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

AbbyKatt wrote:

>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?
>
>

Yes, this is an actual problem with the Gecko servo drives. That's why
encoder
feedback to the computer can avoid these sorts of problems, and save you
both
wrecked workpieces and fried motors/gecko drives. Higher-end servo amps
usually have RMS current limits, so if peak current is being delivered
for several
seconds, the amp will fault.

>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...
>
>

Inadequate lube could do the same. That's why my home machine has current
LED bar graphs on the servo amps. If an axis is binding, I'll see the
bar graph sit
there with a bunch of bars lit and know something is wrong.

>
>
>
>>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.
>>
>>
>
>
>

Yup, my system does all that.

Jon