ALTA Compass calibration

Discussion in 'ALTA' started by Cesar Fernandez, Dec 31, 2015.

  1. Cesar Fernandez

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    Is there any tutorial video on Compass calibration? I follow the instructions on the APP but at times it takes many tries (many flips and turns) for the application to accept it. I have seen people flipping the drone in the opposite direction that the app shows on the screen so, I just want to be sure I am doing it right and that it gets calibrated OK.
    Second question, once the app gives the "OK", or calibration completed, how sure can you be that the drone has a correct calibration and ready to fly using GPS parameters? can one really trust the application?
    Third question: once you calibrate the drone in a particular area, how often you have to re-calibrate? (assuming you have not moved to another area far away?
    Thanks for any advice.
    By the way, I built a very simple device that would allow single operator compass calibration to be done extremely easy. If anyone is interested I would be glad to send the picture on how to build it. No cost and it can be done in minutes.
     
  2. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    I just follow the animation on the iPhone app. It's a lot easier to do the calibration if you have two people. That's especially true if you are going to fly with the MōVI and camera as you need to calibrate with the configuration you're going to fly.

    Yes. :)
    The long answer is: Yes, to the degree that any computer system running software written by humans is trustworthy, you can trust it.

    If you're going to move to a new area that has substantially different surroundings that might affect/distort the Earth's magnetic field, then recalibrate. If you have changed the aircraft and/or payload configuration then re-calibrate. If you have any worries, concerns, financial problems, dietary issues, or political doubts, then.....well....it won't do any harm to recalibrate! :)

    I think you will find that every ALTA owner/operator would like to see images of that!

    Thanks
    Andy
     
  3. Jose Luis Ocejo

    Jose Luis Ocejo Active Member

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    Andy I have been calibrating my Alta solo, detach from the Movi its way easier than calibrating with a Movi/camera attach, I have no issues what is the actual reason to calibrate with the payload? does it really make a difference?
     
  4. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Jose: The question you're really asking (if you don't mind me restating it) is: "Is there anything electrical or or magnetic on the MōVI that might generate any local distortion of the Earth's magnetic field?"

    The answer is: I'm pretty sure it will have some effect, but I don't know how much. Therefore, given the choice of calibrating the ALTA's compass with or without the MōVI, I'd always do it with the MōVI and the camera powered up.

    What you are doing by calibrating the compass is allowing it to sense the three-dimensional Earth's magnetic field, taking into account "deviation" (the amount magnetic north is off from true north -- which is 16 degree East in my neck of the woods) and the "dip angle" or "inclination" (the angle at which the Earth's magnetic field dips down into the ground and heads directly towards the Earth's magnetic poles).

    To do that you move the compass through various circles (e.g. facing the ALTA north and pitching the ALTA over, then East and pitching the ALTA over), so that it can compute corrections to the three axis magnetic field sensors (X, Y, and Z) that act as the compass.

    The more accurately you can "expose" the compass to the surrounding magnetic field (including that generated by the ALTA, the MōVI, and perhaps even the camera), then the more accurately the X, Y, and Z corrections can be computed.

    Hope that helps.
    Andy.
     
  5. Cesar Fernandez

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    Hi Andy, thanks so much. I think you just won "the answer of the year award": concise, precise, to the point and very funny.
    Thanks again.
     
  6. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Glad it helped, Cesar.
    Too bad that it was the "answer of last year..." :)

    Andy.
     
  7. Jose Luis Ocejo

    Jose Luis Ocejo Active Member

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    Thanks Andy I will try to calibrate the Alta with the camera and Movi together, I wonder what the wiz Synapse FF designers have to say about this, is there a way that they or us can measure, or see the data between calibrating the Alta with the Movi/camera and calibrating the Alta solo .
    FF any one?
     
  8. James Adkins

    James Adkins Member

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    If you have a spare $2750 you could just measure the fields with a fluxgate magnetometer and know for sure. They measure fields thousands of times weaker than the earth's.
     
  9. Jose Luis Ocejo

    Jose Luis Ocejo Active Member

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    Hmmmm na.... Il just calibrate the normal way
     
  10. alex ryan

    alex ryan Member

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    Can you post a picture of this one person compass calibration contraption?
     
    Dimitar Krustev likes this.
  11. Cesar Fernandez

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    Dimitar Krustev and MIke Magee like this.
  12. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Good find, Cesar. I didn't know Jeff had done a video.
    I'll add it to a Sticky Thread.

    Andy
     
  13. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    By the way, bear in mind:

    1. If you're going to flying with a MōVI and camera, attach it to ALTA and power everything up before you do the calibration.
    2. As Jeff says, be very careful, where you do the compass calibration -- it's all too easy to "bake" in an error if you calibrate near metal structures, railings, or on concrete with rebar embedded in it.
    3. It does no harm to recalibrate the compass before the first flight of the day (as long as you don't bake in an error by doing a bad calibration), but typically, if you're flying out of the same location as you did for your previous flights, you don't really need to recalibrate.

