Auto-resume after power failure - any con's or comments?
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I have a dumb question
Not so dumb at all…
Does the printer have to re-home?
That depends…
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What kind of power failure do we expect? On a sharp power cut, the idea is that all movements stop immediately, then, with the remaining power in the capacitors (of PSU and Duet boards), the positions of all axes are written to the SD card. However, there are other types of power failures such as „brownouts“ or intermittent outages due to a lightning hitting the line - at least in theory, the mechanical position may then be out of sync with the latest stored coordinates. Luckily, we can ignore this case as we can’t do anything to prevent it.
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Assuming a clear power cut: will the printer mechanics stop on the spot, or will inertia and/or gravity cause further movement of an axis? If, for instance, a heavy bed tends to give way down due to its own weight, the final Z-position differs from the Z coordinate stored in resurrect.g. Obviously, any axis which is suspected to not hold its exact position after a power loss will need to be re-homed.
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Which precision per axis is required to continue a print? In the event of a power loss, stepper motors tend to skip microsteps, I.e. they like to snap into some full-step position. Same thing may happen when they are re-powered. In the worst case, these effects may add-up to a misalignment of two full steps - depending on your printer’s steps/mm, layer height and line thickness, this can be either tolerable or ruin your print. In case of the latter, the relevant axes have to be re-homed.
To conclude: In most cases, re-homing of X and Y is possible without interfering with a print on the bed, so simply do it.
how do you home Z when there's a partial print on the bed?
Good question. I can think of three different solutions:
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Using a probe (such like a BLTouch), you can possibly probe your bed along one or more edges or in some corner - given that the printable area is smaller than the bed’s physical dimensions and the head or gantry are guarantied to not collide with the print object. However, if your printer has multiple Z drives (which is good for tramming or even bed levelling), probing just one corner can result in a bed not being well levelled.
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Using (additional) high-end switches on all Z-drives, you can re-home Z with these. This may be a bit less precise than using a probe, but could be good enough to resume the current print job.
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Trust in god and simply continue to print. Small inconsistencies can be corrected on the fly by adjusting the microsteps via DWC. If, however, your printer tends to not hold its Z-height when left without power, this method is unsafe.
To conclude: as a general solution, #2 is the way to go, but often, #3 does surprisingly well (depending on the mechanics of your printer).
Finally, you can prevent the thread of a power loss by providing some kind of UPS: admittedly, a full-fledged UPS powerful enough to keep a bed heater alive is costly, but in most real-world scenarios, a low-voltage (e.g. 24V) battery bank feeding just the steppers, nozzle heaters and controllers, will save your day. See @deckingman's post above, he has the expertise.
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@infiniteloop Thanks! That's the answer i was looking for!
I have tweaked my resurrect-prologue.g to home XY and hope that Z stays where it was. It is on a Railcore Core XY, so it's tricky to rehome Z.
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That's the answer i was looking for!
Glad I could help. Apropos your ”dumb question”: obviously, my prose was just waiting for the perfect question to be asked 🧐
