What is the working principle of an inductive proximity sensor?

How Does an Inductive Proximity Sensor Work?

If you've been to a factory or seen a production line in action, you may have spotted small, rectangular devices placed close to conveyor belts or robotic arms. These are proximity sensors: the inductive proximity sensor is one of the most common types. But do you know what an inductive proximity sensor actually is? It's simpler than you think! Let's go through the sensor step by step, so you can understand what makes it a great tool for detecting metal objects.

Inductive proximity sensors operate without needing light, sound, or active touching. They simply automate detection through electromagnetic induction; a principle which has stood the test of time. These sensors reliably determine the presence of metals when performing various industrial automation tasks. They prevent robotic crashes when arms are maneuvered near metal parts, and they sort and tally metal products on conveyor belts. To appreciate the value of these sensors in Industrial automation, it is important to appreciate the fundamentals of the sensors, the engineering and physics involved, and how they converts changes in a magnetic field to usable signals.

Core Components That Make It Work

The principle of operation hinges on the various parts of an inductive proximity sensor. It is a system of various parts that need to function in unison. To appreciate the system, consider the fact that there are four fundamental elements which every sensor has. These fundamental parts need to cooperate for the sensor to function.

The first component is the oscillator. Consider this the sensor's magnetic field "power source." It produces a high-frequency alternating current—typically in the kilohertz range—and the current oscillates direction rapidly. This is followed by the detection coil which is normally a loop of thin, high-quality wire (e.g., copper) coiled around a ferrite core. When the oscillator sends alternating current through the coil, the coil performs a significant function: it generates an alternating magnetic field around itself. It is like a little, invisible "bubble" of magnetism surrounding the sensor's front face.

The next stage is designing the amplifier circuit. This piece's job is to "listen" for changes in the in the magnetic field. When something disturbs that field, which we'll cover what that "something" is soon, the coil's electrical properties change too—and those changes are small. The amplifier boosts those small changes to make them stronger for the next part to process. The last part is the output circuit. When the amplified signal reaches this part, the circuit determines if there is a metal object present, it sends a signal which is normally digital (on/off) to the machine it's interfaced with to instruct a conveyor to stop, a robotic arm to move, or a counter to increase a value by one.

Chenwei Automation has designed the core components inductive proximity sensors are built with optimized. For instance, the detection coil are built with quality copper wires to provide constant and accurate detection. The sensor wouldn't function if any of these components were missing, and that's true for all of them. They are aligned with the rest of the systems.

Electromagnetic Induction: The Basics

Having covered the components, let's look at the core component that makes the whole thing work: electromagnetic induction. This concept was pioneered by an English scientist, Michael Faraday, in the 1800s, and is the principle behind the working of generators and transformers. We will look at its working principle in the context of inductive proximity sensors.

The detection coil receives high-frequency alternating current. When current is applied in one direction and then the opposite direction, the coil generates alternating magnetic fields. This is the "invisible" magnetic "bubble" that extends from the proximity sensor to the 30 mm at its most distant point when the sensor is activated. When the sensor is not activated, the field is at rest and constant. The absence of a magnetic field indicates there is a metallic object near the sensor. In this state, the oscillator is active, the coil's impedance is constant, and there are zero fluctuations.

This is where Faraday's law of electromagnetic induction is useful. When a conductor, which is a metal, is exposed a magnetic field, it will have a flowing current due to the changing magnetic field. This current is an eddy current, which is a current that flows in a spiral pattern and circles back on itself. Picture how water swirls in a puddle with eddies. They will have their magnetic field and will have the opposite polarity of that produced in the sensor's coil. This is Lenz's law and part of electrodynamics. Now, instead of a steady magnetic field, the coil will have a "pushback" magnetic field produced by the eddy current.

Changes in the opposing magnetic field influence how the detection coil operates. Do you recall the definition of coil impedance? When the opposing magnetic field interacts with the coil, impedance increases. It becomes more difficult for the coil to accommodate the alternating current sent by the oscillator, and the oscillator may weaken or slow down due to additional resistance. This phenomenon—an increase in impedance or decrease in oscillator output—is [probably] the only thing the sensor's amplifier circuit registers. It is fairly reasonable to conclude that the sensor is indicating, "There is metal object present!"

