The Inductive Sensor That Didn’t Trigger: Small Target Sizing, Not the Sensor
An inductive proximity sensor (M12, 8 mm rated range) detecting a 5 mm steel bolt. The sensor didn’t trigger. The bolt was too small for the sensor’s effective range. The sensor was rated for an 8 mm range on a standard target (a 12 mm steel plate). A 5 mm bolt is a small target — the effective range drops. This is about inductive sensor target sizing, not about the sensor being defective.
The target size correction factor
Inductive sensors are rated for a standard target (usually a square of steel, side = sensor diameter, e.g., 12 mm for M12). Smaller targets reduce the effective range.
| Target size (vs sensor) | Correction factor | Effective range (8 mm rated) |
|---|---|---|
| Standard (1x sensor diameter) | 1.0 | 8 mm |
| 0.5x (small bolt) | 0.5 | 4 mm |
| 0.25x (tiny pin) | 0.25 | 2 mm |
| Non-ferrous (aluminum, stainless) | 0.4-0.6 | 3-5 mm |
What we found
The M12 sensor was rated at 8 mm for a 12 mm steel target. The bolt was 5 mm (0.4x the sensor diameter). The correction factor is about 0.4. Effective range: 8 × 0.4 = 3.2 mm. The bolt was passing 5 mm from the sensor. It was outside the effective range. The sensor didn’t trigger.
What we did
1. Moved the sensor closer. We moved it to 2 mm from the bolt path. The bolt was now within the 3.2 mm effective range. The sensor triggered reliably.
2. Used a smaller sensor. We switched to an M8 sensor (4 mm rated range, smaller target required). The M8 sensor has a smaller sensing face, so small targets (like the 5 mm bolt) are detected at a higher fraction of the rated range. The effective range for the bolt was 2 mm. We set it at 1 mm. It worked.
3. Added a target flag. We welded a 12 mm steel flag to the bolt mechanism. The flag is the standard target size. The original M12 sensor at 8 mm detects the flag. The bolt itself doesn’t need to be detected — the flag does.
The material matters
Inductive sensors detect ferrous metals (steel, iron). They don’t detect aluminum, stainless (some grades), copper, or brass well. If the target is stainless 304, the range drops by 50%. If it’s aluminum, it drops by 70%. The sensor’s datasheet has correction factors for different materials. Check the target material before selecting the sensor.
The inductive sensor rule: The rated range is for a standard steel target. Small targets reduce the range. Non-ferrous metals reduce it further. For a small bolt, move the sensor closer or add a target flag. The sensor that didn’t trigger wasn’t defective — the target was too small for the rated range.