    Andy.
     
  14. Jose Luis Ocejo

    Jose Luis Ocejo Active Member

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    Andy just coin a new term "bake an error":)
    in the video Jeff calibrates a naked Alta solo, very effortless, with out a Movi and camera its got to be pretty difficult solo to do the same loaded even with a light cam setup, Movi powered, or Movi off, with an Alta loaded minimum you need 2 people to calibrate and figure a way for the Movi not to go crazy with the tumbling,
    now we the users only get a "Good" feedback from the app,.... short of buying a Fluxgate like James mention, (im not doing) again I wonder if the folks at Freefly that may have a Fluxgate or a bunch of fancy measurement toys, could do a test calibration with an Alta naked, and a Alta loaded and see the numbers, and see if it makes a diifference that maters or not, calibrating without the gimbal is so easy.
     
  15. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    I'd like to say I could lay claim to "bake in" as an original thought, but it is widely used in the computer world to describe the process of making a temporary parameter the new default.

    As to testing whether the MōVI and camera matters, if you have an iPhone it has a fluxgate magnetometer (compass) in it.

    1. In open outdoor space (and not near the MōVI and camera), use your iPhone and built-in Compass app to get a measurement of which way north is.
    2. Then bring the MōVI and camera close to the iPhone (don't move the iPhone to the MōVI!), power up the MōVI and camera and then see if you can observe the compass on the iPhone changing.
    3. If it changes, then the MōVI and camera (and the electromagnetic fields they create) is inducing a compass error and you must calibrate the compass with the MōVI and the camera mounted to the ALTA and powered up.
    4. If the compass does not change, then you can calibrate the ALTA's compass without the MōVI and the camera.
    Also note, I've created a "quick reference" PDF for the ALTA compass calibration procedure so you can park it in your flight bag and refer to out on location to remind yourself of the procedure. You can see it in the ALTA FAQ 9 here. Hope this helps.

    Andy.
     
  16. Jose Luis Ocejo

    Jose Luis Ocejo Active Member

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    Thanks Andy it is really cool and helpful you did that pdf, thank you, I will try the test you mention with the iPhone,
    back in the Cinestar days, I just to calibrate the copter loaded all the time, with the release of the Toad and the hole it made it so easy to detach the gimbal and calibrate the copter, doing it all together does make sense, I guess the way would be to power the Movi and put the motors in kill mode, use like mini bungie to partially restrain the 3 axis and calibrate away
     
  17. Rolf Ableiter

    Rolf Ableiter Active Member

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    what is the deal about the compass calibration if the copter is flying as it should?

    i did many flights just changing the declination value if needed.
    i calibrated the compass once and didn't do it again.

    if i have a "compass good" and everything is fine while flying why should i calibrate the compass?
     
  18. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Well, the "compass" is not really a compass. It's a 3-axis fluxgate magnetometer that you need to "train" so that it knows which way North is based on the local magnetic field impinging on the ALTA.

    Thus if you fly in a different area where the Earth's magnetic field is different from where you last calibrated, then you need to re-calibrate.

    The declination you mention is only the difference between true north and magnetic north at your location and that's predictable from an isogonic map (see here for an example). But it's entirely feasible that two locations that have the same declination might have different magnetic field strengths or local distortions caused by, say, being near the Cascade mountain range where the mantle is thinner and you're closer to magma.

    Also, coronal mass ejections (CME) from the sun distort the Earth's magnetosphere and that affects the Earth's magnetic field on a minute-to-minute basis, especially when there is a major solar storm going on.

    Having "compass good" merely means that the calibration factors that you last got when you re-calibrated the compass are within acceptable values. It does not mean "the magnetic field in which your copter is now flying has accurately been compensated for in the 3-axis fluxgate magnetometer."

    Apologies for all the words, but you did ask "why should I calibrate the compass?"
    And that's why! :)

    Andy.
     
  19. Rolf Ableiter

    Rolf Ableiter Active Member

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    thanks - that's an answer i like because it is an answer :)

    but still there is a part of the question left to answer: why should i calibrate the compass when the flying behavieur of the copter is "normal", even in GPS mode? is it because i can get in trouble when the ALTA is doing a coming home? Or will i recognize the compass error even in normal flying?
     
  20. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    I think the question your asking can be restated as: If my compass is erroneously calibrated, will it even show up in the way that the ALTA flies.

    The short answers to that are: I don't know. Maybe. :)

    For those flying Mikrokopter Flight Controllers, the compass was used primarily to provide additional yaw stability -- but I don't know how it's used in the Synapse, I'm afraid.

    Andy.
     

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