How Inductive Proximity Sensors Work - Part 1
, by Jim Ryan, 9 min reading time
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Proxes detect metals. They ignore non-metals. How? Let's get into it.
This will be a short series of posts. This first post presents the basic physics that we need to understand proxes. It's a little bit of the electricity and magnetism that you would learn in the second semester of a college physics course. Let's break it into seven numbered chunks.
(All images below, except where noted, taken from Samuel J. Ling, et al, University Physics Volume 2, OpenStax, 2021.)
1. Consider an electric current going in a straight line in metal, such as a current running along a straight wire. The current causes a magnetic field to circulate around the wire. It just does. It's an amazing basic fact of physics. The result is like what we see in the following diagram, with "I" indicating the current and "B" the magnetic field. The magnetic field runs in circles around the wire.
Two more things about this: The strength of the magnetic field is proportionate to the amount of current in the wire. Also, the direction of the magnetic field is as shown above, not the opposite direction. There is no reason why that we know of. It's just a basic fact of physics. The field doesn't circulate in the opposite direction until you reverse the direction of the current and then it does.
Hang on a moment. We need a refresher: What is electric current? It's the flow of electrons. (They actually flow in the opposite direction of the current, but that is not important here. You can read about that strange fact on the web. It has to do with Ben Franklin.) Electrons move along the wire, causing a circular magnetic field.
2. Okay, now, let me tax your imagination. Suppose you bend the wire into a circle, a loop, so that the current runs in a circle in the loop. What would the magnetic field look like? Use your imagination to try to figure this out before looking at the image below. It's not easy! The answer is that the magnetic field would look like this:
3. Now take a long wire and bend it into a coil or "solenoid" with current running through it.
Now we have many loops stacked up into a cylinder. What happens to the magnetic field now? Look back up at the single loop and imagine what happens. It's this:
Aha! Somewhat straight lines emerge from the mouth of the solenoid. Running straight along the axis of the coil is a pretty darn straight magnetic field.
So, if the wire is bent into a coil, then the current will now cause a magnetic field to go straight through the solenoid, approximately parallel to its axis. The field will also circulate back around. An inductive proximity sensor has a solenoid in it and the magnetic field circulates back around the sides of the prox, as pictured above. This magnetic field traveling around the outside of the coil accounts for the fact that a metal mount can cause problems. The prox can "see" to the side when its magnetic field hits metal to the side of the prox. Shielding can reduce the field outside the solenoid and reduce the severity of this problem. Okay, but what about the magnetic field coming straight out of the mouth of the coil?
4. If a piece of metal is straight in front of the opening of the coil, the magnetic field will strike the metal's face. The magnetic field pierces straight into the metal. This lets the prox "see" the metal. Here's how.
The metal has electrons that are free to move about inside the metal. That's what a metal is and it is why we use metal for wires and not plastic or wood. Non-metals do not have free electrons. Theirs are all stuck to their respective atoms, whereas in a metal only some of the electrons are stuck to their atoms while others are not. So, if the magnetic field can cause the free electrons in the metal to move somehow, maybe the prox could tell that it was a metal that was present and not a non-metal. Hmm. But how?
Well, it just so happens, as a basic fact about physics, that electrons that are free to move will circulate around any changing magnetic field that happens to be in their vicinity, moving around it in a circle. But the magnetic field has to be changing in magnitude. This is Faraday's Law. So, we use an AC current created by an oscillator in the sensor's hardware. Think of it as a DC-AC converter. Being AC, the current is changing in strength at all times and its magnetic field is therefore changing in strength at all times. Then, when the magnetic field strikes the metal, the electrons, since the magnetic field is constantly changing, will move. In fact, since the current is AC, it will even cause the magnetic field to reverse its direction, really sending the electrons into a tizzy, making them run in circles and reverse directions in time with the AC. Think about a dog chasing his tail in one direction and then the other direction.
