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Compass Calibration: What is measured?

Discussion in 'Pro/Adv Discussion' started by StumbleBee, Sep 3, 2015.

  1. StumbleBee

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    I regularly check my compass calibration value and have always found it between 1482 and 1500 although it jumps around constantly between those figures. Thanks to posts by many of you I realize the importance of a re-cal every location change. Also, I usually do one even if flying in the same location a week later (with no other location in between). Some might call me obsessive but "there are bold pilots and old pilots but no old bold pilots".

    Anyway, what I'm wondering is what precisely the 1500 value 'goal' means? 1500 what? And how was the 1500 read, is it the sum of values from cardinal points or what? A sort of gausmeter?

    Another thing I wonder is this. What if you did a calibration with your P3 turned 180 degrees during both the horizontal and vertical calibration circles. Would it then have its calibrations reversed and think N is S and E is W? And theoretically, would that even matter since no inputs are compass points?

    I know someone here knows, and then I will too.
     
  2. robinb

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    You do not have to calibrate compass for each location, only if you go more than 50 or morales away.

    You are confusing compass and imu
     
  3. StumbleBee

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    I'm not so sure of that robinb. I've never re-cal'd the IMU because the screen tells me that it is not necessary. What exactly is IMU and how is that measured?
    Yes, I've read that the compass does not have to be done as often as I do, but I'm just making sure. It's a habit I got into decades ago when I did bluewater sailing and lighthouses took the place of a GPS device.
     
  4. ianwood

    ianwood Taco Wrangler
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  5. BudWalker

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    The MOD value is the square root of the sum of the squares of the X, Y and Z numbers.
    MOD = ( X**2 + Y**2 + Z**2) ** 1/2.

    The compass has three separate magnetometers that measure magnetic flux. These three magnetometers are aligned in the X, Y and Z axes the Phantom. I.e., Z points up/down, X points ahead/backwards, and Y points left/right. The earth's geomagnetic field is inclined in most locations so it is not aligned with any of the X, Y, Z axes. This inclination and the direction the Phantom is pointing cause the geomagnetic field to be projected onto each of the axes. If the phantom is pointed directly magnetic north then Y=0 and the geomagnetic field is projected onto X and Z, If it's pointed NE then X=Y and not equal 0. The MOD value is the strength of the geomagnetic field that is derived from the strength of that geomagnetic filed that is projected onto each of the X, Y and Z axes.

    Calibration is the process of adjusting the gain on each of the X, Y, Z sensors so that the MOD value falls close to 1500.

    Not sure what your describing about calibration 180 degrees different. If you were to calibrate the Phantom while holding it upside down then it "shouldn't" make any difference. The X, Y and values will have sign reversed but that shouldn't matter since these values are squared. The calibration process may detect the Z field having a negative value and refuse to calibrate. I think I'll try this later.
     
  6. StumbleBee

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    Thanks for a concise and understandable description of how the value is obtained and why it is important. It all makes sense to me now.
    Yes, you are correct in your assumption about what I meant, I did mean that the unit would be held reverse to what is required. If you're willing to be the guinea pig I'll be watching for your post.
     
  7. N017RW

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    It does not matter if you rotate CW or CCW whist calibrating.

    You cannot perform a 'partial' calibration by only rotating 180 deg.
    The device is 'smarter' than that.

    In fact you may need to rotate a bit more than 360 deg.
     
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  8. StumbleBee

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    Hi ianwood. Thanks for the referral to your excellent primer all about compass calibration. I've read it but it doesn't seem to say that you can actually calibrate too much. It does give examples of what could cause a flawed calibration but doesn't say calibrating could do more harm than good. Or are you saying that excessive calibration may increase your odds of getting a 'bad' calibration? As long as I check my MOD value afterwards I should be OK, no?
     
  9. StumbleBee

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    Sorry, I didn't make myself clear. I didn't mean that I would only perform half a dance (180 degrees), I meant that I would hold the P3 up instead of down and in instead of out. Interesting to know I can spin the other way! Might offset some dizziness. :D
     
  10. ianwood

    ianwood Taco Wrangler
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    This. And your calibration doesn't go bad over time so there's no point. If it works, keep it. You could arguably use the same one for months if you flew in the same area and didn't make hardware or software changes.
     
  11. StumbleBee

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    I'm on board now. Understanding the 'why' reassures me that I can fly with 100% certainty of having done what is humanly possible to control.
     
  12. BudWalker

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    I tried calibrating upside down. I have a test stand that I can use to precisely point the phantom in 8 cardinal directions. I recorded the X, Y, Z and MOD values for each of the 8 cardinal directions and then did the same after calibrating upside down. I then repeated these two calibrations. I statistically eye-balled the numbers and didn't see any significant differences in the recorded X, Y, Z and MOD values.

    Calibrating upside down is equivalent to calibrating in a location on the other side of earth that is directly opposite my location.

    I then flew it with the upside down calibration by launching in ATTI mode. It's a windy day here and I had to work some to keep it in the same spot. When I switched to GPS it stabilized indicating correct readings from the compass.

    I then did a normal calibration.
     
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  13. III% Streve

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    There has been some indication from a few people that even with a MOD value of as high as 1480ish there will be some strangely unstable take-off conditions. As in a significant lean as the quad leaves the ground. This condition was remedied by redoing the compass calibration. My intent in that thread was not to imply that a more frequent calibration was needed, but more in the direction that, first and foremost, a person MUST look at their MOD value prior t takeoff! And if ,in the process of takeoff ,there is any tendency to lean to any direction, or not remain stable rising, there could be a need for calibration anyway. I was able to duplicate the unstable situation myself, and could almost always predict if a problem would arise by watching the MOD setting as I moved my P3 around the ground. Even in small movements of less than a yard. This info was forwarded to DJI along with video and logs. Hopefully they can do something in the app to help better predict this.