Sensor Sense: ID Sensors for Hostile Environments

March 2, 2010
A metal plate with holes provides the code for optical ID sensors in environments that would destroy a normal ID tag.

Some sensors detect the presence of objects, but they cannot identify the object. For that, you need special identifications systems that make use of RFID, 1D bar codes, and 2D DataMatrix codes. The code is usually printed on a flat plate or paper marker to be read by a camera.

Temperature is not a real concern for such sensors in many manufacturing applications, and these systems work reliably. In cases where temperatures reach 500°F or more, the situation is quite different. For example, temperatures for bake-in processes, paint and adhesive curing, as well as glass or metal-processing applications can easily exceed 700°F.

Another problem is in areas where the printed ID code may become unreadable. Spray-coating applications, heavy dust environments, and extremely greasy or dirty situations may all make a tag unreadable. Reliability for ID systems in these environments is nil.

Fortunately, there are now ways to solve these problems using camera-based systems in conjunction with metal or ceramic code plates that encode a unique ID. These systems are not harmed by temperatures up to the melting point of the code-plate material, and are designed to operate in coating, painting, and generally dirty environments where a normal printed code may become obscured.

The code plates use a defined pattern of holes rather than a printed dot matrix. The holes are large enough to keep paint and other materials from filling them. It’s not uncommon to find code plates drilled with 5-mm (about 0.2-in.) diameter holes. Ideally, the camera system provides a light source that is not susceptible to ambient light: IR is ideal. The light source illuminates the surface of the code plate. As light strikes the code plate and passes through the holes it is effectively “absorbed” making the holes appear dark on a light background. The camera records the image and evaluates the hole-pattern using the appropriate algorithm. Rotational orientation is usually not an issue in these fixed applications. Cameras with the right optics let the distance to the code plate be quite large, moving the camera safely out of the dirty or hot area.

Pepperl+Fuchs (www.am.pepperl-fuchs.com) supplied information for this column.

Edited by Robert Repas

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