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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Good day I hope you are well.
I am a student currently busy with a project where we have to build a coin sorter and we want to try and use an induction proximity sensor to detect 3 different types of coins, they each have different materials and dimensions.
My knowledge on induction proximity sensors is very limited and I was wondering if you could explain to me if it would be possible to detect aluminium, stainless steel and mild steel with one induction proximity sensor. I am under the impression that each of these coins will create a unique amplitude in the pulse generated by the sensor and that we could detect which coin is which in this way. Please correct me if my understanding is wrong and any helpful suggestions will be much appreciated.
Thank you so much.
Kind regards,
Aaliyah
Will an inductive sensor sense titanium? If so what would the correction factor be?
I am using an analog sensor to measure distance to a target (steel). What thickness target will give the best results?
I would like to sense a 304V SS wire that is .002″ diameter. Is there a sensor that will do that?
Does powder coated metal affect the sensor?
The thickness of the coating may de-rate the operating range depending on the thickness of the coating however the sensor will still have no problem picking up the coated target. For example, if the sensor has 4mm of range and the coating is 1mm thick you may see an operating range of the sensor at ~ 3-3.5mm Sn.
Hello, is an inductive sensor capable to sense stainless steel or carbon steel?
An Inductive Proximity Sensor will detect both stainless steel and carbon steel. Keep in mind that there will be a reduction rating when sensing stainless steel. If the reduction is a concern with a standard Inductive sensor there are specialty Inductive sensor such as Factor 1 sensors that will detect ferrous an non-ferrous material at the same operating distance.
SIR WE HAVE AN APPLICATION FOR SENSING ONLY NON MAGNETIC STEEL FROM 5 MM DISTANCE IT SHOLULD NOT SENSE MAGNETIC STEEL PL ADVICE M30 SIZE IS OK
I would recommend using an analog inductive sensor (BAW) and do some experimentation to characterize the differences in signal response between their magnetic and non-magnetic steel samples.
Explanation about Factor 1 sensor was useful. Thanks
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.
What property of the target material creates this effect on the sensing range of an inductive prox?
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.
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
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.
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
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.