Probing failsafe with motor stall

TypQxQ

Hi!
Is it possible to use stall detection to abort a homing with sensors?

I'm using optical on X and Y. It has happened that the pin or the sensor has wrong angle and not being detected.

I use Precision Piezo Orion and it has happened that the cable got loose and the head continue into the buildplate until I cut power.

So I would want to use stall detection to save my bed and belts.
I've already changed belts twice and I need to allways check homing with my fingers near the power button as I don't trust the homing to 100%.

3dmntbighker

@typqxq +1 on this. My Orion has also failed to trigger right and buried the nozzle into the Buildtak more than once. Eventually the Z belts slip, but only after the bed springs are bottomed and the bed mounts are disturbed. Yesterday it refused to stop triggering until I power cycled the whole printer.

I was actually thinking yesterday about a fail safe mechanical limit switch being added somewhere. But stall detect would be an adequate fail safe as well. However I have a geared stepper so I'm not sure this will work.

dc42

Have you considered reducing Z motor current during probing moves? You would probably need to anyway to get stall detection to work reliably.

3dmntbighker

@dc42 said in Probing failsafe with motor stall:

Have you considered reducing Z motor current during probing moves? You would probably need to anyway to get stall detection to work reliably.

I reduce it to 40% on X and Y. But I had not considered reducing it on Z when the piezo probes, because there should be no reason. But since the piezo probe operation is unreliable I guess I need to look into that. Given that I have a geared stepper it could be interesting to get that set to the right percentage. I already doubled the size of the stepper pulley to reduce the torque. The geared stepper was used in an effort to stop the bed settling to the bottom when power is off. This solved that at least. Maybe what I really need is a kill switch on the bed in case the front left spring gets compressed.

deckingman

@3dmntbighker said in Probing failsafe with motor stall:

........................... Maybe what I really need is a kill switch on the bed in case the front left spring gets compressed.

That's kind of what I do although in my case, I don't have springs on the bed but I use the nozzle for probing and the hot end mount allows for some movement in Z - same thing just a different approach.

Anyway, as a back stop, in case the probe switch fails (and they all can), I have a micro switch set to trigger about 2mm after the normal Z probe, should it fail. This switch is connected to a spare end stop and then I use M581 with the T0 parameter to initiate an emergency stop if the micro switch triggers. It sounds like you could use a similar arrangement on your bed.

SupraGuy

@deckingman said in Probing failsafe with motor stall:

(snip) I have a micro switch set to trigger about 2mm after the normal Z probe, should it fail. This switch is connected to a spare end stop and then I use M581 with the T0 parameter to initiate an emergency stop if the micro switch triggers.

I like this idea, I think that I should implement it myself. I originally had a spring-loaded system for my Z axis that would save the bed/hotend in the event of a limit switch fail, but this is probably better, since that one didn't actually shut things down, it just let the axis nut spin after overpowering the spring.

3dmntbighker

I have had some really horrifying nozzle crunches that I still can't believe didn't bend the heat break. The bed was bent down at probably a 20 degree angle under the nozzle.

TypQxQ

Sounds like a good idea for Z with the mecanical endswitch.
How about X and Y?
When my optical switch didn't trigger the motors bent the belt.
3 times...
Bothe steel reinforced and the fibre reinforced ones.
Would lowering the X and Y current while homing help with that maybe?

deckingman

@typqxq said in Probing failsafe with motor stall:

Sounds like a good idea for Z with the mecanical endswitch.
How about X and Y?
When my optical switch didn't trigger the motors bent the belt.
3 times...
Bothe steel reinforced and the fibre reinforced ones.
Would lowering the X and Y current while homing help with that maybe?

Yes to reducing motor current while homing - always a good idea IMO. Because homing usually takes place at slow(ish) speeds, you can reduce the current by quite a lot. You can either simply set the motor currents by using M906 or you can set a percentage reduction by using M913. Both will achieve the same result. Put the command at the start of the homing file(s) but don't forget to either put the motor current(s) back to what you normally use, or set the percentage reduction to 100, at the end of the homing file.

I don't bother with additional switches for homing X and Y as they home very close to the minimum travel - about 2mm from a hard stop, so there really isn't room for a second back-up switch without losing some travel. But should a switch fail, then the reduced current will prevent any serious damage.

I do however have additional switches fitted to all the axes maximum travel - set to trigger just before a hard stop. In normal use these aren't needed, as once the machine has been homed, and providing you have set the axes maxima correctly, then the firmware won't allow moves beyond those maxima. However, I often work on the printer and tend to allow axis movement before the homing has been done, so in the event that I do something stupid, the switches will trigger an emergency stop and prevent damage.

To keep the wiring and configuration simple, all the switches are wired in series and normally closed, and connected to a single end stop on the duet. So if any one triggers, then it will initiative the emergency stop. It's generally best practice to wire switches normally closed, then if a wire falls off, it will fail as if the switch has triggered, which is safer than not see the trigger.

HTH