Re: STG2 Outputs

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

JohnDRoc-at-aol.com wrote:

> Ok, this is going to sound weird.  One amp board works, the other two don't.
> I ran the STG diagnostics program.  I was able to move all 3 axes with it.
> So I went back and switched DAC0 and DAC1 again, so that DAC1 was running the
> X axis instead of the Y.  Oddly enough, it worked!  However, the Y axis, with
> DAC0, still did not work.  This confirms that the settings in the .ini file
> are usable.  So I would say, with some certainty, that it is amplifier
> related.  But I don't know why the amplifiers would work with the diagnostics
> program.  I would say maybe it's EMC, but both DAC's worked on one amp, but
> not the other two.  I'm feeling logically impaired here!  When I try to jog
> the Y axis, it will move in the positive direction then start drifting in the
> negative, until I get a following error (or it crashes, whichever comes
> first).

When you switch the cables from the DAC to the servo amp input, you
also need to switch the encoder cables, so the axis being tested has
both the command out and the feedback in consistent as far as EMC
sees it.  (This doesn't matter with the STG diagnostics, since they
don't close the servo loop.)

If you try to test by ONLY swiching the velocity command cables, then EMC
has the problem of seeing movement on the wrong encoder channel,
which will cause a following error.  EMC has 3 states, completely off
(E-Stop), servo amps powered on, but feedback loop not closed
(E-Stop OFF), and feedback loop closed (Machine On).  Depending
on the error, it can go from Machine On to one of the other two
states.  If the servo amps are on, but the feedback loop is no longer
closed, then the machine will drift.  Mine is tuned up pretty well, and
the drift is very slow, about .0001"/sec, but it could be faster on some
other setups.

I suggest checking the polarity of the motion commands and encoder
counts, to make sure these are consistant for each axis.  If the polarity
was wrong, it would cause the problems you are having.  It would
not be at all unusual for the encoders to read axis movement in the
positive direction by rotation in a different direction on some axes.
You can compensate for this by swapping the A and B wires on the
encoder, or in EMC you can change the sign on the encoder factor
in the .ini file.

Also, the sign (either electrical or numeric) of the DAC output could
be different for different axes.  So, plus might mean move the X
axis in a positive direction, but move Y in a negative direction.
All of this stuff has to be consistent for each axis, or that axis
will run away into the stop.

It seems that the motor and tach signals are consistent, or you
would have runaways even without the computer.

You've never answered the question, how fast is this drift you mention?
If you have a tach or encoder phasing error, but the current limit is set
real low, what you call 'drift' may be a runaway axis in slow motion.
If the drift is over .01" per second, that may be what you have.  If
turning the current limit up just a little causes the 'drift' to speed up,
what you have is a runaway, not drift.

I hope this helps,


Jon