Hard stop homing

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.
 

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.

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.

OK since this has gotten some interest, there are two methods of using hard stops for homing without messing with the drives at all, for DC brushed motors:
1. using hal sensors wired to inputs (be it Mesa or Pico or parallel), more windings around the sensor = more sensitive
2. using REED switches wired to inputs ...., again more windings = more sensitive
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There are two types of hal sensors, those with variable output and the on/off type, latter is easier to use for this use case.
Sensors should be wound with only one motor wire, not both.

I can't wait to try that. I have tons of cheap inductive proximity sensors (well, more like a couple handfuls). Coil one motor power lead around it. Great idea. Cheap and simple. Thanks!