What is an inductive sensor and how does it work?

An inductive sensor is a non-contact proximity sensor that forms the foundation of industrial automation. It is used to precisely detect objects made of metals – both ferromagnetic (such as steel) and non-ferromagnetic (such as aluminum or copper). Its operation is based on advanced electromagnetic phenomena, which makes it extremely resistant to harsh environmental conditions. Because the sensor does not need to physically touch the detected object, it is not subject to abrasive wear, allowing uninterrupted operation over millions of cycles.

The detection process step by step:

• Generating the magnetic field: The heart of the sensor is a high-frequency generator (LC oscillator). The powered coil induces a variable electromagnetic field that penetrates through the sensor face and creates the so-called active detection zone in front of the sensor.
• Inducing eddy currents: When a metal object comes within range of the magnetic field, so-called eddy currents (Foucault currents) begin to flow in its volume, in accordance with Faraday's law of induction. Their intensity depends on the electrical and magnetic properties of the material.
• Damping the oscillations: Eddy currents generate their own magnetic field, directed opposite to the sensor's field. This causes a forced extraction of energy from the oscillator, resulting in a noticeable decrease in the amplitude of its oscillations.
• Signal processing and switching: The sensor's electronic circuit (demodulator and comparator) monitors the amplitude of the oscillator's oscillations in real time. When the amplitude falls below a set threshold, the comparator changes the sensor's output state, informing the PLC controller of the object's presence.

Construction of a typical inductive sensor

A modern sensor is a precision electronic circuit that, despite its small size, must withstand extreme working conditions. Its construction consists of key functional blocks:

• Housing: Made of nickel-plated brass, stainless steel or specialized plastics. It serves not only as a mounting frame but also as an electromagnetic shield that protects the internal electronics from interference.
• Measuring (active) face: The point of magnetic field emission. In industrial versions, this element is made of materials with high impact and thermal resistance, which prevents failures resulting from accidental impacts by the object.
• Coil and ferrite core: A copper winding placed on a ferrite core. This core acts as a magnetic "lens" – it concentrates the field flux in a specific direction, giving the sensor its desired directional characteristic.
• Oscillator (LC generator): A circuit responsible for generating a continuous high-frequency electromagnetic wave (typically between 100 kHz and 1 MHz).
• Signal demodulator: A block that filters out noise and converts the variable oscillation amplitude into a stable voltage signal subject to further analysis.
• Comparator (threshold circuit): This is the sensor's "brain," comparing the signal from the demodulator with a reference value. Thanks to hysteresis (the difference between the switch-on point and the switch-off point), the comparator prevents oscillation of the output signal when the object is located exactly at the boundary of the range.
• Output stage: A transistor circuit (PNP or NPN) that acts as a switch in the control circuit. High-quality NEXOTEC sensors are equipped with advanced protections: short-circuit protection (irreversible damage to the sensor in case of an output short to ground is excluded), reverse polarity protection, and surge filters that protect the electronics from high-voltage pulses occurring in machines when switching large inductances (e.g., motors).

Technology choice: Decision table

Choosing the right sensor is not just a matter of catalog parameters, but above all a matter of anticipating the conditions in which the device will operate over the coming years. The table below will help you decide based on critical application factors.

Key tip for the maintenance engineer:

Remember the safe zone rule Sa. Never design an application in which the sensor operates at the edge of its range (close to Sn). Always assume that the actual, stable operating range is about 80% of the rated value. This eliminates the problem of false signals caused by machine vibration, temperature changes, or slight contamination of the sensor face.

PNP or NPN – which inductive sensor to choose for a PLC controller?

The choice between a PNP and NPN sensor is one of the most important design decisions. It determines whether the sensor's signal "supplies" power to the controller's input or "shorts" the input to ground (0V). Understanding the difference is key to proper communication with PLC input modules.

What is the technical difference?

• PNP sensor (Source): At the moment of detection, the sensor applies the positive supply potential (+24V) to the output wire. This is the standard in most modern industrial installations in Europe and North America.
• NPN sensor (Sink): At the moment of detection, the sensor shorts the output wire to ground (0V). Popular mainly in older installations, in Asia, and in specific applications where the control logic requires low-state activation.

Why is a given type chosen?

