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How Do Inductive Sensors Respond to Stainless Steel?

Posted by Tracy Molnar on Thu, May 22, 2014

Inductive sensors are designed for the detection of metal targets at close range and are a popular choice for use as proximity switches in factory automation applications.  Carbon steel is the typical target material for these sensors, but in some industries, stainless steel is more common.  There are also certain cases when either both types of steel must be detected by the same sensor, or one sensor is needed to be able to distinguish between the two. 

How do inductive sensors respond to stainless steel?For these applications, in order to determine which inductive sensor will give the required results, you need to know how different models respond to stainless steel as compared to carbon steel.

Rated operating distance

An inductive sensor’s data sheet includes a specification called rated operating distance, which indicates the point at which a standard target, when moved toward the active face of the sensor, will cause the sensor’s output to change.  For standard inductive sensors, the target used to determine the rated operating distance is carbon steel.  Also included on the data sheet are reduction factors for non-standard targets such as stainless steel, brass, and aluminum.  These can be used to calculate the reduced operating distance of the sensor for those alternative targets.

Standard inductive sensors

Given the information in the previous paragraph, you now know how a standard inductive sensor will respond to a stainless steel target – its rated operating distance will be reduced by the reduction factor listed on the datasheet.  For example, based on the specs shown below for model NBB8-18GM50-E2-V1, this sensor will detect a steel target up to 8 mm away, but if the target is 304 stainless steel that range is reduced to 5.6 mm (8 x 0.7).  Put another way, for much of its sensing range (from 0 to 5.6 mm) the sensor responds the same way whether the target is steel or stainless steel, but at the far end of its range (from 5.6 to 8 mm) the output will be different depending on which target material is used.

Standard inductive sensors

Reduction factor 1 models

As the name implies, a special design makes it possible for a reduction factor 1 inductive sensor to detect all metal targets at the same rated operating distance.  This is also seen in the technical specs on the datasheet for one of these models, copied below.  A reduction factor 1 sensor is a good choice for applications where either steel or stainless steel targets may be used, but the response of the sensor is required to be the same for both.

Reduction Factor 1 Sensors

Pile Driver FE vs. NFE models

Inductive sensors in the Pile Driver family have a one-piece stainless steel housing (including the sensing face) that makes them unbeatably rugged in applications where they might be subjected to harsh conditions.  Original Pile Driver models are separated into ferrous sensing (-FE) and non-ferrous sensing (-NFE) types, and so are worth mentioning as we discuss carbon steel vs. stainless steel targets.  With these sensors, neither material can be detected by NFE models.  Both are detected by FE models, with a reduction factor for stainless steel.

Pile Driver Inductive Sensors

Extended range Pile Driver models do not include the FE or NFE designation.  Their detection function is similar to standard inductive sensors, with a rated operating distance based on a carbon steel target, and reduction factors for non-steel targets.

Special application NE/FE models

There’s one more sensor we’ll discuss, a special-application model designed to distinguish between ferrous and nonferrous targets.  It has two separate outputs – one is activated for carbon steel targets, and the other is activated for nonferrous metals such as copper, brass, or aluminum.

Special Application Inductive Sensors

You’ll notice that stainless steel is not listed on this sensor’s datasheet for either output, but the sensor is able to detect it.  The nonferrous output is activated in the sensor’s mid-range (from about 5 to 10 mm away from the sensing face), and the ferrous output is activated at very close range (from about 0 to 5 mm away).

Application solutions for stainless steel targets

Based on the sensor operation descriptions just discussed, here are some recommended solutions for applications involving stainless steel targets:

  • If your target is always stainless steel, then you can use any of the sensor types mentioned above except NFE Pile Driver, but pay attention to listed reduction factors when selecting a model so you’ll get the sensing range you expect.
  • If your target is either stainless steel or carbon steel and you want both to be detected at the same range, then you should use a Reduction factor 1 model.
  • If your target is either stainless steel or carbon steel and you want the sensor to give a different output based on which metal is used as the target, an NE/FE model is the best choice, but you’ll need to ensure the target is always in the range where the NE output is activated by stainless steel.
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Topics: Inductive Sensors, Applications

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