Re: following error question

[Jon Elson <elson-at-pico-systems.com>]

Dam wrote:

>   Ray Henry wrote:
>
> >
> >Hi Dale
> >
> >You've got my curiosity aimed at your test bench!  Are you trying to run
> >real servos with step and direction from smdromod?  If so, and you can
> >talk about it, how
> >
> I'm not sure what you mean by real servos.  Servo refers to the
> command/feedback loop,  a servo motor without feedback is just a motor.

Some high-end mills, etc. use true positioning servos for the spindle, and can
be commanded to turn the spindle to 37.525 degrees from the reference, and hold
it there.  They can also be commanded to link the spindle rotation to table
motion, and so moving the table will control the spindle to perform rigid tapping
and similar operations.  This generally is not necessary.

A motor which can have its speed regulated within reasonable tolerances,
and can be crept to the index position for tool changes will work as well.
In that case, the spindle would be set to the desired RPM, and the table
moved in ratio to that motion for rigid tapping or threading.

>   Jumping from full speed
> down to half speed or from half to full would have to drive any stepper
> crazy.

Any stepper directly driven by step pulses (not microstepping or pulse multipliers
that have some smoothing) would not be able to track these large step frequency
changes.

>  Wouldn't make for a very good accel/deccel ramp.  One thing of
> interest is that during a move EMC seems to be attempting to brake the
> motor by throwing in some opposite direction pulses.

WHAT?  It is accelerating up to some speed, and then the direction line
changes state DURING the move?  I can believe some flicker of the
direction line, right at the beginning, and especially at the very end, where
the velocity is nearly zero.  But, if there is flicker of the direction line during
the time direction is sampled for a step (varies by stepper driver design)
it has got to be an error.  Now, some programs only drive direction during
the step pulse, and let it return to zero when there is no step pulse.
This is dangerous, of course, since different drivers need it to be stable
before, after, or all the time.

One possible reason for this reverse pulse is that the gain (P) term is WAY
too high, and even slight sample to sample fluctuations of the encoder pulses
counted in that sample lead the program to think the motor has gotten ahead
of the desired position.  This will lead to instability even in an analog servo
system, and cause a lot of motor and drive heating, and machine vibration.
It will kill a plain stepper system - in other words, no stepper drive without
some smoothing could follow such a pulse stream, it would HAVE to either
stall or skip 4 steps.

>  It's time to
> figure out how to use the logging and plotting so I can analyze what's
> going on.  With my scope it's there then it's gone, it seems to ramp up
> and down nice enough for my application. Also worth noting,  for manual
> incremental jogging smdromod will output a series of pulses and I can
> feel the shaft move and watch the display update in increments of  0.0001"

Sure.  A DSO is almost necessary for tuning analog servos.  The logging
feature of EMC is SO GOOD, you almost don't need it.  And, for a step/dir servo
with encoder feedback, it should be all you need.

It does a fine job with my universal stepper controller, too.  And, that digitally
simulates a number of the features of an analog servo.

Jon