LSM303DLM Magnetometer Skewed Readings

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1

I am currently working with a custom board that has the LSM303DLM module on it. I am successfully reading correct data via the I2C interface and calculating a heading based on the process of this source:
github.com/ryantm/LSM303DLH
I am working in C (not C++) on a dsPIC so I had to make a few minor changes but nothing functional.

The heading I am getting seems to be very weighted towards the south pole. At magnetic north I get a heading of 0, and south a heading of 180. But at what should be about 135 degrees I get a reading of 90 degrees and at about 225 degrees I get a reading of 270.

In other words the rotation of readings is very slow around the north pole and fast around the south pole. Most of the calibrations I have found have just been a scaling of the data which does not fix this problem. And I have tried to follow the process of this source:
pololu.com/file/download/LSM … e_id=0J434
But I had trouble following the math without example data, and my setup is not ideal for getting reliable data at known orientations.

What I am looking for is a step by step set of equations to correct my readings (not just scale them). Also, most of the solutions I have seen reference MATLAB to get and plot the data but I do not have access to this either.

If it helps, here are the maximum and minimum values of each of the readings for my device (not scaled):
x min: -540
x max: 320
y min: -710
y max: 163
z min: -426
z max: 419

Thanks for your help.

2

The magnetometer is nearly useless without proper calibration and from the data that you provide, yours appears to be performing quite a bit worse that the two uncalibrated examples that I’ve used.

See the link below for a discussion of state of the art procedures, including free downloadable, self-contained programs for calibrating the magnetometer. Before using either program, you will have to collect machine-readable magnetometer measurements for as many different orientations as possible (hundreds if not thousands). The programs work for accelerometers as well, providing the device is perfectly still during each measurement.

sailboatinstruments.blogspot.com … ation.html

3

Thanks a lot, this was a huge help. I have one question about the application though. It asks for the norm to to the local Earth’s magnetic field and gives a link where you can find that data.
Here:
ngdc.noaa.gov/geomagmodels/s … alcIGRFWMM

But that page does not call the data the “norm to the local Earth’s magnetic field”. Which piece of data is this? I am using the total field_value/1000 but I am not sure if this is correct.

4

The “norm of the local earth’s magnetic field” is just a scaling constant. You can use 1.0, unless you want the measured value to conform to some particular unit of measurement.