Re: re:Servo not stalling, a problem?

Thanks Jon, very informative.

Did I read somewhere that you have an EMC compatible servo board
available? Have a link?

Ron

--- In CAD_CAM_EDM_DRO@egroups.com, Jon Elson <jmelson@a...> wrote:
>
>
> ballendo@y... wrote:
>
> > By definition, a servo is a "feedback" driven device.
> >
> > ANYTHING which causes the "loss" or "incorrectness" of this
required
> > feedback, CAN lead to axis "runaway". This could be the
unplugging of
> > a connector(or a wire breaking), a software glitch, or a servo
> > drive "error" (not likely in most modern drives, but a "classic"
> > problem). But the careful "balancing act" that IS a servo system
can
> > and does become unbalanced at times. Sometimes it is just a slow
> > moving problem, or a simple adjustment... Or...
> >
> > If (when!?) the axis "runs away", the servo motor will be trying
> > to "get somewhere it can't", either mechanically or
electronically.
> >
> > Usually this means the FULL FORCE of the motor is being applied
where
> > it shouldn't be... Since the motor keeps trying to "get there from
> > here", heat and/or pressure build up and... motor burns out/ drive
> > fails/ machines(or people)get crushed/ tools break/ parts are
ruined/
>
> Since I developed my own servo amps, I've seen nearly all possible
> failure
> modes. Since the servo amp has a number of different sources of
> feedback,
> a failure of one of those generally does not cause a disaster, but a
> controlled
> fault trip.
>
> Failure of the encoder means the CNC control has no idea where the
table
> is,
> or that it is even moving. But, the servo amp has velocity feedback
> from the
> tach, so it will never move faster than the max design velocity, as
the
> velocity
> dac can not supply a larger command signal than that value. Very
> quickly,
> the following error between where the table should be and the
apparent
> lack
> of any motion will cause an E-stop.
>
> Failure of the tach is more serious, but will generally cause a
jerk of
> maybe 1/4"
> or so, and then a following error condition will cause an E-stop.
The
> velocity
> control loop is not designed to be run open-loop, and so it may
cause an
>
> overcurrent trip.
>
> Failure of the armature current feedback will cause a rapid
increase of
> motor
> current. A good amp uses a completely independent method to check
motor
>
> current and shut the amp off if the threshold is exceeded. This
> condition won't
> likely cause much motion at all, just a loud click.
>
> A properly designed servo system is not a disaster waiting to
happen,
> but
> a reliable system. It IS possible for malfunctions to cause an
axis to
> try to
> runaway at high speed, but fairly simple fault detection methods can
> detect
> most of those situations and cause an E-stop. Proper E-stop
circuitry
> should contain measures to remove servo power from the servo amps,
such
> that shorted transistors cannot continue feeding power to a motor.
They
>
> should also have energy dumping features to bring rapidly moving
motors
> to a quick but controlled stop.
>
> Jon