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I have a thin (~1-2mm) rotating plate, where one side is made of metal and one side is made of red plastic and I want to count the number of rotations it makes. The plastic and metal plate are housed in a thin clear plastic housing, and the distance between the metal plate and detector can be up to 5 mm. Top and Side view of thin metal plate with a detector I was thinking about using an induction proximity sensor for this but does it still work with the plastic in front of the metal plate? Similarly is the IR proximity sensor, would it give a strong enough oscillating signal to give a readable output on the oscilloscope? Are there any other simple, cheap options that I didn't consider?

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  • \$\begingroup\$ In case of a magnetic metal, then a magnet on one side and a Hall effect sensor on the other side might work for finding the transitions between metal and plastic. \$\endgroup\$
    – Lundin
    Jul 13, 2022 at 10:11
  • \$\begingroup\$ What is the diameter of the rotating plate? \$\endgroup\$
    – Jens
    Jul 13, 2022 at 11:58
  • \$\begingroup\$ The diameter is 12mm \$\endgroup\$
    – Actros
    Jul 14, 2022 at 9:17

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I was thinking about using an induction proximity sensor for this but does it still work with the plastic in front of the metal plate?

For sure it'll still work. The eddy currents that are in the metal plate (due to the proximity sensor) will not be diminished by anything other than conductive material or ferrous material that might be placed in the way. Plastic will have zero effect.

Similarly is the IR proximity sensor, would it give a strong enough oscillating signal to give a readable output on the oscilloscope?

It's unlikely that this would be successful on the face of it unless the surface of the metal and the surface of the plastic had quite different reflective properties. Of course, you could just put a reflective strip on the disc as most people would in this situation. Might be the easiest solution. However, what you refer to as the "clear plastic thin housing" may let visible light through but, may be opaque at infra-red.

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