[FEATURE REQUEST] Self check of load cell functionality

[rcmaniac25]

Printer model

MK4/XL

Describe the feature

Occasionally, when starting a print, the printer will home and then decide to do an X/Y calibration to ensure functionality before continuing the homing sequence.

I would like something similar with the load cell. Given how important it is (between homing Z, auto bed leveling, stuck filament detection, the nozzle alignment that occurs on the XL, and potential future features), the load cell needs to function or you risk damage to your printer.

What spawned this was I had some TPU get tangled in the extruder gears. I removed the TPU after a lot of struggle and continued the print. At a later point, I installed a MMU3 and during the installation process I noticed a divot taken out of the epoxy that coats the load cell sensor. It worried me and talked with support. They indicated that it may warrant replacement to ensure the load cell continues to function. My printer happened to be out of warranty, so they indicated I should run a load cell test and if it fails, I should get a new heatsink/loadcell unit. It passed so I did not need it. But it worried me that even though filament is not supposed to be in that area, some TPU getting caught in the gears could rip out a chunk of the protective coating. I'm concerned that some situation could occur that causes the loadcell to cease to function during or in-between prints and cause odd side effects and damage the printer.

Ex. imagine mid-print the loadcell ceases to work, suddenly the printer seems to have never ending stream of "stuck filament". Not knowing what's happening, you turn off stuck filament detection and let the print continue. It finishes, you remove the print and start another. It homes X, Y, slowly descends towards the bed... then crashes into the bed and keeps going down. By the time you realize something is wrong, damage has already been done.

The feature, like the re-calibration of X/Y, would offer some indication that the load cell is working. The support person indicated the load cell values should not be consistent and should fluctuate. Watching it myself (no filament loaded), the values sit between -10 and -30. Rough values and may be specific to my printer, but still something. Perhaps (for no loaded filament), if a print is started and the values are greatly fluctuating or exceed some threshold, it instead pauses and requests a load cell test first.

Expected functionality

1. The print process starts.
2. It homes X and Y.
3. It checks load cell values. If the values are within expected ranges or follow a trend line consistent with filament oozing, then start homing Z. If values do not match, move the extruder 1 step forward and back, if the load cell value does not move within given ranges for each move, then alert the user and ask them to run a load cell test.

If within expected values: home z. If not within expected values, and user opts to do load cell test then cancel the print and start the cooldown process so the load cell test can be done. If not within expected values, and user opts to skip load cell test, then run like normal and hope the user is around to catch if something fails.