Re: EMC handling backlash

[Jon Elson <jmelson-at-artsci.wustl.edu>]

Fred Proctor wrote:

>
> > Maybe once we find a reasonably priced linear encoder (which
> > of course directly senses table movement) then it will be
> > feasible to have EMC directly characterize the backlash
> > along the length of each axis (say every 1/10th, 1/100th or 1/1000")
> > and rather than a simple fixed value, the backlash is looked
> > up near that location on every reverse of an axis.
>
> and Jon Elson's reply:
>
> > Theoretically doable.

This is kind of muddled.  Obviously, with a decent linear encoder, there
is NO backlash.  What we got into was temporarily installing a linear
encoder to characterize variations in backlash over the length of the screw.

I don't think anybody does this in practice.

>
> This sounds like standard lead screw compensation, that Jon described.
>
> I'm putting lead screw compensation in the EMC, and I'm curious about
> how it's implemented in practice. Some questions:

Excellent!

> 1. If a CNC has lead screw compensation, does it have backlash
> compensation specified separately? Bidirectional LSC can compensate for
> backlash at the same time, as Doug described, so backlash compensation
> is redundant.

These might be mutually exclusive, although I think both COULD be done.

> 2. How often are the points sampled when building the LSC table? On our
> Fanuc axis controller for the Hexapod, the points are 10 millimeters
> apart. Ideally there would be several, say 10 or so, around each
> rotation of the screw. This works out to every 0.010 inches for a
> typical machine. Collecting these points manually for the whole length,
> both directions, will take forever. Coupled automatically it can be done
> quickly, or at least without being tended.

I think it is often done every inch.  Without very expensive test gear,
it is a manual procedure with gauge blocks and a dial indicator.
Obviously, without a home reference, the compensation is meaningless.

Jon