Re: home and limit switch in series

[Doug Fortune <pentam-at-home.com>]

Joel Jacobs wrote:

> Seems kinda complicated to have so many switches for home/limit.

Three switches per axis.

> Wouldn't this work?
>
> Just two NC switches in series or two NO switches in parallel for each
> axis - only three pport inputs used - one for each axis.  The same switches
> could be used for home and limit.  Each axis moves at HOMEING_VEL in the
> direction of HOMING_POLARITY until it hits a switch.   The axis then would
> decelerate to a stop and slowly back up until it's off the switch.  It could
> optionally continue until an index pulse is recieved from an encoder or a
> hall switch on the lead screw.  This position would become 'machine zero'.
> Then move to HOME_OFFSET at HOMING_VELOCITY and reset axis position to HOME.
>
> Using this method EMC would have to know which way it was moving when it hit
> a limit.  If it hits a limit maybe it should stop and back off the limit
> before entering Estop.  That would reduce the possibility of the machine
> being turned off with a limit switch activated.

Exactly.    But the scheme fails if the machine is turned off (on purpose or by
accident)  if one or more of the axes has tripped (opened) the HL  switch.  Then
upon awakening,  further pulses to Home in the homeward direction will make it
overtravel and grind against the stops with no feedback and possibly damage
something.   Remember, upon awakening, you don't know which of the two
(home or limit) positions you are at.  (If it had tripped the limit switch, travelling
slowly in the homeward direction would untrip the limit switch, giving the hint
that you were now at the far end, and could now rapidly Home.)

Since it is nice to have one (or more) emergency stop buttons arrayed around
the machine, my suggestion was to series wire it (them) with Home-Overtravel
switches (ie overtravel switches only on the Home end) on every axis, coupled
with a "try-to-home-each-axis-sequentially"  (ie non-simultaneously) algorithm
to guarantee no indeterminate states during Homing.   That way you get a lot of
use out of only one input line to the computer, including the emergency stop
function.

This results in 4 axis Home/Limit capability with emergency stop and
reliable cold start capability with only 5 inputs.

Future excercise: Figure out how to retain the above functionality for
5 axis with only 5 inputs!

Doug Fortune
pentam-at-home.com