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Sensing motor stoppage/resistance
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Sensing motor stoppage/resistance

by cmlarsen on Thu Mar 27, 2008 2:33 pm

Hello, would it be possible to determine if a stepper motor jammed? IE if I have a device and it spins, but then it jams, I would like to sense the jamming and allow it to try to unjam itself (by reversing, jittering, etc).
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by darus67 on Thu Mar 27, 2008 6:00 pm

One solution is to put a slotted disc on the motor shaft and have it break
a light beam. Hook an optical sensor to an input pin and look for pulses.
If you stop getting pulses when you think the motor should be turning, it's
probably jammed.

This can also give you feedback on RPM, if you need speed control of the
motor.
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by cmlarsen on Thu Mar 27, 2008 6:06 pm

That would work in some situations, but I do not have that sort of clearance in this project. I am looking for a more elegant solution. Like (i believe) on a normal dc motor, you can place a low value resistor across one of the power leads as voltage divider and check for a currant drop, if the motor stops drawing power. but the stepper motor complicates things since the coils are turning on and off constantly.
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by darus67 on Thu Mar 27, 2008 6:20 pm

Yea, on a DC motor you can monitor the current flow. The current will be
proportional to the load. At no load, i.e. the motor spinning freely, current will
be low. With a high load or stalled motor, the current will be high.

I'm not sure if that sort of technique can be applied to a stepper motor.
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by adafruit on Thu Mar 27, 2008 6:20 pm

its not very easy to do with stepper motors, which is why stepper motors often are paired with position sensors, as darus67 mentions.

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by darus67 on Thu Mar 27, 2008 6:37 pm

If you can't get a chopper disc into your system, perhaps you can get away
with a reflective sensor. Put alternating light and dark stripes on some
rotating part and bounce your light beam off of it.
"He's just this guy. You know?"
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Posts: 246
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Location: Minnesota, USA

Please be positive and constructive with your questions and comments.