. This leads to the error "probe wasn't triggered during probing move".
    So a workaround could be to set the dive height to a very small height but that could be inaccurate since the time between slowing down and triggering is very short.



  • @Eumldeuml Angling the probe will probably make things worse. The emitter and detector are already angled so that the focus point is directly below them. If you angle the probe the emitted IR is going to start skipping right past the detector.

    When the probe reading goes to 0 again, it means that the bed is too close to reflect the IR back to the detector. I'd try digital mode to see what happens. Change the P1 parameter on your M558 command to P8 and test with a piece of tape on a piece of aluminum scrap before you test with your bed.

    Also can you post your M574, M558 and G31 commands from config.g?



  • I've tried it with P8 now and the results are as following:
    On blue tape it gives me a consistent reading of 1000 at 19 mm and below (Z=0 is the height where it gives me the same reading on Kapton + aluminium). But the value jumps between 0 and 1000 erratically when it's over blue tape and between around Z19 and Z25.
    When the probe is over the scuffed Kapton on blue tape, it shows 1000 at around 18 mm and also jumps around if a little higher than that.

    In all cases the red LED on the probe only lit up at very close distance. I guess that this is hardware-related as that shows when it is triggered in analog mode, right?

    I tried homing on all three different surfaces (although with a reduced speed) and it did work on all of them. It Trigger distances are all in the range of 25 to 35 mm above the surface which sounds a little bit excessive. From what I could measure, the consistency is quite bad on all of them (Blue Tape ~0.2 mm; Kapton on Blue Tape ~0.2 mm; Kapton on aluminium ~1 mm).
    I also did the same test for consistency with the standard analog mode and it was accurate to about 0.02 mm.

    My settings are:
    M574 X2 Y2 S0
    M574 Z1 S2 (the two M574's are in seperate lines so that I can set a different S parameter)
    M558 P1 H2 F2400 T48000
    G31 P500 X0 Y-47 Z0.78



  • Might be time to ask @dc42 to chime in.


  • administrators

    @Eumldeuml said in IR probe on Kapton Tape?:

    Kapton on bare aluminium:
    Z0.4-: 537 (triggered)
    Z0.45+: 1000 (error)
    Z0.8+: 466 (surface detected but not triggered)
    Z1.5+: 0

    Looks like your IR probe is too sensitive for use with aluminium because of the 1000 reading, assuming that wasn't caused by direct bright sunlight reflecting off the bed into the sensor. There is a resistor on the IR sensor that could be reduced, to reduce the sensitivity.



  • Is there a way of testing that without modyfing the board itself? A resistor in series with one of the connecor wires perhaps?


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