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Compass Calibration, A Complete Primer

Discussion in 'General Discussion' started by ianwood, Jan 8, 2015.

  1. solentlife

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    A fundamental fact of vector addition of forces is that any change in one or other of the forces causes a change in the final vector result.

    Change of location gives a change in one of the forces that interacts creating the vector result.

    Second - You and another may have agreed - but there is a bigger world out there !!

    Nigel
     
  2. solentlife

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    Even I - despite others claims - know that Geo data is programmed - BUT it is averaged.

    Datcon is acting as designed .............

    That does not in anyway detract from the VECTOR addition in the dance ...

    If Datcon was the total answer - then NO-ONE would be saying to 'dance' ....

    The important factor is not the variation - but the resulting VECTOR of forces to resolve ...

    Nigel
     
  3. sar104

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    Your post neither addresses my question nor makes any sense at all, and I've moved from thinking that you understand this pretty well to realizing that you really don't.

    Imagine three perfect, orthogonal magnetometers rigidly attached to what we might refer to as the principle axes of a lightly magnetized structure that is free to rotate about all three axes, in the presence of an external magnetic field that varies in a known and smooth way (for the most part) in both direction and magnitude with location within a second, fixed, frame of reference (lat/long).

    The goal is to know the orientation of the structure with respect to the fixed frame of reference. If the structure, itself, had no magnetic field, then the total magnetic field vector measured by summing the individual components, measured on the axes of the magnetometers, would simply be the local external field vector. And suitable knowledge of that external field vector at that location allows calculation or the orientation of the structure in the second frame of reference by a simple transformation.

    However, if the structure is also magnetized then the field measured by the magnetometers is a linear combination of the local external magnetic field and the structure's magnetic field. The former changes as the structure rotates, while the latter does not. To fix that, all that needs to be done is to subtract the structure's magnetic field from the measured magnetic field, leaving just the external magnetic field. By rotating the structure around at least a couple of axes one can separate the unchanging internal field from the changing external field, yielding the necessary data.

    That's all that calibration does - it determines the aircraft's own magnetic field (as seen by the onboard magnetometers) so that it can be subtracted from the measured magnetic field to leave just the earth's magnetic field, from which orientation is computed by transformation with pitch and roll. And the relationship between the local earth's magnetic field relative to lat/long is derived from a geomagnetic model, not from direct measurement at that location. The latter would not be possible since there is no absolute direction reference available to the FC to compare with the earth's magnetic field. To be more specific, since "vertical" is an absolute direction while "true north" is arbitrary, combining magnetometer data with accelerometer data would allow direct measurement of the inclination of the earth's field at that point, but not the variation (declination).

    So explain again what you think that recalibrating as a function of location is actually measuring and correcting for.
     
    GadgetGuy, N017RW and BudWalker like this.
  4. N017RW

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    Well said s104.
    And with more foundational support as oppposed to jargon.

    I have, in the past, substituted Calibration with Compensation which better describes the result of the ‘dance’. However DJI chose the former probably out of simplicity.
     
  5. sar104

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    Agreed - it really is compensation. Calibration implies comparing measurements to known values, which is not what is happening here.
     
  6. solentlife

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    I get it now .... OH Wowee !!

    Bud is the developer of DATCON ...

    Right ..................

    Bud me ol'fruit ....

    1. Please explain how your graphs show DJI P3 compass setting as you imply.

    2. Please explain why you persist in saying external is not important and then you produce graphs and data that shows it IS !

    Its one or other me'ol pal !

    Vector result of external and internal is a fact and you can produce all the graphs you like based on YOUR program ... fact is you cannot get away from the fact that magnetism is all around ... and even your program accepts that inalienable fact !

    I have to stop now ... I'm laughing so much it hurts ...

    Sorry ...

    Nigel
     
  7. solentlife

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    So where does it get its information from to compensate ?

    Changing the word does not change its actions.

    Nigel
     
  8. BudWalker

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    You're mistaken. I never said that. What I did say is that the external magnetic effects can not be detected and then compensated for.
     
  9. solentlife

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    I wish you would stop putting another wording or intent to mine ...

    That's not what I am arguing about and I cannot understand why you persist ...

    External flux and internal produce a vector addition of resulting flux that is compensated for ... otherwise there is no point in any calibration dance ................. I did not say that the P3 itself determines variation and compensates for it. I said that the P3 during the 'dance' detects and calibrates based on the vector result of flux it sees during that 'dance'.

    I am running out of ways to word what is basically the same ******** thing !

    There is no compass in the world ..... even a Gyro Compass that is NOT subject to external influence !! (Gyro Compasses are a different ball game all together with Precession and so on ... but point is made)

    Oh bollocks ... I've had enough of this shite ..

    If I stuck a magnet under your AC and you calibrate ... who's going to cry then ? Me ??

    Nigel
     
    #229 solentlife, Oct 21, 2017
    Last edited: Oct 21, 2017
  10. solentlife

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  11. sar104

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    So you have now resorted to the random misuse of scientific words. What you are running out of is ways to word your misunderstanding of the subject.

    If you are not interested in detailed explanations, then simply consider the following: the "compass" needs to calculate the external magnetic field to determine its orientation relative to the world frame of reference and, in order to do that, needs to subtract the internal magnetic field from the overall magnetic field that it actually measures, which is the vector sum of the two. It determines the internal field by the calibration process, by identifying the unchanging magnetic field components when it rotates. It knows the transformation between the external field and the world frame of reference, which varies with location, from its geomagnetic model, not via the calibration process.
     
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  12. solentlife

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    Very funny sar ....
     
  13. GadgetGuy

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    I can personally confirm the last sentence from a crash of my own.
    "the cause was just that the launch site was geomagnetically distorted and the calibration was not flawed"
    Wild J hook flight caused by warming up IMU on top of a giant hidden metal bowl and heavy metal frame of a firepit, located under a wooden table top, hidden by an overhanging table cloth. Calibration was fine. The area of launch was not, and back then, in early P3P days, the DJI GO app gave no warning.

    However, had I known to look at the aircraft heading on the map, before take off, it would have shown the aircraft pointing in the wrong direction on the map! Compass error, from geomagnetic distortions caused by the launching surface the aircraft was resting on!
     
    #233 GadgetGuy, Mar 17, 2018
    Last edited: Mar 17, 2018
    JJErrico likes this.
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