Get to Know Different Photoelectric Sensing Modes

As with many retroreflective sensors, a proximity sensor typically includes both an emitter and a receiver within the same physical housing. But while both opposed and retroreflective modes work by indicating an object’s presence when a beam is interrupted, proximity-mode sensors work in the opposite way, indicating a target object’s presence only when the beam reflects off it and returns to the receiver. One way to think about it: opposed and retroreflective sensors detect objects that break a beam, but proximity sensors detect objects that make a beam.  There are four subtypes of proximity mode, each with a different optical arrangement: diffuse, divergent beam, convergent beam, and background suppression.

Diffuse sensors are the most common, and they receive light directly reflected from an object’s surface, often with the help of a lens. Diffuse proximity sensors offer the benefits of not requiring a reflector and being easy to install, generally for applications in which opposed mode or retroreflective sensors can’t be easily implemented or aligned. However, they tend to return a smaller amount of light (signal loss) and work best at shorter distances than opposed and retroreflective sensors.

Divergent-beam proximity sensors don’t use a lens. They’re less dependent on precise beam angles, but they’re best limited to applications to detect very small targets, such as wire or yarn, at extremely close distances consistently within one inch of the sensor.

By contrast, convergent-beam proximity sensors use lenses to focus both the emitted light and the receiver to the same exact point. This creates a small, well-defined sensing area at a fixed distance from the sensor. They can detect objects of very low reflectivity and ignore targets beyond their depth of field, making them useful for precise positioning of small parts. (One important caveat is that convergent-beam sensors can potentially detect brighter objects beyond their focal point. This aspect has led to an increased market for fixed- or adjustable-field background-suppression sensors, as described in the next bullet point.)

As their name implies, background-suppression proximity sensors ignore anything past a set cutoff. They work by comparing the amount of light returned to two separate receiver elements. Whenever a background suppression’s second receiver registers an equal or greater amount of light than its first receiver, it recognizes a target. Whenever the amount of light received by the first receiver is greater, the sensor’s output signal is cancelled. Background suppression sensors can be useful if objects you want to detect are darker than the background behind them (for example, dark object on a white conveyor belt). Background suppression sensors are available in fixed-field modes, which have a consistent cutoff distance, and adjustable-field modes, which allow users to set the cutoff.