Hard stop homing

Hey, is anyone doing hard stop homing? I'm not finding any combination of LinuxCNC + hard stop homing. Google is not my friend today. Llm claims to know everything about it but I'm not buying that. If google doesn't know about it then llm has got to be fabricating it.

I am ready to try coding a halcomp if it doesn't already exist. I just don't got time to reinvent the wheel if someone else has already done it.

I consider hard stop homing to be the least accurate version of homing, but in today's project it will be a lot better and cleaner than rigging up some homing switches. If I can attain 5mm repeatability I will be happy, but I am confident I can do better than that.

Sorry, I forgot to write, these are DC servo motors. Most hard stop home systems I've seen, used steppers.
I think I will write a hal component homeswitch-hardstop that monitors encoder velocity and latches a home-switch-output when the velocity decelerates faster than decel-limit (or servo motor load exceeds max-load?). The latched home switch output clears when the velocity changes direction and moves farther than min-backoff. These encoders do not have index. The most repeatable home position will be with the motor stalled against hard stop. The instant the motor power is switched off, the axis will likely rebound slightly, probably an unrepeatable distance.

I guess I will need to use the homing method of:
1. Home search velocity toward hard stop. Ouch, not at max power!
2. Home switch output latches on hard stop.
3. Back off home switch; home switch unlatches when it detects reversed motion.
4. Home-latch-velocity toward hard stop.
5. Homing complete; move to home-pos at the speed of home-final-vel.

I see I could use this component as shared home + pos limit switch + neg limit switch.

NWE wrote:

Most hard stop home systems I've seen, used steppers.

Yes, and not reliable at all, and only used on 3D printers.
Definitely not something used on large machines with motors that can do quite some damage.

tommylight wrote:

NWE wrote:

Most hard stop home systems I've seen, used steppers.
 

Yes, and not reliable at all, and only used on 3D printers.
Definitely not something used on large machines with motors that can do quite some damage.
 

You're right, I certainly wouldn't want to try it with big powerful servos. I should have included more context: These are small dc servo motors driving Z1 and Z2 on the Ursviken press brake I'm retrofitting. That is the independent left and right motion of the two fingers on the backstop. The whole thing is so overbuilt, these little dc motors are going to smoke before they destroy anything.

I have been studying the machine, and have come to the conclusion Ursviken had to have been using hard stop homing on these. All the other, more powerful axis have homing switches, and only the R axis has limit switches. I have not yet found anyone who has watched one of these homing, running the original Ursviken controller.

FWIW Tormach does hard stop homing on their ClearPath machines. I retrofitted my Series 3 with them and honestly it seems to repeat pretty well. Certainly better than 5mm. I haven’t done an experiment to precisely measure it, but I’m fairly certain it’s within a .001” - .002”

That is interesting. One of my friends is building cnc plasma tables, he uses his favorite proprietory software (Centroid), along with ClearPath servos. I don't recall whether we ever talked about his homing. Next time I meet him I might remember to ask how his system does it, just out of curiosity.