12 March 2014 - Brenner - Fiedler

Proximity Sensors - Capacitive vs Inductive

Detecting a target that is close to your sensor can be tricky, but so can the material the target is made of. In this episode, Brenner-Fiedler's sensor specialist, Paul ...

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automation life brought to you by printer Fiedler I am your host Jeremy Schubert each week we discussed technologies used in automation this week Paul Oppenheim sensor specialist with Brenner Fiedler is here to discuss proximity sensors thanks for joining us today Paul good to be here Jeremy thank you so Paul when we say a proximity sensor when people are talking about approximately sensor what are they talking about okay so essentially when they're mentioning the word proximity they are meaning some target that is in close proximity to the sensor itself so if you were going to be needing to sense a target that's me fairly close to your sensor proximity sensors what you mean needing to use okay weird nice and straight very straightforward that rarely happens so what are there types of proximity sensors or absolutely although a lot of people use the term proximity sensor are very elusive they might even just say hey I need some proxies generally when they are referring to the true sense of the word proximity sensor they're referring to one of two sensors being other a

capacitive or an inductive type products okay so of course my inquisitive mind wants to know now what's the difference which I mean a capacitive and inductive very good question okay so we'll start with an inductive sensor and or an inductive proximity sensor is one that detects metal targets onlys okay we'll only see metal targets as opposing to nonmetal like plastic paper or whatever the case might be wood okay so capacitive sensor we could even detect would if we were cutting a 2x4 or some capacitive sensor critic welcomed like an inductive sensor would not would not see that would would seem that right so so like so again an inductive sensor is a missing metal only whereas a capacitive sensor is probably kind of surmised with privileged detects anything in close proximity to it okay so in addition to metal it can see wood plastic you know some paper if it's thick enough and whatnot interesting so-so okay so how does an inductive sensor work how does it detect metal so okay so when you have an inductive sensor essentially you have a circuit which consists primarily of an oscillator so you have a voltages being

sent across a coil and it's kind of like flip-flopping back and forth and in doing so it creates a magnetic field now when a piece of metal comes in fairly close proximity to that sensor it starts to collapse the field and induces what's called an eddy currents then there was a circuit inside that inductive proximity sensor that will then detect that eddy current and say uh the fields collapsed and now send a signal saying there is metal presence basically okay now that would not happen obviously with paper or water or so that would not be of that nature isn't without collapse I gotcha so not only is it great at detecting metal it's also great at ignoring things that are not metal exactly good interesting so I got another question based on that what about what about metals like aluminum or copper are those detectable or absolutely yeah it kind of depends on type of proximity says you know the the general proximity sensors when you're working with a magnetic field with magnetism you think of something that a magnet would stick to primarily iron or what we call ferrous material okay okay but there are specially designed ones that will create

different fields that can be sensitive to metals different types so we have like all-purpose metal sensors we also have ones that say like you want to be able to sense an iron target but not sense some aluminum target or if you want to distinguish between the two yeah or other metals have better you can come across applications where I wanted to be able to sense the differences to be seen let's say like silver versus a stainless steel or something interesting okay gotcha and so then this this leaves us out hang in here what's what it has the capacitive work how's this thing detect wood and water and paper right gotcha okay so the cast a little bit different because what you have now is you have a voltage that's being applied to essentially a capacitor now imagine a if you could envision this probably best way to simplify the circuit is if you could think of a capacitor which is normally an element which holds a charge but imagine if you will slicing and 1/2 so you're only charging one of the plates now the other plate would be technically the target itself or so it has a certain amount of density to be

able to handle a charge so I'm picturing the schematic symbol of capacitor where you've got the right line space line right we're cutting it inside that space yes okay basically like that is probably the best ways I describe this and then you are that other plate or the target is the other plate okay now if it's a very thin piece of material that would only be seen of as a dielectric which falls in between the two plates it won't recognize it okay so if you have like it like a sheet of paper towel it might not see that but they have several sheets or whatever something could possibly in terms a static charge or holding possible charge or something it has a density to it it will detect that say oh there's definitely something there okay I gotcha so for anybody doesn't know capacitor you've got basically two charged plates spaced by an insulator called a dielectric and so this sensor works by essentially detecting the change in that dielectric I guess or the change of the charge yeah it's almost like it's like it's like Do It Yourself capacitor if you kill and what and that bolt is always present and no one sees they're like

