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Mode of operation of inductive sensors

How do inductive sensors work?

Inductive sensors are contactless sensors designed for the detection of (conductive) metal objects. How they work: Inductive sensors operate on the basis of a magnetic field that is generated at the front of the sensor in an open magnetic circuit. The detection principle is based on the damping (attenuation) of the magnetic field by the approaching metal object. The amplitude of the internal resonant circuit is reduced by damping until a threshold is achieved and the sensor outputs a switching signal.

The functional principle

The principle of inductive sensor technology is based on an electromagnetic alternating field created via a copper coil embedded in an open pot core made of ferrite. An resonant circuit is created using the capacitor. An electrically conductive metal located within a certain range in front of the sensor causes strong attenuation of the eddy currents of the magnetic field. These changes in the oscillation amplitude are detected by the sensor and the sensor is switched. The special feature of sensors which can be installed flush is the additional outer metal ring which surrounds the coil. This shields the alternating field and bundles it at the front.



Observance of free zones

Free zones refer to areas around the sensor which must be kept free of metallic materials in order not to compromise with the operating principle of the device. With inductive sensors a distance of at least three times the nominal sensing range must be maintained between a metallic material and the sensing face. The correct installation of sensors for non-flush mounting also requires that there is no interference material within a displacement less than the diameter of the detection face.



The sensing range of inductive sensors

The distance at which inductive sensors switch is referred to as the sensing range. Depending on the approaching metal, a correction factor must be applied for the majority of inductive sensors to reduce the sensing range. The following table displays the factors of various metals for the majority of inductive sensors:
Exemplary sensing range
6.00 mm
Cast iron
0.93 ... 1.05
5.58 ... 6.30 mm
Stainless steel
0.60 ... 1.00
3.60 ... 6.00 mm
0.65 ... 0.75
3.90 ... 4.50 mm
0.35 ... 0.50
2.10 ... 3.00 mm
0.30 ... 0.45
1.80 ... 2.70 mm
0.25 ... 0.45
1.5 ... 2.70 mm
For some time now, Factor 1 sensors have been available that offer an identical sensing range for all metals (Factor 1). The switching frequency of inductive sensors usually ranges between several hundred and several thousand switching operations per second. Inductive sensors therefore have a very fast reaction time and can be used for speed monitoring, for instance.
"Inductive sensors are contactless sensors used for the detection of (conductive) metal objects."

Advantages of inductive sensors:

  • Compact size
  • High protection rating (up to IP69K)
  • Very insensitive to environmental influences
  • Very reliable
  • High switching frequencies
  • High measurement accuracy
  • Contactless operation
  • Long service life

Disadvantages of inductive sensors:

  • Only metals can be detected
  • Relatively low sensing range
  • Sensitive to magnetic-fields

Applications for inductive sensors:

  • Position monitoring
  • Actuators
  • Machine tools
  • Hydraulic assemblies
  • and many more

Range of available inductive sensors

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