An inductive proximity sensor is a non-contact device that detects metal objects using an electromagnetic field. It is the most widely used sensor in industrial automation, and for good reason: nothing touches, nothing wears out, and dust, oil and coolant barely affect it.
How it works
1. Electromagnetic field creation
Inside the sensor is a coil driven by an oscillator. Together they generate an alternating electromagnetic field that radiates from the sensing face.
2. Metal detection
When a metal object enters that field, the changing field induces small circulating currents — eddy currents — in the surface of the metal.
3. Signal change
Those eddy currents draw energy out of the field. The oscillator has to work harder, and its amplitude falls measurably.
4. Switching output
A trigger circuit watches the oscillator amplitude. When it drops past a set threshold, the output switches — the sensor has detected metal.
The whole detection chain is electromagnetic. Nothing touches the target, so there is no mechanical wear and no moving part to fail. This is why inductive sensors routinely outlast the machines they are fitted to.
Key characteristics
- Detects metals only. Ferrous metals such as iron and steel give the longest range. Aluminium, copper and brass reduce it, typically to 30–50% of the rated figure.
- Non-contact. No physical wear on the sensing face, and no force applied to the target.
- Reliable in harsh conditions. Dust, oil, coolant and dirt generally do not affect performance, because a non-metallic film is effectively invisible to the field.
- Short sensing range. From about 1 mm on the smallest barrels to around 50 mm on the largest.
- Fast. Switching frequencies up to 200 Hz handle most high-speed production.
Sensing distance by barrel size
Range scales with coil size, which in turn scales with the barrel diameter. Typical rated distances for mild steel targets are:
| Size | Typical rated sensing distance |
|---|---|
| M8 | 2 mm |
| M12 | 4 mm |
| M18 | 8 mm |
| M30 | 15 mm |
| M50 | 25 mm |
| M80 | 50 mm |
Two adjustments matter in practice. First, non-ferrous targets reduce these figures substantially. Second, a shielded (flush-mountable) sensor reads shorter than an unshielded one of the same size, because its field is focused forward rather than allowed to spread.
Shielded or unshielded?
- Shielded (flush) — a metal ring focuses the field forward, so the sensor can be mounted flush in a metal plate without latching on. Shorter range.
- Unshielded (non-flush) — the field spreads sideways, giving longer range, but the sensor needs a clear zone of free space around the face. Mount one flush in metal and it will stay permanently on.
Common uses
- Detecting metal parts on conveyor belts.
- Position sensing in machinery — end of travel, home position, turret index.
- Counting metallic objects at speed.
- Safety interlocks and guard position confirmation in industrial automation.
A practical example: in a bottling plant where the caps are metal, an inductive sensor confirms every cap is in place before sealing — without ever touching the bottle.
When an inductive sensor is the wrong choice
If your target is not metal, an inductive sensor will never see it. Plastic, glass, liquid, wood, grain and cardboard all need a different technology — normally a capacitive sensor or a photoelectric sensor. If you need an actual distance measurement rather than a present/absent signal, use an analog proximity sensor.
Related reading
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