The choice is usually dictated by the PLC controller architecture and safety:

• PNP – Safety first: In a PNP system, if the signal wire is accidentally worn through and touches the machine's metal structure (grounded), a false input activation will not occur. In an NPN system, such a short to ground would be interpreted by the controller as an "object detected" signal, which can be dangerous for machine movement.
• NPN – Legacy and specific standards: NPN systems are often found in very old imported machines or in situations where specialized, cheap sensors with simplified transistor construction are used.

Comparison and wiring

How to wire it to a PLC?

The wiring depends on the configuration of the controller's input card:

• To a PNP PLC (standard): The sensor is connected with brown to +24V, blue to 0V, and black (signal) to the PLC input. The PLC input must share a common ground with the sensor's power supply.
• To an NPN PLC: The sensor is connected with brown to +24V, blue to 0V, and black (signal) to the PLC input. In this case, the PLC input "expects" a short to ground, meaning the common of the controller's input card must be connected to +24V.

Important note: Some modern PLC controllers have Universal-type input cards that automatically recognize whether the signal comes from a PNP or NPN sensor, but when designing new machines it is always recommended to keep one standard (currently almost exclusively PNP) for the entire installation.

Advanced parameters of inductive sensors: Understanding the physics of operation

Beyond range (Sn, Sr, Sa), professional sensor selection requires analysis of dynamic and environmental parameters. Understanding their influence allows you to eliminate errors that most often lead to emergency stoppage of the production line.

Hysteresis (H) – switching point stability

Hysteresis is the difference between the distance at which the sensor switches on (object approaching) and the distance at which it switches off (object receding).

• How it works: The electronics deliberately introduce a "dead zone" to avoid relay or PLC input chatter when the object vibrates minimally near the detection limit.
• Advantages and disadvantages: Too small a hysteresis causes signal "flickering" in an unstable setup. Too large can prevent precise positioning.
• When to choose: In applications with vibration (e.g., vibratory conveyors), choose sensors with pronounced hysteresis (about 10-15%).

Switching frequency (f) – detection dynamics

This is the maximum number of detection cycles (on and off) per second.

• What it depends on: Mainly on the coil's inductance and the performance of the demodulation circuit. The larger the sensor (larger coil), the generally lower the switching frequency.
• Advantages and disadvantages: A high "f" allows counting very small objects on fast lines (e.g., bottles on a belt). The downside is greater sensitivity to electromagnetic interference in the surroundings.
• When to choose: At belt speeds above 1-2 m/s, always check whether "f" is sufficient to register the object.

Temperature drift – stability under difficult conditions

A parameter determining how much the detection range changes depending on the ambient temperature.

• Why it occurs: A rise in temperature changes the resistance of the coil windings and the magnetic parameters of the ferrite core.
• Advantages and disadvantages: A cheap sensor can change its range by 20% with a 50°C temperature change, which in precision systems means a detection error. Premium-class sensors have temperature compensation systems.
• When to choose: In applications near furnaces, injection molding machines, or cold stores – where temperature changes dynamically during machine operation.

Influence of supply voltage (Voltage tolerance)

Voltage fluctuations in the plant network affect the magnetic field generated by the coil.

• Technical significance: Most industrial-class sensors support a range of 10-30V DC. A stabilized output means fluctuations in this range do not affect the constant detection range.
• What it affects: Unstable power supply in low-quality sensors causes the sensor's "switching point" to float along with voltage drops (e.g., when starting a motor).

Comparison table of specialized parameters

How to effectively diagnose and solve problems with inductive sensors?

Maintenance is a race against time. To effectively solve sensor problems, it is worth adopting a structured approach that excludes random actions. The following methodology allows you to quickly find the source of the fault.

Step-by-step troubleshooting algorithm

• Step 1: Visual verification (Checklist): Check the physical condition of the sensor. Is the face not damaged, cracked, or coated with metal chips? Is the cable not worn or crushed in the cable guide? Often this simple mechanical damage is the cause of the failure.
• Step 2: LED check: Most sensors have a status LED. If the LED does not light despite the presence of metal, the problem is the power supply. If it lights but the controller does not respond, the problem is the signal connection or a logic mismatch (PNP/NPN).
• Step 3: Multimeter test: Use a multimeter according to the diagnostic procedure to check whether the sensor is actually supplying voltage at the output. If the sensor "reacts" (LED lights) but the multimeter shows 0V at the output, this indicates damage to the internal output transistor.
• Step 4: PLC input verification: If the multimeter confirms the sensor is working and the LED on the PLC input card does not light, the fault lies with the controller's input card, an interrupted signal wire, or a poorly tightened terminal block.

