Photoelectric Methods of Operation

Photoelectric sensors vary in their operating principles and can be used in a variety of ways, depending on the application. They can be used to detect whether an object is present, determine its position, measure level, and more. With so many types, it can be hard to narrow down the right sensor for your application while accounting for any environmental conditions. Below will give a brief overview of the different operating principles used in photoelectric sensors and where they can be best used.

Diffuse

Diffuse sensors are the most basic type of photoelectric sensor as they only require the sensor and the object being detected. The sensor has a built-in emitter and receiver, so as light is sent out from the emitter and reaches an object, the light will then bounce off the object and enter the receiver. This sends a discrete signal that an object is within the sensing range. Due to the reflectivity being target-dependent, diffuse sensors have the shortest range of the three main discrete operating principles. Background suppression sensors work under the same principle but can be taught to ignore objects in the background using triangulation to ensure any light beyond a certain angle does not trigger an output. While diffuse sensors can be affected by the color of the target object,  the use of a background suppression sensor can limit the effect color has on reliability. Foreground suppression sensors work in the same manner as background suppression but will ignore anything in the foreground of the taught distance.

diffuse

Retro-reflective

Retro-reflective sensors also have the emitter and receiver in a single housing but require a reflector or reflective tape be mounted opposite the sensor for it to be triggered by the received light. As an object passes in front of the reflector, the sensor no longer receives the light back, thus triggering an output. Due to the nature of the reflector, these sensors can operate over much larger distances than a diffuse sensor. These sensors come with non-polarized or polarizing filters. The polarizing filter allows for the sensor to detect shiny objects and not see it as a reflector and prevents any stray ambient light from triggering the sensor.

retroreflective

Through-beam

Through-beam sensors have a separate body for the emitter and receiver and are placed opposite each other. The output is triggered once the beam has been broken. Due to the separate emitter and receiver, the sensor can operate at the longest range of the aforementioned types. At these long ranges and depending on the light type used, the emitter and receiver can be troublesome to set up compared to the diffuse and retro-reflective.

throughbeam

Distance

The previous three types of photoelectric sensors give discrete outputs stating whether an object is present or not. With photoelectric distance sensors, you can get a continuous readout on the position of the object being measured. There are two main ways the distance of the object is measured, time of flight, which calculates how long it takes the light to return to the receiver, and triangulation, which uses the angle of the incoming reflected light to determine distance. Triangulation is the more accurate option, but time of flight can be more cost-effective while still providing good accuracy.

Light type and environment

With each operating principle, there are three light types used in photoelectric sensors: red light, laser red light, and infrared. Depending on the environmental conditions and application, certain light types will fare better than others. Red light is the standard light type and can be used in most applications. Laser red light is used for more precise detection as it has a smaller light spot. Infrared is used in lower-visibility environments as it can pass through more dirt and dust than the other two types. Although infrared can work better in these dirtier environments, photoelectric sensors should mainly be used where build-up is less likely. Mounting should also be considered as these sensors are usually not as heavy duty as some proximity switches and break/fail more easily.

As you can see, photoelectric sensors have many different methods of operation and flexibility with light type to help in a wide range of applications. When considering using these sensors, it is important to account for the environmental conditions surrounding the sensor, as well as mounting restrictions/positioning, when choosing which is right for your application.

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The Foundation of Photoelectric Sensors

PhotoelectricsThe foundation of a photoelectric sensor is light!  Without the light you have a housing with some electronics in it that makes an interesting object to leave on your desk as a conversation starter.  Is all light the same?  Does the light source really matter?  When do you select one over the other?

Red light or red LED light sources are the most favored as they are easy to set up and confirm that the sensor is working properly since you have a bright light that you can focus on your target.  Depending on the lensing the light spot size can vary from a pin point to a spot that can be several centimeters square or round.  It is important that you aim the sensor correctly if you have the sensor installed near an operator so as the light is not shining in their eyes as it can be rather irritating.

There are several misconceptions with the laser light.  Many think that lasers are the most powerful light and can penetrate anything.  Also there is the concern that lasers will cause damage to the human eye.  Lasers in photoelectric sensors are typically available as either a Class 1 or Class 2.  Class 1 lasers are safe under normal use conditions and are considered to be incapable of damage.  Class 2 lasers are more powerful, however it is the normal response of human eye to blink which will limit the exposure time and avoid damage.  Class 2 lasers can be hazardous if looked at for extended periods of time.  In either case viewing a laser light with a magnifying optic could cause damage.

Lasers provide a consistent light with a small beam diameter (light spot) that provides a perfect solution for small part detection.  Although the light beam is small and concentrated, it can be easily interrupted by airborne particles.  If there is dust or mist in the environment the light will be scattered making the application less successful than desired.  In some cases the sensing distance will be greater with a laser light than with a red light.

Infrared LED’s will produce an invisible, to the human eye, light while being more efficient and generating the most light with the least amount of heat.  Infrared light sources are perfect for harsh and contaminated environments where there is oil or dust.  Also infrared through-beam sensors are sometimes capable of “seeing through” a package or object which is sometimes preferred to solve an application.  The ability to see though an object or dirt makes this light source perfect in very contaminated environments when the contamination builds up on the lens or reflector.

In all cases LED’s are modulated or turned on and off very rapidly.  This modulation determines the amount of light a photoelectric sensor can create and prolongs the life of the LED.  In addition, the sensor receiver is designed to look for the modulated light at the same frequency to help eliminate ambient light causing the sensors output to false trigger.

We have determined that all light sources are not the same each with their benefits and drawbacks.  Selection of the light source really depends on the application as often red lights have been installed in very contaminated applications that required the power of the infrared.

If you are interested in learning more about the basics of photoelectrics request the Photoelectric Handbook or visit www.balluff.us/photoelectric.