Re: [CAD_CAM_EDM_DRO] Simple, safe, effective limit switches?

Abby Katt wrote:

>Hi again my ever resourceful allies!
>
>I've been having this terrible recurring nightmare where an encoder
>fails and a servo wedges the table as hard as it can against the end
>of the mill and the little controller that could keeps feeding more
>power and the servo becomes an induction heater.. and... and... I
>lose sleep over it at night..
>
>
>So, let's talk about limit switches.
>
>

Limit switches won't really help. They will, I guess, shut the servo drive
down after the crash happens, but in a true servo runaway condition,
it will be going so fast when the limit switch is tripped that it will
still crash
into the end.

>
>I have an idea of how I want to setup my limit switches, and I'm
>about to buy the stuff for it, but I was wondering what feedback I
>could get first, just in case I'm wrong (or things can be done
>better).
>
>What I was thinking of doing was serial-chaining a whole bunch of
>microswitches in NC (so they pass current through them normally, but
>break the circuit when they hit limits). This would mean, normally my
>current would flow in the circuit, but as soon as any axis hit the
>limit, or if I pushed the emergency stop button (also in series),
>then the circuit would be broken. The chain would drive a big relay
>(20A). The relay would sit on the servo driver side of the power-
>supply capacitor, so that when the relay was off, there would be no
>power for the geckos (not even the resisual in the capacitor, since
>this would be disconnected from them). A second relay could activate
>and this could be used to indicate to the computer that the driver
>had faulted/E-stopped.
>
>

This is a VERY bad idea, unless you provide a suitable energy dump circuit
to remove the energy returned to the Gecko drives from the motors.
This could GUARANTEE that you destroy all the Gecko drives!

>So, is this a good idea? Or will the geckos burn from suddenly having
>motor back-EMF and no cap to discharge it to? Or will it not matter,
>since, diconnected, the servo circuit would not be closed? What about
>the cap, now stranded and connected only to the beefy PSU? Would it
>steam and boil and fill my shins with aluminum electrolytic ionized
>shrapnel?
>
>

The capacitor would not be harmed.

Jon