Update G29.cpp

[Maker-Paul]

Description

Simple update to G29.cpp (ABL) only. Adds automatic probe trigger height determination to set the Probe Z offset. avoids the probe trigger height above Z stop (Z=0) being incorrectly stored in all the bed level mesh values.

Only on machines that do not use the probe for homing when using ABL bed leveling. The probe offset will usually be larger than the home offset in these setups (it doesn't matter if its lower). When the G29 routine starts it sets the Z = 0 or home position using the homing device. But goes on to measure the bed level mesh points relative to the Z = 0 this causes all the mesh points to appear high or low by the difference between the home trigger height and the probe trigger height. This pull request simply reads the probe trigger height (which is the difference between Z home and probe trigger height) and adjusts the Z offset to account for that difference at the start of G29.

Another way of thinking about this is, if the printer has a seperate Z stop, this pull request will allow automatic determination of the probe Z offset from the Z stop. The probe to Z stop offset value is added to any user set home Z offset. The total being the bed to probe Z offset. So G29 maps Z values relative to the bed (Z=0) accurately.

Requirements

No special requirements does not affect machines with only Z stops or machines that use the probe for Z homing.

Benefits

Machines with separate methods for homing and creating the bed level mesh will not include probe Z offset in the mesh values just the bed topology.
Provides highly acurate mapping with all types of probe including proximity types. The user can still set the Z home offset as normal but doesn't have to worry about probe Z offset.

Configurations

definitions required in configuration.h

//#define Z_MIN_PROBE_USES_Z_MIN_ENDSTOP_PIN
//#define USE_PROBE_FOR_Z_HOMING
#define Z_SAFE_HOMING
#define AUTO_BED_LEVELING_BILINEAR

Related Issues

yes

will fix #27680 a simple way.
#28393

may fix other

• [BUG] Z Probe Offset not applied homing using Z min endstop instead of the probe #22653
• [BUG] Z probe offset ignored unless UBL is enabled #21727
• M851 & Z Probe Offset does not work with Bilinear, endstops & BL Touch #21833

Also #28380 identifies an issue where the mesh is incorrectly shifted by the X and Y probe offsets. it is easy to see in the bed level maps I have posted on the #28380 pull request. So should also be merged please.

Please note G29 UBL calculates the mean from the mapped bed Z values and uses that as the Z probe offset. So does not require changeing. However this method means the XY home position may not correspond with Z=0.

[Maker-Paul]

All build checks now pass and the function has been tested. it needed more robust preprocessor directives to pass all the build tests but the code was essentially correct. Had to remind myself of the interaction between compiler directives and in code if statements. So my apologies for the number of commits to get here. I couldn't find a way to rerun build tests without editing the Pull Request.

The first image shows a Bed map control taken two days ago compared to one just done. with the Bed cold.

Legend Red is above 0, Yellow is below 0, Blue is 0
The image below shows the bed is very consistent between two days ago and today.

This image below shows the surface topology after the bed has heated and soaked for 15 minutes (The bed was trammed Hot nearly a week ago)

The Mesh visualizer is a spread sheet I created to display the data provided from the printer with M503.