The Sensor That Chattered on a Shiny Part
We used an inductive proximity sensor to detect a steel part on a conveyor. On the bench, it worked. On the floor, it chattered — the output flickered on and off as the part passed. The problem: the sensor’s rated sensing distance was 2 mm, but we mounted it 3 mm away. The sensor was right at the edge of its range. The part’s position variation (±1 mm) pushed it in and out of the sensing zone. We moved the sensor to 1 mm away. It detected consistently. But wait — the customer had shiny plastic parts too, and the inductive sensor only detects metal. For mixed parts, we needed a photoelectric sensor. The mistake was choosing the wrong sensor type for the application, then mounting it at the edge of the range.
Sensor selection for automation isn’t picking a part from a catalog. The sensor type (inductive, photoelectric, capacitive), the sensing distance, the output type, and the mounting all determine whether it works. This article helps you pick the right one.
The Main Sensor Types
Inductive Proximity
Detects metal objects only. Uses an electromagnetic field. No contact, no physical wear. Short range (2–15 mm for standard sizes). Good for detecting metal parts, positioning, and limit switching.
Best for: Metal parts (steel, aluminum), fixed-position detection, dirty environments (sealed). Doesn’t see non-metal parts.
Photoelectric (Through-Beam, Retroreflective, Diffuse)
Uses light (LED or laser). Detects any object that blocks or reflects light. Longer range (100 mm to several meters). Three modes:
- Through-beam: Emitter and receiver separate. The beam between them is broken when an object passes. Longest range (up to 10 m). Most reliable (doesn’t depend on surface reflectivity).
- Retroreflective: Emitter and receiver in one housing, with a reflector. The beam bounces off the reflector. An object breaks the beam. Medium range (1–3 m). Less alignment than through-beam.
- Diffuse (proximity mode): Emitter and receiver in one housing, no reflector. The light bounces off the object itself. Short range (50–300 mm). Range depends on the object’s reflectivity (shiny = far, black = short).
Capacitive Proximity
Detects any object (metal or non-metal) by changing a capacitance. Senses through non-metallic walls (plastic, glass). Short range (2–10 mm). Used for level detection (liquid in a tank through the wall) or detecting non-metal parts.
| Sensor Type | Detects | Range | Best For |
|---|---|---|---|
| Inductive | Metal only | 2–15 mm | Metal part detection, limit switches |
| Photoelectric (through-beam) | Any object | 0.1–10 m | Converters, part present, long range |
| Photoelectric (diffuse) | Reflective objects | 50–300 mm | Short-range detection, boxes, labels |
| Capacitive | Any material | 2–10 mm | Level detection, non-metal parts |
| Fiber optic | Small objects | 1–50 mm | Tight spaces, tiny parts |
Step 1: What Are You Detecting?
The object determines the sensor type.
- Metal part: Inductive (simplest, cheapest). Or photoelectric (if mixed materials).
- Box or carton: Photoelectric diffuse (or through-beam for a conveyor line).
- Liquid level: Capacitive (mounted through the tank wall).
- Clear plastic or glass: Photoelectric through-beam (diffuse doesn’t see clear objects).
- Small part (under 5 mm): Fiber optic sensor (small sensing head).
Step 2: Sensing Distance
The sensor must see the object at the actual mounting distance. Don’t mount at the edge of the rated range.
Inductive: Use 50% of Rated Range
The datasheet lists a “rated sensing distance” (Sn). Mount the sensor at 50% of Sn for reliability. If Sn = 4 mm, mount at 2 mm. The other 50% is for part variation, mounting tolerance, and temperature drift.
Mounting at the rated range (4 mm) means the sensor is right at the edge. Small variations (the part stops 0.5 mm further than expected) cause chattering. The 50% rule is standard practice.
Photoelectric: Range by Mode
Through-beam: the object must break the beam reliably. The beam width is small (a few mm). Mount the emitter and receiver aligned. If the object is small (a thin wire), use a focused beam or a fiber optic.
