Ask the experts: How to select an inductive proximity sensor

Josh Kingsley:
All right. Our first one is going to come from Jerry on LinkedIn. I think this is going to be a great way to start. His question is what's an inductive proximity sensor?

Matt Simms:
That’s a nice softball question. But, if you're unfamiliar with these kinds of electronic sensors, it's a good place to start. The two instructive words are inductive and proximity. We'll use proximity first. So proximity sensing means an electronic sensor that does not require the target to actually contact the sensor physically in order to be detected. That's the proximity part. If you compare it to a limit switch, where the target has to actually physically contact the switch operating lever to make the switch change state, in a proximity sensor, it's electronic and the target only has to come in proximity of the sensor.

The inductive part means it's a sensor designed to detect metallic objects. So, inductive proximity, metallic objects, where the target only has to come within a certain distance of the sensor in order to be detected.

Josh Kingsley: 
Got it. All right. Thank you for the level set on that one. I appreciate you getting the audience all on the same page. Thanks for that question, Jerry, to start us out. 

Josh Kingsley: 
We're going to stay on LinkedIn for this next one, and this one's coming from Darryl. His question is how does an inductive proximity sensor actually work?

Matt Simms: 
Another good question. It’s probably a good way to start by looking at an image of the building blocks of an inductive sensor. 

So, you see a couple of component parts of the sensor. In the front end is what we call the transducer. Basically, it's the sensing part of the device. It's a coil of wire in a ferrite core, wrapped around a ferrite core, that creates a field. Behind that sits an oscillator circuit. That oscillator circuit that you see in the block diagram creates an electromagnetic field basically at radio frequency levels. That oscillator circuit feeds up to that coil. That coil and core shape the field as it emits from the front of the sensor.

When that target then comes in to the sensing environment, that electromagnetic field from the sensor induces what are called eddy currents, in the surface of the target material. Those currents start to flow and that dampens our oscillator-induced signal. That damping is detected in our detector circuit. That's that third block in the block diagram there.

So we sense that the electromagnetic field from the sensor is being dampened by the presence of the target. Then the detector circuit recognizes that as a valid detection and it switches the output for the sensor. So a sensor turns on or off depending on the type of sensor it is based on that chain of events.

Josh Kingsley: 
Got it. Perfect. Thank you for bringing that graphic up and explaining that. 

Josh Kingsley: 
All right, the next question we have is going to come from Facebook. This time it's from Sarah. It looks like she's throwing out a scenario. The scenario is changing from a limit switch to a non-contact device in the application. What's the potential for interference from other metal in the area where the sensor's going?

Matt Simms: 
That's a great question. I'll show you that in the framework of a sensor like this. This is a real sensor and a real application. We'll talk about this in a second. So the limit switch application, very typical. Common to be transitioning an application on a machine from a contact device like a limit switch to a non-contact device like a proximity sensor.

The limit switch, like I said earlier, requires the target to actually contact the switch lever. Once you've got that sorted out, you really don't have to worry about what's going on out in the sensing environment.

In this case today, you've got an electronic device like this sensor, and you can see it here mounted on this bracket. So, this is designed to sense metallic objects that we talked about, but you've got obviously metal nuts that mount this sensor onto a bracket like this bracket here. You're not so much worried about those things.

The sensor is designed to avoid any objects like that, the tube the sensor's built in, the nuts, the mounting bracket, that sort of thing. Where you need to be concerned is the metal that's in the target detection zone. That's the greater concern.

Josh Kingsley: 
We have a ton of questions coming in. Thanks to Eaton nation for that. This episode of Ask the Experts is brought to you by the E57G inductive proximity sensor. Make sure to visit eaton.com/E57 to learn more about the product.

Let's jump over on this one. We're going to go to LinkedIn. This is a pretty high-level question. What's the basic principle of a proximity sensor?