Think about this again. It's very fortunate for us because it allows us to make proxes. It's amazing. When a constantly changing magnetic field goes straight into the metal, electrons on or near the surface of the metal start circulating around the lines of the magnetic field. They just do. That's just a basic fact of physics.
The electrons circulating around like this are called "eddies" or "eddy currents". They are themselves little tiny electrical currents. Therefore, like the single wire loop we looked at, they create a magnetic field that goes straight through the axis of the eddies. (Go look at the single wire loop again.) The direction in which the eddies' magnetic field travels is opposite to the direction of the solenoid's field. That's just a basic fact of physics. Here's an image of the whole kit and kaboodle: a solenoid's magnetic field striking a square piece of metal, causing eddies that oppose the solenoid's field. (Picture adapted from Javier Garcia-Martin, et al, Non-Destructive Techniques Based on Eddy Current Testing, Sensors 2011, 11(3), 2525-2565)
Here's another good diagram of the whole system.
5. Let's stop for a moment and take a look at that. As the metal moves into view of the sensor - striking distance of the sensor's magnetic field - the free electrons in the metal are affected by the field. Electrons are caused by a magnetic field to move perpendicular to it. This is explained by Lenz's Law that electrons will continue to move perpendicular to the magnetic field as the field changes. Think about this for a moment. This means that they will go in a circle perpendicular to the field and to the solenoid's axis. But Lenz's Law tells us more. It tells us which circular direction they will go: the direction pictured above.
6. Due to the direction in which the electrons in the metal circulate, something amazing happens. First let's review what we saw. The circulating electrons - eddies - are a current. Therefore, they generate a magnetic field of their own, just like the loop of wire we saw above. But the magnetic field that the eddies generate will oppose the direction of the solenoid's field. Go look at all the pictures above to convince yourself of that. Take your time. Now, here's the thing. The electrons are causing a magnetic field of their own to go back up along the solenoid's axis in an opposite direction to the solenoid's field. The two magnetic fields mix and result in a net magnetic field of lower strength. The eddies lower the strength of the magnetic field inside the solenoid.
7. Here is where the rubber meets the road. Because the strength of the magnetic field in the solenoid is therefore changed by the electrons in the metal, the current in the solenoid is affected. You have seen that changing magnetic fields have an effect on electrons, and this is no exception. The current in the coil is affected by the change in the coil's magnetic field. This change in the current is detected by the sensor because the sensor has a device in it that detects current changes. The sensor goes into the ON state, signifying that metal is present.
Put some wood or plastic in front of the sensor and nothing happens. These materials do not have free electrons. Their electrons can't move around the solenoid's magnetic field. They can't generate eddies. The sensor stays OFF: no metal detected.
Conclusion
Let's go over that the last few points again because this is where the action is. The solenoid's magnetic field oscillates to and fro thanks to AC. This will cause the electrons in the metal to circulate in one direction now and then the other, back and forth, striving to oppose the solenoid's constantly changing magnetic field. The solenoid makes the electrons dance, as it were. The electron's circular dances are called "eddies" and they create oscillatory magnetic fields. These cause the net magnetic field in the solenoid to change, which causes the current in the solenoid's wire to change. The sensor detects that its current is being altered and goes into the ON state, signifying metal present.
Electrical engineers can read the patterns in the eddies and make sophisticated proxes in this way. For instance, they can tell whether the eddies are isolated in metal shavings and ignore them if so. At least, our sensors can do these things. Engineers have even been able to read the eddy patterns to discern whether there are cracks in the metal. See the article in Sensors mentioned above.
By the way, we have seen electric currents create magnetic fields and vice versa. This is called "induction". A current "induces" a magnetic field and vice versa. This is why they're called "inductive" proximity sensors.
The physicists who discovered the laws: Michael Faraday (1791 - 1867) and Heinrich Emil Lenz (1804 - 1865)
UPDATE: Here is Part 2 of this series.