RE: [CAD_CAM_EDM_DRO] Servo heat-sensors or paranoia?

Les,

Unfortunately, Geckos don't have a continuous current timeout-limit. I
was thinking about this recently after I read an article here about
someone who was peck-drilling and fried a motor. I think they were using
Steppers, but it made me think about servos. Take this example:

Z axis is moving up and down.
X and Y are stationary.

Lets's say that in drilling in, we don't quite match up with the
original center-drill marking, or that the surface is slightly curved,
so our large drill decides to gnaw in offset and go off at a slight
angle... (I've seen this happen when I was drilling steel manually).
The Z moves up and down fine.. The X and Y have no signal to move, so
they won't break on a servo-lock loss (they'd need to be commanded to
take steps) therefore over a very short time the servo gain becomes max
- as if you ripped the encoder off. The motor gets full power applied to
it, but it just can't budge the bed that 2 or 3 steps it's out. Since
it's within the +-128 step lock of the servo, the servo never faults.
Everything looks fine.. Z moves up and down as per normal, but the X or
Y motor is silently cooking at max fry...

Am I wrong? Or is this exactly what could happen with a Gecko 320?

The other terror is the idea that someone twiddles the table-gibs a bit
tight, but they can still move.. So you start out on a 3 hour milling
project all the time unaware that the servo amps are compensating for
the extra-friction just fine.. Except for the steady 10degrees/min temp
rise of the motors...

>It is certainly not paranoia to be thinking a lot about e-stop sytems

though.

>Such a system should:
>1) work even if the control computer does not
>2) protect against shorted amps and runaway
>3) protect against a stalled spindle due to excessive feed or a damaged

cutter

>4) cause a safe rapid shutdown in a power failure
>and many other factors, if you want to discuss them.

My E-stop system so far encompasses some limit switches and a nice big
red E-stop button. I have a feeling I'll be using it.. But as I'm
running into more and more stuff, I find myself wanting to integrate a
PIC microcontroller into it, as it'd be a lot simpler than all of the
chips/transistors needed for a latching shutdown circuit that gives
indication to which gecko failed. I have it set up at the moment so that
E-stop forcibly disconnects the power supply and drops the cap across a
hefty wirewound resistor. Similarly, if the computer drops it's enable
line the same thing happens. Unfortunately, the geckos will soon lose
power after E-stop (good) and thus the drive-fault light which indicated
which gecko faulted will go out (bad) - so I just won't know which one
faulted. This is where the pic may come in..

I've been meaning to post pics of the conversion for a while...
~Abby