Matt Simms: 
Yeah. Again, the basic principle is that you use the electronics in the sensor circuit. So, this is a typical sensor circuit board to create an electromagnetic field. Tthat's one step. The second step is you need to somehow direct that field into an area in the vicinity of the sensor where you want target detection to occur.

That's the purpose of this coil and core. So, if you create an electromagnetic field, the core and the coil shape that. In this case, here's a tube. You take that. Now that core and coil shaping creates the field directly in front of the sensor so that that creates the target detection zone.

So, again, the principle is the creation of electromagnetic field. Then that electromagnetic field is disturbed by the presence of that target, the way we described earlier. We are able to detect that disturbance.

Josh Kingsley: 

All right. So next one that we're going to run with is also from LinkedIn. The question is where would you use such a device? So I think what we're really trying to find out are what are some good common applications for inductive prox sensors?

Matt Simms: 

Sure. So these sensors typically, again, they're used for sensing metallic objects. One of the primary areas where they're applied is sensing the moving parts of a piece of equipment, a machine where there are moving pieces, there are gates, there are shafts rotating, there are arms moving back and forth, there are slides, there are hydraulic cylinders moving things back and forth to create a movement of a product or a movement of an object that's being manufactured, conveyed on this piece of equipment

So, those internal moving parts of the machine, typically those components are metal or metallic. And so, an inductive proximity sensor can be used to sense that a cycle of some part of the machine has been completed. For example, if that cylinder has pushed the ram out to the extended position, detecting that, or that has retracted back to the base position, detecting that.

When you're talking about actually looking at materials that are produced, for example, on a piece of equipment like bolts in a machine like that or stamped parts in a stamping machine or a press, again, many of those are metallic objects, and a proximity sensor can be used to detect those. For example, when they're being manufactured, counting them at the end of the process, looking at a particular feature, something that's being welded onto one of those components.

So, you can look at the internals of a piece of equipment or a machine and you can also look at the components that are being produced by that machine or moved by that machine if they're metallic in nature.

Josh Kingsley: 
Got it. 

Josh Kingsley: 
All right. So here's another one from LinkedIn, coming from Dan. Is this an analog or digital type of setup?

Matt Simms: 

So, this is a good starting point. So the most common wiring type when you enter the world of inductive proximity sensing is a two-wire wiring system. Most commonly you're coming from something like a limit switch where there are just two wires and your power to your sensor and your power to your load are the same.

So imagine that sensor is a limit switch and you got your load. When the switch is off, the circuit is open. There is no current flowing. You put an inductive sensor into that circuit, a two-wire inductive sensor, and all of a sudden the sensor needs some power to stay alive and operate in order to detect the target and turn that load on or off.

So you have what we call a residual current. That just means that that circuit in an inductive sensor mode, or an application, will be always drawing a small amount of current, where in the case of a limit switch in that two-wire circuit, the circuit's open and it's not drawing any current when the circuit's off.

So, then there's another graphic you guys can pull up that shows a three-wire circuit. So that was a two-wire limit switch wiring. This is more common, three-wire circuit. The third wire's the output wire. So the power to the load and the power to the sensor are separate.

In this case, this is a PNP output. So when the sensor switches on or off, that current flows from the sensor through the load. Then if you pull up the next graphic, we've got an NPN graphic that shows, again, a sinking circuit. It's not as common in North America, more common in other parts of the world, but certainly present. Here in North America, again, typical three-wire circuit.

Then you might also hear sensors referred to as four-wire devices. Sometimes you hear five-wire, you hear six-wire. Mostly what that means is a sensor that’s got power wiring plus multiple output circuits. So,in this case, an NPN circuit and a PNP circuit in the same sensor, that would be a four-wire device. If you had, for example, two output circuits on a two complete solid-state transistor output circuits on a sensor, you might see a six-wire sensor. But the most common are three-wire devices and two-wire devices when you talk about replacing something like a limit switch.

Josh Kingsley: 
Got it. I can definitely tell that that question comes up a lot. Thanks for that really thorough explanation. Thanks for the question, Luanne.

