Product type
A proximity sensor detects that an object has arrived at a position, without touching it and without a mechanical lever to wear out. The sensing element sits behind a face, energised continuously, and the output changes state when a target comes within range. Because there is no contact, the sensor does not wear, it needs no force to operate, and it switches far faster than a limit switch driven by a physical lever.
A proximity sensor detects that an object has arrived at a position, without touching it and without a mechanical lever to wear out. The sensing element sits behind a face, energised continuously, and the output changes state when a target comes within range. Because there is no contact, the sensor does not wear, it needs no force to operate, and it switches far faster than a limit switch driven by a physical lever.
The great majority of these are inductive: they generate an alternating magnetic field at the face and detect the eddy currents induced in a metal target. That makes them reliable in oil, coolant, dust and washdown, where an optical device would struggle, but it also means they respond only to metal, and to different metals at different distances. Capacitive types detect almost any material, including plastic, liquid and powder, at the cost of being easier to trigger by accident.
The selection question is therefore rarely which sensor in the abstract. It is what the target is made of, how close the face can physically get to it, how fast the target passes, and what the surrounding metalwork does to the field. This page works through those, and marks the boundary against the neighbouring sensor families that are often reached for instead.
An inductive sensor drives an oscillator whose coil is wound around a core behind the sensing face. When a conductive target enters the field, eddy currents circulate in it and damp the oscillation; the sensor detects the change in amplitude and switches its output. Nothing is measured about the distance itself - the output is a yes or a no, at a point set by the internal threshold.
The damping depends on how well the target conducts and how magnetic it is, which is why the rated sensing distance is quoted against mild steel and reduced for everything else. Aluminium and copper damp the field at a shorter distance, and the reduction factors published by the manufacturer express exactly this. A target thinner than the specified minimum, or one that is plated, behaves as neither.
Because the field extends beyond the face in all directions, a sensor can also be triggered by something other than its intended target. Two units mounted face to face or side by side can interfere, and an unshielded sensor embedded in metal has part of its field absorbed by the surrounding housing. Mounting rules are therefore part of the measurement, not an installation detail.
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
|---|---|---|
| Sensing principle | Inductive, capacitive and magnetic types | Which materials can be detected at all, and how forgiving the sensor is to stray targets and to its surroundings. |
| Rated sensing distance | Quoted against a standard mild-steel target, from a few millimetres upward | The air gap the installation can tolerate. It is the starting figure rather than the working figure, because material and mounting both reduce it. |
| Shielded or unshielded | Shielded types can be mounted flush in metal; unshielded types reach further but need a clear surround | Whether the sensor can be embedded into a bracket or has to stand proud of it. This is decided by the mechanical drawing, not by the sensing requirement. |
| Housing form and size | Threaded cylinders in common metric sizes, and rectangular or block forms | Whether it fits the space available and can be adjusted and locked in position after setting. |
| Output type | PNP, NPN, two-wire, three-wire and four-wire, in normally open or normally closed form | How the sensor connects to the input card and what the controller sees when the target is absent. A mismatched output polarity is one of the commonest reasons a new sensor appears not to work. |
| Connection | Integral cable, or a connector in a common industrial size | Whether the sensor can be unplugged for replacement without rewiring, which matters on a machine that carries many of them. |
| Supply and switching frequency | The operating voltage, and the maximum number of switching operations per second | Whether the sensor keeps up with the machine speed. A fast target can pass through the field and out again between two switching cycles. |
| Environment | Ingress protection, weld-spatter resistance, and immunity to the fields of nearby welding equipment | How long the sensor survives where it is mounted, which on a welding cell can be measured in weeks for an unsuitable type. |
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|---|---|
| Target material | It sets the real sensing distance, because a target that is not mild steel triggers the sensor at a shorter gap than the published figure | State the target material, and its plating or coating if it has one. |
| Target size and thickness | A target smaller or thinner than the reference reduces the distance further | Give the smallest dimension of the target along the sensing axis, not only its area. |
| Mounting constraint | Flush or non-flush, and how much metal sits around the sensor | Send a sketch of the bracket and of the sensor position relative to neighbouring metalwork. |
| Switching speed against machine speed | The sensor has to resolve the fastest pass and the shortest dwell | Give the line speed and the time the target remains in front of the sensor. |
| Distance to other sensors | Adjacent sensors can trigger each other | State how close the sensors will be, and whether they can be alternated or shielded. |
| Ambient conditions | Coolant, swarf, washdown, dust, weld spatter and ambient temperature all act on the sensor face | Describe the cell the sensor sits in, not the plant in general. |
| What the signal drives | The input card voltage and polarity, and the logic the program expects | Confirm the input type before ordering, so that the output type is fixed by something other than preference. |
The rated sensing distance is a laboratory figure. The working distance is what remains after the target material, the target size and the mounting have each taken their share, so it should be treated as an upper bound rather than as a setting.
