Inductive proximity sensor targets – Material does matter

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 1 mm 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 8 mm square; for an M12 sensor with Sn = 2 mm it would be 12 mm 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

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.