Re: Re: How to establish repeatable home position w/steppers?

Doug,

The system described by Carey is VERY common on commercial machines.
Usually uses a hall effect sensor (looks like a short piece of
threaded rod with wires coming out one end)as the first switch and
some kind of optical switch on the ballscrew shaft. (often uses the
encoder index pulse)

Microswitches are available in differing quality levels. The
"generic" switches we all see everywhere can be made to repeat
reliably to .001". Higher cost specialty microswitches (read much
higher cost!!) can do better.

Some thought should be given to the homing routine itself if you are
after accurate results. Many of the "low-cost" p-port software
available to us does very little to ensure an accurate home. Most
just hit the switch, then back off some pre-determined amount. This
will not be very repeatable!

A (much) better home routine is to slam into the switch at some
arbitrary speed (just lookin' for the ballpark here) going a few
steps past as we decel to a stop (the switch is still tripped). Now
reverse direction and set a slow! speed away from the switch.
Step,check for state change,step,check state,etc. until the switch
state changes. Then go a few more steps (Set as a debounce parameter
in the software)looking for another state change. If the switch STAYS
off (or on), you have found the home position. Otherwise, reset the
debounce count to 0 and continue checkin'. Microswitches bounce and
the hysteresis will getcha, if you don't allow for the debounce. Now
the software can reset the postion counters and/or move to where ever
needed.

Another variation is to slam at fast speed, back off a bit, then
slowly go back into the switch. Although this will trip the switch as
accurately as above, it means we have "homed the backlash in", since
the next move will be away form the switch. It also means we have a
switch in the tripped position where vibration could un-trip it when
we try to make our first move.

One commercial pcb milling machine (using steppers) just homes to a
mechanical stop! Very repeatable, and since steppers dont care if
they're stalled, also very reliable. Don't try this with servos! :-)

Hope this helps,

Ballendo