Matrix of most common problems and solutions

Golden rules of prevention – how to avoid failures in the future?

• Mounting with margin: Never mount the sensor at a point where the object "rubs" against the face. Keep a minimum safe distance of 1-2 mm.
• Cable isolation: Always route sensor signal cables (low voltage) in separate ducts from motor power cables (400V/inverters).
• Careful tightening: When mounting threaded sensors, use a torque wrench or tighten by hand – tightening force exceeding the manufacturer's recommendations damages the housing and seals.
• Avoiding loops: Coil overly long cables into a figure-eight shape, not a tight loop – this avoids creating additional inductance that can pick up interference from the surroundings.

Inductive sensors in practice: Scenarios, substitutes and "real-life" solutions

In industrial automation, theory often collides with reality. Below are examples of problems encountered on production floors and strategies for dealing with them when an inductive sensor fails or when a different approach is required.

"Real-life" scenarios and how to solve them

Problem: "The sensor works when the machine is cold, but loses signal when it heats up."

Diagnosis: Temperature drift or thermal expansion of machine components. A metal object may, under the influence of temperature, minimally change its position or magnetic properties (though the latter to a lesser degree).

Action: Check whether you are at the Sa limit (safe range). Reduce the mounting distance by 0.5 mm or replace the sensor with a model of higher temperature stability class.

Problem: "Metal filings keep settling on the sensor and causing false triggers."

Diagnosis: The sensor's magnetic field attracts filings. This often happens on lathes and milling machines.

Action: Use a sensor with a "factor 1" function or, if that does not help, consider mechanical shielding (e.g., making a plastic cover that separates the sensor from the flow of chips but lets the magnetic field through).

Problem: "The sensor gets mechanically damaged every 2 weeks."

Diagnosis: The object hits the sensor during feeding.

Action: If you cannot change the object's path, replace the inductive sensor with an all-metal sensor or hide it behind a Teflon (PTFE) plate. Teflon is transparent to the magnetic field and very resistant to impact and high temperature.

What to replace an inductive sensor with? (Alternatives)

• Capacitive sensors: Detect everything (plastic, metal, liquid, wood). When to use: When you need to detect the presence of a food product (e.g., water bottles) or liquid level through a tank wall.
• Photoelectric (optical) sensors: Detect at much greater distances (up to several meters). When to use: When the object is non-metallic or when you need to detect an object from a distance an inductive sensor will never reach.
• Ultrasonic sensors: Insensitive to the color or transparency of the object. When to use: Ideal for measuring liquid levels or detecting objects with very different shapes and materials (e.g., film, glass, metal).
• Laser (triangulation) sensors: Extreme precision (down to microns). When to use: When you need to verify that an object is correctly seated in its socket (presence/position check).

Alternative selection matrix

What can you do yourself as "preventive" measures?

• Regular inspection: Add cleaning sensors with compressed air once a week to the maintenance (PM) schedule.
• Inventory management: Always keep at least 1 spare sensor in stock (of the same type/connector).
• Documentation: Take a photo of the correct mounting (distance from the object) and paste it in the control cabinet. This saves service technicians a lot of time at night or during weekend shifts.
• Log analysis: If you have a PLC controller with an error logging function, set alerts for too-frequent switching – a failure is often preceded by several days of unstable operation.

Diagnosing an inductive sensor with a multimeter – a complete engineering guide

Multimeter diagnostics is the fastest method of verifying a sensor without needing to disconnect it from the machine. This procedure will allow you to rule out a sensor fault and focus on the PLC controller or wiring.

Preparing for measurements

For correct diagnostics you need a multimeter set to the DC voltage range (DC V). If the sensor is mounted, it is most convenient to work directly on the M12 connector.

• Pin 1 (Brown): +24V DC supply.
• Pin 3 (Blue): Ground (0V).
• Pin 4 (Black): Signal output.

Step 1: Verifying power supply (First link in the chain)

If the sensor has no power, it will not work. Place the multimeter probes on pins 1 and 3:

• Correct reading: 20V - 30V DC (typically 24V).
• No voltage: Check the power supply, the input circuit fuse, or the cable continuity (a break in the power wires).

