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Sensing ranges of position sensors

How is the sensing range of sensors defined?

Definition: The sensing range of position sensors is the displacement between the sensing face of the sensor and the approaching measurement object that triggers a signal change in the sensor. The sensing ranges for inductive sensors are determined with the aid of a square of steel (Fe 37), a so-called standard target according to DIN EN 60947-5-2. As a rule of thumb, the larger the sensor diameter, the larger the sensing range. For measurement objects made of other materials, a correction factor must be used in determining the sensing range. The sensing range is also dependent on the size of the measurement object. As a rule of thumb, at least the forward surface of the sensor must be covered in order to reach the sensing ranges as given in the data sheet. If the objects to be tested are smaller, this reduces the sensing range or even renders the object unmeasurable.

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 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 figure displays the factors of various metals for the majority of inductive sensors:
Sensing range graphicnduktiven Sensoren und Faktor 1 Sensoren im Vergleich
"The sensing range of position sensors is the distance between the sensing face of the sensor and the approaching measurement object."

Manufacturers specify up to four different basic types of sensing range:

  • Sn: nominal sensing range or rated sensing range
  • Sr: real sensing range

Sn: nominal sensing range or rated sensing range

The theoretical distance at which the sensor switches. Manufacturing tolerances, the temperature and other external influences are not taken into account. Many sensor manufacturers only specify this sensing range distance in their data sheets.
  • Su: useful sensing range
  • Sa: operating distance or assured sensing range

Sr: real sensing range

The actual displacement between the sensor and an object at which a switching operation is triggered in the sensor assuming that certain defined criteria apply (as per DIN EN 60947-5-2). Here, factors such as temperatures, potential difference, object size and object material are specified in a testing process. To comply with the DIN standard, the sensor must react within 90% to 110% of the nominal sensing range.
This can be expressed as follows:

Sn * 0.9 ≤ Sr ≤ Sn * 1.1

 

Su: useful sensing range

The useful sensing range additionally takes the entire permitted (and specified in the data sheet) working range of the sensor into account. The sensor must always react at displacements between 90% to 110% of the real sensing range, irrespective of fluctuations in temperatures and voltage.
This can be expressed as follows:

Sn * 0.81 ≤ Su ≤ Sn * 1.21

Sa: operating distance or assured sensing range

The operating distance is the displacement at which the sensor always switches taking all of the tolerances and ranges specified in the data sheet into account. It lies within 0% to 80% of the nominal sensing range. Only at this displacement between the sensor face and the object does the sensor switch under all circumstances and error tolerances. This makes the operating distance the most important distance, and that is why it is referred to as the assured sensing range.
The following definition is applied:

Sa ≤ Sn * 0.8

Products with differing sensing ranges

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