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Thread: How to determine where is COG on Z-axis

  1. #11
    Full Blown Hucker janoots2's Avatar
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    Quote Originally Posted by mitsaras View Post
    If I understand you correctly, you simply measure the vertical distance from the GPS to the center of the IMU (or Naza, I presume?), and use that as the "Z" value? You ignore the vertical location of the COG?
    Sorry but I am new to this...
    Correct, that's what works best on my setup at least. There was a thread a while back with others talking about it too. Yesterday I tried it out and confirmed that yes (at least on my setup), keep the IMU z axis at 0, and then measure appropriately for the GPS z axis (and x and y) off the middle of the IMU - or - the unit is much less stable. Test it out both ways though to find out what works best. My unit still flew and was controllable with the true COG inputted in, but a lot less stable
    Courage is the foundation to success

  2. #12
    Pilot Extraordinaire
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    I am with Borris on the c of g, all the lines and strings might work but a simple balance point is exactly what c of g is.

  3. #13
    Defies Gravity mitsaras's Avatar
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    Quote Originally Posted by janoots2 View Post
    ...keep the IMU z axis at 0, and then measure appropriately for the GPS z axis (and x and y) off the middle of the IMU...
    aha... so DJI gear considers itself to be "the center of the universe"!!!


    thx for your help!

  4. #14
    Full Blown Hucker BorisS's Avatar
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    guys pleas don't just enter 0 for the Z axis value for the IMU there is a reason why you can change that value. Its not rocket science to find a close value.

    Setp 1:


    CXA1.jpg

    1. I choose a imaganery line (in this case blue) which i take as an starting point to consider were theheavier parts are. Blue line is on the booms, because of the prespektiv it dosent work out in this foto.
    • Whats over the blue line ? The motors and the props and some plates and the IMU plus the Herkules ESCs.All probably adds up to 4x KW 139g a piece so 556 g. plus props and plates IMU ESC lets say another200 grams. Total 756 grams.
    • Under the blue line 4 s 6000 mah Nano Tech little under the line the attachment of the PH gimbal withthe GoPro on, Ladinggear reciver Gps etc. Feeling wise i already know I am heavier under the blue line.Nano Tech 623g Gopro 94 grams Microtilt PH may 100g, receiver GPs extension plate etc. maybe another100 grams. Total 917grams



    2. Simple test am I right with my assumption japp do to the tilt backward there is more weightunder the blue line. Thus my COG has to be somewhere under the blue line.
    • Now to find this exact your frame might give you the chance to hold it on a different spot to balance it out or you might not have the chance to do that than just make an assumption.
    • You can always take the birfd down and increase or decrease the value and see if the flight behaviorget better
    CXA3.jpg

    3. So i persume that my CoG on the z-axis is probably were the yellow cricle is. I have a good starting point.
    • The IMU is what ever the measurement is over the CoG thus i need a negatice values. Lets say the length of the green line is 7 cm thus -7 I will enter in the WKM assistant. 7cm from the CoG to the middle of the IMU
    • The GPS on the Carbon plate is once the Lipo is on horizontal so the GPS modul is spot on on the CoG thusthe value 0.
    In those cases were 0 for the IMU z-axis value works the reason might be that the IMU is already close to the to the CoG. Check your bird you know the weight of your components make an assumption work front there.

    Boris





    Last edited by BorisS; 06-23-2012 at 02:52 PM.
    CS8 conversion to X8 DJI WK-M Herkules III V3 ESCs
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  5. #15
    Full Blown Hucker janoots2's Avatar
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    I'm not saying just enter 0, but if you're unstable when you enter your cog in, it would be a solution to try. My cog is -11 cm from the imu, so its not even close to 0. Input this onto my system and it is very unstable. Keep the cog at the imu at 0 and it's rock solid. Works for me at least. Maybe ill try something in the middle to see if i can even get it better. It is a little twitchy in the wind, thats why i tried to dial it in better in the first place.
    Last edited by janoots2; 06-23-2012 at 04:27 PM.
    Courage is the foundation to success

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