[grbl Issue#1624] Soft limits do not prevent contact with limit switch.

#1 – jyelon 于 2020-04-23

I was looking into this matter a little deeper. Here’s some homing code from limits.c:

“
#ifdef HOMINGFORCESET_ORIGIN
setaxisposition = 0;
#else
if ( bitistrue(settings.homingdir_mask,bit(idx)) ) {
setaxisposition = lround((settings.maxtravel[idx]+settings.homingpulloff)*settings.stepspermm[idx]);
} else {
setaxisposition = lround(-settings.homingpulloff*settings.stepsper_mm[idx]);
}
#endif
“
So the important thing to remember about this code: the machine origin is ALSO one of the soft limits. Wherever you put the machine origin, you also put one of the soft limits. It’s therefore essential to put the machine origin wherever you want the soft limit to be.

In case 1, the machine origin (and hence soft limit) is placed at the backoff position. In case 3, the machine origin (and hence soft limit) is placed right up against the limit switches. That’s probably only a millimeter of difference, but it means that in case 1, the soft limits really do prevent contact with the limit switches, and in case 3, they don’t.

It’s obvious from looking at the code that somebody deliberately coded case 1 and case 3 to be different from each other. But why? I just don’t understand why you would sometimes want to put the soft limit at the backoff position, and at other times put the soft limit in direct contact with the switch. It seems to me that only one of those could be right.

#2 – jyelon 于 2020-04-24

Ah, crap, I just realized this is a dead repository for GRBL. Closing issue.