Inductive Sensors Selection Guide

An inductive sensor is a proximity sensor that detects metal objects entering its electromagnetic field without contact. Sensing distance is chosen by housing and output type.

What is an inductive sensor? An inductive sensor is a proximity sensor that detects metal objects entering the electromagnetic field it generates, without any contact. It does not detect non-metallic objects; for plastic, wood or liquids, a capacitive or photoelectric sensor is used. On machines it is used for position, counting and speed detection.

This guide explains what to look at when choosing products in the Inductive Sensors category.

How to read the sensing distance

The distance in the product name (such as 2 mm, 8 mm or 15 mm) is the rated sensing distance (Sn). In the definition based on EN 60947-5-2, this value is measured with a standard target: a square carbon steel plate 1 mm thick. The side of the plate is equal to the diameter of the sensor's active face or three times Sn, whichever is larger. Three practical consequences follow.

• Sn does not include the manufacturing tolerance (±10%) or the drift caused by temperature and supply voltage. The target must stay within the safe operating distance (Sa) given in the datasheet; one manufacturer's terminology guide gives half of Sn as the best operating point.
• If the target is not steel, the distance is shortened by a factor. For an M12 series in the same guide, the factors are 1.00 for steel, 0.70 for stainless steel, 0.40 for brass, 0.30 for aluminum and 0.25 for copper. The factors vary noticeably from series to series; check the datasheet of the model you choose.
• If the target is smaller than the standard plate, the distance drops again. If small parts such as screw heads or pins are to be detected, leave a distance margin accordingly.

Housing size and head type

Cylindrical housings are referred to by their thread size and have separate subgroups: M8 inductive sensors, M12 inductive sensors, M18 inductive sensors and M30 inductive sensors. The larger the housing, the greater the sensing distance; in this category, values such as 1.5 mm and 2 mm are seen on M8 housings and 10 mm and 15 mm on M30 housings.

For tight spaces there are ultra-mini models with 4 mm, 5 mm and 6.5 mm housings, square-type inductive sensors for screw mounting on a flat surface, and rectangular housings with an 8 x 8 mm cross-section. In the steel-head group the sensors have flat heads, and their switching frequencies are 100 Hz, 200 Hz and 300 Hz.

For a non-flush sensor, the metal-free zone to be left around the head and the minimum spacing between two sensors mounted side by side are given in the datasheet; if these distances are not observed, the sensor is affected by the surrounding metal or by the neighboring sensor. A non-flush sensor can be embedded in a non-conductive material.

Output type, number of wires and connection

• DC 3-wire: it has two supply leads and one output lead. With NPN, the load is connected between the output and positive; with PNP, between the output and negative; choose the one that matches the connection of the PLC input card. In the common color code, brown is positive, blue is negative and black is the output.
• DC 4-wire (NO+NC): the same sensor has one normally open and one normally closed output.
• DC 2-wire: DC 2-wire inductive sensors are connected in series with the load like a mechanical contact and can be used in both NPN and PNP circuits. In return, a small leakage current flows in the off state and the voltage drop across the sensor is higher when conducting; in the terminology guide based on EN 60947-5-2, unless otherwise stated, this value is given as below 8 V for 2-wire DC models and below 3.5 V for 3-wire DC models.
• AC 2-wire: AC 2-wire inductive sensors are listed with supply ranges such as 20-250 V AC and 90-250 V AC. They are connected in series with the load and must not be connected directly to the supply without a load; check the load current limit in the product data.
• Analog: analog inductive sensors give a 0-10 V or 0-20 mA signal that varies with distance; they are used when a position measurement is needed instead of presence/absence information.

Cable models mostly come with 2 m of cable, and connector models come with an M8 or M12 plug. The suffixes in the code are read differently depending on the brand: in the Lanbao names in this store, -E1 indicates an M8 and -E2 an M12 connector model, whereas in Isıso names, -E0, -E1, -E2 and -E3 correspond to the output type (NPN NO, NPN NC, PNP NO, PNP NC) and connector models carry the letter C at the end; verify the connector size on the product page. Mating plugs and pre-assembled cables are under Sensor Connectors and Cables.

Switching frequency

The frequency value shows how many times per second the sensor can detect and release, and it is measured by passing standard targets at a distance of half of Sn. In this category there are values such as 1.5 kHz and 2 kHz for M8 housings, 300 Hz and 500 Hz for M30 housings and 20 Hz for AC 2-wire models. When counting revolutions on a gear, compare the number of teeth passing in front of the sensor per second with this value.

When an inductive sensor is not enough

• If plastic, glass, liquid or granular material is to be detected, use Capacitive Sensors.
• If a longer distance or detection independent of the material is required, use Photoelectric Sensors.
• If the position is to be taken by mechanical contact, use Limit Switches.

Brands in this category

• Lanbao: cylindrical housings with the LR code and square and rectangular housings with the LE code; the product name states the head type, sensing distance, housing size, output and frequency. The analog, steel-head and rectangular groups are available only from this brand.
• Isıso: cylindrical housings with the IM code and cubic housings with the IP code; the product name states the housing, output type and distance.

Order notes

Sensors are sold per piece; for a few products shown as made to order, the lead time is confirmed separately. When replacing a faulty sensor, type the code on its label (such as LR12TAF04DPOY or IMB8-M18-E2) into the site search; if the code is not found, choose an equivalent by matching the housing, head type, distance and output type.

Frequently asked questions

What is the difference between NPN and PNP inductive sensors?

The difference lies in which pole the output transistor connects the load to. In an NPN sensor, the load is connected between the output lead and the positive supply, and the sensor pulls the output to negative when it detects. In a PNP sensor, the load is between the output and negative, and the sensor supplies positive to the output. The sensing characteristics are the same; the connection style of the PLC input card or relay determines the choice. When replacing a sensor, get the same output type as shown on its label.

Does an inductive sensor detect aluminum and stainless steel?

It does, but from a shorter distance. The distance in the product name is for a standard steel target; for other metals it is multiplied by a reduction factor. The factors one manufacturer gives for an M12 series are 0.70 for stainless steel, 0.40 for brass, 0.30 for aluminum and 0.25 for copper. For example, with a factor of 0.30, a rated distance of 4 mm falls to about 1.2 mm on aluminum. The factors vary by series; for the exact value, see the product datasheet.

What happens if a non-flush sensor is embedded in a metal surface?

The field of a non-flush sensor also spreads sideways. If the head is embedded in metal, the surrounding metal affects this field; the sensor may give a detection signal even when there is no target, or the sensing distance changes. With this type, the metal-free zone specified by the manufacturer must be left around the head. Flush mounting is permitted in non-conductive material. If mounting flush with a metal surface is required, choose a flush-head model.

Can a 2-wire inductive sensor be connected to a PLC input?

It can, but two values must be checked. A 2-wire sensor also passes a small leakage current when off; this current must stay below the level the PLC input regards as off. When conducting, there is a higher voltage drop across it than with a 3-wire sensor; the voltage remaining at the input must be above the level the input regards as on. Both values are given in the datasheets of the sensor and the input card.

What do the cable colors on an inductive sensor indicate?

On DC 3-wire sensors, in the common color code, brown indicates the positive supply, blue the negative and black the switching output. On 4-wire models with NO+NC outputs, there is a fourth conductor for the second output. On 2-wire models, the two leads are connected in series with the load. Since colors can vary by manufacturer, verify the connection from the diagram on the sensor housing or in the datasheet; whether reverse connection protection is provided is also stated there.