Machine Design

Sensor Sense: Seeing the light

All photoelectric sensors use a light source to signal the presence or absence of an object.

There are three basic methods of photoelectric sensing. All sensors have a light source or transmitter (TX) and a light detector or receiver (RX). Both thru-beam and retroreflective sensors are usually dark-on sensors. They turn on when objects block the light putting the receiver in the dark. Diffuse-mode sensors usually turn on when the receiver sees light reflected from the object. This is called light-on sensing. All three types are available with opposite switching if required.

So one method of classifying these sensors is by how they use that light.

The first photoelectric sensors were known by the general public as electric eyes. The eye was basically a lens that focused light onto a photosensitive device. Early light sensors were selenium cells whose resistance varied with the amount of light hitting their surface. Today light detectors typically are silicon photodiodes or light-sensitive transistors offering greater sensitivity and faster response than older photocells.

Objects passing through the light beam cast shadows across the light detector to trigger the electronics. The closure of either dry contacts or an electronic switch indicates passage of the object.

In technical parlance the old photoelectric eye would be called an opposedmode dark-on thru-beam sensor. Opposed-mode sensors have separate light sources and receivers. The thru-beam term denotes an object detected as it passes through the light beam. Thru-beam sensors offer the most reliable method for sensing opaque objects. Opposed-mode sensors also work at the longest range because the light source aims directly at the receiver.

Retroreflective is another type of thru-beam sensor where both light source and receiver reside in the same housing. Light from the source reflects back to the receiver via a special reflector. As in the opposed-mode sensor, an object passing through the light beam interrupts light hitting the sensing element.

Retroreflective sensors work best in limited spaces. The reflector is very thin and can mount just about anywhere. Because the light must travel to the reflector and back, a retroreflective sensor lacks the range of an opposed-mode sensor. But it is still considered a long-range device.

Diffuse-mode sensors detect light reflected by objects passing before the sensor. They work particularly well detecting reflections from clear materials like plastic or glass. Different kinds of targets may require various light-beam patterns to aid detection. Convergent beams excel at detecting objects that are small, have low reflectivity, or a rounded shape. Divergent beams handle clear materials at close range along with shiny or vibrating targets.

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