Re: stepper backslash trouble...

[Fred Proctor <frederick.proctor-at-nist.gov>]

EMC users,

Rafael Bello wrote about problems with stepper motors:

>
When having backslash correction, the steppers perform it well in +
direction, but in the - one emc claims an "axis following error" and
goes to "estop reset".
<

I have seen this and in my case it was due to the value of [AXIS_#]
MIN_FERROR being 0.000 in the .ini file. This should be the same as
[AXIS_#] FERROR for the usual case of a constant following error, e.g.,

[AXIS_0]
...
FERROR = 0.100
MIN_FERROR = 0.100
...

to set up an abort and servo shutoff (estop reset) when the following
error exceeds 0.100 inches. To fix your problem, try setting MIN_FERROR
to be the same as FERROR.

The introduction of the MIN_FERROR value was to enable
speed-proportional following error. This came up when Matt Shaver was
setting up an EMC on a machine that had no amp fault feedback, and whose
amps were blowing fuses for some reason. At the time there was only a
single value for the following error, FERROR, and we had to set it high
enough so as not to trip during rapid moves. When moving more slowly,
though, the servo lag was much smaller and FERROR could have been set
lower. It would have been nice to set it lower, since this would have
caught the amp fault early enough that the part might not have been
scrapped when the amp tripped while cutting. 

If MIN_FERROR is present in the .ini file, it enables speed-proportional
following error. The following error FERROR applies to rapid moves (for
the speed specified in [AXIS_#] MAX_VELOCITY), and goes proportionally
down from there. At half the MAX_VELOCITY, it will kick in when the
servo lag exceeds half the FERROR, etc. Note that there must be some
minimum value for the following error, below which it's always OK. This
is MIN_FERROR. If it's set to 0.000, then really slow moves, or the
beginning of moves, will trip an error since the effective following
error limit is 0.000. I found this depends on the direction of the move.
So, set MIN_FERROR to be something like a tenth of the FERROR and you'll
be OK, e.g., 

[AXIS_0]
...
FERROR = 0.100
MIN_FERROR = 0.010
...

The problem with the gains being clobbered by steppermod.o has been
fixed, as per a previous email.

--Fred