Retroreflective sensor detects moving pill bottles.

Background suppression, sharp cutoff, polarization, and transparent object detection are all examples of new technologies that give photoelectric sensors the ability to solve challenging sensing applications.

Before examining these technologies and their applications, it’s important to understand a concept called operating margin, otherwise known as excess gain, because it plays a key role in these solutions. Operating margin is the ratio of the electrical signal (usually voltage) available to the minimum signal required to trigger the amplifier and output of a sensing device. For example, a margin of one means that the sensor is on its threshold of operation. Conversely, a margin of 20 or more enables the same sensor to “see” many objects, even those with a flat black finish.

No photoelectric sensor should be applied in an industrial setting at an operating margin of one. In such situations, small changes in the environment, such as airborne dust or dirt accumulating on the lens, can impair operation of the device. Hence, it is advisable to install photoelectric sensors where they can maintain an operating margin of at least two.

Margin is usually described graphically. When operating margin is plotted against distance, the result is a traditional bell curve. Because of different design objectives, each sensor exhibits its own curve. Be aware, however, that gain can be adjusted by a “sensitivity” knob on a photosensing device. Decreasing gain from its maximum setting causes a loss of operating margin.

Background suppression

In the past, a common dilemma in photoelectric sensing was the need to detect an object against a nearby or more reflective background. Interference caused by that background often made the use of a conventional diffuse sensor ineffective. But, the advent of a special type of diffuse sensor now makes this problem easy to solve.

This device, a background suppression sensor, is different than other types in that it has two photodetectors in one housing. One photodetector senses objects while the other “looks” straight ahead, sensing background, Figure 1.

The sensor receives a signal from the photodetector looking at an object and subtracts its value from the signal received from the background photodetector. The background value is thereby eliminated, and the remaining signal is then amplified.

The distance at which a background signal is suppressed is adjustable, and this adjustment replaces the traditional gain setting on a standard diffuse sensor. The user, in effect, creates a different margin curve for each distance setting. With this adjustment, a background suppression device is generally effective from a sensing distance of zero to the maximum sensing distance set on the device.

For example, with the device set at 8 in., Figure 2, a white background at 8 in. or more is ignored, while a black object can be detected at a distance of 7.7 in. or less. This is a difficult sensing scenario because a white background is highly reflective, whereas a black object is highly light absorbent.

A background suppression device can also solve a number of other difficult sensing problems. For example, magazines moving on a belt conveyor can be detected by such a device looking straight down from above. Even if the background conveyor belt is bright white, the sensor still functions as well as if the belt was black.

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