Background check with photoelectric sensors

Photoelectric sensors exist in three varieties—transmitted beam (also known as through beam), retro-reflective, and diffuse (proximity mode). Diffuse mode photoelectric sensors, unlike transmitted beam and retro-reflective sensors that operate based on the blockage of their light beam, rely on sensing light reflected off a target object.

By Control Engineering Staff February 1, 2003

Photoelectric sensors exist in three varieties- transmitted beam (also known as through beam), retro-reflective , and diffuse (proximity mode).

Diffuse mode photoelectric sensors, unlike transmitted beam and retro-reflective sensors that operate based on the blockage of their light beam, rely on sensing light reflected off a target object. Diffuse sensors are therefore more sensitive to target characteristics, such as color, surface irregularities, shape, position, and reflectivity. For this reason, transmitted beam and retroreflective photoelectric sensors offer more reliable sensing regardless of target characteristics. However, use of beam-break sensors not only requires access to both sides of the application, but also demands extra labor, cost, and space for the separate receiver unit or reflector.

Although diffuse photoelectric sensors offer less sensing distances than transmitted beam and retroreflective modes, they have sufficient range to address a greater number of applications than they could a decade ago. Diffuse mode sensors-especially background suppression type-are ideal for applications requiring the sensor to see a target very close to a reflective background.

The three types of photoelectric sensors are transmitted beam (shown on the left), retro-reflective (center), and diffuse (right). Transmitted beam requires a sender and a receiver. A retro-reflective sensor contains both sender and receiver in one package, but it requires a reflector. Diffuse sensors contain all necessay elements in one package.

True background suppression is particularly effective when the target and background have similar reflectivity (i.e., light returned to the sensor from the target is roughly equal to the light reflecting from the background), or when sensing dark targets against a lighter, more reflective background.

Sensing distance of background suppression sensors is categorized as either fixed or adjustable. Fixed background suppression has a stationary focal plane set by the manufacturer; and the application must be set up to accommodate the fixed sensing distance. In adjustable background suppression, however, sensing distance can be ”dialed in” to suit the application. Turning a potentiometer on the sensor changes the angle of an internal mirror and, therefore, the focal distance. Adjustment of a fixed-focus sensor equates to a change in sensitivity, whereas adjustment of a background suppression sensor moves the focal plane while sensitivity remains constant

Separate target, background

Separation between the target and background is an important factor to consider when applying background suppression sensors. The required gap between the target object and background varies according to their reflectivities. As a rule, the greater the difference in reflectivity between target and background, the greater the separation gap must be for the sensor to distinguish between the two. For example, a black target is non-reflective, so it must be moved closer to the receiver to compensate for the light absorption. In general, a minimum separation distance of 6 mm (0.24in.) is recommended for reliable background suppression.

Another concern is the direction of target travel in relation to lens orientation, particularly in high-speed sensing applications. Depending on the housing style, photoelectric sensor emitter and receiver lenses can be arranged either side-by-side or in an over-under configuration. If the sensor’s optics are side-by-side, target movement parallel to the lens ( that is, movement in the horizontal plane-either left-to-right or front-to-back) at close range may cause the target to go undetected as it enters the sensing field without covering the emitter and receiver simultaneously. By the same token, with over-under lenses, targets moving vertically may go undetected.

Background suppression applications can be found in a broad range of industries, but can be spotted most often in material handling and packaging facilities. When the target rests against a pallet, carrier, conveyor, or other item that cannot be moved or removed, background suppression sensors are usually the best solution.

Rockwell Automation supplied much of the information used in this article. .

-Control Engineering Staff
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