These are the types an enquiry for a proximity sensor usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a proximity sensor |
|---|---|
| Photoelectric sensors | Optical, so they detect almost any material at distances an inductive sensor cannot reach, and they can distinguish light from dark or detect a transparent object. They need a clear line of sight, and they suffer in dust, mist and washdown, which is precisely where an inductive sensor is unbothered. |
| Position sensors | Report where something is, often as a continuous value over a travel, rather than reporting that it has reached one point. Where the control system needs the actual position, a proximity switch supplies only the end-of-travel confirmation. |
| Displacement sensors | Measure a small movement as a continuous distance, which is what a run-out, a deflection or a thermal expansion check needs. A proximity sensor collapses that information into a single threshold, so it can confirm a limit but cannot report how far away the part is. |
| Level sensors | The same physical principles pointed at a material surface rather than at a machine part, and packaged for tanks and hoppers. A proximity sensor inside a hopper is a point-level switch; where a continuous level is wanted, the level family is the right one. |
| Speed sensors | Count the rate at which targets pass and output a frequency proportional to speed. A proximity sensor can supply the pulses, but it cannot report the speed itself - that conversion happens in the controller or in a dedicated speed sensor. |
| Other industrial sensors | A basket category for duties that no specific family covers, such as magnetic cylinder switches or safety-rated interlocks. Where an application carries a certification requirement, a general-purpose proximity sensor is not a substitute for it. |
The two families most often confused here are inductive and photoelectric. The choice usually settles itself on one question: is the target metal, and is the environment dirty enough to blind an optical sensor?
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
Available products
LJ12A3 · Proximity Sensors
M12 inductive proximity switches for non-contact metal detection, in shielded 2 mm and unshielded 4 mm versions with NPN, PNP, two-wire DC and two-wire AC outputs.
Also in Industrial Sensors
Every type we quote for under Industrial Sensors. A type with no link yet is still quoted for - it is described on the Industrial Sensors overview, and gets a page of its own as soon as a model is published against it.
Frequently asked
The published figure is measured against a standard mild-steel target of a set size. A different material reduces the field damping, a smaller or thinner target couples less of it, and mounting the sensor in or near metal absorbs part of it. All three reduce the working distance, and they compound rather than simply add.
They can, but their fields interact and one may hold the other switched. The safe separation depends on the sensor size and on whether the types are shielded, and it is wider for unshielded types. Where the spacing is fixed by the machine, alternating the sensors or choosing a shielded version are the usual answers.
Inductive types respond to metal only; capacitive types respond to almost anything, including plastic, glass, liquid and powder, because they sense a change in capacitance rather than in a magnetic field. Capacitive sensors are more easily triggered by build-up on the face and by humidity, so they are chosen where a non-metal target has to be detected and an optical sensor cannot see it.
It needs an input that matches its output type. A PNP sensor requires a sinking input and an NPN sensor a sourcing input, and the two are not interchangeable without a relay or an interface module. This is worth confirming before ordering, because a polarity mismatch produces a sensor that is correctly installed and visibly does nothing.
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