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The compass....misunderstood or easy scapegoat?

Discussion in 'General Discussion' started by n6vmo, Apr 24, 2015.

  1. n6vmo

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    I have read many 'fly away' posts blaming a faulty or uncalibrated compass as the cause.

    But what does the compass actually perform for us in the Phantom.

    I am under the impression that the compass only provides azimuth data that keeps the aircraft pointed in a particular direction, whether its hovering or moving horizontally, until the operator yaws it in another direction.

    Having used GPS in the desert for 20 years, I have noted that just before navigating to a waypoint, I have to travel a short distance before the GPS knows my motorcycle's azimuth or orientation. The GPS can't tell the azimuth of the aircraft in a hover, until the craft has moved horizontally over the earth for a short distance.

    HL is a good example of this in action. In HL, the orientation or azimuth of the aircraft is ignored and the GPS uses its data to bring the craft back to home point.

    So, IMO, blaming the compass on a 'fly away' is incorrect. Maybe those that know the workings of the NAZA controller and compass can chime in and help explain.
     
  2. dirkclod

    dirkclod Moderator
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    I really can't explain what you ask but I have to disagree with ya on saying a compass won't cause a flyaway .
    I had mine to and it was because I had moved the location of my tracker and didn't recalibrate the compass afterwards.
    That is the only time that has ever happened and that's why I disagree with that . I did find it with the tracker and it only caused me to have and replace the shell but that was the only thing that could have caused it . IMO ;)
     
  3. ianwood

    ianwood Taco Wrangler
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    1. Most people have no idea how the compass actually works or what calibration does.
    2. Not only is it possible for the compass to cause your bird to fly out-of-control, it is the number one cause of out-of-control flight.
    3. Compass calibration does not cause flyaways. If you don't follow the recommendations and calibrate incorrectly, you could experience what appears to be a flyaway.
    4. Avoiding calibration when the recommendations are to calibrate can lead to less accurate flight and possibly TBE.

    EDIT: I keep reposting this link:

    Compass Calibration, A Complete Primer
    http://www.phantompilots.com/threads/compass-calibration-a-complete-primer.32829/
     
    #3 ianwood, Apr 24, 2015
    Last edited: Apr 24, 2015
  4. dirkclod

    dirkclod Moderator
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    Well I will stand corrected then :) Was just watching it at about 130' up and 100' out and it took a left and was gone .
    Only thing I had changed was moving the tracker and not recalibrating .o_O
     
  5. ianwood

    ianwood Taco Wrangler
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    To answer the question of what exactly the compass does, it does what an analog compass does for us. It tells us which way we are heading. As stated above GPS has no concept of heading. It can only determine "course over ground" by comparing two or more GPS points. GPS course can be thrown off by many things.

    The compass likely also plays a critical role in the overall stability of the bird. Unlike the accelerometer, it provides a vector to gravity that isn't influenced by inertial movement. This falls under a category called "sensor fusion" in which multiple sensors are used together to determine a single outcome.

    One way to see how the compass plays a role in balance is in TBE videos. The gimbal horizon will go off at the same time the TBE takes place.
     
  6. dirkclod

    dirkclod Moderator
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    Maybe it should be a stickie.
     
  7. ianwood

    ianwood Taco Wrangler
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    If you moved it from or to close proximity to the compass, then yes, that would be a strong possibility. Glad you got a replacement!
     
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  8. dirkclod

    dirkclod Moderator
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    I do seem to have a lot of issues huh ! ;)
     
  9. ianwood

    ianwood Taco Wrangler
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    I think we all have! All we can do is learn.

    I am lucky enough that I am working on a number of "sensor fusion" projects in my day job so I've been taking a "crash" course on the compass, accelerometer, gyro, barometer, etc.
     
