Help please - Linear Scale only homing

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22 Mar 2022 06:31 #237996 by whatwasIthinking
Pic attached - with luck.
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23 Mar 2022 22:46 #238171 by andypugh
That doesn't _look_ like an absolute encoder.

Though it is possible that it sends a burst of pulses at switch-on to make it so.

You _might_ be able to just use the HOME_ABSOLUTE_ENCODER option in the INI file.
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03 Apr 2022 03:59 #239125 by whatwasIthinking
Everything homing nicely. Thanks for your help.

FYI.
- My first mistake - not knowing that you need an absolute encoder to do switch-less homing
- 2nd - some settings in my ini making the encoder look like it was an absolute encoder when it wasn't

@Andy, if I can ever do anything to help the cause - please shout out. I have skills in mechanical design CAD/FEA, have imported 3 CNC machines from China and my day job is in IT. Current project is a 1.5kw laser hybrid to do wire deposition and subtractive.
Cheers Pete
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05 Apr 2022 15:57 #239352 by andypugh

@Andy, if I can ever do anything to help the cause

How's your C / C++ / Python / Tcl

There's plenty of things to look at in the issues list. 
github.com/LinuxCNC/linuxcnc/issues
 

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14 Apr 2022 04:27 #240205 by whatwasIthinking
Non existent sorry. Pascal / LISP back in my day but that was then.

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25 Apr 2022 19:08 #241237 by strahlensauger
I also have linear scales with an index signal.
On the x-axis it is in the middle and on the y-axis it is at both ends.

At the moment I home with a combination of  limit/home switch. The switches are at the far end, so I know
in which direction I have to look for the home switch, but I would like to home the index signal of the scale.

What would be a good strategy to find the index mark, especially when it is in the middle, for I don't know the position
the mill was shut down.
So I might search in the wrong direction.
 

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25 Apr 2022 19:52 #241244 by andypugh

What would be a good strategy to find the index mark, especially when it is in the middle, for I don't know the position
the mill was shut down.
So I might search in the wrong direction.

 

LinuxCNC will search for a home switch in one direction, then move off the home switch in the other direction looking for an index.
So, the trick is to have a secondary track (probably with a proximity sensor) that extends to just before the index mark. 
If the machine is "off" the track it moves towards it, then off to the index. If it is already on the track it goes straight to the reverse move. In either case the system will then find the index. 

For the Y I would suggest a short target and an inverted signal. 


 
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26 Apr 2022 21:16 #241336 by strahlensauger
Thank you for the quick answer, but how do I set up a second track?

I thought homing is controlled by the three variables
HOME_SEARCH_VEL, HOME_LATCH_VEL and HOME_USE_INDEX.
and just could search in one direction.
Can the mill change the direction by the signal of a switch or proximity sensor?
Or do I have to calculate the sign (the direction) of the variables before travel begins?
And how do I put that in the ini or hal?

 

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26 Apr 2022 22:10 #241343 by andypugh
The trick is to provide a switch in addition to the index on the scale.

If the home switch is already set at the start of homing then the system goes directly to moving at HOME_LATCH_VEL until the switch goes false, and then it looks for the index. You can use this behaviour to let it find the index.

It does need an extra sensor, and a long target that is true for all of one side of the index.
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27 Apr 2022 10:14 #241369 by strahlensauger
Now I got it,
I thought I have to tinker with the ini,
thanks again

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