Engineers have several choices to sense the presence of objects or information: machine vision systems and photoelectric sensors have been common selections. Recently, manufacturers have enhanced UV sensor capabilities, improving feedback, speed, range, and setup, which make them suitable for many of the same applications as machine vision systems and other sensors.

In addition, UV sensors have their own unique niche. Unlike photoelectric sensors, UV sensors easily detect invisible or nearly visible objects; glue, plastic wrap, clear containers, labels, bottle seals, or other objects that are the same color as background material. They are also suitable for fastening and joining applications, such as noticing excess glue in assemblies.

Each sensor option is best suited to specific applications. Photoelectric sensors emit a beam of light that the target returns to the receiver. The beam's intensity depends upon the target, which often acts as a reflector. This design, however, presents a problem with clear or colorless objects and when the background and target material are the same color. When both the target material and the background reflect the same amount of light, the receiver cannot distinguish between them.

UV sensors emit a beam of light, typically between 320 and 380 nm and offer more than 100,000 hours of operation. Older versions used discharge lamps with lifetimes of about 4,000 hours.

UV sensors look for fluorescence, not reflected light. When the light hits the target material, it glows (fluoresces) in a color in the visible spectrum. The sensor detects the visible light and converts it into an electrical signal that gives a numerical reading or triggers an indicator. Even though the visible light comes back as one pixel of information, these sensors can examine 3,000 or more objects per second. UV sensors can also detect the presence of labels on packaging; however, they are unable to sense orientation. Until recently, UV sensors could miss objects that fluoresced below their factory set detection threshold. These sensors used special algorithms to alter the response threshold.

Newer sensors eliminate this problem with manually adjustable parameters. A numeric display indicates the threshold level, which is useful in troubleshooting. In an automotive application, for example, seat frames are painted with a UV mark to indicate whether they underwent certain steps in the manufacturing process. Engineers initially set the sensor's detection threshold at 15. A reading of zero would indicate no mark was present, while a reading above 15 registers the mark. Sometimes, however, the seat frames are poorly marked and the sensor cannot perceive enough light to read the mark. Because the sensor can be adjusted, operators can lower the threshold to less than 15, thereby reducing rejects.

For range, typical UV luminescence sensors need to be within 90 mm or less of the target for accurate readings. This short range can be a problem in certain applications, such as wood processing with an optimizing saw. UV crayon marks indicate the location of defects in the wood. Sensors feed this data into a controller, which determines how to cut the wood most efficiently. However, this application is inherently dirty. Sawdust and other debris quickly cover a sensor that is placed one inch or less from the operation. Longer-range sensors eliminate this problem.

Sensitivity can now span almost the entire sensing range of a UV sensor, instead of only at a distance, often through the use of interchangeable lenses. However, in a factory setting the versatility of a single, wide range lens is useful, such as in applications where the sensor must detect objects across a conveyor, as well as objects up close. DW

EMX Industries, Inc. provided information for this article.