RE: [CAD_CAM_EDM_DRO] Re: Safety -- servo vs. stepper

What I hope a discussion like this generates is really an
informal FMEA (failure mode effects analysis).

As a product designer I insist on having a FMEA for all designs...
and I think this is equally appropriate for hobby cnc applications.

Granted the major safety aspect might be the unhealthy state of depression
when your machine self-destructs due to an unforseen failure mode... :)

So here's my example FMEA for EMC servo using the stg card. I will try to
list any failures that would cause machine damage or safety risks in order
of probability.

Failure mode effect

1)Spindle stops during machining......... damaging forces due to continuing
motion

2)Mains power loss....................... immediate estop from latching
relay

3)encoder failure........................ immediate estop from following
error

4)servo amp short........................ immediate estop from following
error

5)computer lockup........................ possible runaway; estop on
overspeed or limit switch

6)program error.......................... estop on overspeed indication from
tach

7)i/o card failure....................... worst case estop on overspeed or
limit switch


For me the worst scenario right now is (1)... trying to cut with a stalled
spindle
due to motor failure or just a too agressive programmed feed. To that end I
have designed
a rotation sensor in the hardware estop system. I am not coverd as much as I
would like
on (5), (6), and (7). A watchdog could take care of that.

One observation on the following error estop...this should happen even on a
gecko
step to servo system. I think the Gecko drive faults on 256 counts or
something like that.

An FMEA is most effective if a group of people compile a list of failure
modes rather
than just the builder.

So can anyone add to the list?


Les

Leslie M.Watts
L M Watts Furniture
Tiger Georgia USA
(706) 212-0242
http://www.lmwatts.com
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http://www.lmwatts.com/shop.html
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