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Clear Object Detection with Photoelectric Sensors

Posted by Michael Turner on Thu, Jan 31, 2013

The most important requirement for a photoelectric sensor when detecting clear or transparent objects is that the sensor must be able to detect minute variations in the received light level. Over the years, photoelectric sensors with varying degrees of sophistication, in a multitude of shapes and sizes, and offered by a numerous array of manufacturers, have been designed to do just this. The most successful designs, however, rely on traditional principles.

Retroreflective mode for clear object detection

Retroreflective mode sensorThe most common photoelectric sensor technology in use for clear object detection is a special variant of the standard, retroreflective sensor.

A retroreflective mode sensor for clear or transparent object detection works in almost the same way as the standard retroreflective mode sensor does. The underlying principle is the same; a beam of light is emitted from the sensor. This light is then directed back to the sensor’s receiver by a reflector. When an object is positioned between the sensor and the reflector, a portion of the emitted light is absorbed or scattered by the object. If the amount of light received begins to fall below a set threshold, the output changes state.

For opaque or dark-colored materials, the amount of light the object attenuates can be significant. However, when the object is transparent or clear, the amount of light that returns to the sensor from the reflector is only slightly less than when no object is there. Therefore, the sensor must be incredibly sensitive.

To recognize small amounts of light attenuation, a simple teach-in process must be performed; the sensor is taught the amount of light received from the reflector. Through the use of a low-hysteresis circuit, the retroreflective mode sensor is able to detect minor changes in light that are typical of clear objects.  

Major Benefit:
Long sensing ranges

Major Disadvantage:
Sensitive to environmental influences such as excessive dust, dirt accumulation, and rapid temperature fluctuations.

Divergent Beam Diffuse Mode

Divergent beam mode sensorAnother photoelectric sensor technology used for clear object detection is a special variant of the standard, diffuse sensor. But unlike the diffuse sensor, the divergent mode sensor does not use a lens to collimate or narrow the emitted light. As a result, the divergent mode sensor produces an extremely wide beam. By spreading the optical power over a much larger area, the sensor can detect slight changes in the light level received.

Major Benefit:
No reflector required

Major Disadvantage:
Sensing range is very limited.

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Topics: Applications, Photoelectric Sensors

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