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Mid band resonance>

Posted by leadinglights 
Mid band resonance>
February 22, 2021 12:37PM
I am trying to understand a problem I am getting with a stepper motor which is stalling unexpectedly. The stepper swaps between two print heads on a 3D printer and can be seen on [www.youtube.com] The motor in question is the NEMA11 model 11HS20-0674S just above and to the left of the print heads.

The problem is that the stepper stalls at full speed approximately halfway through its travel, when it has already accelerated to full speed; full speed being 520 motor RPM. Just driving the heads as shown in the video the movement is very quick, about 0.5 seconds to complete the swap, it is also very difficult to stop the movement of the stepper with one finger, even applying a lot of pressure. This all changes as soon as filament is fed through the short Bowden tubes when the stalling happens about 1 in 3 or 4 transitions - the only real change being the addition of a very small mass of filament.

Could this be mid-band resonance instability? The driver board is a pololu/stepstick type DRV8825 board, the current being set to 0.95A and is supplied from a 24V supply and quarter stepping is selected. Would a different board make a difference or a dynamic damper on the stepper motor?

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