Short answer
An inductive proximity sensor creates an alternating magnetic field; a conductive target perturbs that field and changes the evaluation circuit. Rated sensing distance is normally referenced to a defined steel target, so stainless alloys, aluminum, brass, copper, and thin or small parts can switch closer than the catalog reference. A larger, flat target couples more strongly than a small fastener or curved edge.
What to look for
Hysteresis is deliberate separation between the operate point and release point. After the output changes, the target must move measurably away before the sensor resets; vibration, shaft runout, or a borderline target therefore does not make the output chatter at one threshold. Flush and non-flush mounting also change the usable field and required metal-free clearance.
For false or intermittent detection, inspect target travel, alignment, stand-off, looseness, wobble, and metal chips on the sensing face. A changing sensor indicator with a stable PLC input suggests wiring, connector, or input-interface trouble; a steady indicator despite motion points to target or sensor conditions. Verify supply quality and output behavior using the approved schematic and equipment manual.
When the symptom appears in the plant
A bottling plant used an inductive proximity sensor rated for a 10mm sensing distance to count aluminum cans. However, it constantly missed counts. The technician realized that 10mm is the rating for a standard mild steel target. For aluminum, the sensing distance is reduced by a correction factor of approximately 0.4, meaning the true sensing distance was only 4mm. The sensor was mounted 7mm away. Adjusting the physical mounting closer to the cans fixed the counting issue.
Five checks before replacement
- Identify the target material; apply the manufacturer's correction factor to the nominal sensing distance (e.g., Steel=1.0, Stainless=0.7, Aluminum=0.4).
- Measure the physical distance between the sensor face and the target object in its nominal position.
- Check for hysteresis issues: Ensure the target moves far enough away from the sensor to drop below the release point (usually 10-20% greater than the sensing point).
- Verify the sensor is not flush-mounted into metal if it is an unshielded (non-embeddable) type, as surrounding metal will cause it to stay constantly on.
- Ensure the target object is physically large enough; sensing distances are based on a standard target size (typically a square piece of steel the size of the sensor face).
Never assume the sensing distance on the box applies to all metals; it is almost always specified for mild steel.
If you need to detect non-ferrous metals (aluminum, copper) at longer ranges, specify a 'Factor 1' inductive sensor which senses all metals equally.
Vibration can move brackets over time, slowly pushing targets out of the sensing range.
Bench checklist
- Confirm the supply at the device terminals.
- Confirm the meter is in the correct mode and position.
- Confirm the input range matches the signal type.
- Confirm the reference/common is intentional.
- Record the reading before changing the wiring.
Reference families
- IEC 60947-5-2 Standard
- Sensor Manufacturer Application Notes
Always verify the current edition and the exact device manual for the installation.
Official references
These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.
- IEC 60947-5-2: Proximity switches IEC
Official proximity-switch standard landing page covering sensing range and operating-distance requirements.
- Proximity Sensors Omron
Official technical guide explaining inductive sensing, eddy currents, and impedance change.
- Inductive Proximity Sensor Specifications Rockwell Automation
Official data manual covering sensing range, material correction, wiring, and selection.