Help please - Linear Scale only homing

The best solution if you don't have an absolute linear encoder (few do) would be to obtain one with the index mark close to one end.

I know all of mine are near the middle but this makes sense because they were intended for use with a DRO unit on a manual machine. In that application, an index near the middle of the travel allows you to discover it after power up without having to crank the handle all the way to the extreme end of travel.

Actually I'm glad my scales have an index signal. I have already seen linear scales without index signal.

 

Looks like the recommended solution is to add physical homing switches.

The Ursviken press brake I'm working on has one index near top of travel for both left and right ram. Then it has a prox sensor near the bottom of travel, only reachable if the tooling is removed, with limited documentation. It appears these might be for bottom limits to avoid over-extending the hydraulic cylinders.

If I were to try homing to only the index signal... has anyone done this successfully? I searched around a bit and did not find much hope.

I'm thinking of maybe trying to add a home-switch-out-pin to each of the two encoders in the Mesa firmware, on index this pin would always go high during positive counting/ negative during negative counting.

Just bringing this up in case anyone has a better solution. I could always add more sensors/switches. I'd like to complete this retrofit without adding additional sensors to the various axis.

LinuxCNC does support index only homing and seems a
reasonable option on a linear scale with only one index at the end of motion.
 
You would need to guarantee that the axis was not past the index mark
when homing starts...

Thanks, so the secret sauce I seem to be missing, is how to do index-only homing? On another axis I'm homing to home-switch+index so I see the homing latch on index only after the limit switch is found. Here I think I need to latch on the first (and only) index without first finding the home switch?

Ensuring we never try homing from the wrong side of the index seems like it should be easy on this machine. The operator is already required to park the machine with the upper beam on blocks before shutdown, which always puts it well under the index. I may also program a prominent warning into the GUI to display at homing, reminding the operator that the ram must be more than 1" down from top before homing.

NWE wrote:

Thanks, so the secret sauce I seem to be missing, is how to do index-only homing? 

Huh... maybe I just need to use the index signal in place of a homing switch. Sometimes the obvious is obscured by the thought pattern.

Using the index as home in hal is not what I meant, LinuxCNC does support homing to index:

linuxcnc.org/docs/html/config/ini-homing.html#_configuration

See homing type = index only
 

Perfect, thanks! I skimmed over that page too fast trying to find index only homing and overlooked that detail. I guess I was depending too much on searches which were not getting me anywhere.