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Technology Overview

Magnetic proximity sensors are ideal in applications where inductive sensors do not provide enough sensing range, where photoelectric sensors are not ideal due to harsh environments, or where mechanical tolerances can cause issues with target repeatability. Magnetic proximity sensors are not affected by outside influence from aluminum, stainless steel, plastic, wood and water, all of which can influence sensing range and reliability of other technologies.

ifm’s magnetic sensors are based on the Giant Magneto-Resistance (GMR) principle which enhances the performance and durability in industrial applications.

GMR cells consist of three layers of material – two layers of cobalt (magnetizable) and one layer of copper (non-magnetizable). Without a magnetic field applied, the atoms of these layers do not allow electrons to pass through because of the high resistance between the layers. However, when a magnetic field is applied, the atoms align and the electrons can now pass through. The sensing element uses this physical property to generate a signal that can be used to indicate the position of a magnetic target.

The benefit of GMR technology is that this alignment occurs on a molecular level, so there are virtually no moving parts to wear out and switching frequencies above 500 Hz are achievable.

Sensing range

The sensing range of a magnetic sensor is based on the size of the magnetic field generated by the magnet. The size of the magnetic field is a combination of the magnet’s physical size, shape and material. The sensing range specified on the sensor datasheets is based on a magnet with 103 mT magnetic strength. The following chart shows sensing range for various magnets.

* indicates the sensor / magnet combination has not been tested for sensing range

Approach curves

The alignment of the magnet axis in relation to the sensor axis affects the sensing range and switching behavior of magnetic proximity sensors. Note the differences in the following four images.

1. Sensor axis and magnet axis are on one plane. The magnet approaches the sensor axially. The output changes state as soon as the magnet has reached the switch-on point.

2. Sensor axis and magnet axis are on one plane. The magnet approaches the sensor laterally. The output changes state when the magnet is in the lateral active zone of the sensor.

3. Sensor axis and magnet axis are 90 ° opposed. With this orientation, two magnetic fields are formed and therefore, two switching points. Exactly in the middle of the two curves, the magnetic field breaks down. In this area, the sensor remains undamped and depending on the passing speed, the signal can be interrupted.

4. Sensor axis and magnet axis are 90 ° opposed and the magnet passes laterally. As in the above case, two fields are formed with two switch points and an area in the center where the field breaks down. In this area, the sensor remains undamped and depending on the passing speed, the signal can be interrupted.