[There is now a better method for removing offsets in flight. See this discussion. - WJP]
The picture shows a UAV DevBoard connected to Jordi's magnetometer breakout board.
First, since my discussions tend to get long winded, if you want to "cut to the chase", you might want to read this.
I recently integrated Jordi's HMC5843 magnetometer breakout board with the UAV DevBoard (UDB).
Jordi's board is really great, having the onboard bidirectional level shifters and builtin pullup resistors made it possible to simply connect Jordi's board directly to the I2C pins in the programming connector. Thank you very much, Jordi.
So, heli and quadcopter pilots who are using the UDB would do well to buy Jordi's board, my plan is to integrate it into the heli firmware that is being developed for the UDB. There is now a magnetometer demo and instructions for the UDB.
But that is all besides the point of this discussion....
Along the way to implementing the magnetometer demo, I realized the hardest part was going to be determining the offsets of the magnetometer. The integration of the magnetometer itself into the DCM algorithm was trivial, but what to do about the offsets?
The HMC5843 has a self calibration feature, so you can determine the gains just fine. But the first magnetometer I bought had rather large offsets. So I bought 3 more (1 more from Jordi, and 2 more from Sparkfun). It turned out that all 4 units had some offsets. Even if they did not have offsets, it would still be possible for stray magnetic fields in the aircraft to create offsets.
So, some sort of procedure is needed to null the offsets.
I thought of several manual ways to do that, and I read several postings on the subject....
Then I wondered if there might be some way to use the direction cosine matrix to automatically compute the offsets in flight. And there is....even for dynamically changing offsets, such as from power leads to the motors...
The basic idea is the following...
When the aircraft is rotating, the offset fields rotate with the aircraft, while the earth's magnetic field is stationary. So it should be possible to use the direction cosines to separate the two fields.
The basis of the idea is the same as flipping the magnetometer exactly 180 degrees along one of the axis and averaging the two readings. The result is the offset.
The idea can be extended to rotations other than 180 degrees, even small rotations, by using the direction cosine matrix and some vector algebra.
The resulting theory and implementation is described here, and it has been tested. It works rather well.
It was interesting to watch the computations of the offsets during the tests. A few random turns along a couple of the axes was all that was needed to compute the offsets. So, you can either null the offsets prior to takeoff by rotating your aircraft a bit, or simply let it happen automatically during the first few turns of the flight.
It was also interesting to see what would happen if I deliberately manually set the offsets to the wrong values, they would quickly reset to the correct values after a few turns.
The bottom line is that now both the calibration and the nulling of offsets can be done completely automatically, without any manual operations or entry of measurements. It is possible to extend any DCM-based IMU, such as the ArduIMU, to use this method to simplify the integration of a magnetometer.