Mark,

Just one question though. How will you figure out, automatically, which axis
caused the collision with the pier?

You can't reverse both motors, since if the RA caused the hit, for
example, reversing the DEC which was in a good position, might get into more
collisions. If the AP was slewing with both motors, perhaps it's best to stop,
issue the alarms, and let the human figure out how to untangle the traffic
mess. Stopping means another relay/breaker, to cut off all DC power to the
mount. Drastic, but better than putting a ding in the pier :-(

Actually, I faced this same quandary, when I designed my variation of the
Mel Bartels stepper tracking system (Scope.exe). However, my circuit version
knew which axis signals were sending stepper pulses to each motor, so it was
simple to reverse individual motors, while activating (optional) user selected
alarm devices. In your approach, you only know that the some part of the scope
has started to strike the pier, so figuring out which way to reverse, is
difficult. It will handle pier detection during tracking - one axis reversal -
but I don't see how it can handle two axis slewing with reversal. Best to stop
the mount altogether, in that situation.

Joe