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.

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.

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.

i used hard stop homing on many machines with mitsubishi/yaskawa servo drives. all of them have a "torque limit" input and a "torque clamped" output. so just connecting

joint.*.homing => servo-*.limit-torque
joint.*.home-sw-in <= servo-*.torque-clamped

does the trick.

and here's the video when i'm using "torque-clamped" for motion.probe-input. with z axis driven by 2mm pitch sync belt on 20 teeth pulley it is able to resolve silkscreen paint thickness on a PCB...

tommylight wrote:

Or on weak machines where any motor can do damage.  I had an accidental "hard stop homing" on my old wooden CNC.  The limit switch failed and the motors ripped the end off the gantry before I could stop it.
 

tommylight wrote:

There are reasons why brushed DC motor powered machines have two sets of limit switches, one set does normal limiting while the other set is "extreme limits" that will cut all power to motors.
Brushless DC and steppers motors have no need for such measures as if the drive fails it usually causes the motors to lock and burn quietly, if all other safety measures fail.

Normally, yes. To date I've seen two Ursviken Pullmax press brakes. Both had DC servomotors all over and a shocking lack of hard limit switches. They are heavily built and very precise press brakes, but given the way the old controls were set up, the wording in the manual, and also a reprimand I once received from one of their dealers when I asked how to initialize the controls, I get the idea the users were expected to hire Ursviken techs to 'repair' the machine if any axis so much as ran into a limit switch. I consider the old Ursviken controls quite difficult to clear limit errors.

Instant runaway is certainly one of the possible hazards of brushed DC servos when the h-bridge shorts over one side. However, I have not yet seen that happen, but I will not say it can't. The few failures I have run into so far, the h-bridge was shorted completely and popping fuses instantly. I think what happened in those cases is the closed loop feedback detected the servo was a few encoder counts too far off and tried to correct by reversing the motor (by switching on the still good side of the h-bridge), causing a short and blowing fuses. Theoretically, the servo might run away far enough, fast enough to cause a positioning error, disabling the servo amp (and switching off the h-bridge) but with power still applied to it, and with half the h-bridge shorted, the motor could then run away uncontrolled.

Any way, I don't like brushes. Back when I was busy with smaller repair projects, I messed with sooo many brushed DC motors that needed brushes which were next to impossible to match up. I usually filed down bigger brushes to fit, but that is messy work. If I'm working on a project like this and have any reason to suspect a motor is weak or otherwise worn, I strongly recommend my customers to go with AC servos if possible.