7 min read
Sensor selection looks simple: check the distance, pick a type, wire it. Yet the most common cause of a line stopping at random is a sensor chosen or positioned wrongly. The problem is usually this: the catalogue sensing distance is stated for a standard steel target under ideal conditions.
| Type | What it detects | Typical range | Weakness |
|---|---|---|---|
| Inductive | Metal only | 2–40 mm | Cannot see non-metals |
| Capacitive | Metal, liquid, granules, plastic | 5–30 mm | Affected by moisture and dirt |
| Photoelectric (retro-reflective) | Anything that breaks the beam | 1–10 m | Dust and steam cut the range |
| Photoelectric (diffuse) | Objects reflecting from their surface | 10–500 mm | Range drops on dark, matt surfaces |
| Ultrasonic | Anything that reflects sound | 20 cm–6 m | Affected by temperature and draughts |
Why the catalogue range does not hold on site
An inductive sensor's rated distance is given for a standard mild-steel target. With stainless steel the range drops to about 70%, with aluminium to 40%, with brass to 50%. On a diffuse photoelectric sensor the decider is surface colour: a white glossy surface gives the catalogue range, while a matt black one can cut it to a fifth.
Flush or non-flush?
Inductive sensors fall into two groups. A shielded (flush) type can be mounted embedded in metal but has a shorter range. An unshielded (non-flush) type gives more range but requires a metal-free zone around it; otherwise it sees the surrounding metal and signals continuously. This choice must be made before the mounting position is fixed.
PNP or NPN? The PLC decides
A sensor's output type must match the PLC input card it will connect to. European systems commonly use PNP sensors with sinking inputs; Far Eastern equipment often uses NPN. Mixed, the signal simply never appears and the fault is mistaken for a failed sensor. Confirm the PLC input card type before ordering.
The environment changes the choice
- Washed-down lines need IP67 or better with a stainless body.
- In welding areas, choose a weld-field immune type resistant to spatter.
- In dusty environments, prefer retro-reflective or through-beam over diffuse photoelectric.
- On vibrating machinery, an M12 connector version instead of a cable tail prevents cable breakage.
Tell us the material to detect, the distance and the environment; we will settle the sensor type and mounting position together. We supply the Omron sensor range from stock.
Frequently asked questions
The sensor works intermittently — what could cause it?
The three most common causes: the target sitting at the edge of the catalogue range, dirt building up on the sensor face, and position drift from vibration. A fourth is electrical: a sensor cable sharing a trunking route with power cables picks up noise. Measure the distance first, clean the face next, and if that fails look at the cable route.
Which sensor suits detecting stainless steel?
An inductive sensor detects stainless, but at about 70% of its mild-steel range. Choose a model with a larger rated distance to absorb that drop. Where space is very tight, factor-1 sensors (giving the same range on all metals) are an alternative; they cost more but simplify selection.
How is transparent material detected?
A standard photoelectric sensor cannot see clear glass or a PET bottle; the light passes through. Special photoelectric sensors with transparent-object detection, or a capacitive sensor, are used instead. Ultrasonic sensors are also common here because transparency does not affect them.
Does cable length affect a sensor?
Yes, in two ways. A long cable drops voltage and can take the sensor below its supply limit. It also acts as an antenna and picks up noise, especially where it runs beside drive cables. Over long distances, increase the cross-section and use shielded cable. On analogue sensors, 4–20 mA is far more reliable than 0–10 V at length.