G30 question

jens55

When generating a bed map, one option is to probe the bed multiple times at one spot until two successive measurements are within a certain range. Is there a way (hopefully without a lot of macro wizardry) to have a G30 command do multiple probes?
I run a BLTouch on one of my printers and I find that the z zero setting is not necessarily the same from probe to probe. Now I must admit it is early in terms of me trying to track down the issue but it would seem that the quickest way to fix that is to do multiple probes to establish z zero.
I can probe a given spot multiple times and the result is, IIRC, within 0.02 mm or thereabouts. At first glance this should probably be good enough (although I would have hoped for better repeatability) but I find that my first layer is somewhat inconsistent with the nozzle being too close to the bed on one print and then a bit too high on another print. My first layer is generally 0.2 mm and I re-establish z zero between each print.

As I am writing this, it occurred to me that I should really heat the bed up, once the bed has reached it's operating temperature I should turn off the bed heater and run the G30 command and then turn the bed heat back on.... I haven't tried to do this yet.
Another option would be to set z zero the normal way, then print out a single layer flat sheet and adjust baby stepping for best first layer and then not turn off the motors between prints and not setting z zero again. I also haven't tried that yet.

How consistent are other people with a BLTouch finding their z zero probing?

moth4017

@jens55

i think you need this in your config
; Z sensor
M558 P8 C"!io3.in" H15 F2000:300 T12000 R0 A10 S0.01

that a look at the link below to see the function

https://docs.duet3d.com/User_manual/Reference/Gcodes#m558-set-z-probe-type

OwenD

@jens55
As @moth4017 said, M558 will allow everything you're asking for.
You'll need to use P9 for BL Touch and B1 to turn off filters during probing.
The BL Touch is susceptible to electrical noise so turning off the heaters helps.
You should set your trigger height with the bed at the same temp.

jens55

@moth4017, thank you for your suggestion. My config.g contains the following line for the probe:
M558 P9 C"^zprobe.in" H3 F60 T6000 A10 R1.25 S0.01 B0. I will try to switch the B0 to B1 to turn off the heat to see if that improves things. I will also reduce the S parameter to S0.005 and see what happens.
I had actually forgotten about this configuration setting and had not noticed any repeat probing action during the G30 command. With a reduced S parameter it should become clearer if the printer is verifying z zero with more than just one measuring attempt.

jens55

@OwenD, thanks ... I will play a bit with things tomorrow.

Phaedrux

@jens55 said in G30 question:

had not noticed any repeat probing action during the G30 command.

Do you have any additional M558 commands somewhere in your macros that modify that?

jens55

@Phaedrux, good question - I don't think so but life has gotten in the way of running tests. Maybe I will have a chance tomorrow.

jens55

I am happy to report that with a tighter S parameter the G30 command does in fact probe multiple times like it is supposed to. Doing a repeatability test with 10 measuring points yields a deviation from mean of 0.003 with the maximum deviation being +0.005 and -0.004. This is with an 'S' parameter of 0.005.
I think I had a maximum S deviation set to 0.01 because a tighter setting did not allow my bed scanning macro to complete. I have yet to re-do a bed map to verify that 0.005 for S will work.
I will need to make sure the homez.g macro only runs after the bed has reached operating temperature.
Thank you for all your help!

dc42

@jens55 the minimum useful S parameter depends on how much backlash you have in your Z axis. If backlash in not the limiting factor then the Z steps/mm will be. For example, if the Z steps/mm is 200 then the S parameter should be at least 0.005.

jens55

@dc42, thanks. I am set up for 400 steps/mm but that is a good thing to keep in the back of my mind (where it will probably get lost among clutter and cobwebs
Other people might find this tidbit of info interesting too.