Josh Kingsley: 
So, I'm going to throw two questions at you at once here that I think are going to tie together. So this is a follow up from Chetin on LinkedIn. The first is a follow up, which is what is a sensing distance of an inductive sensor? 

Matt Simms: 
Sure. So, detection distance, just as a basic concept in the world of inductive sensing ... And I showed you a tubular type of sensor in some of the earlier examples. There are other form factors of sensors. This is a cube style, 40-millimeter device. You can see here's another one that's a limit switch size device.

Each one of those has different range performance characteristics, but most typically you'll see sensors that have ranges out to a maximum of about 100 millimeters or four inches roughly in the world of industrial inductive proximity sensors. To get a four-inch or a 100-millimeter range, you need to have a fairly large sensor device, because typically the sensor can see about as far as its diameter, physical diameter.

So in this case, this cube is 40 millimeters square. The maximum sensing range of that device is roughly 40 millimeters. This tubular sensor is 30 millimeters in diameter. As far as it can see is roughly 30 millimeters. So think about it in those terms. If you need 100 millimeters sensing range, the sensor's got to be 100 millimeters in diameter. It's just physics at work.

Josh Kingsley:
Got it. Perfect. All right. Got a couple of questions that are cut from the same cloth. The first one is operating temperature range. This is coming from LinkedIn. Then we also have Jorge on LinkedIn saying what happens to an inductive proximity sensor over temperatures?

Matt Simms:
So, just as a general topic. So if you look at a typical inductive sensor for an industrial application, it's got a pretty broad temperature range, typically -25 degrees Celsius to +55 degrees Celsius, or even -40 C to +70 C or +80 C. So pretty broad temperature range.

The electronics in a circuit like an inductive sensor, these electronics can be sensitive to temperature. So those same international standards organizations that define the target characteristics to give you a sensing range, that 10-millimeter range to that mild steel target. They also define how much your sensor range can change over that temperature band. That's what you see in that graphic.

So, the international standard is a sensor can vary up to 10% across that full temperature range. So that 10-millimeter sensor can range from 9.1 millimeters to 10-11 millimeters over that full temperature range. What you see in this graphic is it's more common the average sensor varies about 5%. You can see that over the statistical data we developed over time shows a variance of about 5%, and you can see it. 5% is what you see at the more extreme ends of the temperature spectrum. So your range is going to go out. That 10-millimeter sensor's going to see 10.5 millimeters at the extreme cold and extreme hot.

Just as an aside, so how we do that is when we develop a sensor, we do extensive, what we call, temperature compensation across this full range, because if you just take a standard electronic circuit and you don't compensate for temperature, you might have a variance of 30%, 40%, 50% in range, which is not practical. Obviously, if your 10-millimeter sensor only sees five millimeters when it's cold and sees 18 millimeters when it's hot, that's not great.

Josh Kingsley:
Got it. Perfect.

Josh Kingsley:
All right, next question. Here's a quick one from Dan. Digital outputs are designed to be used in DC circuits only or in AC or DC?

Matt Simms:
It depends. So most typically those transistor outputs are used in DC circuits, but AC/DC sensors are quite common where the sensor is rated. It's got, for example, a solid-state transistor output and it can be wired for AC or DC operation. So those output circuits can be used in both AC and DC applications.

Josh Kingsley:
All right. Perfect. 

Josh Kingsley:
All right. The next couple of questions are asking about some more details on the iProx series. Then also just what are some things that set iProx sensors apart from other ones that are available out there.

Matt Simms:
 I'd say, going back to the point about shock and vibration that was asked a few minutes ago, one of the things we have become very good at over time, because we've been selling sensors and applying them in difficult applications like those stamping equipment applications, presses, those kinds of things, we've learned how to develop high-shock and high-vibration performance ratings in a sensor. So that's an area where if you have those types of applications, we typically have sensors that can survive in the most difficult places because of that shock and vibration performance.

Josh Kingsley:

Got it. I think that's all of the questions we've gotten online.