Another poor soul not sure about inductive sensors and G29.



  • I have just setup a new mount for my extruder and I included an inductive sensor on it, shown here: (imgur) http://imgur.com/a/YUhL5. I connected it up in mode 5 with the power coming from one of the always on fan 12V source.

    I can get Z probing to stop properly. So it definitely senses the bed.

    I have set it up like so:

    M558 P5 I1 Z1
    G31 P1000 X-51 Y30 Z3.7

    On the G31 command, just to explain what I did, I used my scale (shown in picture) to measure the distance from the nozzle to the centre of the inductive sensor, and it was to the left, hence -51mm. I did the same with Y, and being that I would have to move Y in the positive direction I figured it should be positive 30mm. On the Z side, I found the sensor triggered at about 3.3mm, and I had to move the nozzle about 0.4 down, so I added these for 3.7. I think I might be mistaken on how I have this.

    Now when I go to setup for G29, I use:

    M557 X10:190 Y10:135 S20

    and when G29 goes ahead it proceeds to the front right of the bed and then crashes into the bed - the light of the sensor definitely turned on.

    I am at a loss as to where I can proceed.

    Additionally, I manually tested 15 positions on the bed - each along X 52, 110, 185 and Y 10, 40, 70, 100, 130 by moving to the pair of X,Y and doing a Z home, and moving down on a 0.006" feeler gauge and recording it. The results I am not sure what to make:

          x     52    110    185
    y   130   -0.9   -0.6   -0.5
        100   -0.8   -0.4   -0.4
         70   -0.8   -0.4   -0.3
         40   -0.7   -0.4   -0.2
         10   -0.6   -0.4   -0.2
    
    

  • administrators

    In your G31 command:

    1. P1000 in your G31 command is wrong. Try P100.

    2. The Z parameter should be the nozzle height when the probe is triggered, which from your description I think is 0.4mm.

    3. A positive Y offset means the probe is in the +Y direction from the nozzle, in other words behind it. Is that correct?



  • When I run G31 on its own, I get 1000 as a response. The Z probe value shown in sensors in the web interface also shows 1000. I have never seen 100. Can you clarify why you say to use 100?

    If I fix the Z parameter, and the parameter is "good" is that how it won't go into the bed?

    It was 0.4mm at the first test I made. Its that last table that has me a little worried, obviously I am measuring a point on the bed which is 25% away from the nozzle, relative to the size of the bed. Is there a way I can progressively test if I get the distance right without it just banging into the bed?


  • administrators

    You can use P1000 if you like, but then it will have to get 8 consecutive readings from the sensor to register as triggered.

    I think the problem you had before was that with the Z parameter set to 3.7, after Z homing the Z=0 position was 3.3mm below the bed.

    To get a larger trigger height, lower the Z probe a little. You currently have it about 3.3mm higher than the nozzle, but 1.5 or 2mm is sufficient.

    The variation in trigger height with position is a concern. It's mostly a left-to-right effect. One possible explanation is that the bed plate is dish-shaped, so that the left-right gradient at the left hand side is more in a downwards direction than at the right hand side. Or the rails that the carriage is supported on are bowed, so that the head doesn't remain horizontal as it moves along the X axis. Either way, the relative heights of the probe and the nozzle above the bed vary with position. Inductive probes are generally quite large, so they can't be mounted close to the nozzle. The greater the distance between the nozzle and the probe, the greater the effect of any varying bed or print head tilt of this kind.



  • Can you explain how the trigger value works?

    It seems that the dive height parameter (H) is responsible for it hitting the bed. I tried to set the value to H1 (from a default of H5) - although I don't rightly know what this value is beyond its name as none of the documentation mentions it outside of its name.

    Now when I try to run G29, it moves to the first position and starts moving down, is triggered (as evidenced by the light on the sensor and the interface shows 1000 afterwards). But it reports "Error: Z probe was not triggered during probing move"

    Now I just did a small test that might help my situation. I moved the sensor to the nearest front left position of the bed (close to 0,0) and used a caliper and measured 2.93mm height from the bed to the bottom of the sensor. I moved the nozzle to the same position near as sight can reckon and it was roughly 0.508mm in height from the bed. I further moved the nozzle to the front right but not the sensor due to the extent available) and zeroed the the Z with the probe and confirmed that 0.508mm across the front of the bed on the nozzle.

    An equivalent series of commands that I am using right now to test out the bed levelling
    M558 P5 I1 Z1 H1
    G31 T5 P100 X-51 Y30 Z0.4
    M557 X10:190 Y10:135 S20
    G29

    Is this an appropriate set of commands? I have not modified them yet to consider the new test mentioned above.

    I am not entirely certain what you mean by a larger trigger height.

    Does any of this assume a variable signal from the inductive sensor, it appears only fixed as 0 or 1000.



  • Hi…i am a new user here. As per my observation the g31 command should be P100.Also the Z parameter should be the nozzle height that is 0.4mm.

    hdi pcb


  • administrators

    The trigger height is the Z command in the G31 command so it should be the height the nozzle is from the bed when the inductive sensor triggers. This is seen on the we interface when the Z probe value changes from 0 to 1000.

    The trigger height is not the difference between the bottom of the inductive sensor and the bottom of the nozzle. It is the distance from the nozzle to the bed once the inductive sensor has triggered.

    You can measure this by seeing how much further you need to move the nozzle down, once the the sensor has triggered, until it is just touching the bed.

    As for the commands you are using I would set the dive height to at least 4mm to start with (The H command in M558)


  • administrators

    See https://www.duet3d.com/forum/thread.php?pid=14334#p14334 for an explanation of how the trigger value works.



  • I have read a few more things, is the dive height supposed to be the Z height it moves to before it starts diving? Equivalent to G1 Z[dive height]?

    Now I have the command M558 P5 I1 Z1 H10 R2 F60 setup, and confirmed when I read it back with M558 alone.

    Am I to understand that G30 S-1 (as documented https://duet3d.com/wiki/Setting_up_automatic_probing_of_the_print_bed under setting up probe parameters) should move to the the H10 height (and Z height in M31) and start moving down from there?

    In my case, it starts movings down immediately (I slowed it down with F60) and EVERY time it goes into the bed with the trigger hit.

    Now, I have gone back and lowered the Z probe to a much closer to the nozzle height, but still just slightly above it.

    G30 S-1 still hits the bed.

    I am really not sure what else I can do.

    The procedure for homing Z seems to accept the trigger just fine.



  • Just had something interesting happen - I changed the probe from type 5 on the probe connector to type 4 on E0 and everything else the same - it stops now.

    Any reason why S30 didn't read from my type 5 probe when homing Z was fine?



  • Further update, I have used the procedure "Checking the trigger height" on https://duet3d.com/wiki/Using_mesh_bed_compensation on 5 locations. I used a 0.038mm feeler gauge.

    The bed is highest at the back left, and slopes down every other position.

        x      y   value
       52    120    2.58
      182    120    1.68
      182      0    1.59
      100    100    2.19
       82      0    2.26
    
    

    Now am I right to suspect that this can only be slightly improved with bed levelling given the offset on the probe to the nozzle?

    Is this variance too much?



  • It appears that some of the variances can be made tighter, but I am going to work on getting some more clips - there is the distinct possibility that this bed is bowed along the line from the back left to front right.



  • It appears that I am a lazy duck. I had some cork under the heated bed and it was raising certain areas around screws, so I cut it back. Now the values range from 1.59 to 1.69 (1.69 is an outlier near an edge), which is well within a layer height. I am going to proceed tomorrow to try out the rest of the procedure.

    Thank you all who have given info.


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