Photoelectric Sensors and Background Suppression

E-mail JJ Thiara

In many cases, the selection of a sensor, particularly in difficult applications, is the result of trial and error. Although the applications for diffuse-mode photoelectric sensors are diverse, the most effective selection and use of the technology requires an understanding of the operating principles and how the fundamentals relate to the application. In no area of diffuse sensing is this truer than in background suppression.

Diffuse-Mode Photoelectric Sensors
This group of sensors—unlike transmitted-beam and retroreflective sensors, which detect when their light beam is broken or blocked—relies on sensing light reflected off a target. They 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 the target characteristics. The use of these beam-break sensors, however, requires access to both sides of the application and demands extra labor, cost, and space for the separate receiver unit, or reflector.

On the other hand, advances in optics have made diffuse photoelectric sensors more viable as a long-range solution. Although they do not offer the sensing distances of transmitted-beam and retroreflective modes, they have enough range to address a greater number of applications than a decade ago.

The diffuse sensing mode has become much more prevalent with the development and refinement of background suppression—the ability of a sensor to see an object and ignore the reflective surface directly behind it. This is a contributing factor to the diffuse mode's emergence as the preferred method of photoelectric sensing. But how does background suppression work and, more importantly, how and when should it be used?

Background Suppression
Although other diffuse-mode sensors, such as fixed-focus and sharp-cutoff models, offer a degree of background suppression, true background-suppression photoelectrics are designed specifically for applications requiring the sensor to see a target very close to a reflective background. This background suppression is particularly effective when the target and background have similar reflectivity (e.g., light returned to the sensor from the target is roughly equal to the light reflecting from the background) or when dark targets are to be sensed against a lighter, more reflective background.

Background-suppression technology, in its true form, uses light triangulation to create a distinct focal plane that is the effective sensing area. Targets beyond the focal plane will not be detected. Unlike fixed-focus and sharp-cutoff sensors that achieve background suppression through their inability to see the background, true background-suppression sensors actively sense both target and background. If we liken the operation of these sensors to human eyesight, a fixed-focus or sharp-cutoff photoelectric cannot see the background because it is outside of its field of vision. Background-suppression sensors see the background but choose to ignore it. They achieve this by virtue of dual receiving elements, R1 and R2. If an object is located between the focal plane and the receiver, the beam falls on receiver R1. If the object moves out of the focal plane, the beam falls on receiver R2. So presence of a target is based on a comparison of the light seen by the two opto-receivers. If the amount of light at R1 is less than or equal to that at R2, the sensor output changes state, indicating target presence. Conversely, if the light at R2 is greater than that at R1, the sensor output remains de-energized.

The 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, so the application must be set up to accommodate the fixed sensing distance. In adjustable background suppression, however, the sensing distance can be dialed in to suit the application. Turning an adjustment knob on the sensor changes the angle of an internal mirror and, therefore, the focal distance. Unlike fixed-focus sensors that are sensitive only at the focal point, background-suppression models are sensitive to objects anywhere in the focal plane. So the adjustment of a fixed-focus sensor equates to a change in sensitivity, whereas the adjustment of a background-suppression sensor moves the focal plane while sensitivity remains constant.

Application Considerations
Separation between the target and background is an important factor to consider when applying background-suppression sensors. This gap varies according to the reflectivities of the target and background and is expressed as a percentage of the sensing range. 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. So the best-case application would be a 90% reflectivity background with an identical target, and worst case would be a black target of 6% reflectivity against the 90% background. In simpler terms, the black target is nonreflective, so it must be moved closer to the receiver to compensate for the light absorption. In general, a minimum separation distance of 6 mm (0.24 in.) is recommended for reliable background suppression.

Another concern when applying background suppression sensors—or any other diffuse sensor, for that matter—is the direction of target travel in relation to lens orientation, particularly in high-speed sensing applications. Depending on the housing style, the 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 arrangement (i.e., movement in the horizontal plane—either left-to-right or front-to-back) at close range can 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 can go undetected.