I received a call the other day from a customer who wanted to use a magnetic field sensor on a cylinder, and evidently was requiring very precise results. He asked, “what is the hysteresis of your sensors? I notice that it is listed in your catalog as a percentage and I need to know the exact value in millimeters.” My response was, “well it depends”, upon which he was not overly pleased. I then continued to explain my answer which leads me to the contents of this posting.
As a magnetic field’s strength is increased, or as the magnet moves closer to the magnetic field sensor, a point will be reached where the value will be high enough to satisfy the circuitry in the sensor and the output is turned on. Any increase in this value does not change the output. As the magnet is moved away from the sensor the field is weakened and the output is turned off. Hysteresis is the differential between when the output is turned on and the output is switched off.
Some sort of hysteresis is found in just about every sensor including some of those old style “clickety-clack” limit switches, especially the precision type. Typically the published value of this differential is expressed in a percentage of sensing distance or in this case the magnetic strength. Hysteresis is not a bad thing; in fact it can be rather helpful, unless you like writing a lot of extra PLC code. If you have a machine that vibrates or shakes, and a magnetic field sensor or any sensor for that matter did not have any hysteresis, then the output would be triggering on and off making a great flash while creating havoc with your production and machine control.
In a magnetic field sensor there are several factors that affect the hysteresis value:
Magnet material – there is a wide variety of materials used to make magnets which would perhaps justify another blog posting. I would say it is safe to assume that all of the cylinder manufacturers do not use the same magnet from the same supplier.
Size of the magnet – this naturally depends on the size of the cylinder.
Magnetic strength – there are a lot of variables with this item.
Distance between the magnet and the sensor – How thick is the cylinder wall? What is the distance from the magnet inside the cylinder to the cylinder wall? What is the distance from the outside of the cylinder wall to the sensing face of the magnetic field sensor?
Temperature – cold magnets are stronger than warm magnets. Perhaps this is a small percentage, however; if you are trying to sense a position with micron accuracy everything adds up.
What is the sensor type? In my previous postings, I have covered the different types of sensing technology used in magnetic field sensors. In this case, reed is the worse, next is Hall, followed by magnetoresistive (AMR), and the sensor with the least amount of hysteresis and the most precise sensing point is giant magnetoresistive (GMR).
There are probably more factors some that may even have more of an impact then the ones listed, however these are the main factors that come to my mind.
So what is the hysteresis of your magnetic field sensor? Well, it all depends! To find out the best answer is to test the sensor in your application, and for best results, test it on more than one cylinder; you could obtain different results.
To learn more about magnetic field sensors, click here.