BudWalker Posted at 2018-7-19 11:18
Thank you, but I'll continue here. Your comment that others may become confused by this conversation doesn't make sense to me. Others can always ask questions or ignore this.
The article you sited
ac.els-cdn.com/S1474667015350400/1-s2.0-S1474667015350400-main.pdf?_tid=e777d530-2eaf-4b5b-8254-40a83a2ca165&acdnat=1532007753_714244eb3a8376741804917c14edb90b
Bud Walker,
Post 2:
You say: “Just to be clear all this discussion comes down to one key statement. A compass calibration does not and can not detect/compensate for any magnetic effects that do not rotate with the AC. I.e., if a magnetic effect is external to the AC it can not be compensated for by the calibration.”
Another misunderstanding. No – this argument is not about compensating for magnetic effects that do not rotate with AC. This argument is about measurement of geo-magnetic field and accounting for geo-magnetic field properties in a given location and than compensating for magnetic effects within AC. I do agree with you that magnetic effects or disturbances which are external to AC cannot be compensated for by the calibration. But geo-magnetic field properties metrics, in particular geo-magnetic field strength as measured in all three axis, is knowledge required as a prerequisite to successful compensation to account for soft/hard iron disturbances within Spark.
This article describes a method of calibrating magnetometer when true geo-magnetic field strength is unknown (field strength is geo-location dependent). Algorithms which are location independent and are commonly used require very elaborate set-up and rotating magnetometer in three axis and are very complex (Spark’s calibration is 2-axis calibration, horizontal and sideways, and is damn simple – can’t use such algorithms). Authors of this article came up with the alternate algorithm which is simpler and is 2-dimensional. This algorithm does not require measurement of the geo-magnetic field strength in a given location to result in accurate calibration.
Please note start of chapter 2.4 “Calibration does not require location information”. The key assumption to have successful calibration is knowledge about exact radius of the circle – it must be known quantity. Radius of the circle is proportional to the total strength of the true magnetic field. True magnetic field is highly geo-location dependent.
This is why most magnetometers should be re-calibrated when the geo-location changes. The compensation for hard/soft iron distortions calculated in one place may be invalidated by different properties and strength of geo-magnetic field in different location.
However, authors of this article do propose a method where calibration may be accurate and independent of the geo-location.
In summary:
- Widely used algorithms to calibrate magnetometer must take into account magnetic field strength in a given location. This is prerequisite before adjusting for hard/soft iron distortions. Location-dependent strength of geo-magnetic field can be either computed from tables based on location parameters or can be measured on the spot.
- Algorithms which do not require location information are complex and require elaborate set-up and 3-axis measurements (Spark is not using such algorithms for obvious reasons).
- The article I quoted provides simpler method which also does not require data about geo-magnetic field strength as a prerequisite to successful calibration.
I do not know which algorithm Spark uses. I did not know that non-location specific calibration methods existed until I started research based on conversation with you. I did know that for regular and simple calibration algorithms you do require measurement of geo-magnetic field in calibration location as a prerequisite. I do suspect that Spark uses simple algorithm and the simple ones do measure geo-magnetic field strength and than provide method of calibration in 2-axis which do account and adjust for hard/soft iron distortions. My suspicion is based on simplicity of Spark design – it is not a laboratory-quality equipment. While advanced technology, it is also fairly cheap. In addition, DJI does recommend re-calibration after moving long distances (the question is – do you believe them here – I do).
Mirek
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