I have a setup problem that I think should be a fairly common issue
that I would like to bring up to the development team.

 I have a milling machine that uses open loop steppers to operate the
screws, and scales to monitor table position.  I want to use the
scales to monitor the table position when the drives are disabled, and
I am manually moving the machine.  This will mitigate the need for an
MPG.

 My problem is, I don't see a good way to integrate the direct
reading scales into the HAL component 'axis'.  I can't use
motor-pos-fb, because the backlash is added to it to determine actual
joint position.

 What I think needs to happen is joint-pos-fb needs to become an
input.  Then I can have the option of tying my scales to that.
Machines without scales would use (motor-pos-fb+lashcomp) ->
joint-pos-fb.  I suppose a separate following error pin could be
calculated too, one for the servo loop, and another for the secondary
feedback (scale).

I think this fits nicely with the existing line of logic, and
accurately describes the mechanics of a joint.

Thoughts?

Brian

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