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Compass calibration

Discussion in 'Standard' started by RayTerry, Mar 11, 2018.

  1. RayTerry

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    In response to a question about compass calibration in a recent thread, the answer was, if the drone doesn't rotate in hover, the compass doesn't need to be calibrated. Wouldn't the drone be programed to maintain a constant compass heading? Even if the compass was off it wouldn't make any difference.

    I guess I still don't know what the compass does.
     
  2. Fly Dawg

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    The compass determines AC orientation. Otherwise it would not know North, South,East or West.
     
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  3. Thunderdrones

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    Yes but the compass is only 1 part of your navigation system, you also have GPS. All systems need to work together to get your drone to fly where you and the software tell it to go.
     
  4. Fly Dawg

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    A) No, not during manual flight. You control the orientation of the AC and the compass should respond accordingly.

    B) If the compass is off, it can make a HUGE difference. All of the location and directional information are calculated together internally within the AC.
     
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  5. sar104

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    That statement was referencing one observed behavior that can result from an uncalibrated compass - uncommanded rotation of the aircraft during hover while it tries to reconcile divergent compass and IMU yaw data.
     
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  6. RayTerry

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    Wow! You're way over my head.
     
  7. sar104

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    To answer what the compass does - it determines the horizontal axes of the earth frame of reference (latitude/longitude):

    At startup the GPS system determines the aircraft location while the compass locates magnetic north relative to the orientation of the aircraft.

    The FC uses the aircraft location to look up the magnetic declination at that location, with which it adjusts the yaw value to be relative to true north rather than magnetic north.

    The FC initializes the initial yaw to this value, and then uses a combination of rate gyro data (primary) with compass data (secondary, to correct for rate gyro drift and bias) to keep track of yaw during flight. The compass data should only be needed to apply small, low-rate corrections to the IMU-computed yaw - if those two start to disagree significantly then the FC declares a compass error and switches to ATTI mode because it can no longer trust its heading, which is needed for active navigation.
     
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  8. sar104

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    Sorry - see post #7.
     
  9. RayTerry

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    As I said, you are over my head. I take off after the GPS has determined my home location, then I fly manually to where ever. Except for fixing my home GPS location, how does the GPS or the compass have anything to do with my flying manually. Then I activate RTH and it comes home to the GPS home location. In simple layman terms, how does the compass have anything to do with what I've done so far, except control the rotation of the drone while it is landing? Please be simple. I am a simple man. It sounds like the compass is doing something while I'm flying manually, but I don't know what.
     
  10. sar104

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    GPS mode, which I assume is what you mean by manual, requires the flight controller (FC) to know which direction it is facing (its yaw, or heading). It needs that in order to know in which direction (relative to the aircraft) to apply thrust in order to hold position, hold course, or return to home.

    Contrast that to ATTI mode in which all thrust commands are simply relative to the aircraft orientation, and the FC doesn't attempt to control its motion beyond that. For example, if you apply just elevator with a cross-wind then the aircraft will move forwards, but also get blown sideways by the crosswind, and so its track (actual path) will differ from its heading (direction it is facing). In GPS mode the aircraft would ensure that its track and heading were the same by applying lateral thrust (via roll) to correct for the crosswind.
     
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  11. RayTerry

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    Thanks. I had assumed that flying manually in GPS mode, you were actually flying manually, as in a toy drone. Apparently, I have some help. Thanks again. I appreciate your patience.
     
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  12. RunningWithScissors

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    I followed what sar104 said.
    No, really I did.
     
  13. hph

    hph

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    Looky, this is really simple. If in doubt, calibrate the doggone thing. Not having a calibrated compass is one reason for flyaways.
     
  14. Meta4

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    Not calibrating the compass won't make it fly away at all.
    That's just superstition.
    A good compass doesn't lose calibration.
    Under normal circumstances there should never be any need to calibrate the compass.
     
  15. hph

    hph

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    You definitely need to define "normal" here. FX, if you change flying locations, you should always recalibrate compass. If you've been around electrical equipment, like ground transformers, you should recalibrate.

    Read the manual!!!
     
  16. N017RW

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    You should pay more attention to the institutional knowledge here based on thousands of users and 100s times more hours of usage.

    The manual is constantly revised and may be subject translational and other errors.

    It also helps to research and understand just what compass compensation does.

    Don’t recompensate unless prompted by the app.
     
  17. hph

    hph

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    It's not "compensation," it's "calibration." OK, now I'm guessing you were the author of some drone manual(s), so you know exactly what BS to write, huh?
     
  18. sar104

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    Even though it has been stated in various (though not all) versions of DJI manuals, there is no basis for calibrating when you change location because nothing in the calibration process measures anything useful about the external magnetic field.
     
  19. N017RW

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    Calibration is an easier concept for the masses.
    Compensation is what is actually occurring.
    The process of rotating the magnetometer in two axis allows for the characterization of the hard and soft iron distortions created by the components in and on the aircraft. Then an algorithm creates a polar compensation matrix correcting them.

    I guess it’s easier to accuse folks of propagating BS than actually looking into it.

    If you wish here’s just one article to read...

    Hard and soft iron magnetic compensation explained
     
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  20. hph

    hph

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    Then why does my P3A require me to recalibrate the compass before flight when I am in NC altho it was previously calibrated in SC? And vice-versa?
     
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