Re: [CAD_CAM_EDM_DRO] Runaway servo questions

R Rogers wrote:

>While running the mill today and thinking about the runaway servo issue should an encoder fail. I wondered if an electronic device could be designed that mounts between the servo leads and the A/B signal from the encoder. It would monitor the pulse train to the servo and the signals returning from the encoder. Then be tied into the E-stop circuit of the control. If a pulse stream was present and a A/B signal stream was absent it would break a connection and make the E-stop circuit drop out. Having hard limits at the extremeties of the envelope seem to be the only protection available and could result in alot of damage before it ever got there. That would be great if something would monitor and trip in a very short distance in the event of a lost encoder. Or would a dithering servo situation make this impossible? If someone designed one that worked, I'd be the first customer. Someone mentioned that Geckos fault after a 128 step/count discrepancy. I've heard and seen Marriss mention
> the runaway servo issue and dont recall him saying it will stop in a short distance. He stresses hard limits at the ends of the travels.
>
>

What Allen-Bradley did on the 7320 CNC control was VERY sophisticated.
But, it was also necessary
in the days when encoders had light BULBS in them, and they burned out
every three months or
so. First, they used differential encoders, and compared the A to A/, B
to B/ and Z to Z/ with exclusive
or gates. If any of these signals was the same polarity as its
complement for more than a microsecond,
that caused an E-stop. Also, it generated a velocity signal from the
encoder, and compared that to the
magnitude of the tachometer. If the tachometer signal exceeded the
encoder velocity by 10%, it caused
an E-stop. This was a pretty exhaustive safety system.

Your description above may not be workable. In certain cases, there can
be dither or VERY small
moves from the CNC that don't require a physical move (because they are
within the deadband).
Using the encoder signals to detect encoder failure is pretty hopeless.
You need some other means
to measure position or velocity as a check on the encoder. (Possibly
armature voltage could be a
good enough velocity reference, but you'd have to bring this
differential signal with huge voltage
pulses from the drive down to a level that could be used to compare to
the encoder or step
pulses.)

Geckos will not fault in most cases of a failed encoder. The drive will
see no movement, and go
up to the current limit. It will ram the end of travel, and if nothing
shuts it off, it will just sit
there, cooking itself and the motor. In some cases, the abrupt stop at
the end of travel will
"surprise" the Gecko current limit with a sudden increase in motor
current, and the drive
will trip into fault status. But, that is WAY to late, of course.

The analog servos on my Bridgeport are set in a wierd way so they WILL
fault. I have the
current limit set close enough to the overcurrent trip threshold so that
if the amps ever go to
full current limit, they will fault. In normal servo operation, the
amps should NEVER reach
current limit, as that introduces a hard non-linearity in the transfer
function (input to output
relationship) that can destabilize the servo loop. The current limit is
really just to protect the
motors, amps and other drive components. So, if the amp ever gets out
of the linear part
of the transfer function, it is already out of control in some manner,
and a trip is probably a
good thing to have happen. I can short the tachometers with a
screwdriver, and the amps
go "click". Or, I can unplug an encoder while the machine is supposed
to be holding position,
and it moves about 1/8" and the amps go click. I occasionally develop a
bad contact somewhere,
and the same thing happens immediately when I try to start up the
machine. I have NEVER had a
runaway, and things would likely get broken if I did.

Jon