Is Stall detection a reliable safety feature w/ MB6HC
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Working through a new printer design which if the Z axis were to run away for some reason or a dumb thumb ocurred could cause serious damage. Probably will implement some type of break away or pin shearing feature incase but redundancy would also be nice too so would stall guard be an option. To give perspective the Z will be actuated by 2 mema 23's - 2.5v, 2.8a, 4.5mh, 1.26nm .9deg driving 2mm lead ball screws up to possibly 500 rpm....so lots of force multiplication and potential carnage.
Duet controlled Lathe, micro mill, 3d printer and 1992 Haas VF2 VMC
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Is Stall detection a reliable safety feature
safety?
I would say no.The force multiplication would be a downside making the back emf tricky, but on the plus side, the high rotational speed might help. Really only one way to know for sure is to try and configure it and see what happens.
I've actually been using stall guard for probing my Z axis lately. Originally because the bltouch pin broke, but I've left it in place because it works pretty reliably and consistently. 2mm lead screw, 2:1 reduction, and high steps per mm, high rpm. Just nema17 though and I don't use it during a print, just homing.
It is a bit tricky to tune, and you do have to let it crash into something, so not for the weak willed or those with good judgement.
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Thanks for the input and good point on the torque multiplication reducing sensitivity. That reminded me, I think I asked a similar question regarding stall guard a couple years ago. Forgot until now.
I may experiment with a cheap 2mm lead screw and bang on that to see if it can be tuned. Regardless its probably best to have some form of mechanical fail safe.
Probably not a bad idea for folks to check out this little calculator that quickly gives you force using a ball or lead screw for any axis. It can get nutz with low lead numbers
https://www.lintechmotion.com/sizing/Thrust-Force/thrust-inputs.php -
@3DPMicro Another problem with stall guard is that it can be affected by motor temperature, which can make it unreliable. Dropping the motor current when homing can also be a useful safety feature to implement (and it's free and easy to do).
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@deckingman said in [Is Stall detection a reliable safety feature Dropping the motor current when homing can also be a useful safety feature to implement (and it's free and easy to do).
I use that technique on my Duet 2 micro mill. I'll make a note to implement that in 3.
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One problem I had in tuning stall detection for homing is that I had to lower the current LESS than I was because it would stall too easily. It's a balancing act.
Plus if you intend to use stall detection during an actual job operation you're tuning for a different current entirely.
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i'm in the same boat . my current (not yet implemented) solution is to use 3 optical switches . 2 switches close to the edge of the axis on each side (around 10mm before dead stop) , another switch for homing which will be approx 30mm of the edge , before the "edge switch".
2 edge switches should never be triggered , and will be connected to EMO / power disconnect of some sort .
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double post
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My main concern is the 2 motors getting out of sync (Z axis) or 1 running and 1 not. I recently had that happen on another machine and it tore stuff up big time before I could stop it and that's what got me thinking. This new build has way more power but that's an unwanted byproduct of trying to have an insanely accurate and repeatable Z
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stacked optical switches seems like a viable option?
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@hackinistrator said in Is Stall detection a reliable safety feature w/ MB6HC:
i'm in the same boat . my current (not yet implemented) solution is to use 3 optical switches . 2 switches close to the edge of the axis on each side (around 10mm before dead stop) , another switch for homing which will be approx 30mm of the edge , before the "edge switch".
2 edge switches should never be triggered , and will be connected to EMO / power disconnect of some sort .
That would only work close to "home" though. Correct?
Duet controlled Lathe, micro mill, 3d printer and 1992 Haas VF2 VMC
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@Phaedrux said in Is Stall detection a reliable safety feature w/ MB6HC:
stacked optical switches seems like a viable option?
Only close to "Home"?
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@3DPMicro NOPE.
Not safe. At all.
put some safety switches on your bed. -
@3DPMicro close to axis limits .
i don't have anything else that can crash , except axis limits ( for each axis) . well maybe the printed part in case of warping , but that's nothing serious . -
Thanks. I may implement the dual switches . Looks like there are solutions except for if 1 motor moves and the other doesn't say if it was at the bottom of travel going towards home. I think I have an idea that can take care of that mechanically/electrically. If it gets too far out of wack it will open/close a circuit that kills power or activates an emergency stop
