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Tech Tip: Photoelectric Sensors for Object Detection
(VID-PH-0004)
Learn about the different types of photoelectric sensors and which are appropriate for your application.
Photo electric sensors are classified as Through Beam, Reflective, Diffuse, and Diffuse with Background suppression. The Through beam sensors have a transmitter at one end and a receiver at the other end so they will have the longest range and are going to be the most accurate, but they are also gonna also cost more because you have twice the electronics and twice the cabling. Reflective Photoelectric sensors shoot a beam at a reflector so the one sensor head acts as both the transmitter and the receiver which makes them less expensive but also means they have a shorter range and will be a little less accurate. While the through beam and reflective sensors are looking for an interruption of the beam, diffuse photoelectric sensors actually look for the object to reflect the beam. This is great for those situations where you don't have access to the far side of the object. The down side of this is it is heavily dependent on the reflectivity of the object passing by and has a shorter detection range and is therefore less reliable. Background activity can also produce false readings with this kind of sensor. Which is where Diffuse with background suppression comes in. There are a number of different ways these work, but one of the more popular methods is a sensor with two receivers built in. The transmitter sends energy out, the beam bounces off of the target and hits one of the receivers. When the object isn't present, then energy bounces back at a different angle so most of the beam hits the other receiver telling the sensor that this is not the object we want. These background suppression sensors have about the same range and are about the same cost as regular diffuse sensors, but they perform better in many applications. Fiber Optic Photoelectric sensors are basically the same animal as regular photoelectric sensors. The main difference is, the optics are separate from the electronics. In this through beam sensor example, you can see the two optic heads which feed back to the sensor electronics through these fiber optic cables. Here's a reflective sensor head where both optics are in one head. Again, it is the same photo-electric sensor, the optics have simply been remoted from the electronics. These sensors even come with a special tool for cutting the fiber optic cables. You can get a feel for just how small these things are when I hold them right next to the regular photo-electric sensors. As you can see, these are great for squeezing into small tight installations where you can't fit traditional sensors. They are also great when you have an environment that might mess with the electronics like high heat, electronic interference, things like hat. because with these you can put the electronics out of the way and only expose these passive optics to the hostile environment. Of course the larger heads give you the greatest range out to a couple meters which is comparable to the regular photo-electric sensors. And since the optics are so small, that means the beam is going to be small, which means you can detect smaller objects. As you can see, there are a number of very definite advantages to these fiber optic photoelectric sensors. Photo Electric sensors are available in AC powered models, in fact if you go to the AutomationDirect webstore, select sensors, click on photo-electric sensors, the Parametric search allows you to search on operating voltage. Of course you can also search on the type of sensor, the kind of output it includes, the logic level and the max sensing distance in feet meters and millimeters, and you can even specify what kind of connection the sensor has. This parametric search makes selecting the photo-electric sensor you need really quick and easy. A diffuse sensor is normally 'light-on', since its output is on when its own light source is reflected from a target. Reflective and thru-beam sensors are normally 'dark-on', since the output is active when the light beam is blocked. Some sensors can be configured to either a 'light-on' or 'dark-on' mode of operation. Diffuse sensor range is dependent on the reflectivity of the target. In some cases, a dark object may reflect light better than a light-colored object. Diffuse sensors are specified by the distance at which they detect white card stock. Photoelectric sensors come in different shapes and materials from plastic to stainless steel suitable for wash down applications. Just pick the one that works for your application, connect it to your PLC and you're ready to go. Check out the How To Videos to learn how to wire these up and program them in a PLC. If you have any questions, please contact AutomationDirect's free,, award winning tech support during regular business hours. They will be happy to help. And don't forget the forums. There are lots of folks there that love to share their years of experience. Just don't post any support questions on the forums, AutomationDirect's support staff doesn't monitor the forums on a regular basis.
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