Re: home and limit switch in series

[Gene Warren <gwarren-at-execpc.com>]

Joel Jacobs wrote:
> 
> Seems kinda complicated to have so many switches for home/limit.  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
> optionaly 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.
> 
> Well, thats my .02
> 

    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.

     Never, never depend on the processor to stop the machine in a
runaway axis condition; it won't do it.  Never rely on operator response
time to stop a machine by smacking the E-Stop button; he or she won't be
fast enough all the time.

     End of Travel switches are always wired in series and always wired
normally closed and always wired to each axis contactor.  Any break in
this E-Stop string will stop ALL axis movements.  Again, this is for
safety.

     For anyone who doesn't know what a contactor is, it's a heavy duty
relay.  It's purpose is to allow power to the motor.  It does not run
the motor; your motor controller does that.  When you deenergize the
contactor (E-Stop condition), you could care less what the motor
controller does because the motor isn't going to turn.  A non turning
motor is a safe motor.

    Joel, this is not aimed at you, this is aimed at everyone.  Anyway,
end of sermon.

    HOMING:

     My experience with homing (servo systems) have been this.  Each
encoder we use has a Z-marker.  This Z-mark is one pulse per
revolution.  We tell the machine to home, it moves at a preset speed
(set with parameters) in one direction until it opens the Home Limit
switch, then it continues until the Z-marker produces a pulse, then
the axis reverses direction a predetermined distance from the Z-Marker. 
This reversal, among other things, helps take up backlash.

    The predetermined distance is also set in parameters.  It is
determined with the machine.  A cylinder (an upside down drill bit will
work) is inserted in the spindle, the spindle is lowered until the
cylinder fits into a hole in the work fixture.  This is home.  Let the
machine back up until it stops. The machine axis position display will
read zero. Jog the machine until the cylinder just fits into the hole in
the work fixture.  See what the machine axis position indicates.  Add or
subtract this amount to the predetermined offset parameter.  After the
machine finishes the new homing procedure, the axis position display
goes to zero.  The cylinder fits into the hole in the work fixture. This
is home.  Every time.

     The industrial machines use this procedure to maintain
repeatability.  Limit switches are not reliable over a period of years
to maintain repeatability.  They wear and get dirty with coolant and
other shop dirt.  This procedure, as you can see, more or less gets rid
of the inaccuracies of the limit switch and relies only on the encoder
and ballscrew for repeatability.

     I understand the difference between absolute home and work fixture
home.  I just used the work fixture home as an illustration.  The upside
down drill bit may be too coarse for some work.  You can also sweep the
hole with a micrometer to get really accurate.

Gene