sensing range explanation for inductive sensors

In the datasheet of our inductive sensors we offer two sensing ranges.
The sensing range s_n and the safe sensing range s_a , this article explains how to handle and reach them reliable and what is the reference target material and size for it.

Inductive sensors detect metallic objects at a specific distance known as the switching distance. According to the international standard EN 60947-5-2, this is defined as the distance at which a standardized calibrating plate=standard target, moving towards the sensor’s active surface, triggers a signal change.

The standard target is a square metal target made from Fe 360 (ST 37) mild steel to determine the rated sensing range Sn. The standard target has a thickness of 1 mm and the surface length is either the diameter of the sensing face or 3x Sn – the larger of the two values is to be taken.
Examples
Rectangular sensor Q12-06NPS KW1 Active area diameter: 12 mm Sn: 6 mm 3 x Sn: 3 x 6 mm = 18 mm
The standard target has a surface length of 18 mm.
Cylindrical sensor IME12-02BPSZC0K Active area diameter: 12 mm Sn: 2 mm 3 x Sn: 3 x 2 mm = 6 mm
The standard target has a surface length of 12 mm.
Cylindrical sensor IME12-08NPSZC0K Active area diameter: 12 mm Sn: 8 mm 3 x Sn: 3 x 8 mm = 24 mm
The standard target has a surface length of 24 mm

Sensing range in practice

To compare individual sensors, the data sheet always gives the theoretical value of the sensing range Sn. In practice, however, there are several factors that can reduce the effective usable sensing range:
• Size and material of the object to be detected (if not standard size and not ST37)
• Component tolerances (component manufacturing tolerances such as resistors, manufacturing tolerances, etc.)
• Ambient conditions (temperature change, voltage fluctuations, EMC interference, etc.)

• Other properties in the vicinity which are not part of the detection (parts of machines, etc.)

A bigger metal target does not increase the sensing range, but a smaller object greatly reduces the sensing range. The same applies to the thickness of the metal target – objects thicker than 1 mm do not affect the sensing range, but thinner objects do.

What happens when using smaller or larger targets?
The smaller the target is, compared to the standard target, the smaller the real sensing range. But increasing the target beyond the standard target does not increase the effective operating distance!
A bigger metal target does not increase the sensing range, but a smaller object greatly reduces the sensing range.
The same applies to the thickness of the metal target – objects thicker than 1 mm do not affect the sensing range, but thinner objects do.

Material of the object to be detected
The sensing range is always from a target made of ST37. For targets made of metal, which are not made of ST37, the sensing range decreases depending on the material used. This is described by the reduction factor. The reduction factor varies from sensor to sensor. It is generally listed in the relevant data sheet.
Examples of reduction factors:
Material Reduction factor Steel (St37) 1
Stainless 0.65 ... 0.9
Brass 0.25 ... 0.6
Aluminum 0.20 ... 0.5
Copper 0.20 ... 0.4

Example calculation:
Sn = 8 mm aluminum target, reduction factor of 0.2
8 mm x 0,2 = 1.6 mm
The effective sensing range is reduced to 1.6 mm.

Nominal Sensing Range Sn
The sensing range Sn (also called "rated operating distance" according to EN60947-5-2) is the distance at which an axially to the active sensing face approaching norm target triggers the output. Production tolerances and changes due to external influences (such as temperature, changes of the supply voltage) are not considered.

Operating sensing range Sa
The distance at which a response of the proximity switch is ensured under the specified temperature, installation and voltage conditions. It is between 0 % and 81 % of the rated operating distance Sn.

To reach a reliable sensor function we strongly recommend to use the safe operating distance s_a for choosing the correct sensor and consider the necessary minimum target size, thickness and material.