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Moving Compass. What to keep in mind

Discussion in 'General Discussion' started by lightingbryan, May 15, 2014.

  1. I just bought wider taller legs. These legs will not put the compass at the same distance to the original distance. Is the direction most important (as in make sure the compass is 100% perpendicular to the nose) Or is height from leg screws most important. I am not familiar with how the compass operates. So I just want to understand what does or doesn't need to happen.

    Thanks!
     
  2. RobFallen

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    Kinda wondering this too, I have just put my stock legs back on after using the wider ones
     
  3. Why did you take your wide ones off?
     
  4. RobFallen

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    they just seemed quite flimsy and I got a GPS tracker bracket that fits the standard legs and wanted to use that. The wider ones do give a wider stance and a little more height but I don't think it's needed and I don't think they will survive impact like the stock ones do, just my opinion though
     
  5. d4ddyo

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    Height is not important. what is important is the orientation of the compass.
     
  6. N017RW

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    It seems the designers wanted to keep it as far away from the motors as possible.

    Or not so much about height as separation.

    It could even be a design compromise due to other competing engineering requirements.

    Otherwise it would be simpler (remember KISS) to mount it inside.
     
  7. By orientation do you mean nose to back left to right. So if it was slanted on the leg (not quite perpendicular to the nose, then and only then would it be an issue?
     
  8. derrickduff

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    It's a digital compass, the orientation should not be important. What is important is that it is secure when you calibrate the compass. Magnetic north can vary by a few degrees in neighboring cities, and can vary by 10s of degrees state to state, country to country. Furthermore Magnetic poles are opposite in the North and South Hemisphere. The compass doesn't know where it's at or what orientation it's in. It only knows that A certain direction is where magnetic north is pulling. The compass calibration is exactly for this reason. If there was no compass calibration we would have to physically adjust the compass every time we went to an area with a different magnetic north. Thankfully we don't have to do that.

    Long story short, If you move the compass to another set of legs make sure it is secure and perform the compass calibration.
     
  9. This seems very logical and glad to hear your response.
    My only question is does the compass send instruction to the brain (naza) on where North is in conjunction with where the nose is pointed. So if you turned the compass upside down and rotated it the compass can easily find north, but the compasses relationship to where the nose is has changed. Thus the orientation does in fact matter.

    So does it actually matter of does the compass worn in some other way? (I hope it does!)
     
  10. ElGuano

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    BE CAREFUL. The above is incorrect. The orientation is paramount for the Phantom compass. Forward MUST point forward. When looking at the front of the Phantom head-on, the compass is installed on the back-left leg. That orientation MUST be maintained if you move the compass to a different location. E.g., if you move the compass from the back-left leg to the back-right leg, the compass must be screwed onto the outside of the new leg, in order to maintain the orientation.

    If you get this backwards, the Phantom will think that the nose is pointed in the opposite direction, and when it tries to adjust its attitude, it will drift even further off-course from where it was intending to turn, and you end up with an increasingly large and fast spiral called the "Toilet Bowl Effect" that soon becomes almost impossible to control.

    So if you're playing around with the compass, it's a good idea to use a sharpie and place a black dot on the "forward" end of the compass, so no matter where you move it, you always have a visual indication for which side should be pointing forward.
     
  11. Great info. It kinda scares me. Is it so finicky that it's not worth moving. Aka does it have to be perfectly adjusted or it will toilet bowl? Meaning if I was careful it won't be that hard. Or does it take lots of skill to get right.
     
  12. ElGuano

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    It's not a big deal, you're just in for a mistaken "omg flyaway!" moment the first time you get it wrong.

    It doesn't need to be perfectly aligned, it will tolerate ~20 degrees rotation either way, and some people rotate the compass slightly to correct for the magnetic declination of their region. As you get it more and more out of true, TBE becomes more prominent until it won't maintain a stable hover (past 45 degrees or so).
     
  13. N017RW

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    Good points ElGuano.

    If you look at these devices at the component or module level (PCB) you'll see the either an arrow indicating forward [motion] or other orientation reference vectors for alternate mounting options. (i.e. coordinate systems). Since the DJI unit is enclosed it's not visible.

    Another thing to consider is that the Phantom's designers likely never expected users to 'adjust' these devices and made the installation fixed by virtue of its mounting enclosure so additional instructions or information on orientation were not deemed necessary for the 'user'.
     
  14. sar104

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    Correct - compass orientation is critical. The compass calibration procedure is performed to expose the compass sensor to the full 3-D local magnetic field, not to teach the FC the orientation of the compass relative to the aircraft. The variation in magnetic north (declination) is supposed to be corrected for in the Naza FC (lookup table or model calculation based on location), although obviously there are some unresolved issues with that for the Phantom.
     
  15. The wider legs are not perfectly strait down. They arc towards the ground. So if I was to mount the compass the face would be tilted slightly down towards the ground. Does this matter? or is it just reading North on a flat plane?
     
  16. sar104

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    Its 3D orientation matters because the magnetic field is not flat (tangential to the earth's surface). Tilting the compass on any axis will confuse the FC.
     
  17. CarlJ

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    I have the new taller wider legs, and mounted my compass on the same leg as the orignal as far down as the cable would allow. The posters stating that the location is important are correct. If not mounted in the right location your bird isn't going to fly right.

    Good luck!
     
  18. Wish me luck! Thanks for everyones help!
     
  19. derrickduff

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    I apologize for putting out bad info. I assumed the calibration was to adjust for local declination as this is something that can vary year to year. This it appears to be a poor design. A lot of people with extreme declinations seem to be having issues.