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Compass failure

Discussion in 'General Discussion' started by Gotcher6, May 9, 2014.

  1. Gotcher6

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    In the event of a compass failure while in flight, I know GPS would be useless for navigation since the orientation of the drone is lost. But if you let go of sticks, it still should hover in place using GPS, right?

    Also, if you lost your compass in-flight, the Naza mode return home feature would still work, right?
     
  2. ElGuano

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    That's an interesting question. I believe if your compass goes bad, your GPS stops working, as in, it flies in ATTI mode. That would also mean RTH would NOT work (the Phantom might know where it is via GPS, but wouldn't know what direction it's pointing, so it wouldn't know which way to go to head home).

    You can test this out by unplugging the compass and loading up Assistant. You get "GPS not present" messages and a host of functions like IOC/RTH become grayed out. So I think the compass is integrated with the function of the GPS.

    It also depends on the TYPE of compass failure. If it stops responding, the above would apply. If it starts giving random/wacky values, you're in for much more of a ride.
     
  3. Gotcher6

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    I had a wire break loose from the plug of the compass connector cable. I have a new cable ordered, but meanwhile I repaired the cable with some electrical tape. It seems pretty secure, but I'm concerned my temporary fix will come loose in flight, resulting in a complete compass failure. I think I should probably stay grounded until the new part arrives.
     
  4. gavinski

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    Would this apply to compass interference as well? I just returned to this place where I seemed to be experiencing some compass issues:

    http://www.phantompilots.com/viewtopic.php?f=5&t=9771

    This time around I had no problems flying for the first ten minutes up at around 10 meters, but when I brought it down to about 3 meters, the lights flashed yellow + red a couple of times (indicating ferro-magnetic interference), and then started getting "jittery" and not fully responsive. I quickly brought down and ended the flight before it got any worse.

    I am wondering is there is something buried here that is causing the problem? I don't think I will fly in this location any more.
     
  5. ianwood

    ianwood Taco Wrangler
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    What happens as result of a compass failure depends entirely on how it fails. Most likely it won't fail in some graceful way that the Phantom will recognize and simply switch to ATTI. More likely it will provide bad readings and the Phantom will get thoroughly confused and possibly fly away until you switch to ATTI.

    If the ribbon cable is severed, it all depends if the connection is analog or SPI/I2C. If it's analog, all three compass values will float like static on a TV. If it's digital, then the lack of communication could be detected and it would switch to ATTI.
     
  6. tanasit

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    Interesting question indeed.
    I think if you switch to HL mode when the compass fails, it will still fly back to home position (assuming its home point is properly set) regardless of its orientation but as soon as it gets to the 10 meter circle parameter (where it turns into CL mode), it will be anybody guess.

    Proper compass & GPS path should look like below:

    [​IMG]
     
  7. ElGuano

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    It's definitely possible to build a system that can intuit position and direction based on historical sampling, but it's very unlikely anything would be built this way. I've never flown a NAZA with a compass failure in-flight, but:

    Say you're a Phantom. You're compass is busted so you have no idea what direction you're pointing, but from the GPS you know you're 20m to the NE of home point. You get put in HL mode, and the pilot pulls back on the stick. What direction do you start flying? In other words, if you don't know which way you're pointing, which motor do you engage to head northeast? You could be facing the homepoint, in which case you'd need to go forward. Or you could be facing NE, in which case you'd need to go backwards. Without a cardinal reference, you're lost.

    To get it to work, you'd have to blindly engage the motors to go in one direction, then check the GPS after a few seconds to see if your position has changed outside of the 2.5m accuracy range. And if you guessed wrong, you might need to stop completely and start flying in the opposite direction. Again, it's possible to build a system that way, it just doesn't fit the operational paradigm of the Naza...just imho.
     
  8. darwin-t

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    In a related note, how would one know if Phantom needed it's magnetic orientation to be redone?

    I have rear speakers in my car. I carry my Phantom in the back seat, but wonder if the trunk would be a better choice. How close can you get to a speaker without causing a problem?
     
  9. tanasit

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    GPS does NOT require compass which basically only shows "true north".
    A network of 24 satellites orbits the Earth twice a day, transmitting signals back to earth. A GPS receiver locks onto signals from three or more satellites (NAZA uses 6 or more) to determine your location, using a method called trilateration. Your receiver calculates the difference between the time a satellite sent a signal and the time your system received it. Using the information gathered from several signals, the receiver triangulates your exact position. It can even determine how fast you're going and how long it will take you to reach your destination. GPS receivers are extremely accurate, many within 50 feet (15 meters) [source: Garmin].

    A GPS receiver provides you with your location, distance/directions, routes and tracking. Tracking means that the GPS unit will provide you with a breadcrumb trail, in case you need to retrace your steps. You'll likely need to initialize and configure your GPS device before you can use it.

    https://www.youtube.com/watch?v=_vfzAL5L29Y
     
  10. ElGuano

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    That's precisely why I say it's definitely possible to build a system that will intuit orientation based on position. If you buffer historical information, you can back into your present orientation with acceptable accuracy. But a Phantom doesn't work that way; it relies on the compass for orientation.

    When working with any practical application, you have to separate the technical and theoretical capabilities of a GPS system from the capabilities of the instant implementation.
     
  11. Garysam

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    This may not be the best place to post this question so I apologize in front. I currently live in Minnesota Minneapolis compass Declination here is about 0.59 so we for the most part have not experienced these issues.

    However this July having a family reunion it's about two hours south of Baltimore Maryland around the Lexington Park area it's has a compass Declination of -11.05 This sounds like potential big problems going to be very close to the ocean renting Beachhouse.

    Top this off I will only have about two or so months of pilot practice, owner of phantom 2 for about a week now. Suggestions ideas and concerns I should have. I feel like possibly I should not even attempt to fly. Hoping I can get some suggestions and workarounds.
     
  12. ElGuano

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    Honestly, I wouldn't worry about it. Take your time setting up, make sure you're not getting any error codes, and do a compass calibration before you fly and you'll be fine. The complaints about the P2's declination issue are about relatively minor flight behaviors. You're not going to run into declination issues in the continental US that will cause you to TBE or lose control.
     
  13. Garysam

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    Thanks for your reply it's words of encouragement LOL I will let you know how things go.
     
  14. sar104

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    A breadcrumb trail is only useful if you know which direction to head to reach the next breadcrumb. Without a compass the GPS is completely blind, because it does not know which direction the craft is facing except by trial and error - i.e. make a random move (e.g. forwards) and monitor change in location to derive track, then adjust pitch/roll accordingly and iterate until track is correct. The assertion is that the NAZA does not have that functionality, even though such a feedback loop is theoretically possible.