Magnetic Field Sensors – Are Magnets Different?

It’s 3:00 AM and your machine has come to a screeching halt because, according to your PLC, the cylinder that holds a part in place is neither extended nor retracted.  After looking at the cylinder, you see it is extended just as it should be however; the cylinder-mounted sensor is not detecting the magnet.  No problem, you have another magnetic field sensor that will fit but it does not work either, so what is the problem? Another bad sensor? Maybe not, it could be the sensor and the magnet is incompatible.

Cylinder manufacturers commonly use two different types of magnets.  Although they look the same, they are completely different.  The most commonly used magnet in pneumatic cylinders is the axially magnetized magnet.  With this magnet the north and south poles are next to each other in the axial plane as shown is compatible with reed switches.  As an axially aligned magnet approaches, the reed elements attract the magnetic flux lines and draw together by magnetic force, thus completing an electrical circuit. 

The second most common magnet used is the radially magnetized magnet.  Instead of the magnetic poles next to each other, one pole is the inner diameter and the other pole is the outer diameter.  This magnet works well with the Hall Effect sensor.  A Hall Effect sensor is looking for a magnetic pole; it does not matter if it is north or south. 

Axially and Radially Magnetized Magnets

Let’s assume you have a cylinder with an axially magnetized magnet and you replace the reed switch with a Hall Effect sensor.  In some cases, the Hall Effect sensor may work however, you may get double switch points.  Remember the Hall sensor is looking for a magnetic pole it does not care which one, so as one pole travels under the magnetic field sensor the output will change and then it will change again as the other pole travels under the sensor, the result is double switching.

This is the first in a series as we look at the differences in magnetic field sensors.  In future postings we will compare the reed switch, Hall Effect and the sensor that could be the universal solution.


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