# Will a magnetometer give distorted values if there is a magnet near?

I am not entirely sure how a magnetometer works in IMU units, so I was wondering how a magnet near the sensor affects readings?

• My magnetometer was damaged by a magnet, but I think the readings are slowly returning to normal. Let's hope.
– user23597
Jul 4, 2016 at 5:50

Will a magnetometer give distorted values if there is a magnet near?

Yes, effectively.

In fact no, as the reading it gives WILL BE the field at that point BUT the field will be distorted compared to what you would see when the magnet is absent.

The effect of magnets at a distance is a somewhat complex subject - especially when there are several presnt and/or where ther are "magnetic materials" present but, as a guide.

• At a distance large compared with magnet dimensions the field falls as the cube (not square) of the distance.

• Very rough but useful rule of thumb: A top-end rare earth magnet will produce a field of about 1 Tesla at a distance of 1/2 it's thickness from the centre of its pole face. So eg a 20mm thick circular magnet will produce ABOUT 1 T at 10mm from the centre of its pole face. This is more useful in motor or alternator applications than when using magnetometers.

• So are you saying if the magnet stays in the same place in relation to the magnetometer, the reading with be quantitatively different, but qualitatively usable, as it always reflects a relative change? If this is so, presumably moving the magnet's position would then change the magnet field distortion and consequently distort the readings entirely? Feb 6, 2015 at 10:15
• @Kontur I wasn't specifically saying that but it may be true. I do not know what magnetometer you are referring to or the application or general environment - but as you note - you may be able to make use of the vector sum of magnet + desired field. Whether this is usable may depend on relative strength and field directions. If the desired field is the earth's field then relative X Y Z angles will probably be affected equally. If it was from a low power local source then the x y z vector sums may now be at new and different angles and interpretation my be difficult. Feb 6, 2015 at 14:36
• As MAC notes - saturation MAY occur. Feb 6, 2015 at 14:37

The IMU magnetometer is made to measure the magnetic filed of the earth, which magnitude is a few 10 - 60 uT (micro-Tesla). You cannot measure it near to a large magnet that may generate 1 T or more: the magnetometer will saturate.