The Inductive Sensor That Didn’t Trigger (Small Target)
We used an inductive sensor (article 108) to detect a small screw (a Ø5 mm screw head). The sensor was rated for an 8 mm sensing distance (on a 12 mm steel target). On the floor, the sensor didn’t trigger (the screw was too small). The problem: the sensing distance depends on the target’s size. The sensor’s rating (8 mm) is for a target that’s 3× the sensor’s diameter (e.g., 36 mm). A small target (Ø5 mm) has a reduced range (the sensor only sees it at 1 mm, not 8 mm). We either moved the sensor closer (1 mm) or used a bigger target (a steel flag, Ø12 mm). We moved it closer (1 mm). The sensor triggered. The mistake was using the rated range (8 mm) on a small target (the range was reduced).
Inductive sensor range vs target size — the target matters. This article covers the derating.
The Rated Range (Sn)
An inductive sensor (article 108) has a rated sensing distance (Sn, e.g., 8 mm). But that’s for a standard target: a steel square, 3× the sensor’s diameter (e.g., 36×36 mm, 1 mm thick). A smaller target (or a different material) has a reduced range.
Step 1: The Target Size (Derating)
A small target (e.g., Ø5 mm) has a reduced range. The sensor only sees it at a fraction of the rated range (e.g., 1 mm, not 8 mm). The derating depends on the target’s size (vs the sensor’s diameter). For a target that’s 1× the sensor’s diameter (e.g., Ø12 mm target on a M12 sensor), the range is ~50% (4 mm). For a smaller target (Ø5 mm), it’s ~10% (0.8 mm).
The range rule: The rated range is for a 3× standard target. A small target has a reduced range. The sensor that didn’t trigger was rated 8 mm on a Ø5 mm screw (only 1 mm real). Move it closer (1 mm). Or use a steel flag (bigger target).
Step 2: The Material (Steel vs Aluminum)
The material matters. Steel (ferromagnetic) has the full range. Aluminum (non-ferromagnetic) has a reduced range (~50%). Stainless (304) is less (~70%). For an aluminum target, derate the range (by 50%). For a steel target, full range.
Step 3: The Approach (Face-On vs Side)
The sensor detects the target face-on (the target’s flat face, to the sensor’s face). A side approach (the edge of the target) has a reduced range. Mount the sensor face-on (the target’s face, not edge).
Step 4: The Flag (Bigger Target)
For a small part (a screw, a small pin), add a flag (a steel plate, mounted on the part). The flag is bigger (e.g., 20×20 mm). The sensor sees the flag (full range). The part (small) triggers the sensor (via the flag). Don’t rely on a small part (use a flag).
| Target | Range (vs rated) |
|---|---|
| Standard (3×, steel) | 100% (rated) |
| 1× (small, steel) | ~50% |
| Ø5 mm (very small) | ~10% |
| Aluminum | ~50% (derate) |
| Flag (steel plate) | 100% (bigger target) |
A Range Checklist
- What is the rated range? (Sn?)
- What is the target size? (mm?)
- Is it 3×? (Standard?)
- Is it small? (Derate?)
- Is the material steel? (Or aluminum?)
- Is it face-on? (Not edge?)
- Is there a flag? (For a small part?)
- Does it trigger? (Test?)
- Is the gap right? (Closer?)
- Is the repeatability good? (Gauge?)
The Bottom Line
Inductive sensor range vs target size derates the range. The sensor that didn’t trigger was rated 8 mm on a small screw (1 mm real). Move closer (or add a flag). Derate for the material (aluminum). The sensor that triggered wasn’t the most sensitive — it was matched to the target.