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  10. dirkclod

    dirkclod Moderator
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    See that's just why I love this place !! It's the only place I have to find folks that do know about this stuff for sure . Do have a bunch though here that's hell on.....well nothing really :(;)
    You do be careful with that fusion stuff ian !! That's how the Hulk came about ! :)
     
  11. SteveMann

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    I am in the camp of "Compass cannot cause a flyaway".

    I have built two 3DR kits where I had to compile and install the firmware from source code, so I assume the NAZA controller is similar.

    The compass is ONLY used to calculate a heading to move between two waypoints. Where it is and Home are two waypoints. When you tell the Phantom to go home, it calculates the heading it needs to go from "here" to "home" then starts moving in that direction. The MC is constantly recalculating the heading to correct for wind, mostly, but no matter how bad your compass information is, it will eventually get to "home".
     
  12. dirkclod

    dirkclod Moderator
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    Well ya in da wrong camp cause I did see mine do it o_O
     
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  13. SteveMann

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    Tell me the mechanics of how bad compass data could possibly cause a fly-away?
     
  14. kirk2579

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    This confuses me . if it does not matter what the compass information is why have it.
    I say that based upon your comment that " but no matter how bad your compass information is, it will eventually get to "home.

    at some point that has to fail in my wild *** guessing.

    if it is saying quad is heading 270 degres but in reality is is maybe at 150 degrees heading. wouldn't that mess something up in naza as to what and where!!

    I do not know , I am just asking

    why did they udgrade the compass in V3 to anti static version if not important to navigation
     
  15. ianwood

    ianwood Taco Wrangler
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    Depends what you're calling a flyaway but there are plenty of examples of Phantoms flying completely out of control as result of TBE. As I'm sure you know, TBE is a disagreement between compass and GPS.

    I'll also throw in the idea that compass data is used in conjunction with accelerometer and gyro to enhance orientation stability. 9DOF vs. 6DOF. To support my theory, look at videos of TBE. You'll see two important things taking place:
    • Deterioration of gimbal roll axis stability. It is directly proportional to the amount of TBE taking place. The more TBE, the more the gimbal does not maintain the horizon. If the gimbal was relying solely on accelerometer and gyroscope data, it wouldn't have this problem. It is using compass data to improve vertical stability.
    • There is a strong banking angle. In bad TBE, the bank angle is strong enough to cause a loss of altitude. The Phantom seeking, missing and seeking in a circular pattern only accounts for a portion of that banking. It is also caused by a decreased precision in the gravity vector, much like what happens with the gimbal. Putting it simply, bad compass data makes the Phantom drunk and it cannot stay upright properly.
     
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  16. Meta4

    Meta4 Moderator
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    There's never been any suggestion that the newer compass was an upgrade or better in some way. And there were no reports that there was a problem with the older units.
    The reason may have been as simple as cheaper production or just a new supplier.
     
  17. Hughie

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    If that was true DJI and its resellers would not have included it on it list of bullet point reasons to buy it
    http://store.dji.com/product/phantom-2

    They certainly do want the buyer to think it is an improvement over the old one. Whether it represents a real functional improvement, who knows.
     
  18. ianwood

    ianwood Taco Wrangler
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    Could be a cost reduction but it could also be a design change for improved performance. It is advertised as anti-static (important for a compass). I think the original was encased in mu-metal which also has anti-static properties.

    I am planning to pick up one of the new ones and try it out. They're cheap. I now log total magnetic flux (what DJI calls the mod value) on all my flights so it would be easy to compare the two across a similar flight.
     
  19. Meta4

    Meta4 Moderator
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    There was never a technical suggestion that the newer compass was an upgrade or better in some way. But the marketing material implied it was.
    I suspect it was pseudo-scientific meaningless marketing fluff.
    It succeeded .. most people assumed it must be an improvement .. it's anti-static! Although the original was too (but never advertised that way).
     
  20. Hughie

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    I agree that they never qualified why is was an advantage or upgrade, but it certainly was implied as advantage or upgrade in their marketing material. No question.