Difference btween last home position and new home position?

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25 Feb 2021 01:38 #200050 by turbostew
I really would like to know how much I am off from the previous home position when I re-home. Is there a simple way of doing this? It seems this would be a popular feature but my googling is striking out.

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25 Feb 2021 02:49 #200054 by MaHa
If you place a calibrationring on the machining area, centerpoint with dial gauge, G28.1 in MDI, #5161 and #5162 will contain X and Y position, machinecoordinates G53. Save this, then rehome.
G0 G53 X#5161 Y#5162 to the previously saved positions. Maybe select G54 and set X Y to zero. Check the deviation with dial gauge.

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25 Feb 2021 02:57 #200055 by JohnnyCNC
I have a DRO with glass scales on my mill from it's manual mill days. I just home, zero the DRO, home again. I think any solution is going to involve the addition of some additional measuring hardware.

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25 Feb 2021 05:11 - 25 Feb 2021 05:12 #200056 by turbostew
More hardware if I were concerned about mill accuracy, I am concerned about router accuracy. My accuracy needs are well within my home switch accuracy. I already have "accurate" home switches, why can I not use them for measurement? I wish I could use the home switches as probe inputs easily.....
Last edit: 25 Feb 2021 05:12 by turbostew.

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26 Feb 2021 10:21 #200180 by rodw
I think you could write a custom component that monitored joint.N.homed so that when it goes true, save joint.N.pos−cmd
you could then read this_home_pos and copy it to last_home_pos after setting a value to home-pos -error.

linuxcnc.org/docs/2.8/html/man/man9/motion.9.html

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26 Feb 2021 11:08 - 26 Feb 2021 11:08 #200183 by pippin88
I have often thought this function should exist.

For stepper based machines it would be a reasonable way to show that steps have been lost.

Probably should be a separate function than usual homing to avoid confusion.

Just a simple message that says expected home position correct, or home switch triggered too early / late by X mm/steps
Last edit: 26 Feb 2021 11:08 by pippin88.

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26 Feb 2021 16:50 - 26 Feb 2021 16:52 #200204 by turbostew
Thanks all for the feedback!

pippin88,

rodw wrote: I think you could write a custom component that monitored joint.N.homed so that when it goes true, save joint.N.pos−cmd
you could then read this_home_pos and copy it to last_home_pos after setting a value to home-pos -error.

linuxcnc.org/docs/2.8/html/man/man9/motion.9.html


So joint.N.pos-cmd is never "reset" during a session? It is not simply zeroed on a home? Probably reset on a new session. If it is never zeroed during a session, your idea seems like it could work....
Last edit: 26 Feb 2021 16:52 by turbostew.

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26 Feb 2021 19:35 - 26 Feb 2021 19:36 #200217 by turbostew
I have searched for a couple hours. Can anyone give me a python code snippet to read joint.N.pos−cmd?????
Last edit: 26 Feb 2021 19:36 by turbostew.

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01 Mar 2021 22:07 #200743 by andypugh

turbostew wrote: I have searched for a couple hours. Can anyone give me a python code snippet to read joint.N.pos−cmd?????


Typically that isn't how you do it.

You create a HAL component (possibly in Python, possibly in C) which creates its own HAL pins.
Then you connect the HAL pins in HAL, and the values are available inside the component.

Though I second the opinion that this monitoring could much more easily be done internally to LinuxCNC.

I think that you would need to add a "previous_motor_offset" here:
github.com/LinuxCNC/linuxcnc/blob/master.../motion/homing.c#L81

Then set that to the joint->motor_offset here:
github.com/LinuxCNC/linuxcnc/blob/master...motion/homing.c#L220

Then run a check that previous != and new != previous here:
github.com/LinuxCNC/linuxcnc/blob/master...otion/homing.c#L1230
raising a warning if the error is above some configurable limit.

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