Step 2: Testing the signal output – the "golden method"

This is the most important step. You need to know whether the sensor is PNP or NPN:

• For PNP type (most common in Europe): Place the black probe (-) of the multimeter on pin 3 (ground). Place the red probe (+) on pin 4 (signal). Test: With no metal present it should be 0V. When metal is brought to the sensor face, the voltage should jump to the supply level (~24V).
• For NPN type (popular in Asian machines): Place the red probe (+) on pin 1 (+24V). Place the black probe (-) on pin 4 (signal). Test: With no metal present it should be 0V. When metal is brought close, the voltage should jump to ~24V (between the positive and the signal).

Step 3: Verifying NO/NC logic

If, after performing the above tests, the sensor works in reverse, check whether you have mixed up the NO/NC type:

• NO (Normally Open): The signal only appears after metal is brought close.
• NC (Normally Closed): The signal is always present and disappears when metal is brought close.
• Note: If you have an NC sensor, at rest (no metal) the multimeter should show 24V, and 0V when metal is brought close.

Step 4: Testing under load ("Live" diagnostics)

Common mistake: the sensor works "dry" (measurement without a connected PLC input), but after connecting, the signal disappears. Why?

• Problem: The sensor's output transistor is damaged. "Dry" shows the correct voltage but lacks the current capacity to drive the PLC input (the so-called "weak output").
• Test: Connect a small LED or a load resistor (~1-2 kOhm) in series with the multimeter. If, when the load is connected, the voltage drops sharply to zero – the sensor needs to be replaced.

Most common diagnostic pitfalls

• Ground loop: If you measure voltage relative to the machine housing instead of pin 3, you may get false readings. Always measure directly on the sensor pins!
• Electromagnetic induction: If the cable runs near a frequency inverter, you may see a "floating" voltage on the multimeter (e.g., 4V-8V) despite the absence of metal. This is a sign that the cable is picking up interference – replace it with a shielded one.
• Contamination: If the multimeter shows 24V even without metal, check whether a magnetic chip has stuck to the sensor face.

The physics of detection: What happens "under the hood" of the sensor?

Understanding that an inductive sensor is not a simple switch but an advanced radio device (operating in the kHz range) changes the approach to diagnostics. The sensor generates an electromagnetic field that is "damped" by eddy currents in the metal. If the object is made of a material with a different magnetic permeability than steel (e.g., aluminum), the damping is weaker, requiring a range correction by the engineer.

What can surprise you in the field?

• Effect of object thickness: Thin sheets (below 1mm) may not dampen the field enough to trigger the sensor. If the object is too thin, the sensor "won't notice" it even if it is physically within range.
• Travel speed: With very fast objects (e.g., in nut-counting systems), the sensor may have trouble "noticing" the object if the switching frequency (f) is too low.
• Face temperature: A heated PBT (plastic) face can soften, causing the face to become "dented" inward when hit by a chip, permanently changing the device's operating parameters.

Diagnostics: Methods you won't find in the manual

Beyond standard voltage measurement, maintenance engineers use:

• Oscilloscope test: If the sensor is "going crazy," connect an oscilloscope to the signal wire. Look for voltage "spikes." If you see high voltage jumps at the moment nearby contactors switch on, you have proof of EMC interference, not a sensor fault.
• Insulation resistance test: In humid environments (washdowns, food industry), the cable may have "leakage." An insulation meter will reveal whether moisture is causing current flow between the wires, which manifests as unstable sensor operation.
• Checking power supply "noise": If the 24V DC supply has a lot of ripple (e.g., a damaged capacitor in the supply), the sensor may "float" – that is, change its range in real time.

What can surprise you when working with sensors?

• Influence of surrounding metal: Mounting the sensor in a "deep hole" in a steel plate causes the sensor to "see" the walls of the hole. This drastically reduces the detection range of the actual object. Always leave clearance!
• Motor magnetic fields: Large electric motors (especially with direct starting) generate strong stray fields. If the sensor is too close to the motor housing, it can generate false signals.
• Cables in cable guides: Bending the cable thousands of times a day causes the copper inside the insulation to crack. Always use cables with increased flexibility (High-Flex).