You can't.
This is why I home to Z MAX, have my Z probe tied to Z MIN and, on my cartesian 3D printer, installed physical barriers to keep the head from crashing too hard into the bed.
You will get head crashes.
I've given up trying to create a crash proof system. Either the Z min detector fails or the idiot operator (me) finds yet another way to stumble past the safeguards.
What I have done instead is to minimize the damage done when a head crashes.
- ...Use a glass plate and Magigoo (or similar). A soft surface like a build material or tape will get torn up if the head is drug along the bed surface.
- ...On my home built cartesian unit I created a french cleat system so the head literally pops off if the head touches the bed.
- ...On my delta I use magball arms so the arms pop off if the head hits the bed too hard.
