Inductive Proximity Switch, M12

An inductive proximity switch detects a metal part without touching it. A coil in the sensing face generates an alternating field, a metal target entering that field absorbs energy from it, and the circuit that keeps the coil oscillating registers the change and operates its output. Nothing moves and nothing wears, which is why the switch is used for end-of-travel detection, counting and position sensing on machines where a mechanical limit switch would be replaced every few months.

The switches on this page are M12 threaded bodies in a nickel-plated brass housing with an ABS sensing face. Two sensing distances are offered: 2 mm on the shielded version, which can be mounted flush in metal, and 4 mm on the unshielded version, which must stand clear of it. Four output forms cover NPN, PNP, two-wire DC and two-wire AC, and the action can be normally open, normally closed or changeover.

Sealing is IP65 and the operating temperature range is -25 to +65°C. The DC versions run from 6 to 36 V, so one part covers both 12 V and 24 V control systems, and the AC versions run from 36 to 250 V.

How an inductive proximity switch works

The sensing face contains a coil that forms part of an oscillator, running at a frequency well above the switching rate of the output. When a metal target enters the field, eddy currents are induced in it and draw energy from the circuit. The oscillation amplitude falls, a detector circuit senses the change and operates the output. The switch point is set by how much energy the target absorbs, which is why the target material matters as much as the distance.

The two ratings that govern where the switch is mounted are the rated sensing distance and the setting distance. The rated distance is measured against a standard iron target of a defined size in a laboratory arrangement. The setting distance is the range over which the switch can be positioned in practice, and it is always smaller, because the surrounding metal, the supply voltage and the temperature all shift the operating point. The material supplied for these switches recommends mounting at 85% of the rated distance for that reason.

Shielded and unshielded versions differ in where the field goes. A shielded switch has a metal ring around the coil that concentrates the field in front of the face, so it can be buried in a metal surface without the surrounding metal damping it, at the cost of range. An unshielded switch has no such ring, so its field spreads and reaches further, but a metal surface beside the face will damp it and the switch will operate continuously.

How to select the right switch

Decide first how it will be mounted. A tapped hole in a metal bracket with the face flush calls for the shielded version. A position where the switch stands clear of surrounding metal can use the unshielded one and gain the longer distance. This choice is usually made by the machine design rather than by preference.

Then match the output to the control circuit. NPN suits a sinking input on a PLC or a controller; PNP suits a sourcing input. Two-wire DC is for a case where only two conductors are available in the existing wiring. Two-wire AC is for an existing AC control circuit, and its response frequency of 25 Hz is a limit worth noting: on a fast counting duty it will not keep up, whereas a DC version switches at 0.5 kHz.

Then check the load current. The output current differs by form, from 200 mA on the two-wire DC version to 400 mA on the AC version, and a load near that figure should be checked rather than assumed.

Finally set the position on the machine. The distance at which the switch actually operates depends on the target material, the surrounding metal and the supply, so the useful approach is to mount at a proportion of the rated distance and then adjust with the machine assembled rather than to set a nominal figure on the bench.

Limitations and points to confirm

An inductive switch only responds to metal. A plastic, glass, ceramic or wooden part passes through the field without being detected, and a non-metal target needs a capacitive or a photoelectric sensor instead. Where the target is metal but recessed behind a cover, detection through the cover works only while the total gap stays inside the sensing distance.

The published sensing distance is not the distance in service. The material gives stainless steel as switching at about 0.8 times the rated figure and brass at about 0.4 times, and it also recommends mounting at 85% of the rating. Those two effects multiply, so a brass target on a 4 mm switch operates nearer to 1.4 mm than to 4 mm. A mounting position taken from the catalogue figure alone is the usual reason a switch that worked on the bench fails to operate on the machine.

Two items in the supplied material are incomplete. The model table lists a three-wire AC form whose code repeats the two-wire AC code, so that form is named on this page with the discrepancy stated rather than left out. And the temperature effect on sensing is quoted twice with different tolerances depending on the range considered. Both are published as given rather than one being chosen.

The material also states that the mounting bracket is not supplied, so a bracket has to be provided by the machine builder. In a strong magnetic field, screened cable is recommended rather than the standard cable.