Magnetic Proximity Switches: Sensing Through Walls and Stainless Steel

An inductive sensor stops at the first piece of metal. A photoelectric sensor stops at the first opaque object. A magnetic proximity switch does neither — it responds to a magnetic field, and a magnetic field passes straight through stainless steel, aluminium, plastic, glass and brass as though they were not there.

That single property solves a class of sealing problem that nothing else addresses cleanly, and it is why magnetic switches survive in positions where other technologies are replaced every few months.

What that actually buys you

The sensor goes outside. The magnet goes inside. Nothing penetrates the wall.

Consider a pneumatic cylinder. The piston position needs to be known. An inductive sensor cannot see through the aluminium barrel. A magnetic switch mounted on the outside detects the magnet in the piston with no penetration, no seal and no wetted part. The same logic applies to a stainless steel vessel with a float inside, to a sealed gearbox, and to any hygienic enclosure where a fitting would create a cleaning problem.

There is a second benefit that matters in dirty plant: because the sensor does not need line of sight and does not care about the material between it and the magnet, dust, product build-up, paint and condensation are all irrelevant. A magnetic switch behind a layer of cement dust works exactly as well as a clean one.

The D33C range

Our magnetic proximity switches come in M12, M18 and M30 bodies, NPN three-wire, 30 mA, short-circuit protected:

• D33C-12MD-NM-3S — M12
• D33C-18MD-NM-3S — M18
• D33C-30MD-NM-3S — M30

Plus speed and direction variants, carrying an SD suffix:

• D33C-18MD-NM-3S-SD — M18 with speed and direction
• D33C-30MD-NM-3S-SD — M30 with speed and direction

The output rating is the thing to watch

This range is rated 30 mA, not the 200 to 300 mA of the D30C inductive or capacitive ranges. It is designed to feed a controller input.

Connecting one directly to a contactor coil or a relay is the single most common misapplication of this series. It will either fail immediately or, less helpfully, work for a while and then fail. If the signal must switch a load, route it through a PLC output or an interposing relay driven by something rated for it.

Speed and direction in one sensor

Detecting rotation direction requires two signals in quadrature. Traditionally that meant two sensors, mechanically aligned to a precise offset — and holding that alignment in an industrial environment, through vibration and maintenance, is genuinely difficult.

The SD variants provide both channels in a single housing, with the offset fixed at manufacture. One sensor, one mounting, no alignment to lose. For reversible conveyors, hoists, and any drive where reverse rotation is a fault condition, this is the more robust engineering answer.

Our guide to speed and underspeed monitoring covers the target geometry and controller requirements.

Where magnetic switches earn their place

• Pneumatic and hydraulic cylinder position — piston magnet detected through the barrel
• Level switching in stainless or plastic vessels — magnetic float inside, sensor outside, nothing wetted
• Hygienic applications — no penetration means no crevice and no cleaning problem
• Door, hatch and guard position — magnet on the moving part, sensor on the frame, tolerant of alignment
• Rotation and speed monitoring in wet or dusty plant where optical sensing fails
• Explosion-hazard adjacent areas where keeping electronics outside the enclosure is preferable
• Through-wall sensing generally, wherever drilling is undesirable

The constraints, honestly

You need a magnet on the target. Where one is already present — a cylinder piston, a magnetic float — this is free. Where it is not, you are adding a component, and it must be fixed securely enough to survive the application.

Ferrous material between sensor and magnet blocks it. The technology passes through non-ferrous walls. A steel wall will shunt the field. Check the wall material before designing around this.

Stray magnetic fields interfere. Near large motors, welding equipment or magnetic separators, consider whether the environment is magnetically clean.

Magnet strength and orientation matter. Sensing distance depends on the magnet, not only the sensor, and magnets have polarity. A magnet fitted the wrong way round will not be detected, which produces a puzzling commissioning fault.

Choosing between magnetic and inductive

Magnetic when you need to sense through a non-ferrous wall, when the environment is wet or dirty enough to defeat other technologies, when a magnet is already present on the target, or when you need direction as well as speed.

Inductive when the target is already ferrous metal and directly accessible, when you need higher output current to drive a load, or when adding a magnet is impractical. It is also cheaper.

Specifying

Tell us the wall material and thickness the sensor has to see through, whether a magnet is already present and if so its type and orientation, the required sensing distance, whether direction sensing is needed, and what the output feeds. For cylinder applications, the cylinder make and bore usually identifies the magnet arrangement.