EMC handling backlash

[Doug Fortune <pentam-at-home.com>]

Jon Elson wrote:

> Doug Fortune wrote:
>
> > I am looking at a big old mill with average-worn
> > acme screws (ballscrews maybe someday).
> >
> > I am assuming with high precision linear scales on the
> > tables, then whether its acme or ballscrews shouldn't
> > matter that much to the final achievable precision (if
> > I gear down the  motors to be compatable with the
> > encoder resolution).
>
> This is mostly true for simple, 1 axis at a time positioning
> work, but for any multi-axis moves, especially arcs, the
> backlash will cause the table to jump every time the direction
> is reversed.  So, when boring circular holes with an end mill,
> you'll get lumpy star-shaped holes, with error equal to
> twice the backlash, I think.  You most likely will get
> severe hunting action, where the motor is constantly
> rocking back and forth, overshooting one way and then
> going back the other way.
>
> Why this happens is that when the table position needs to
> be moved just a hair the other way, the motor picks up
> speed crossing the backlash, while the encoder doesn't
> sense any movement.  Then, the motor takes up all the
> backlash, and the Acme screw suddenly moves the table
> too far before it can stop.  Then the process repeats in
> the other direction.

Shouldn't this be built into EMC?  EMC should know
it takes X steps to go D distance, or in particular S steps
to go B (backlash distance).   It should also know
when it is reversing directions for that axis.

Then when it does reverse directions, it should rapidly
take up the slack (because it knows it can go S steps
with no table movement), and by the time it goes B
distance, the velocity has slowed down to the appropriate
rate that that axis is supposed to be going.  Until that
happens, the other axes have been stopped.

Yes, its a bit complicated, but isn't that supposed to
be the purpose of EMC knowing the backlash in each
of the axes?

Doug