Re: home and limit switch in series

["Joel Jacobs" <jj-at-netexp.net>]

----- Original Message -----
From: Gene Warren <gwarren-at-execpc.com>
To: Multiple recipients of list <emc-at-nist.gov>
Sent: Wednesday, March 29, 2000 3:23 AM
Subject: Re: home and limit switch in series


>     Here's my .02.   You will NEVER see an industrial machine wired this
> way.  A three axis industrial machine will have a minimum of nine limit
> switches.  One for each axis Home Limit (3) and two End of Travel for
> each axis (6).  The End of Travel switches are not there for
> informational purposes, they are there to stop the machine right now.
> Period.  These End of Travel (E-Stop) switches do NOT go into any
> processor (unless to tell the operator exactly which limit switch was
> activated), they go to contactors to stop ALL axis movement.  This is
> for safety.  Anyone who has ever seen a runaway axis appreciates the
> E-Stop.  So a safe industrial machine will still only have three inputs,
> one for each home switch.

Hi Gene,

I can sure see where this would be required for an industrial machine.
Especially one with servos - a transistor shorts in the driver and zoom, off
it runs with no other way to stop it.  I think the processor could be relied
on for a stepper system though since there is no fault condition that could
cause a runaway.

EMC wants to see limit switches though - maybe it would be simpler to do as
you describe and have EMC look at at one home switch per axis only...  Do
the limits with hardware if you're running servos and maybe have an ini
entry for MAX_OFFSET from the home switch - a software limit so to speak for
us stepper guys. Then we would only need one switch/axis.  In either case it
would be nice for EMC to have index inputs (Z marker) for accurate homing.

Doug:
If done this way and it awakend on the home switch it would surely know
which way to get off it!

Joel

PS. Does anyone have any info on the accuracy (repeatability) of hall
switches?  Could they be used as home switches?  How about opto-interupters?
They use opto-interupters to home disk drives...