SZP mounting

vaike_peeter

What is the suggested height for touch probing? In documentation one page suggest "around 3mm above the tip" and another page "1 to 2 mm above the tip".

On https://docs.duet3d.com/Duet3D_hardware/Duet_3_family/Duet_3_Scanning_Z_Probe in Mounting section it is
"The bottom of the SZP coil should be around 3mm above the tip of the nozzle. The G31 Z trigger height can be set to around 2mm, so the coil is 5mm from the bed, to avoid the nozzle contacting the bed while keeping the coil close enough to the bed to get an accurate reading."

Then I read from https://docs.duet3d.com/User_manual/Tuning/scanning_z_probe_calibration in Mounting section:
"The bottom of the SZP coil should be around 1 to 2mm above the tip of the nozzle. The G31 Z trigger height can be set to around 2mm, so the coil is 4mm from the bed, to avoid the nozzle contacting the bed while keeping the coil close enough to the bed to get an accurate reading."

fcwilt

@vaike_peeter

It is confusing.

The closer the better in that the readings can be more accurate.
The farther the better in that you are less likely to have the coil strike the bed.

On my system I use 3mm - it seems to provide the required accuracy while being a bit more forgiving than 1 to 2 mm.

Frederick

gloomyandy

@fcwilt There are other considerations.

Depending upon how you want to use the probe you need it to return valid readings for all of the heights above the bed you wish to use it. But this is not always possible. If the coil is close to the bed then it needs a higher drive current to function, but this higher drive current often means that you get invalid readings if you move the probe to be some distance above the bed. This is particularly important if you want to use the probe for touch homing as the probe will need to operate very close to the bed but also needs to be usable when some distance from it. The current solution is "two stage homing" in which you use a lower coil current to "home" at a point that is actually say 3mm above the bed followed by a seconded stage touch operation using a higher drive current.

From the large number of tests I've run on both scanning and touch probing I'd say that having the coil mounted at about 4mm above the nozzle tip is a good compromise. Just for full disclosure most of my tests have been using a Fly toolboard and coil. It looks like the results with a Duet SZP are very similar, but touch probing is still an experimental feature so we are still gathering data.

droftarts

@vaike_peeter The SZP probes are generally most accurate around 4mm from the bed. We have, in the past, recommended the probe is 2mm above the nozzle, and mesh bed scanning with the nozzle 2mm above the bed (or coil 1mm above the nozzle and scan 3mm above the bed; it really depends on how warped your bed is). With the advent of using the SZP for setting Z height, where 4mm above the nozzle is ideal (as recommended by @gloomyandy, who worked on developing the touch functionality), I think 3mm above the nozzle is probably the best compromise, and is what @dc42 is also using. Beacon and Cartographer also recommend 2.6mm to 3mm to the nozzle.

I'll update the documentation with this new recommendation.

Ian

vaike_peeter

@droftarts
I followed @gloomyandy Discord tutorial and managed to get my SZP working. When calibrating everything was OK, but now I get some random CAN errors.
Error: CAN response timeout: board 120, req type 6061, RID 288
Error: Expansion board 120 reconnected

I have to redo my cabling and see if this helps. Currently waiting connectors.

Many thanks for clearing this out.

gloomyandy

@droftarts Just to be clear, the key thing to test for touch probing is....

With the nozzle in contact with the bed. Determine a coil current that gives good readings (probably using the auto calibration process).
Now raise the probe and see at what height above the bed you no longer get valid readings. A value slightly lower than this is in effect the maximum dive height you can use for touch probing.

From what I've seen the closer the coil is mounted to the nozzle tip the narrower is the working range of the coil (and hence the dive height for touch probing is closer to the bed).

The tricky part is that you need to get the probe to within that dive height. At the moment the best way seems to be to use the two stage process described above. However the problem is that the first stage relies on you manually calibrating a trigger point that works reliably and that gets you within that dive height. The closer the dive height is to the bed the more likely it is that issues like temperature drift are going to cause problems and possibly a crash into the bed. Heating the nozzle (to allow more accurate touch probing even with plastic on the nozzle), may make it more likely that you get temperature drift in this first stage.

I'm not sure but I don't think dc42 has been testing with the full two stage process, I think he has been using the switch probe on his toolchanger for initial Z homing and then just testing the homing touch homing operation (that's also how I ran a lot of my tests). I think we still need to gather more data on the best compromise.

Note that there is a possible alternate stage 1 homing process. This is to basically set things up with the drive current needed for touch homing and then to simply keep lowering the probe say 1mm at a time and then check to see if the probe has started to return valid readings yet. This should bring the probe to a point that it can now be used for touch probing. We don't currently have any way to actually trigger the probe based on a change from invalid to valid reading, but it may be something to consider. If it turns out that temperature drift for initial probing is a problem it may be that a 3 stage sequence would be best. Use normal threshold homing to get to a point say 6mm above the bed, then switch drive current and drop 1mm at a time until we start to get valid probe readings.

Finally you probably want to start the touch probe operation as close to bed as possible. This is because any "rough spots" in the Z motion system can cause a false touch trigger, which will require an increase in the touch threshold to avoid. This in turn results in a higher force touch, which is more likely to cause damage.

fcwilt

@gloomyandy

Yes, indeed there are.

I was speaking from my personal experience BUT as I have no intention of using the SZP for setting the Z=0 Datum, I have done no testing of that feature.

I'm "old school", I install BLTouch units for setting the Z=0 Datum.

Frederick