Proximity Sensor

What is a proximity sensor?

A proximity sensor is a sensor that detects without physical contact whether an object is present within a given distance and passes this on as a switching (on/off) signal to a controller such as a PLC. Depending on its operating principle it responds to metal (inductive), to almost any material (capacitive), to a magnet, to light or to sound. Because there is no mechanical contact, the sensor does not wear, does not spark and lasts for millions of switching cycles. The requirements for these devices are set out in IEC 60947-5-2, the proximity switch part of the IEC 60947 low-voltage series. The current edition dates from 2019 and has been adopted in Europe as EN IEC 60947-5-2:2020.

🧠 How does a proximity sensor work?

The first industrial inductive proximity switch was developed and launched by Pepperl+Fuchs in Mannheim in 1958, at the request of neighbouring BASF, to replace sparking mechanical contacts in hazardous areas. Five operating principles are now common:

• Inductive — an oscillator generates a high-frequency electromagnetic field in front of the sensing face. A metal target induces eddy currents that damp the oscillator; the electronics switch the output once the damping crosses a threshold.
• Capacitive — the sensing face forms one plate of a capacitor. A target with a higher dielectric constant than air raises the capacitance, so you can detect plastic, wood, granules and liquids, even through a plastic or glass wall.
• Magnetic — a reed contact, Hall element or magnetoresistive element responds to a permanent magnet, such as the piston magnet of a pneumatic cylinder, through non-magnetic walls.
• Photoelectric — an emitter sends out (usually red or infrared) light that returns via the object (diffuse), a reflector (retro-reflective) or reaches a separate receiver (through-beam).
• Ultrasonic — the sensor emits sound pulses and measures the echo’s time of flight. Transparent, shiny and dark objects are no problem, but directly in front of the sensor lies a blind zone in which nothing is detected reliably.

🔄 Which type of proximity sensor suits which material?

📏 What do Sn, Sr, Su and Sa mean?

IEC 60947-5-2 defines four operating distances, measured with a square standard target of 1 mm thick mild steel (Fe 360) whose side equals the diameter of the sensing face or three times Sn, whichever is greater:

Always design to Sa, not Sn. A practical rule of thumb is to let the target pass at roughly half of Sn.

Inductive sensors also need a correction factor for metals other than steel. Manufacturers quote different values, but the order of magnitude is: mild steel 1.0; stainless steel about 0.6–1.0 (typically 0.7); brass 0.35–0.5; aluminium 0.3–0.45 (typically 0.4); copper 0.25–0.45 (typically 0.3). An M18 sensor with Sn = 5 mm therefore only sees aluminium at about 2 mm. So-called factor-1 sensors detect all metals at the same distance.

🔧 Which versions and connections are available?

• Flush or non-flush — a flush (shielded) sensor can be embedded level with surrounding metal and has a shorter range. A non-flush (unshielded) sensor needs a metal-free zone around its head but reaches roughly one and a half to two times the distance. Typical Sn flush/non-flush: M8 1.5/2.5 mm, M12 2/4 mm, M18 5/8 mm and M30 10/15 mm.
• NO or NC — a normally open output switches on when a target is present, a normally closed output when it is absent. Mind the behaviour on a broken wire: with NO the controller sees “no target”, with NC it sees “target present”. Choose the contact so that a cable break fails towards the safe side.
• PNP or NPN — a PNP output switches +24 V to the input, an NPN output switches it to 0 V. PNP is the standard in Europe.
• 2-, 3- or 4-wire — a 2-wire sensor sits in series with the load and has a leakage current; a 3-wire sensor uses brown (+), blue (0 V) and black (output); a 4-wire sensor adds a second or complementary output (white).
• NAMUR — per IEC 60947-5-6, a 2-wire current signal at 8.2 V with two switching states: below 1.2 mA and above 2.1 mA. Because some current still flows in the “low” state, wire breaks and short circuits can be detected. Widely used in ATEX zones with intrinsically safe isolators.
• IO-Link — over the standard M12 connector and an unshielded cable up to 20 m, the sensor supplies not only the switching signal but, depending on the type, also the measured distance, signal quality, switching counters and temperature, and you can set switch points remotely.
• Housing and IP rating — cylindrical threaded housings in M8, M12, M18 and M30 are the norm, alongside rectangular formats. Most sensors are IP67; for high-pressure washdown in food processing you choose IP68/IP69K and a stainless steel housing.

