Pepperl+Fuchs Blog

Photoelectric Sensors Are One Way to Detect Clear Objects

Posted by Sean Miller on Tue, Nov 19, 2013

Many kinds of sensors can detect plastic bottles or glass...

Photoelectric sensors are a popular sensor type for solving many applications, but, depending on the application, using light can sometimes have its disadvantages.

Clear object detectionAdvantages of photoelectric sensors

Photoelectric sensors have a distinct advantage over inductive and ultrasonic sensors. They can operate at much longer ranges and maintain a very low light spot. This is a desirable feature for use in closed spaces or when target size is small.

Disadvantages of photoelectric sensors

The disadvantage is that light can penetrate (burn through) certain targets such as white paper or clear objects such as plastic bottles or glass. Standard photoelectric sensors are usually not suited for clear object detection, but we do offer models that are specifically designed for detecting clear targets.

Retroreflective mode

Most of our photoelectric sensors that can be used in these applications operate in a retroreflective mode, meaning that they require a corner cube reflector. The function is similar to standard retroreflective models with the exception of the circuit. These transparent-object detection models have a special circuit with a low hysteresis function that can reliably detect the minute change in light that is received when a transparent object passes in front of it. We currently have the following models that are capable of detecting clear objects using the low hysteresis retroreflective circuit:

Diffuse mode

We do offer some diffuse mode sensors that have some usefulness for detecting clear objects. Our model ML100-8-W-200-RT/103/115 can detect transparent objects with good reliability due to its divergent beam pattern. The only limitation with this sensor is the 200 mm range and the wide beam. This does produce a large beam and is about 170 mm x 170 mm at a 200 mm range, which is significantly larger than a retroreflective sensor model, but it does not require a reflector.

Ultrasonic sensors

Ultrasonics can be used as well since color or transparency does not influence them. However, they do have limitations when compared to a photoelectric sensor. One limitation would include the sound cone emitted. A photoelectric sensor uses a small light spot that can be placed on smaller targets with reliable detection. An ultrasonic sound wave will expand outward and can give false signals if another object is off to the side of the desired target. That is why they are commonly used in large diameter tanks as there are no side targets to give false signals. So tight spaces are not ideal for ultrasonics, but a photoelectric sensor would be a good alternative as they transmit a fine diameter light spot. 

Light grids

The LGS light grid series can also be used to detect transparent objects due to the sensitivity of the LED receivers, but we advise testing these in the application as they are not specifically designed for clear objects.

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Topics: Ultrasonic Sensors, Light Grids, Photoelectric Sensors

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