Inductive Proximity Switches — The Workhorse of Factory-Floor Metal Detection

Inside the sensor face is a coil driven by a high-frequency oscillator. The coil generates an electromagnetic field that radiates out from the active face. When a piece of metal enters the field, eddy currents are induced in the metal — these eddy currents extract energy from the oscillator, damping the oscillation amplitude. A trigger circuit detects the damping and switches the output transistor. No moving parts, no contact wear, completely sealed against dust and oil. Different metals damp the field with different efficiencies — this is why the spec sheet lists a 'standard sensing distance' for iron and a derating factor for stainless steel, aluminum, brass, and copper.

PNP (positive-switching, the European convention) is the default everywhere in the world except some Japanese and Korean factories that standardize on NPN (negative-switching). PNP sources +V from the output wire to a sinking PLC input card; NPN sinks 0 V from the output wire from a sourcing PLC input card. Both are PLC-friendly, just wire to the correct input card type. If your existing factory uses Mitsubishi or Omron PLCs imported from Japan, check the input card — it may be NPN-sinking. For new installations with Siemens, Allen-Bradley, Schneider, or Beckhoff, default to PNP unless you have a reason not to.

Normally Open (NO) means the output is OFF when no target is present and turns ON when a target enters the sensing range. This is the everyday default — counting parts on a conveyor, detecting a cylinder home position, confirming a tool change. Normally Closed (NC) does the opposite: ON when no target, OFF on detection. NC is used for fail-safe logic — if the sensor breaks or the cable falls off, the PLC sees the same OFF state as 'target detected', so the system stops rather than continuing to run blind. Many proximity switches ship with both NO and NC outputs on separate wires so you can pick at install time.

Three-wire (the most common): three separate wires for +V, OUT, and 0V. The output transistor switches independently of the supply current, so the sensor draws constant power and the output is a clean digital signal. Two-wire: the same two wires carry both supply current and the switched output (wired in series with the load, like a contact). Simpler wiring (saves one core in the cable, useful in retrofit jobs where cable count matters), but has a residual leakage current of 1-2 mA in the OFF state that some sensitive PLC inputs interpret as ON. Specify two-wire only when you know the input handles the residual current.

Inductive: metal targets only, short range (0.6-15 mm), cheapest, most robust. Use for the 80% of factory-floor metal-detection jobs. Capacitive: detects anything (metal, plastic, liquid, wood) but sensitive to ambient humidity and moisture; use for level detection in tanks or non-metallic part presence. Photoelectric: longer range (up to 50 m), works on any visible surface; use when the target is too far for inductive or when it's non-metallic. Laser: same as photoelectric but with sub-millimeter spot size; use when you need to detect a small feature (hole, edge, thin sheet) at any range. Inductive wins on cost and robustness, but only for metal.

Inductive proximity switches have no moving parts and are fully sealed against dust and oil. Lifespan in clean indoor industrial environments routinely exceeds 100 million operating cycles — typically 10+ years of three-shift operation. Failure modes, in order of frequency: (1) cable damage at the strain relief from constant flexing, often misdiagnosed as sensor failure — replace the connector first. (2) Mechanical impact on the sensor face from a crashing tool or workpiece. (3) Solvent attack on the PUR cable jacket in degreasing baths — switch to PVC or fluoropolymer jacket for chemical lines. (4) Actual electronic failure of the oscillator/trigger circuit — quite rare.

Catalog 'standard sensing distance' (Sn) is specified for a target of standard ferromagnetic mild steel, with the sensor at room temperature. Real-world derating: assured sensing distance (Sa) is typically 81% of Sn (covers manufacturing tolerance + temperature drift), and for non-standard metals you apply further correction factors (stainless steel ~ 75%, aluminum ~ 50%, brass ~ 50%, copper ~ 40%). Design for the assured Sa with safety margin — for an M12 sensor with Sn = 4 mm, design for a target gap of 2-3 mm to stay well inside Sa.