Earths magnetic field effecting the data out of the FXOS8700CQ

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Earths magnetic field effecting the data out of the FXOS8700CQ

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lukelynch
Contributor III

Hello, i am hoping someone can help me with this.

 

I know the earth has a magnetic field and it can vary from 25ut to 65ut.

 

With no magnet around my magnetometer i get a reading of 17ut (earth's magnetic field). I then put a magnetic exactly 100mm away along the z-axis. I then get a reading of 100ut, this seems perfectly fine to me.

 

I then put my magnetometer  in a new location with no magnet near it and get a reading of 22ut (earth's magnetic field). I then put the same magnetic exactly 100mm away along the the z-axis. I then get a reading of 125ut.

 

Now i see something is not right. As the earth's magnetic field increase, the strength of the field around the magnetometer increase even with a much stronger magnet in place. so it looks like the magnetic field of the magnet and the magnetic field of the earth are combining together. It does  not seems to be linear either.

 

Is there away of subtracting the earth magnetic field from the output of the magnetometer, to just leave me with the data of my magnet?

 

If anyone has had this problem, or knows how they can help, please contact me.

 

Regards,

Luke.

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michaelestanley
NXP Employee
NXP Employee

Luke,

There are a couple things at play here.

  1. Soft iron distortion resulting from ferrous materials in the vicinity of your sensor can warp the fields being measured.  This manifests as apparent non-linearities in individual axes.  If those ferrous materials are fixed spatially relative to the sensor, then magnetic compensation software can remove the effects.  If not fixed, there isn't a lot you can do.
  2. I assume you probably did the experiment in an indoor environment?  Many buildings have major effects on the magnetic field, which can vary quite a bit over just a few inches.  I see this simply by moving a sensor about my work desktop (there are ferrous materials in the fixtures of my wooden desk).

Check out the blog I wrote on the topic several years back at Hard and soft iron magnetic compensation explained .

Regards,

Mike

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lukelynch
Contributor III

Michael i dont seem to have access to see this post? Can you open my access somehow?

Thanks,

Luke.

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michaelestanley
NXP Employee
NXP Employee

Luke,

For some reason, the web masters seem to be requiring that you log into your Freescale account to gain access.  I will ask to have that fixed.

Mike

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lukelynch
Contributor III

Any update on this Michael?

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michaelestanley
NXP Employee
NXP Employee
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lukelynch
Contributor III

THank you for the link.

I have a question in relation to the soft iron as I think this is where my problem is arising.

My magnetometer is fixed and can not be moved in its location. I have a magnet about 100mm away which is pivoting about a point,  this is what I am trying to detect.

I am only using the z axis of the magnetometer. How am I meant to account for the soft iron.

I Can send you pictures and explain better it if you give me your email address?

THanks,

Luke.

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michaelestanley
NXP Employee
NXP Employee

Luke,

You can contact me at mike.stanley @ freescale.com.

If your magnetometer location is fixed in space, you are not going to be able to take full advantage of our hard/soft iron compensation.  At best, a one time calibration before fixing the sensor in 3D space.  But send me your pictures and more information and let's see where it goes.

Mike

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lukelynch
Contributor III

Hello Mike. I just emailed you now. Thank you.

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