Inductive proximity sensor

An inductive proximity sensor detects metal without touching it. It is the most common sensor in automation: checking a cylinder position, counting gear teeth, detecting a metal part on a conveyor or confirming a clamp is closed.

How it works

Inside the sensor, an oscillator drives a coil wound on a ferrite core, creating a high-frequency magnetic field in front of the sensing face. When a metal object enters the field, eddy currents are induced in the metal, which take energy from the oscillator. The sensor's circuit detects that drop in oscillation amplitude and switches the output.

Because it relies on eddy currents, an inductive sensor only detects metal. Plastic, wood, glass, cardboard and liquids are invisible to it. That is exactly what the metal/non-metal sorting line on the PLC Sim Lab home page uses.

Sensing distance (Sn)

The rated sensing distance Sn is measured with a standard target: a square of 1 mm mild steel. Typical values: M8 ≈ 1.5–2 mm, M12 ≈ 2–4 mm, M18 ≈ 5–8 mm, M30 ≈ 10–15 mm. In practice you should design for about 80 % of Sn (the assured operating distance) to allow for temperature and tolerances.

Correction factors for other metals

So an M18 sensor rated 8 mm detects aluminium only at about 3 mm. "Factor 1" sensors are available that see all metals at the same distance.

Flush vs non-flush

• Flush (shielded): the face can be mounted level with the surrounding metal. Shorter range, but well protected.
• Non-flush (unshielded): the coil projects forward, giving about 1.5–2 × the range, but it needs a metal-free zone around the head.

Keep the specified spacing between two sensors facing each other or side by side, or their fields interfere.

Wiring

Most are 3-wire DC types (10–30 V DC): brown +24 V, blue 0 V, black output. Choose PNP or NPN to match your PLC input (see PNP vs NPN). 2-wire types connect in series with the PLC input like a switch, but they have a small leakage current and voltage drop, so check that your input accepts them. Most sensors have an LED that lights when they detect.

Other specs to check

• Switching frequency: how many detections per second (hundreds to a few kHz), important for counting gear teeth.
• Hysteresis: the switch-off distance is slightly larger than the switch-on distance, preventing chatter.
• Protection: IP67 is typical; IP68/IP69K for wash-down.
• Connector: M12 4-pin quick-disconnect makes replacement fast.

Troubleshooting

1. Sensor LED on, PLC input off: input COM not connected, or PNP/NPN mismatch.
2. Sensor LED never on: no 24 V on brown/blue, target too far, or target is non-ferrous at the edge of range.
3. Flickering: target at the edge of the range, vibration, or another sensor too close.

For non-metal targets use a capacitive sensor or a photoelectric sensor.