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fix(api): Fix liquid getting homed into pipette after certain protocols end #17285

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merged 1 commit into from
Jan 16, 2025
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fix(api): Fix liquid getting homed into pipette after certain protocols end #17285

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14 changes: 7 additions & 7 deletions api/src/opentrons/protocol_engine/execution/hardware_stopper.py
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Original file line number Diff line number Diff line change
Expand Up @@ -65,7 +65,7 @@ async def _home_everything_except_plungers(self) -> None:
axes=[MotorAxis.X, MotorAxis.Y, MotorAxis.LEFT_Z, MotorAxis.RIGHT_Z]
)

async def _drop_tip(self) -> None:
async def _try_to_drop_tips(self) -> None:
"""Drop currently attached tip, if any, into trash after a run cancel."""
attached_tips = self._state_store.pipettes.get_all_attached_tips()

Expand Down Expand Up @@ -134,9 +134,9 @@ async def do_stop_and_recover(
PostRunHardwareState.HOME_THEN_DISENGAGE,
)
if drop_tips_after_run:
await self._drop_tip()
await self._hardware_api.stop(home_after=home_after_stop)
else:
await self._hardware_api.stop(home_after=False)
if home_after_stop:
await self._home_everything_except_plungers()
await self._try_to_drop_tips()

await self._hardware_api.stop(home_after=False)

if home_after_stop:
await self._home_everything_except_plungers()
Comment on lines 136 to +142
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In the old code:

        if drop_tips_after_run:
            await self._drop_tip()
            await self._hardware_api.stop(home_after=home_after_stop)

If home_after_stop was True, the self._hardware_api.stop(...) call would home everything, including the plungers.

I think this used to be safe when this code was originally written. The line immediately preceding it, self._drop_tip(), would have ensured that there were no tips attached, and therefore no liquid for the home to accidentally aspirate.

But that assumption stopped holding for Flex protocols with PAPIv≥2.16; since then, self._drop_tip() has been a no-op (see the comments in _try_to_drop_tips().) So this self._hardware_api.stop(...) was accidentally homing with tips possibly still attached and liquid possibly still in them.

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With my fix here, nothing in this file should ever home the plungers anymore.

We could try to be a little fancier than that. We could home the plungers specifically in the cases where the robot did drop tips. I haven't done that here because it doesn't seem worth the complexity. The plungers will home when the next run begins, anyway.

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no, i agree with this fix. let's just not home the damn things, it's too hard to pick the right behavior

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