Index Homing odd offset

CNCCustoms wrote: This morning when I homed the machine, Z was off by 3.4mm which is very strange!

Could this be a tool or coordinate system offset?

How have you determined that it is 3.4mm off?

It seems every time I restart the PC and home then things change.

The tool change position changes. Which is absolute and does not change with any offsets...and so does the position of a 4 Inch stackup on the mill table (according to my probe)

When I rehome and do the checks... the new position repeats to within 0.002mm.

I am using inductive sensors as the initial home signal trip. Could it be that the sense position is changing? Would debounce help? But that makes no sense?

Oh... I'm always measuring in G59.3 offset with the probe

What are you doing? You have mentioned index, and you have mentioned absolute encoders.

I have absolute encoders on my drive servos. I initially wanted to forgo homing by
using the absolute encoders but realised that linuxcnc does not track the absolute encoder empirically. So if ,for example, my z would fall a bit while brake was applied when shutting down, linux would not track that.

That is when decided to use index homing instead.

Then realised I need to use closed loop step gen for index homing to work.
I subsequently setup and tuned my system for closed loop using position control step gen.

I got index homing to work (or so I though). The issues I describe above ensued. Hence the questions around Step gen in position control vs velocity control.

After some trouble shooting it turns out that my servo drives being setup for absolute encoders could be the issue. It seems that when in absolute encoder mode, it doesn't pass the actual index through to linux but rather an inferred index (which could vary in position).Well that is my theory anyway.

Ive set my servo drives up to use the absolute encoders as incremental encoders and that seems to have solved the issues i was having (holding thumbs)


Does this seems like it could have been a reasonable cause?

I am assuming index homing works for stepgen systems setup in position control , closed loop?

Thanks guys. Hopefully my experience helps someone down the line.