Re: [CAD_CAM_EDM_DRO] RE: Request for comment - G320 wiring

Alan Rothenbush wrote:

> >
> > Subject: RE: Request for comment - G320 wiring
> >
> > I looked at your schematic and have one comment - E-Stop switches are
> > normally hardwired into the system, and should not require that any
> > electronics be functional. As drawn, your system has the potential of
> > failing in the "run" position if the electronics fails. This would not meet
> > OSHA safety requirements. If the Geckos are like the Camtronics, you don't
> > want to suddenly open the lines to the motors, but you probably want to
> > interrupt the power supply to them with a switch contact. Once you have this
> > hard-wired override, then you can run additional signals to the PIC (or to
> > the computer) to inform your control of what has happened.
>
> I agree to having had second thoughts about the EStop inputs as well.
>
> I've used PICs in NASTY environments and they've always come up smelling like
> roses. Note that the electronics would have to fail in a particular and
> unusual manner for things to fail "on".
>
> OTOH, there may not be as nasty a place as a shop floor. And Murphy does lie
> in wait for the proud. So I guess I'll rethink this bit.
>
> Here's another question or two for you then, if I may.
>
> 1. Is it permissable to power the relays through an EStop switch and still
> be considered "safe". I would REALLY like to avoid having all the motor
> current going through the EStop switch.

Oh, no. You DON'T want motor current going through the E-stop switch, as these
switches are usually not rated for much current. Also, generally, there are many

things controlled by the E-stop, like spindle, axis motors, coolant and the
computer. I have electronics in my E-stop control, but definitely not the
computer. I also have a watchdog timer that will cause an estop if the
computer doesn't service the motion hardware in time. You do want simple
circuitry for this function, and circuitry that at least is fail-safe if power is

lost.

> 2. Is there any point, short of laziness/convenience, in having separate
> XYZ limit switches ? It would seem to me that the OR of the limit switches
> would equal an ESTOP .. or do machines usually have an extra set of switches
> set just "outside" the limit switches to act as ESTOP sensors ?

That is common in larger machines. Also, some machines limit motion in
the axis that is at the limit. Commoned switches would prevent other
axes from moving when one was at limit.