How It Detects Metal Objects (The Key Trick)

We have established that metal object sensor [ed] current [s] and disrupts the sensor magnetic field. How does the sensor translate this disruption and signal the controller to indicate "object present" and "object not present?" This is the function for the remaining circuitry and the reason inductive proximity sensors are trusted for use in the industry.Let's look at an example. Picture a device over a conveyor belt that transports metal nuts. If a nut isn't near the sensor, the magnetic field around the nut don't distort. The coil's impedance stays low, the oscillator's operational strength stays high, and the amplifier doesn't register meaningful changes. The output circuit keeps the "normal" state, and the system sends a "low" signal to the conveyor system, to tell it to continue moving.

A metal nut slides under the sensor, entering its detection range. Eddy currents are induced in the nut, and an opposing magnetic field is generated. The coil's impedance changes, and the oscillator's current diminishes. The amplifier detects the change in current (or the decreasing impedance) and enlarges it. This enlarged signal is sent to the output circuit, which is configured with a "threshold." This is a line that, when crossed, initiates an action. The threshold is crossed, and the state of the output circuit changes: it sends a "high" signal to the control system. This signal may instruct the conveyor to pause for a moment so a robotic arm can grab the nut, or it could increment a counter that keeps track of the processed nuts.

Here's something you need to know: Inductive proximity sensors only pick up on certain materials—and that's mostly metals like steel, aluminum, and copper. They won't pick up non-metals like plastic, wood, or glass at all, since those materials are weak conductive materials. In many factories, though, this is a big plus. For instance, if you're packaging metal parts in plastic bags (which are non-metal), the sensor can ignore the bag and only pick up the metal. Chenwei Automation's inductive proximity sensors are particularly good at this. They can distinguish even closely positioned metal and non-metal objects, and this prevents production from being disrupted by false signals.

Another thing to understand is that the type and size of the metal affect the detection range. A large steel plate, for instance, will be recognized from a greater distance than a small aluminum screw, since larger metal surfaces produce stronger eddy currents. However, this is rarely a problem. Most industrial sensors are designed to work with a variety of metal types, so you won't need to swap sensors every time you change the part that you want to detect.

Practical Working Logic in Industrial Scenarios

It is important to understand how the different components work in a science context in order to get an overview of the logic behind various Industrial applications in a production plant. We will utilize a few illustrations to get the working principle. This explains the popularity of the inductive proximity sensors in the field of automation.

Let's use a car parts manufacturer as an example. There may be an inductive proximity sensor made by Chenwei Automation used on an assembly line that attaches metal bolts to engine blocks. The sensor is on the robotic arm that holds the bolt. Before the arm moves to screw the bolt to the engine block, the sensor needs to confirm that the arm is indeed holding a bolt. If the arm is empty, it will just waste time spinning or possibly damaging the engine block. Here's what happens. When the arm picks up a bolt, the bolt is now within the sensor's detection range. Eddy currents form on the bolt, the coil's impedance changes, and the sensor sends a "bolt present" signal to the robot controller. The controller will then tell the arm to move to the engine block and start the screwing process. If there is no bolt, the sensor sends a "no bolt" signal, and the controller stops the arm, thus preventing a screw up.

Another example would be a packaging line for metal cans. Cans are transported on a conveyor system and need to be filled with soda and sealed. An inductive proximity sensor is affixed close to the filling nozzle. As a can gets positioned under the filling nozzle, the sensor identifies the can's metal structure and triggers the filling system to be activated. As the can is filled, the sensor detects the absence of the can and signals to the filling nozzle to stop. This system is built to detect cans and refill, ensuring no soda is wasted even during high-speed production.

Once again, the needed response time of the sensor system is in the milliseconds range. This is crucial to maintaining the desired production rates. If the sensor system took even a second to respond, it would signal a fault in the production process. To combat such issues, Chenwei Automation designs their sensors to respond in real time to the movement of high-speed robotic systems and conveyors. The most sensor technologies can be disrupted in dusty environments, but not the inductive proximity sensors. They tolerate the dirt and moisture and continue to operate under harsh conditions. The sensors only stop functioning when the magnetic fields interact within the sensor to deactivate it.

Another thing worth mentioning is that these sensors are "non-contact." This means that they can detect a metal object without having to touch it. This is an advantage over mechanical switches which touch parts that wear out over time. Inductive proximity sensors don't touch anything, which is why they can last a greater number of years, and require very little maintenance. This is important for factories that operate on a 24/7 basis because it allows for greater uptime and reduces the cost of replacements.