Setting Z Trigger Height

TheIndianDude

Hi,

I'm following the "Test and Calibrate a Z probe" tutorial on the Duet Wiki (https://duet3d.dozuki.com/Wiki/Test_and_calibrate_the_Z_probe) and I'm running into minor inconsistencies with the trigger height. The wiki says that after we bring the nozzle down to touch the build plate or just grab a piece of paper (I'm using a sheet of printer paper), to reset z to 0 (G92 Z0) , move z up 5 to 10 mm (G1 Z5) and run a G30 S-1 to get the trigger height and to repeat these series of steps 4 or 5 times.

I'm trying to get my Z trigger height near the bed 0,0 position (sensor is 5mm in from both X and Y edges) as I want to see the height map relative to the bed origin. After I get the nozzle to the right height, I send G92 Z0, move Z up 5mm, run the G30 S-1 and get a trigger height value. I then move Z down by the trigger height value to make sure it doesn't over travel and that there isn't excess pressure on the paper, re-zero Z, and repeat. The second time, the trigger height is completely different. Here are the values I'm getting:

1st try: Trigger height 0.855mm
2nd try: Trigger height 0.840mm
3rd try: Trigger height 0.837mm
4th try: Trigger height 0.848mm
5th try: Trigger height 0.795mm
6th try: Trigger height 0.822mm

Would these values be considered within margin of error for the IR probe? The website for the probe (https://miscsolutions.wordpress.com/mini-height-sensor-board/) states that the repeatability should be approximately 0.01mm, but I'm getting a variance between 0.003mm and 0.06mm which I think is a massive swing.

Any ideas how I can bring the numbers closer together? Also, what should I use as the trigger value? The last obtained trigger height?

Phaedrux

First, I'd suggest using the center of the bed for the Z0 point rather than a corner. Same for homing and setting Z0 before doing a mesh. This is especially important if using any Z axis tilt correction.

Trigger height is going to depend a bit on the surface you're using. Just as a control, try placing a piece of matte paper on the bed and testing the trigger height against that and see if it's more consistent.

Slowing down the dive height can increase consistency a bit, so try that.

Reducing ambient light that could be interfering can help.

Personally I take 10 samples, throw out the highest and lowest and then average them if they are spread out, or take the mode value if one occurs repeatedly.

TheIndianDude

@Phaedrux Thanks for the tips. I'm using the BuildTak flexplate with a black coating on the sheet steel, so I don't think it's related to transparency/translucency of the bed material, but the surface is rough (to enable better adhesion I suppose), so I will use a piece of matte paper to see if that changes anything.

My standard Z homing is based on the center (updated in homez.g and homeall.g to make sure probe is at bed center). I will move to the bed center and try tuning the trigger height.

My dive speed is set to 10mm/s. I'll try slowing it to half that (been meaning to do that anyway, so now's as good a time as any)

I like the idea of throwing out the outliers and taking an average or modal value.

Thanks for the tips! Will update the thread based on the results.

TheIndianDude

I followed your advice and I am now getting consistent readings within 0.005mm which is well within the margin of error for the probe. The key was the probing speed and the dive height (Set to 5mm/s and set to 5mm). Once I slowed the probe down and also didn't raise the Z height above 5 mm, the readings were consistent.

Thanks for the advice!