Re: phase stepping

[Ray <rehenry-at-up.net>]

Hello John.

As I understand freqmod, it figures out how many steps it has to go in what
length of time and sets a rate generating routine.  Steps that can't be
spit out immediately get put into a buffer.  To many pulses in this buffer
and you get the following error.  In the software this looks a lot like a
servo system and in fact uses the PID tuning stuff.

1 - I suspect that since your problem is with several quick starts that your
acceleration is to high.  Try setting it between 2-5 and see if that makes
a difference.

2 - EMC seems to have a problem with scheduling pulses that it's placed in
the buffer back into the pulse stream.  I tend to think of it this way.  It
may not be completely true. Some combination of the [EMCMOT] PERIOD, and
[TRAJ] MAX_VELOCITY,  parameters sets the upper bound of the frequency
generator.  Anytime the trajectory planner wants to go faster than that
upper bound, it starts piling up pulses in the buffer.  Then when the
frequency generator has slack time, it begins to feed these pulses back
into the stream.  I believe that you can create some slack overhead by
setting [TRAJ] MAX_VELOCITY larger by a few percent than you set the same
variable in each of the axis definitions. 

3 - You can also rather easily overrun the frequency generator by demanding
a lot of pulses per unit with INPUT_SCALE.  This problem becomes one of the
mechanical/electrical design of the device you want to run with the EMC. 
Design a high speed wood or foam router with much fewer pulses per unit
than you would an EDM machine.

BTW - Since the release of January, we have found a bug in the code that
sometimes trashes the EMC when you first issue a motion command.  I'm sure
that someone on the list can describe exactly where in the source code the
changes need to be made to fix this release.  This was more of a problem
with steppermod than freqmod but you might watch for it.

Hope this helps

Ray


On Fri, 07 Sep 2001, John wrote:
> Hi
> I'm a hardware designer trying to build a router table.
> 
> I've set up redhat 6.2, rtlinux 2.2a, and the January emc 
> sourceforge distribution. 
> 
> I have the simulations running ok, now added  several 
> steppers running  phase stepping using freqmod.o  
> (phase step both matches my stepper driver, and seems
>  more noiseproof and sensible).
> 
> I'm having trouble in two areas:
> 
> when jogging with the arrow keys, hitting the key several times close
> together generates a "following error" and reverts to ESTOP reset.
> Can someone explain (or point to documenation) to help me understand 
> which ini file veriables are relevant to this mode, and what they do?
> There are kind of a lot, most seem to be servomotor related.
> 
> secondly, I'm using the minimillio as EMCIO.  Is this wise? I have no 
> parallel port hardware at 0x278. 
> (but this could change - as a PS1 video game controller handpiece looks 
>  an interesting option here..) 
> 
> regards, john