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 concept involves an oscillating electromagnetic field generated by the sensor that drive eddy currents through a metal target. This is commonly known as damping of the oscillation. When damping occurs the sensor is able to detect the change in the electromagnetic field.

There are several factors to keep in mind when using inductive proximity sensors, including the size, distance, and material of the object.  Some very small objects do not provide enough electromagnetic losses to be detected, while large objects are easily detected. The nominal sensing range or 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 how well the metal can create losses in the electromagnetic field.  These properties affect how well the sensor can detect the metal. Typically ferrous metals have more electromagnetic losses than nonferrous metals which is a key difference.

Common Metals Reduction Factor

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. 

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

FE-Only and NE-Only Sensors

If a sensor needs to detect only ferrous metals, a ferrous only sensor can be used. Since ferrous metals have increased electromagnetic properties compared to other metals they are easily able to be 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 create electromagnetic losses as well.  The sensor can look for these specific characteristics in the power loss and only give an output if a nonferrous metal is within sensing range. 

Reduction Factor 1 Sensors—Nonferrous or Ferrous Metals

Pepperl+Fuchs Reduction factor 1 series sensors is able to detect ferrous and nonferrous metals at the same distance without a reduction factor. The sensors are able to do this by implementing a two coil, air core system to detect eddy currents and magnetic losses to determine the trip point. The conductivity, permeability, and other electromagnetic properties of different metals do not affect sensing distance but the size of the target is still a factor. Since the sensor doesn’t have limitations on metal type, it will give an output at its Sn value.

For detection of unique metal like gold, titanium, alloys, and others, the reduction factor 1 sensors should be able to do the job with consistent results for all metals. 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 sensor is needed, determining the properties of the unique metal is imperative.

Topics: Inductive Sensors, proximity sensors

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