Delta Z-Probe Negative offset

I have a home made Delta printer. I have implemented my own FSR based bed leveling sensors. I can do a 3 point bed leveling scheme that works well. But, I would like to be able before each print to have the probe touch the bed and then create a negative z offset for that touch. So, when the FSR is triggered, the nozzle is at bed height and I need it backed off a bit. I have tried several methods but am not having much luck. What would be the best method to have the bed get probed for height, then set the height in script to back off .1 or .2mm. ?

Currently when I do it manually, after doing calibration and homing, I will use M306 Z0 to set the new Z position. But I would like to automate this.

Thanks

Imported from wikidot

Wouldn’t just doing a G30 Z0.2 do the trick ?

I am a little confused about G30 with a Z parameter. I see it used on one page for Deltas, but then in other places I see documentation that says NOT to use the Z parameter for G30 with a Delta.

That being said,I have tried that. 0.2 is intended to be the positive offset from the probe to the hot end. However, since my probe IS the hotend, I actually need to back off the other Z direction, thus almost need to use a negative Z value. I have found out that Smoothieware does not seem to accept negative Z values for this.

G30 Zn just probes the bed, then wherever the bed is touched, sets the current Z value to Z.
You can do

G30 Z-0.2
G1 Z0
And it will go to the bed, then back off 0.2mm

I’ll give it a try when I get home tonight. I could have sworn that I was trying that last night without luck (it appeared to not like negative values). I could very well be wrong though. I’ll update once I try it out. Thanks.

Even if it didn’t like negative values ( which I’m pretty sure is not a concern ), you could do

G30
G92 Z-0.2
G1 Z0

or even

G30
G1 Z0.2
G92 Z0

Thank you. I’ll try these out.

One quick question comes to mind. Does a homing G28, or any other command eliminate these offsets? If I perform this offset and then store to SD (M500) will it retain this offset?

Yes, G28 would wipe these out. You really need to store gamma_max or use M306/M206 etc, to store your Z height across resets.

Ah. Perhaps that is what I was doing. I would set the offset, then do a home, G28 and lose that offset.

So, I could get the head into position like you mention above, then set the Z0 with a M306 Z0 and an M500 to store across a reset if needed.