Inductive Proximity Sensor: How It Works & Applications
An inductive proximity sensor detects metal objects without physical contact, using a high-frequency electromagnetic field. Eddy currents induced in the target metal change the oscillator amplitude, triggering a switching output. Inductive sensors are the workhorse of machine automation — limit switch replacement on conveyors, end-of-stroke detection on cylinders, position confirmation on rotary tables.
How it works (60 seconds)
- An oscillator drives a coil at the sensor face, generating a high-frequency electromagnetic field.
- When metal enters the field, eddy currents form in the target.
- The eddy currents draw energy from the oscillator, reducing its amplitude.
- A trigger circuit detects the amplitude drop and switches the output.
- Output is typically NPN sinking, PNP sourcing, or NAMUR (analog current change for hazardous-area applications).
Key parameters
- Sensing distance (Sn) — typically 1 mm to 40 mm depending on size. Bigger sensor = bigger range.
- Reduction factor — sensing distance varies by metal: steel 1.0× (rated), stainless 0.6-0.8×, aluminum 0.4×, brass 0.5×, copper 0.4×. Always specify steel target unless stated otherwise.
- Form factor — M5/M8/M12/M18/M30 cylindrical, or rectangular block style. M18 is the most common general-purpose size.
- Output type — NPN (sinks current to common), PNP (sources current to load), NAMUR (intrinsically-safe). Match to your PLC input requirement.
- Switching frequency — how fast the sensor can detect: 100 Hz to 5 kHz typical.
- Shielded vs unshielded — shielded sensors can mount flush in metal; unshielded need a metal-free zone around the face but have longer range.
NPN vs PNP wiring
A common confusion. The right choice depends on your PLC input card.
- PNP (sourcing) sensor — when triggered, the sensor sends +24V to the PLC input. Use with PLC inputs that expect +24V high (most modern PLCs default to PNP).
- NPN (sinking) sensor — when triggered, the sensor pulls the PLC input down to 0V. Use with PLC inputs expecting 0V low (common in Asian-designed equipment, Mitsubishi, older Omron).
In North America and Europe, PNP is the default in 2026. In Japan and Korea, NPN remains common. Check your PLC input card before specifying sensors.
Common applications
- End-of-stroke detection on pneumatic cylinders
- Position confirmation on rotary indexing tables
- Speed sensing (counting rotations on a metal target wheel)
- Limit switch replacement on conveyors and gantries
- Bin level detection (metal bin walls)
- Tool break detection (tool tip vs sensor face)
- Workpiece presence on machine tool tables
Frequently asked questions
What is an inductive proximity sensor?
An inductive proximity sensor is a non-contact device that detects metal objects using a high-frequency electromagnetic field. When metal enters the field, eddy currents reduce the oscillator amplitude, triggering a switching output. Used as a replacement for limit switches on conveyors, cylinders, rotary tables, and any application where metal presence/absence needs detection without physical contact.
What is the sensing distance of an inductive proximity sensor?
Sensing distance ranges from 1 mm (M5 sensors) to 40 mm (M30 sensors), with M18 size offering 5-8 mm typical. The rated distance assumes a steel target the same size as the sensor face. Stainless steel reduces range to 60-80%, aluminum to 40%, copper to 40%. Always specify steel target unless stated otherwise. Choose sensors with Sn ≥ 2× your maximum mechanical tolerance.
What is the difference between NPN and PNP proximity sensors?
NPN (sinking) sensors pull the output down to 0V when triggered; PNP (sourcing) sensors push +24V to the output when triggered. PNP is the default in North America and Europe; NPN is common in Asian-designed equipment. Match to your PLC input card type — using NPN sensor with PNP-expecting input (or vice versa) gives no signal.
What is a shielded vs unshielded proximity sensor?
Shielded (flush-mount) sensors have the field concentrated forward; they can mount flush in surrounding metal without false triggering. Unshielded sensors radiate sideways and need a metal-free zone around the face — but offer longer sensing range for the same size. Use shielded for tight-spaced installations; unshielded when you need maximum range and have clearance.
How do I wire an inductive proximity sensor to a PLC?
Three-wire DC sensors (most common): brown to +24V, blue to 0V, black to PLC input. The black wire carries the switching signal. For NPN sensors, the PLC input expects 0V when triggered (sinking). For PNP sensors, the PLC input expects +24V when triggered (sourcing). Always check the sensor and PLC datasheets for confirmation; mis-wiring won't damage either but won't give a usable signal.