Re: freqmod.o in EMC started, now questions

Tim Goldstein wrote:

> From: "Tim Goldstein" <timg@...>
>
> OK, with the EMC Board of Knowledge online I have gotten EMC to come up with
> the new freqmod.o module for steppers and it does run the machine with
> faster movements.
>
> Now for the questions:
> What does the P and the FF1 parameter you play with really do?

P is the proportional gain. So, error is multiplied by P to give the
output velocity request. FF1 is applied before the trajectory velocity
goes into the above calculation, and it adds a term for rate of change
of the required velocity. It helps sluggish drives by effectively increasing
gain when the trajectory has a change in velocity.

> Why do my steppers sit and buzz after a move? I actually think I know this
> answer which I think is the steppers are hunting back and fourth about
> .0002" trying to get where the feedback position matches the expected
> position, but the steps are not fine enough to get a zero difference. I do
> see the actual positions oscillating a few ten thousands. So, the real
> question is not why, but how do I get them to just sit still when at rest?

I gather you are using encoders with stepper motors?
If so, you need to reduce P. This is actually a crude method, and not
really desirable. A better way is to hack the code to allow a deadband.
You figure out what this range of hunting is, in terms of linear units,
and if the absolute value of the position error is less than this figure, you
set error equal to zero. Assuming this is a small range of a few ten-thousandths,
it should have almost no effect on accuracy or smoothness.

Jon