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Bit late in the discussion.
A mid-homing switch is usually very simple: there is a bar on one
of the nozzles that presses on a switch whenever the nozzle is
down or up. So the polarity of the switch tells the controller
which way to move to balance when homing. Can't say whether Duet
can do that, but would be surprised if not. Controllers usually
work like that way anyways, in case the machine got powered up
with the axis on the limit, i.e., the homing switch already being
pressed.
In this scene (hope it animates) you barely see the black shiny
bar and how the switch works (credit Mike Menci's machine):
_Mark
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Just be aware that homing to max requires the head to be somewhere in X/Y where the other nozzle doesn't ram into the table (this one is "homing to min" so to speak). Conversely, you can't home X/Y first, as you might have one of the nozzles extended down and it might collide with an obstacle.
When powering up the machine in any state (perhaps after
a mishap), this will usually require manual care, i.e.
repositioning the (unpowered) head in X/Y by hand or by jogging
etc. before initiating the homing cycle.
_Mark
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It just seems too complex, and there is certainly no support in OpenPnP.
Another way is just adding a spring that balances the nozzles.
Power off motors, let them snap to balance, power on, make a
controlled small move off balance, power off again. Maybe add one
or two more such iterations. With each iteration, relaxation will
be from a more precisely known position and result in a repeatable
balanced homing position.
_Mark
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> Are you homing with a single mid travel switch, and depending on the initial polarity of the switch's signal, you know which way to drive to approach mid travel, then you stop at the state change of the switch?
It is not my machine. It is Mike Menci's. He has a Smoothieware
controller and I
made a fix in that firmware to allow for mid-homing. I don't
think Duet has this problem.
I think there is only one switch, and yes it works as you
describe, with a second iteration to be more repeatable and
precise (as seen in the animation).
Once you have the reference homed position, you can add soft-limits usually both on the controller side and on the OpenPnP side (those slightly narrower) to prevent excessive Z motion. Sometimes there are also hard mechanical limits. The Z motor is usually weak (to be light), so it would simply stall against the limits.
But you should be aware that the real danger lays with (fast)
sideways X/Y motion that crash into obstacles that are in the
reachable Z range of the nozzles. This can happen during setup of
the machine, when homing is not yet setup correctly, when using
manual jogging or when coordinates of nozzle tip changers, feeders
etc. are not yet right. Once in production the problem goes away,
all coordinates are correct and preprogrammed motion is always
going to Safe Z first.
_Mark
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Like I said it was Mike's old head. I don't know how he negated
the other nozzle.
But this is irrelevant. Any type of "Peter's head" (negating shared axis) can have such a bar mounted that presses on a switch from the mid-position to one side only. Alternatively, make an optical light-weight variant.
Obviously the analog feed-back works too, but then why not just
add a trimpot+comparator, and you get the same deal (one side down
= 0 the other down = 1). No added complexity in the controller or
in OpenPnP.
_Mark
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Just received PCBs today, so will get those assembled tomorrow, but the membrane pot output works exactly as expected, just feeding into a 'scope.
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