RE: EMC and ragged steps ?

["Tim Goldstein" <timg-at-ktmarketing.com>]

Doug,

Might be news to you, but definitely not a new issue. steppermod was the
only stepper motion module and when Matt Shaver converted a Bridgeport
series 1 using EMC he discovered that the pulses were ragged enough to cause
acceleration and resonance problems at higher pulse rates. As I know the
story Fred Proctor and Matt spent a fair amount of time at Matt's shop
looking at it on a scope. A way to rectify the problem was developed and
that is freqmod. Now the pulses are much more even, but this has introduced
the problem that the task that is creating the pulses is consuming a lot of
the cycles when you decrease the period (increase the pulse task frequency).
If you set the PERIOD parameter low enough you will lock up the system and
if you are just under that frequency you run into what Ray Henry illustrates
as the dragging his ass up hill in the dessert. The system runs, but there
are so few CPU cycles left for the non-realtime tasks that mouse movement
and screen updates are just dragging. So, for an given processor speed there
is a limitation to the frequency you can run the pulse task at. Now in
non-technical terms (or the way I know about it) what this frequency does is
establish the timing for the stepper pulses. Just to illustrate I am going
to use some bad ASCII art. Each character is a beat of the frequency and
each X is when a step pulse is put out.
If we are moving slow it will look like this:
X..................X..................X..................X..................
X
If we want to speed up a little it would look like:
X................X................X................X................X
And even a little faster:
X.............X.............X.............X.............X.............X
If you site down and figure out the percentages of change between each speed
it is quite small and the motor has no problem accelerating up or down with
the changes.
But now let us assume we are moving fast:
X...X...X...X...X...X...X...X...X...X...X...X...X...X...X...X...X...X...X
And we need to go faster:
X..X..X..X..X..X..X..X..X..X..X..X..X..X..X..X..X..X..X..X..X..X..X..X..X
Now if you figure out the % change it is very large. At some point the
possible speed jumps become too large for the motor to instantaneously
accelerate to and you get a stall.

This whole issue with the steps in the speed range is also a well know
issue. The Ultimate solution is to use external hardware to generate the
pulse so the frequency can be high enough without stressing the main CPU
that the change between jumps is still tiny even at very high rapids. This
is why Jon Elson is working on the external module.

So, my original replay was truly not 100% accurate. I don't know which
motion module DeskNCRT is based upon. If it is based on steppermod then it
is likely to exhibit the same ragged pulses that EMC with steppermod is
known to exhibit. If it is based on freqmod then while it may have
relatively even pulse spacing it could be causing stalling due to the jumps
in speed range.

Tim
[Denver, CO]

> -----Original Message-----
>
> It was written in CAD/CAM/EDM/DRO a few days ago in response
> to a guy unhappily running DOS/EMC ...
>
> > I would suggest trying another software package. Knowing that
> >DeskNCRT is based upon EMC and that ragged steps are an issue
> > with EMC I would guess that is what you are experiencing.
>
> This is news to me.  I don't think anyone else has reported
> EMC having 'ragged steps'.  Perhaps he means just DOS/EMC has
> ragged steps.
>
> Doug Fortune
> http://www.cncKITS.com
>
>