Re: EMC handling backlash

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

Doug Fortune wrote:

> > 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.

Well, if the system uses open loop steppers, or encoders on the
leadscrew, then EMC does this, but if the backlash is substantial,
like a worn Acme screw, then the results are poor.  You are demanding
the motor and leadscrew to move a finite distance in an infinitesimal
amount of time, and that is not possible.  If the backlash is on the
order of .001", like a slightly worn ballscrew, then it doesn't do too
badly with it.  But, a worn Acme screw can have .015-.050" of
backlash.  Even after adjusting the nuts, you may be stuck with .007"
or so, due to greater wear in the center of the leadscrew itself.
That's what I got on my old Bridgeport after fixing the nuts.

> 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.

Especially with steppers, they can't accelerate and stop
very quickly.

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

When the leadscrews are worn, the amount of backlash
VARIES over the length of the screw!  It varied from less than
.001" at the ends, to .007" or more at the middle, on mine.

Jon