okay now there's so many micro farad's or Pico farad's that are certainly buildup okay it's a very small change that the internal circuit can see that changes the ah there's something present there interesting yeah so how is this is useful I mean obviously you can detect weird things that an inductive sensor couldn't sure so let's kind of go back to the inductive sensor let's think of some applications where that would be more appropriate over counter sensors so if you're looking at an inductive sensor one example would be if you want to sense metal behind something that's nonmetal okay now you know previous podcast we talked about photoelectric sensors yeah and especially I'm talking about let's say a reflective full electric sensor with a transmitter receiver built into the the same housing and just be sending light out and whatever hits bounces back right well you can never be able to sense let's say metal behind paper because it only sees the paper so to speak as well then the metals there are not there it still thinks it just sees the paper okay so in that regard a photoelectric sensor would not be

appropriate but a proximity sensor would okay because it's only sensing that metal other examples where proximity sensors are cleverly used or an applications that proved a lot of dust buildup oil a lot of debris am i inside of a CNC machine or so okay we're just our food let's say sugar flour something where you want to be where you don't it doesn't harm the sensors so much or rather more importantly impair its ability to sense something if it's got nonmetallic stuff caked on it okay still see through that for lack of a better term because it's only looking for metal okay so now I'm thinking a step ahead and thinking of it like this bakery where there's flour everywhere and building up on the sensor that's not gonna that it the inductive sensor is not gonna see that it's still anybody see the metal buzz I maybe the capacitive would have an issue with this exactly okay right okay because that it's so is there anywhere else that maybe you're a capacitive is an interesting application I suppose it's using a metal as opposed to like an inductive yeah motoi even sure absolutely since they kind of

measure we were talking about let's say if we're looking at maybe the density of a target now getting back into the concept of capacitor linked charge the capacitances are great if you need to see through let's say a fairly less dense material let's say like give a thin piece of glass or plastic and you want to see water behind that let's say I wanted to look inside a plastic bottle and know if there was detergent in an earth absolutely but do I need to go by detergent on the way home right I installed capacitive sensor at home for this okay most of us don't but do I have enough absolutely so maybe a more bit of an industry where again maybe a photoelectric sensor would not work if you're looking through like a site to were a sight mount of a glass or something bad you can see if there's a chemical inside of a bat and the fat itself it might not have any glass or psyche to but it might be made of plastic okay and it could been seen through that plastic and then see the density of the liquid ball okay and we're in your using the word see very like rate as it's just a descriptor see detect okay so like a

photo I wouldn't be able to see through that or detect through that it's just going to it's gonna that's the first thing it hit right right you got it I gotcha okay I got just so any other interesting things to know about these is there a typical size or shape for an approximate e or either its type yeah generally proximity sensors are what we call a typical barrel shape okay a lot of them can be as small as me about five millimeters in diameter some as large as maybe thirty millimeters a little over an inch or so in diameter okay again they don't again for the large size they don't have that much of a detecting distance again why we're talking about proximity sensor but the other applications okay were very specific for I gotcha and I've imagined that the normal things we've talked about in terms of an NPN or PNP type output I think that we've talked about normally closed versus normally open and when we did the relays weight so reference past podcasts for more information about this type of output characteristics so cool well I think that's all the time we have this week Paul thank you very much for joining us

again so spirit gonna be here again then for me that helped out and thanks everybody for listening to Brenner Fiedler's this automation life if you have questions about what you just heard or if you have a topic that you'd like to hear discussed please email us at tech that's te CH at BRF a calm and mckenna didn't you be sure to continue tuning in each week we have more exciting upcoming episodes

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