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

I understand. Seems they have a current limit (320) but perhaps
not a transient peak limit adjustment.

In my case I am more concerned with large forces breaking things
rather than motor temperature. Things break fast-stalled motors need
relatively longer time to overheat. (I am moving 500+kG ant very high
speeds)

In any event you could use a very low value series resistor in the servo
line to sense current. With a couple op amps, RC filters, and perhaps a
flip flop you could easily make a little circuit that would send a logic
signal
to the e-stop system both with a continuous current limit and a higher
allowed
peak current limit with time set by the rc network. Or of course a PIC could
do this nicely as well.

I thought the Gecko units had this...they should.

Your system of disconnecting the supply and dumping the cap sounds fine.
I have to do something more drastic due to the extreme mass and speed of
a large router. I disconnect the servos from the amps and short them through
a power resistor. 600 Watt transient voltage protectors take care of the
inductive
voltage spike. It is set to brake as quickly as possible without
mechanically
damaging something...a couple g or so.

Les

Leslie M.Watts
L M Watts Furniture
Tiger Georgia
(706) 212-0242

Main page:
http://www.lmwatts.com
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-----Original Message-----
From: AbbyKatt [mailto:cnc@...]
Sent: Tuesday, December 07, 2004 4:04 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: 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...