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Q&D Magnetometer Calibrations #2524
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Posted at 2017-01-07 by ClearMemory041063 Result of the QD calibration of the LSM9DS1 magnetometer using the 8 Gauss scale.The calibration results:Statistical results of using the calibrations
For the 8 Gauss range the use of the Gain term reduced the variance in the data. R1 uses raw readings. R1= sqrt( X1^2 + Y1^2 +Z1^2) |
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Posted at 2017-01-10 by ClearMemory041063 Magnetometer, Turntable, Wireless, Plot Circle, CalibrateThe hardwareA magnetically agnostic turntable is used to spin a three axis magnetometer. The magnetometer readings are read by a Pico (or equivalent) and transmitted wirelessly to a web page (served by the Pico). Puck? The webpage has the following elements:
As the data from the magnetometer are acquired the selected axis pair are plotted as an asteroid belt around the origin of the graph. The offset sliders are adjusted to center the asteroids. The cursor circle and the gain sliders are adjusted to visually best fit the data to a circle. The values in the gain and offset boxes are then used for the magnetometer calibration values. It is conceivable that local distortions in the magnetic field could be detected and visualized with such a system. |
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Posted at 2017-01-11 by ClearMemory041063 In Depth Magnetometer Calibration Using Excel and Solver( Best fit so far!) The mathematical modelGiven raw magnetometer readings X, Y , and Z, Results
Magcal 4 Gauss 5 Hz 11Jan2017
http://diydrones.com/forum/topics/magnetometer-soft-and-hard-iron-calibration |
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Posted at 2017-01-11 by ClearMemory041063 (Found the error in the previous post, a copy and paste in Excel) In Depth Magnetometer Calibration Using Excel and SolverThe mathematical modelGiven raw magnetometer readings X, Y , and Z, Results
Magcal 4 Gauss 5 Hz 11Jan2017
http://diydrones.com/forum/topics/magnetometer-soft-and-hard-iron-calibration |
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Posted at 2017-01-11 by ClearMemory041063 Used a long cable to move magnetometer away from the table and computer with improved results.
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Posted at 2017-01-12 by ClearMemory041063 See attached spreadsheet in ods format. Attachments: |
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Posted at 2017-01-07 by ClearMemory041063
Q&D Magnetometer Calibrations
The Ideal Magnetometer
From an ideal three axis magnetometer with readings X, Y and Z from orthogonal axis, we can compute the Radius R of a sphere. For easy coding let us represent the three components X, Y, and Z in a vector M=[X, Y, and Z]. This M[0] is the X axis reading, M[1] is the Y axis reading and M[2] is the Z axis reading. The radius R = Sqrt(M[0]^2+M[1]^2+M[2]^2).
For an ideal magnetometer the radius R is a constant for a given external magnetic field.
In practice each axis of the magnetometer will not be centered and may have a different gain from the other two axis. Centering consist of finding an offset value to subtract from the raw readings for each axis. Gain adjustment finds a value that makes the output of each axis equal when positioned in the same magnetic field.
Coding note
I’m using a LSM9DS1 IMU chip connected to a Pico. Since there is recent interest in the Puck magnetometer, I will try to point out where there are differences and give my best guess as to how to use this technique with a Puck.
How the calibrations are implemented
We start with a raw reading from the magnetometer. Mr[i], where Mr is the raw reading vector containing raw values for the X, Y and Z, axis. To get the calibrated reading in vector Mc, the following equation is used:
Mc[i]=( Mr[i] – Moffest[i] ) * Scale * Mgain[i],
Where Mc[i] is the calibrated vector component,
Mr[i] is the raw vector component,
Scale is used to convert ADC count to engineering units such as Gauss. (Can omit for Puck),
Mgain[i] is the gain factor for the vector component,
and i is the component index 0...2.
The raw magnetometer readings from the LSM9DS1 IMU are in ADC counts and the Scale (there are 4 different ones) is the full scale engineering units divided by 32768.
For example 4 Gauss/ 32768 = 0.0001220703125 = Scale.
The min/max Method
For the LSM9DS1 I used the minmax class in a loop that can be stopped using a menu system.
Due to budget I don't own a Puck yet, so this code may need some enhancement.
For a Puck the Puck.on('mag', function() { ... }); would be used something like this:
Running the code
The idea is to run the code and then point each end of each axis towards the magnetic pole. For my location in North America it is North and about 60 degrees below the horizon due to magnetic dip. This process is continued until the readings for each axis stop changing.
At the start: (readings are in raw ADC counts)
After readings stop changing:
At this point for the LSM9DS1, I used a different program to write the offsets (avg)to ROM. For the Puck copy the offsets (avg) to the Moffset[].
To evaluate the calibration data were collected using code like this:
The magnetometer was pointed in many different orientations as data were collected. R1 is the radius from raw data, R2 is the radius from raw data - offset, and R3 is the radius from the gain times the raw data - offset. Excel was used to perform descriptive statistics on the radius values R1, R2, and R3.
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