monitor extruder stepper voltage to detect filament errors

[RandyL00123]

RandyL00123
RandyL00123
15 Mar 2023, 11:11

I started thinking of how stall detection works, and I was wondering if it's possible to monitor the extruder stepper voltage. It seems to me that if you have a filament run out, the voltage would suddenly drop significantly as the motor force would drop. On the other end of the spectrum, a filament snag would cause the motor voltage to spike before the extruder started to grind, leading to a reduction in motor torque as the filament disappeared from under the drive gear. So, is it possible to monitor the extruder stepper voltage, and have an acceptable range? Any voltage outside of that range would trigger a problem.

[dc42]

@RandyL00123 in principle it might be possible to do this by monitoring the StallGuard register in the driver. However, back emf detection usually only works well at low motor current (relative to the rated current) and high speed. This is because the component of back emf you are looking for is the one that is in phase with the motor current; but the resistance of the motor windings also causes a voltage drop in phase with the current, and the resistance (and hence voltage drop) is temperature-sensitive. High speed makes the back emf you are looking for higher, and low current makes the voltage drop that you need to ignore lower.

So I think you would need a highly-geared motor to get the motor rpm high enough during normal extrusion, and it would need to be larger and heavier than normal so that it can be run at low current.