Release home switch then return to almost the trigger position?

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29 Jul 2020 23:56 #176424 by nick-parker
Hi folks,

I've got my LinuxCNC conversion of a Shizuoka ST-N with Summit-Dana ATC 99% working with hall effect axis limits.

One of the last issues is the hysteresis distance on my max Z endstop. I have my +Z endstop trigger height just a tiny bit below the +Z hard stop and I want that to be my home position Z=0 because that's where it needs to be for the tool changer.

Unfortunately the endstop has a fair amount of hysteresis distance (0.15 inch or so), and Axis complains if I try to move anything with the +Z stop still triggered, so at the moment I have my offset set so Z=0 is 0.15 inches below the trigger height.

Is there any way to either:

1) Configure LinuxCNC to ignore the +Z endstop state unless I attempt to move +Z? That's how other firmwares I'm used to work and it'd be the simplest solution

or

2) Configure LinuxCNC to move out past the hysteresis position of the endstop before returning to a home position above the hysteresis distance but below the trigger distance? This seems silly but I would happily take it.


Thanks!
-Nick

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30 Jul 2020 00:28 #176427 by cmorley
If you posted your config file's INI it would be easier to understand what you currently have. Also please mention what version of linuxcnc your are using.

after homing linuxcnc must move within soft limits - so your tool change position must be inside the soft limits.

soft limits must be inside the limit switches.

your homing switch _can_ be outside the soft limits by any amount you would like.

beyond that I would need to know specifics about your hardware - like are you using shared home/limits or a separate home switch.

Not sure if you have seen this:
linuxcnc.org/docs/2.8/html/config/ini-homing.html

but assuming shared limit/home switch (figure 2)
The distance between D and A/B would be adjusted for switch hysteresis.

Chris

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30 Jul 2020 00:34 #176429 by cmorley
Also having the latch velocity negative or positive can change the position a (relative) lot if there is a big (travel) difference between on and off of the switch.
See figure 3 for that.

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30 Jul 2020 21:48 #176567 by nick-parker
Thanks for the help cmorley!

I'm not sure if this is what you were recommending but you gave me an idea that totally works:

I switched the Z axis latch direction so instead of "rapid approach, back off to unlatch, approach again to set trigger position" it's just "rapid approach, back off to unlatch and set trigger position there" and then I was able to set my home offset to very slightly less than the switch hysteresis distance so the axis moves back toward the switch after unlatching, exactly like I wanted!

It's perfectly lined up for the toolswitch claw now at Z=0. Thanks!

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31 Jul 2020 04:54 #176603 by cmorley

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