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Tech Tip: Proximity Sensors for Object Detection from AutomationDirect
https://www.automationdirect.com/proximity (VID-PX-0006)
Learn about the different kinds of proximity sensors or proximity switch for object detection and which is best for your application.
The quick disconnect cable with the diagnostic LED used in this video is an "EVCxxx" part number.
Proximity Sensors come in four flavors. Inductive, Capacitive, Magnetic and Ultrasonic. Inductive sensors detect metallic objects, capacitive sense non-metallic objects and magnetic proximity sensors detect magnets. This video will focus primarily on Inductive and Capacitive Proximity sensors. Magnetic sensors are covered in a separate video. Ultrasonic sensors are used for object detection BUT they are also used for distance sensing, so we'll look at those in more detail in the distance measuring video. Detection ranges for these sensors max out around these numbers. Some proximity sensors do have analog outputs so they can do some distance sensing which is really helpful when setting up the system to make sure the sensor is in a good range to detect the object. Note that the ones that operate over larger distances are going to be the larger units. There are two different mounting options for these barrel style sensors. One is optimized for flush mounted applications it's threaded all the way up to the face it's called a shielded sensor - and one sticks up like this it's called un-shielded. The shielded flush mounted sensors typically have a shorter sensing distance. The reason for this is the sensing region actually starts below the face of the sensor kinda like this. So when you flush mount one of these in a metal bracket, you're choking out a portion of the sensing region and reducing the range. That's usually not a big deal, because if you are flush mounting a sensor, it's probably because the object is sliding right over the sensor so distance is not a big issue. Can you connect proximity sensors in series? Sure! Why would you want to do that? Suppose you have three sensors and you only want to know when all three sensors are active at the same time. Normally you would run all three into the PLC and make that decision in software, but what if the sensors are a long distance from the PLC and you don't want to spend the money running three sets of cables? Or maybe you are running low on PLC Inputs. Well, then this is a neat trick to have in your tool box. If you have a three wire NPN sensor, then wire it like this: Connect the sensors and PLC to 24Vdc. Connect the first signal wire to the PLC and that negative terminal of that sensor to the signal wire of the next sensor. Do that again with the third Sensor. Finally, connect the third sensors negative terminal to the negative supply. Now the PLC won't get an active input signal until all three sensors are active and provide a path to the negative supply. How many sensors can you put in series? That depends on the sensors. If you look at the spec sheet, you can see a parameter called Voltage Drop. That's the voltage drop across the sensor. Looks like this one is 2.5 volts. So if I have 10 sensors in series, they would need 25 Volts but I only have a 24Vdc supply, so that isn't gonna work is it? So the maximum number of sensors in series depends on how much voltage you have available to drop across all of the sensors. When placing sensors side by side, a good rule of thumb is to leave at least one sensor diameter of space between them so they don't interfere with each other. Keep an eye on the switching frequency of the sensor. Looks like this one is 5000 hz but you might see some that are 100 hz. So if you are trying to detect objects moving by faster than 100 hz like gear teeth for example and you are using a 100Hz sensors, then you are going to miss some of the objects right? So sure the switching frequency of the sensor is high enough to capture the objects you want to measure. I like for my switching frequency to be at least twice the rate of the objects I am trying to detect. How do you know which type to use? NPN or PNP? That depends on what you are connecting the sensor to. The main thing to know is sinking and sourcing are always paired up. If the PLC is sourcing current from the positive supply, then the sensor must be sinking current to the negative supply. If the PLC is sinking current to the negative supply, then the sensor has to provide the source of the current from the positive supply. How do you remember which is which? Well, if the sensor is sinking current to the negative supply, then it is an NPN think of N for negative. If the sensor is sourcing current from the positive supply, then it is PNP think of P for positive. The sensors that come with wires already connected are handy and tend to cost less, but the sensors with quick disconnect cables allow for quick replacement of sensors. So if you have an application where short down time is critical then consider using quick disconnect cables it'll save you money in the long run. Proximity sensors typically have indicators to tell you when the sensor is active. I love that, but it is not always easy to see when the sensor is mounted and sometimes I really wish I could tell if the power was connected correctly too. Well, look at this. You can get cables with indicators built in that show you the supply status and the status of both outputs. They are much easier to see than the indicator on the neck of the sensor and now you can tell if the system is wired correctly at a glance. There are also various length patch cables with the LED indicators that you can just insert temporarily to do test and diagnostics. Can you use a proximity sensor in a vibrating environment? You can, but keep the vibrations below 55Hz and a max amplitude of 1mm for no longer than 30 minutes on any axis. Will a magnet trip a proximity sensor? It can. If you have a capacitive sensor the answer is yes since they detect non-metal objects. If you have an inductive sensor the answer is - it depends on how much metal is in the magnet. The TV spec tells you how long it takes the sensor to become stable. Looks like this one is 30 milliseconds, so don't expect stable results until 30 milliseconds after the sensor is turned on. You can typically expect proximity sensors to repeatedly detect objects within plus or minus 5%. The smaller short range sensors will be the most accurate. If you only need to detect the presence of an object, then larger longer range sensors are fine. The best way to tell what kind of connector a sensor uses is to look at the last character of the part number: an H is a 12mm 4-pin connector that all of AutomationDirect's M12 cables are compatible with. This is the same connector as Allen Bradley's Micro QD and Turck's eurofast., An F suffix is the 8mm connector which may be 3 or 4 pins. Make sure you select the correct one The three pin connectors are what Turck calls picofast. Speaking of connectors, these multiport junction boxes are really handy for combining groups of 4, 6 or 8 sensors into a single cable. That makes cable management for a bunch of sensors so much easier. Also, don't forget to order mounting brackets with the sensors. They are inexpensive and they save you the trouble of finding and making your own. With all the different form factors, output types, cabling options etc, choosing a proximity sensor can be a little overwhelming. The good news is the AutomationDirect website has a fantastic parametric search that will help you find the sensor you need quickly. For example, if I click on Inductive proximity sensors because I want to detect something metal - I see that there are 266 different kinds today. But if I narrow that by selecting a size and sensing distance, I can quickly get that down to just 6 options. That parametric search is a huge time saver. That ought to be enough to get you started with proximity sensors. Check out the How To Videos that show you how to connect these to a PLC and then program the PLC to use them. If you have any questions, please contact AutomationDirect's Free award winning tech support during regular business hours. They will be happy to help you. 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 there. AutomationDirect's support staff doesn't monitor the forums on a regular basis.
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