Keep position after M81 power down

jphilly

Whenever I power down with M81, the board reports vin under-voltage which clears the head position. I get that's the default because it's assumed something will move with no power but there's no option to keep positioning data after a planned power down.

Besides having to re-home after power downs I'd like to be able to pause a print, power down, and resume later with the saved position data. (It'd require some cooling and extra heating on the heaters to avoid a fault but that's sort of separate)

I have 24v on vin and 5v on usb so after vin < 9v it's still on.

fcwilt

@jphilly

Did you read about M916?

M916 Documentation

Frederick

jphilly

@fcwilt Yeah, problem is that requires re-homing, which is unnecessary if the printer doesn't move when powered down.

fcwilt

@jphilly

Why is that a problem?

The system saves all the information needed to resume.

Isn't that the goal - to resume?

What is the downside to having to home again?

Frederick

Phaedrux

@jphilly said in Keep position after M81 power down:

which is unnecessary if the printer doesn't move when powered down.

Well when the motor re-energizes it will snap to the nearest full step, which isn't huge, but isn't nothing.

If you're parking the printer in a known location and don't care if the positions is off by a step in any direction, then you could create a macro with G92 X Y Z command to set the current position manually to whatever the parked position is.

I have a macro that does exactly this for the cases where I'm testing and the printer hasn't moved and I don't really want to rehome again.

dc42

@phaedrux said in Keep position after M81 power down:

Well when the motor re-energizes it will snap to the nearest full step, which isn't huge, but isn't nothing.

It's worse than that, it snaps to the nearest multiple of 4 full steps that matches the power up position of the microstepping counter in the driver.

The power fail recovery mechanism allows you to assume the positions haven't changed, if you really want to; but I suggest you home at least X and Y again after power up.

jphilly

@dc42 So would there be a way to save the z position so it doesn't have to rehome or would it have to be parked? Z homing is the main problem since I likely wouldn't be able to raise the bed fully once there's a print on it.

The only definite position I could drop it to is z max which would take a longer time than possible with power outage given 2mm lead screws

Phaedrux

@jphilly Homing to zmax is an option, but the distance between Z0 measured at the start of the print to the actual position where the zmax endstop triggers must be measured accurately.

For what it's worth, I have 1mm lead screws and after a power outage I don't rehomed to zmax even though I could. I just find the approximate height of the print, check it against the sliced model to find the exact layer it was on and then use that to set the Z position with G92 and resume the print. There may still be a slight line visible on the print, either due to inexact position or simply just the effects of cooling and shrinkage. But if that's not important at least you have a saved print.