Inductive Proximity Sensor Targets – Material does matter

Written by: Jeff Himes

From some of the previous blogs, it has been indicated that a number of variables can affect the actual sensing range achieved by an inductive proximity sensor.  One of those variables mentioned was target composition or target material.  Let’s review how various target materials can affect an inductive sensor’s performance.

In general the best target for an inductive proximity sensor is a flat piece of ferrous metal.  Per the IEC specification the “test target” for an inductive proximity sensor is a piece of carbon steel (Fe360) that is 1mm thick.   The size of the target is square and for a standard range sensor it is typically equal to the size of the sensor’s active surface. For an M8 sensor with Sn = 1.5 mm, it would be 8mm square; for an M12 sensor with Sn = 2 mm it would be 12mm square, etc.

In the real world target material types can vary.  As indicated above the target material that will result in the best sensing range is ferrous steel.  Other non-ferrous target materials can still be detected by an inductive sensor, but normally at a reduced sensing range.  To give you an idea of the amount of reduction, a chart of “correction factors” is given below.

Typical Material Correction Factors 
Material Correction Factor Type of Metal
Steel (Fe360) 1.0 Ferrous
Stainless Steel 0.6…1.0 Non-Ferrous
Aluminum 0.30…0.45 Non-Ferrous
Brass 0.35…0.50 Non-Ferrous
Copper 0.25…0.45 Non-Ferrous

The chart provides approximate “correction factors” as most metals are compounds rather than pure so the effective sensing range achieved will vary depending on the composition.  As and example – if a standard inductive sensor had an effective sensing range of 10 mm with a ferrous steel target, that same sensor would have approximately a 3.0 mm to 4.5 mm effective sensing range with an non-ferrous aluminum target.

The main concept to keep in mind is that an inductive proximity sensor can detect a variety of ferrous and non-ferrous metals – but not necessarily at the same sensing range.  Normally a non-ferrous target material will result in a shorter sensing distance.

If you need to overcome this target material sensing reduction, a number of options exist:

1. Rather than using a standard range sensor, look for an extended range model such as a double or triple sensing range version.

2.  If the application will allow – use a non-shielded sensor rather than a shielded sensor (provides longer sensing range).

3.  Many sensors exist today that can detect ferrous and non-ferrous models at the same distance.  These are sometimes known as “Universal” or “Factor 1” sensing types.

Sensor selection can be chosen to minimize the affects of varying target materials as long as the target sensing characteristics are understood in the beginning.

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About Shawn Day

Balluff Inc. Market Manager ~ Object Detection
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9 Responses to Inductive Proximity Sensor Targets – Material does matter

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  3. Henry Menke says:

    I see. Sounds like the conductivity of the metal is key. Steel is a relatively poor conductor, so the I^2*R power losses from the eddy currents are higher than for better conductors like aluminum or copper.

  4. Henry Menke says:

    What property of the target material creates this effect on the sensing range of an inductive prox?

    • Jeff Himes says:

      As the iron contect in the target material varies, the magnetic properties and the eddy current generation properties vary as well. An inductive sensor generates a magnetic field that causes eddy currens to form on its metal targets. As the target moves closer these eddy currents increase and cause the signal amplitude to decrease. When the signal amplitude decreases to a certain level – the inductive sensor “triggers”. The higher iron content the target material has the more eddy currents it generates and the longer sensing range it achieves. As the iron content in the target metal decreases so does the detection range of the target.

      • steven says:

        Hi Jeff Himes,

        I’m about to test with that kind of sensor.
        I wonder, can any random steel object be
        used to measure if calibrated correctly?
        And does mass play a role in this?

        thank you in advance,

        Steven

      • Henry Menke says:

        Steven: In order to closely replicate and compare the specified switching distances of inductive proximity sensors of different types and different manufacturers, you will want to use the standard target for an inductive proximity sensor. This is a square shape made from mild steel. The thickness of the standard target should be 1 mm and the side dimensions should be equal to the diameter of the active face of the sensor OR three times the nominal switching distance…whichever is greater.

      • LCBP says:

        uhm, i don’t have inductive sensors to try sensing copper wire such as those used in outlets(3.5mm2). my question is will it change the signal amplitude based on the wire’s eddy current produced? thanks you in advance

      • Henry Menke says:

        LCBP: For a standard inductive proximity sensor, copper is a difficult target. Even though eddy currents are produced in the copper, the electrical resistance is low, so the power loss is low. Therefore a copper target that is close to the sensor will “fool” the sensor into thinking it is looking at a steel target at a longer distance. The effect is that the copper is not detected until it gets very close to the sensor…in other words, the sensor does not have as much sensing range when looking at copper. Looking at copper wire is even more difficult, because you have very little metal present to generate eddy currents and power losses for the sensor to detect.

        One answer could be what is called Factor 1 or “F1” inductive sensor technology. These sensors use two or even three sensing coils and more advanced internal signal analysis in order to “know” what target material the sensor is looking at. Once the sensor identifies the target material, the threshold of detection is dynamically adjusted so that the sensor will always switch at the same sensing distance, regardless of the material type. For sensing typical copper household electrical wire, you might try an M08 threaded tubular Factor 1 sensor. Another idea, if you are trying to detect wire breakage, is to look at inductive ring sensors (wire passes through the center) or optical window (frame) sensors.

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