Pepperl+Fuchs Blog

Metal Detection Using Inductive Sensors

Posted by Edwin Stoll on Thu, Sep 27, 2018

Inductive sensors are a type of proximity sensor that use an electromagnetic field to detect metal. The oscillating electromagnetic field generated by the inductive sensor drives eddy currents through a metal target. The eddy currents in the metal target generate an opposing electromagnetic field that resists the field being generated by the sensor. This causes a power drop in the sensor, indicating the nearby metal, and an output signal is given.

There are several factors to keep in mind when using these industrial proximity sensors, including the size, distance, and material of the object. Some very small objects do not provide enough of an electromagnetic profile to be detected, while large objects are easily detected. The nominal sensing range, commonly referred to as the Sn value, is a standard value for defining the operating distance. The Sn  value does not take into account production tolerances or changes through external influences, such as voltage and temperature. Conductivity, permeability, and other electromagnetic properties determine a metal’s ability to generate electromagnetic fields. These properties affect how well the proximity sensor can detect the metal. Typically, ferrous metals have more electromagnetic resistance than nonferrous metals, which is a key difference.

Ferrous (FE)/Nonferrous (NE) Standard Inductive Proximity Sensors

Metals can be sorted into two categories: ferrous and nonferrous, otherwise known as magnetic and nonmagnetic. The most ferrous metals are iron, cobalt, nickel, and manganese. These metals have stronger electromagnetic properties than most, so the electromagnetic fields created by the eddy currents are typically stronger than those in other metals. Ferrous targets are easily detected by any standard inductive proximity sensor.

When detecting nonferrous metals, such as aluminum, copper, and brass, with a standard inductive proximity sensor, a reduction factor should be implemented. A reduction factor is a number usually ranging from 0 to 1 that describes how well metals can be detected by the proximity sensor. For example, a standard inductive proximity sensor has a reduction factor of 0.3 when sensing copper, as seen in the table below. This means that the sensing distance is reduced to 0.3 of the effective sensing range when detecting copper, a 70 % decrease.

FE-Only and NE-Only Sensors

A ferrous-only sensor can be used if an inductive sensor is needed to detect only ferrous metals. Since ferrous metals have increased electromagnetic properties compared to other metals, they are easily identified by the sensor.

If a sensor is needed to detect only nonferrous metals, a nonferrous sensor can be used. The electromagnetic properties of these metals have less resistance, and they do not generate electromagnetic fields as well. The sensor can detect these specific characteristics in the power drop and only give an output if a nonferrous metal is within sensing range. 

Reduction Factor 1 Sensors—Nonferrous or Ferrous Metals

Reduction factor 1 proximity sensors use multiple frequencies to calculate the precise position of a metal target within the Sn range. The conductivity, permeability, and other electromagnetic properties of different metals do not affect sensing distance on the reduction factor 1 proximity sensors. This is a result of Faraday’s law, Lenz’s law, and Maxwell’s equations. These rules and equations of electromagnetics make it possible to switch the frequency of the sensor and calculate its electromagnetic properties, which allows the metal’s distance from the sensor to be calculated. Since the reduction factor 1 sensor does not have limitations on metal type, outputs will be given at the sensor’s Sn value.

Reduction factor 1 sensors should be able to consistently detect unique metals like gold, titanium, alloys, and others. If a standard inductive sensor is chosen, there might be a reduction factor involved, depending on the metal type. If a ferrous-only or nonferrous-only industrial sensor is needed, determining the properties of the unique metal is imperative.



Topics: Inductive Sensors, proximity sensors

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