Diffuse: the range depends on the object’s reflectivity. A white paper reflects well (range 300 mm). A black plastic reflects poorly (range 50 mm). Test with the actual part. Don’t assume the rated range works on black parts.
The sensor mounting rule: Never mount at the rated range. Use 50% of the rated distance for inductive. For diffuse photoelectric, test with the actual part (especially dark or shiny surfaces). The sensor that chattered was mounted at the edge of its range. Move it closer, and it detects consistently.
Step 3: Output Type (NPN vs PNP)
The sensor output must match the PLC input. Two types:
- NPN (sink): The sensor switches the output to ground (0 V). Load between +24 V and the sensor output.
- PNP (source): The sensor switches the output to +24 V. Load between the sensor output and ground.
Most PLCs accept either (via DIP switch or jumper). But mismatched wiring = no signal. Check the PLC input card type and match the sensor. Don’t mix NPN and PNP on the same input card.
Also: normally open (NO) vs normally closed (NC). NO: output ON when the part is detected. NC: output OFF when detected. Most applications use NO (part present = input ON).
Step 4: Switching Frequency
The sensor must respond fast enough for the application. If parts pass at high speed (10 m/s on a conveyor), the sensor must switch fast enough to catch each one.
- Inductive: Fast (1–5 kHz). Catches parts up to high speed.
- Photoelectric: Fast (1–10 kHz). Through-beam is faster than diffuse.
- Capacitive: Slower (10–100 Hz). Not for high-speed counting.
For high-speed part counting (600 parts/minute = 10 Hz), most sensors work. For very high speed (10,000 parts/minute), use through-beam photoelectric or fiber optic.
Step 5: Environment (IP Rating and Temperature)
The sensor must survive the environment.
- IP rating: Standard sensors are IP67 (dust-tight, waterproof). For washdown, use IP69K. For oil/mist, use an oil-resistant sensor.
- Temperature: Standard sensors work 0–60°C. For ovens or freezers, use an extended-temperature sensor.
- Vibration: Sensors on vibrating equipment (vibratory feeders) need vibration-resistant models. Standard sensors can fail from constant vibration.
Step 6: Mounting and Adjustment
How the sensor mounts affects reliability.
- Bracket: Use an adjustable bracket (slot or swivel) so the sensor can be aimed. Don’t bolt it rigidly — you’ll need to adjust it on the floor.
- Target for inductive: The inductive sensor detects metal through a certain distance. If mounting through a non-metal bracket, that’s fine. If mounting through a metal bracket, the bracket itself triggers the sensor.
- Background for diffuse: A diffuse sensor sees everything in its range. If there’s a fixed background (a wall, a rail) behind the part, the sensor detects the background too. Use a through-beam instead, or adjust the sensor to ignore the background.
A Sensor Selection Checklist
- What are you detecting? (Metal, non-metal, liquid, small part?)
- Pick sensor type: inductive, photoelectric, capacitive, or fiber?
- What is the mounting distance? (Use 50% of rated range.)
- For diffuse: test with the actual part (reflectivity)?
- Is the output NPN or PNP? (Match the PLC.)
- NO or NC? (Based on the logic.)
- Switching frequency fast enough? (Part speed?)
- IP rating for the environment? (IP67 standard, IP69K washdown.)
- Temperature range?
- Is the mounting adjustable?
- For through-beam: can emitter and receiver be aligned?
- For diffuse: is there a fixed background to avoid?
The Bottom Line
Sensor selection for automation starts with what you’re detecting. Metal = inductive. Any object on a conveyor = through-beam photoelectric. Level through a wall = capacitive. Then mount at 50% of the rated range, match the NPN/PNP output to the PLC, and use an adjustable bracket. The sensor that chattered wasn’t defective — it was mounted at the edge of its range on the wrong sensor type. Move it closer, pick the right technology, and it detects every time.