Two dynamic properties appear in every datasheet. The switching frequency is the maximum number of switching operations per second: hundreds of hertz up to a few kilohertz for inductive sensors, considerably less for capacitive and ultrasonic types. Hysteresis is the difference between the switch-on and switch-off points, limited by the standard to 20 % of Sr and in practice usually below 15 %. It prevents chattering caused by vibration or slow-moving targets.

🏭 How do you select a proximity sensor for a conveyor?

Suppose you want to detect aluminium tray bases, 100 mm wide, on a conveyor running at 1 m/s in a washdown environment.

1. Identify the material — aluminium is a metal, so an inductive sensor is possible. Calculate with a correction factor of about 0.4.
2. Determine the distance — the tray passes at 2 to 4 mm. With a factor of 0.4 and designing to Sa (0.81 Sn), you need an Sn of at least 4 mm / (0.4 × 0.81) ≈ 12.3 mm. That points to a non-flush M30 (Sn 15 mm) or a factor-1 sensor in M18.
3. Check the speed — at 1 m/s a 100 mm tray damps the sensor for 100 ms; with an equal gap between trays that amounts to about 5 switching operations per second, comfortably within the switching frequency of an inductive sensor. Do check the PLC scan time and input filter, though.
4. Choose the output — PNP, NO, 3-wire, matching the PLC input card; or IO-Link if you want to monitor contamination and switching counts.
5. Choose the mechanics — stainless steel housing, IP69K, and an M12 connector rather than a fixed cable for quick replacement.
6. Test — after mounting, verify the switch-on and switch-off points with the real product and record the distance in the maintenance documentation.

🛠️ What are the most common faults?

With IO-Link sensors you can watch signal quality and temperature drift before the sensor fails, a simple form of predictive maintenance. Once a sensor is on a network it also becomes part of the security scope: if parameters can be changed remotely, they can also be changed by someone who should not.

🔐 Can you use a proximity sensor as a safety component?

A standard proximity sensor is not a safety component. It can stick in the “on” state, be fooled by a stray piece of metal or give a wrong signal after an internal fault. For guard door monitoring, position monitoring of hazardous movements or other safety functions, use only sensors designed to IEC 60947-5-3: proximity devices with defined behaviour under fault conditions (PDDB). These safety sensors have defined fault behaviour, often two OSSD outputs, and a certified level under ISO 13849-1 or IEC 61508. They connect to a safety relay or safety PLC, just like a light curtain. The choice follows from the risk assessment and the functional safety required by the Machinery Regulation.

❓ Frequently asked questions

What is the difference between an inductive and a capacitive proximity sensor?

An inductive proximity sensor detects only metals through an electromagnetic field and is unaffected by dirt, oil and water. A capacitive proximity sensor responds to the dielectric constant and therefore also detects plastic, paper, granules and liquids. Capacitive sensors are more sensitive to moisture and build-up and often need adjustment.

How far can an inductive proximity sensor detect?

The sensing distance of an inductive proximity sensor depends mainly on the diameter of its coil. Typical values are 2 mm for a flush M12 and 10 mm for a flush M30; non-flush and extended-range versions reach roughly double. For reliable switching, design to the assured operating distance Sa, which is at most 81 % of Sn.

Why does my proximity sensor detect aluminium at a shorter distance than steel?

A standard inductive proximity sensor is calibrated on steel. Non-ferrous metals such as aluminium and copper damp the oscillator less, so the sensing distance drops to about 30–45 % of Sn. For mixed metals, use a factor-1 sensor that detects all metals at the same distance.

Should I buy a PNP or an NPN proximity sensor?

A PNP proximity sensor suits a sinking PLC input and is the standard in Europe. An NPN proximity sensor belongs with a sourcing input and is mostly found on Asian machinery. With the wrong type no current flows through the input, so the proximity sensor never switches the PLC input.

What is the benefit of a proximity sensor with IO-Link?

An IO-Link proximity sensor supplies diagnostics such as signal quality, switching counters and internal temperature in addition to the switching signal. You can set switch points centrally, and when a sensor is replaced its parameters are restored automatically. The wiring stays the same: an unshielded three-core cable with an M12 connector.

Is a proximity sensor suitable for guard door monitoring?

An ordinary proximity sensor is not suitable as a safety component for guard door monitoring, because its behaviour after an internal fault is undefined. Use a safety proximity sensor to IEC 60947-5-3 with a certified safety level. It connects through a safety relay or safety PLC.

📌 In summary

A proximity sensor detects objects without contact or wear; the right type, the assured operating distance Sa and the material’s correction factor determine whether it works reliably. Then choose the output (PNP/NPN, NO/NC, IO-Link), housing and IP rating, and for safety functions use only sensors designed to IEC 60947-5-3.