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magnetic speed pickup question
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Posted 9/14/2022 11:27 (#9843932 - in reply to #9843790)
Subject: Experience with shaft sensors

Southern Minnesota
I have experience with metal proximity sensors. Generally they did not work well sensing off of the teeth of a sprocket. The reason being that there needs to be a clear distinction between sensing metal and not sensing metal.

Since the teeth of the sprocket are tapered in that regard the results are not stable or predictable. Rather than sensing off the sprocket you might have better results sensing off something different.

Raven industries made a special wheel just for such purposes. We called it a NinJa wheel. It fit on a 1" shaft with a set screw and had four portions of metal each followed by a gap of no metal. That meant that as the wheel turned the sensor "saw" a fairly long metal area followed by an equally long nonmetal area. This produced 4 or 8 pulses/revolution depending on whether the display was watching the falling, rising or both conditions of the waveform.

Shapes other than this NinJa wheel could work as well. The important thing with a metal proximity sensor is to have a distinctive gap between the metal and not metal areas. Something as simple as a piece of metal that turns with the shaft but has a large gap of no metal before it passes the sensor again might work fine.

I have also used the magnetic type sensors. I had one that worked fine with a corn dryer and a Calc-N-Acre for shaft speed for several years. Upon startup one season it went wacky. I replaced the sensor but no change. I looked at the magnet which was glued to a rotating shaft. Apparently when some work was done on a bearing in that area, the magnet got broken and consisted of two pieces beside each other. Replacing the magnet cured that problem.

You mention that your sensor is magnetic. The ones I've seen have the magnet on the rotating portion. I'm not sure if the sprocket would be interfering with the magnetic field or not. In my situation the magnet was just attached to the shaft and there was no sprocket in the immediate area.

Edited by tedbear 9/14/2022 11:54
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