Advanced hard and soft iron magnetometer calibration for dummies

If you bought the cheap magnetomter module like HMC5883L you can not use it without calibration. Measurement of magnetic field will be subjected to distortion. There are two categories of these distortions: the hard iron distortions and the soft iron distortions. The hard iron errors refer to the presence of magnetic fields around the sensor (magnets, power supply wires) and are related to measurement offset errors, while the soft iron errors refer to the presence of ferromagnetic materials around the sensor, which skew the density of the Earth's magnetic field locally and are related to scaling offset errors. You can read more information about these distortions here.

In other words, to get the correct magnetometer data you should get the calibrated magnetometer data. One of the ways to resolve this problem: you should apply the bias to the vector of the non calibrated magnetometer data (X, Y, Z coordinates) and then multiply the transformation matrix by this resulting vector:

Picture 1

In this case the magnetometer calibration is the process of getting the transformation matrix and the bias. To get these data you can use the MagMaster program.

You can download MagMaster here.

Example of using the MagMaster

Hardware:

Software:

  • MagMaster (placed in MagMaster folder)
  • MagViewer (placed in MagMaster folder)

Firmware:

  • Arduino Sketch (placed in MagMaster folder)

Picture 2

Picture 3

Picture 4

Connect the magnetometer module to the arduino board via I2C bus (picture 4). Upload the arduino sketch to the arduino board (see "Arduino_Code" folder in the "MagMaster" folder). This arduino sketch requires the HMC5883L library, copy the folder "HMC5883L" (placed in "Arduino_Code" folder)  to the folder "C:\Program Files\Arduino\libraries".

Then run the MagViewer.exe, select the serial port of the arduino board (the boud rate of the seraial port should be 9600 bps) and click "Run MagViewer". Now you can see the coordinates of the magnetometer data vector in 3D space on a real time (picture 5, video 1, 2). These data are not calibrated yet.

Picture 5

Video 1

Video 2

If you see the points of the magnetometer vector coordinates in 3D space the arduino board and the PC connection works right.

Now close the MagViewer window and run the MagMaster.exe (picture 6). Select the serial port of the arduino board. The green strings X, Y, Z will indicates the coordinates of the magnetometer vector.

Picture 6

Place the magnetometer module as shown on the picture 8.1 and click "Point 0" button of the "Axis X+" groupbox. For the placement of the module you can use the wooden bar or the paper box (picture 7). If you can not connect your magnetometer device to PC then you can use any other way to get to know the magnetometer data. You can enter these data in the program manually.

Picture 7

Place the magnetometer as shown on the picture 8.2 and click "Point 180" button of the "Axis X+" groupbox and so on. You should do in the following way:

  • Picture 8.1: "Point 0", "Axis X+"
  • Picture 8.2: "Point 180", "Axis X+"
  • Picture 8.3: "Point 0", "Axis X-"
  • Picture 8.4: "Point 180", "Axis X-"
  • Picture 8.5: "Point 0", "Axis Y+"
  • Picture 8.6: "Point 180", "Axis Y+"
  • Picture 8.7: "Point 0", "Axis Y-"
  • Picture 8.8: "Point 180", "Axis Y-"
  • Picture 8.9: "Point 0", "Axis Z+"
  • Picture 8.10: "Point 180", "Axis Z+"
  • Picture 8.11: "Point 0", "Axis Z-"
  • Picture 8.12: "Point 180", "Axis Z-"

Picture 8

You should fill the table. After that click "Calculate Transformation Matrix and Bias" and get the required matrix and bias (picture 9).

Picture 9

The transformation matrix and the bias are got. Now you can calculate the calibrated magnetometer data in your device on a real time with using the matrix and the bias as shown on picture 1. The example of the arduino sketch of using this calculation you can find in the "Arduino_Test_Results" folder.

You can apply the sphere radius stabilization algorithm to your program (use for it the Arduino_Radius_Stabilization folder placed in MagMaster folder).

The calibrated magnetometer vector coordinates in 3D space with the radius stabilization shown on the picture 10 and video 3, 4.

Picture 10

Video 3

Video 4

Using of the arduino is only example, you can easy adopt the arduino code for any other system and use the MagMaster program with it.

Views: 119790

Comment by Yury Matselenak on June 12, 2014 at 10:02am

About orientations.

For exapmle, this orientation:

equivalent to this picture:

Comment by Yury Matselenak on June 12, 2014 at 10:06am

You should not change the sign of the z axis.

Comment by Don Howes on June 12, 2014 at 11:18am

Yury Thanks

I just position the board with components up. Sorry I must have mistaken the Z axis in that position

Don

Comment by Harry on June 28, 2014 at 5:24pm

Man, this is a brilliant program.  Thanks for offering it.  I have a board with the HMC5883L and I got my matrix and bias and I'm working on using the example code.  I like how you have a diagram for each position.  That really really helped. 

I am having one problem though.  I stuck the calibration code into my program and the Z axis won't move.  It must be canceling itself out somewhere?  Maybe my sensor alignment is swapped somewhere?

Comment by Yury Matselenak on June 29, 2014 at 12:57am

Harry, can you put the results of calibration and your code here?

Comment by Harry on June 29, 2014 at 8:38am

Comment by Yury Matselenak on June 29, 2014 at 10:50am

Harry, I think you have a problem with the magnetometer device orientation during the calibration process. There is screenshot of my calibration results:

In my location the magnetic field is directed at the 70 grad angle to the earth plane:

So, in my case, if I place the magntometer with Z axis directed to the sky I will get the negative value of the Z coorinate of the magnetometer vector (number 3 on the picture above). If the Z axis of magnetometer is directed to the earth I will the positive value of Z coordinate (number 6). These rules are the same for X and Y axis (numbers 1,2,4,5 on the picture).

Comment by Yury Matselenak on June 29, 2014 at 10:57am

The angle of magnetic field in your location you can find here:

http://www.ngdc.noaa.gov/geomag-web/

Comment by Yury Matselenak on June 29, 2014 at 11:03am

For the result testing I recomend to use the code in folder "Arduino_Test_Results" of MagMaster archive and the MagViewer.exe program. It useful for the result testing.

Comment by Harry on June 29, 2014 at 12:02pm

Thank you, Yuri.  It's working now.  I rotated it 90 degrees and ran the calibration again.  It's brilliant.

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