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  1. #121
    Join Date
    7th February 2009
    Location
    Arlington, TX
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    1,257
    Quote Originally Posted by SparkyVTFlyer View Post
    Radians on roll.
    That's the important one.

    While your plot looks wild, it is hard to tell anything from just pitch and yaw. When I plot that for my data it also looks odd but the tilt tracks the quaternion version perfectly till ejection. (I computed tilt from pitch and yaw by squaring them and taking the square root of the sum.)

    Click image for larger version. 

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  2. #122
    Join Date
    8th February 2009
    Location
    O'Fallon, IL
    Posts
    108
    I was out for a few days on vacation. I got back and tried your formula with interesting results. I combined pitch and yaw a few different ways and they showed the same thing.

    I'm thinking there might be a speed limit to your method. I tried it for one flight that only rolled twice on the way to apogee and your equations were spot on. However, when I tried it with a flight that spun like a top on the way up, then it quickly got out of control. See the plots below.

    I'm not sure what to make of it.

    Click image for larger version. 

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    Sparky

    NAR #85720, L3
    Tripoli #12111, L3
    2017 Impulse: 86,532 N-s
    2017 Flights: 26

  3. #123
    Join Date
    7th February 2009
    Location
    Arlington, TX
    Posts
    1,257
    When you say "spun like a top" it makes me wonder about the roll and sample rates. The simplified algorithm I presented (not mine, found in another thread here) uses the small angle approximation.

  4. #124
    Join Date
    8th February 2009
    Location
    O'Fallon, IL
    Posts
    108
    Peak roll rate was about 1400 degrees per second. Average roll rate through most of the flight was about 700 dps. Datarate at about 800 samples per second through the flight.

    Small angle approximation should be fine within 10 degrees for sine and 14 degrees for tangent, and none of the individual cycle rotations violated either. So I'm not totally sure why it isn't working.

    I also take advantage of small angle approximation. Combined with fixed-point math I got around 400-450 cycles per second written to an SD card on an ATMega328. I double that now with the Teensy3.5, which pretty much pegs the output data rate of my sensor package (even over I2C).
    Sparky

    NAR #85720, L3
    Tripoli #12111, L3
    2017 Impulse: 86,532 N-s
    2017 Flights: 26

  5. #125
    Join Date
    7th February 2009
    Location
    Arlington, TX
    Posts
    1,257
    While that is good enough to keep the small angle approximation error below 1%, there is still some error. I have no idea what the limitations of this algorithm are as I haven't found any description of its derivation.

  6. #126
    Join Date
    21st April 2009
    Location
    New Jersey
    Posts
    141
    Quote Originally Posted by SparkyVTFlyer View Post
    Peak roll rate was about 1400 degrees per second. Average roll rate through most of the flight was about 700 dps. Datarate at about 800 samples per second through the flight.

    Small angle approximation should be fine within 10 degrees for sine and 14 degrees for tangent, and none of the individual cycle rotations violated either. So I'm not totally sure why it isn't working.

    I also take advantage of small angle approximation. Combined with fixed-point math I got around 400-450 cycles per second written to an SD card on an ATMega328. I double that now with the Teensy3.5, which pretty much pegs the output data rate of my sensor package (even over I2C).
    Here is an example of using your Arduino code with my Arduino Nano, MPU-6050 (16G) , ADXL-78 (70G) and BMP-180 for recording data.
    I also had a 70cm GPS transmitter using Lora SX-1278 with Adafruit Feather M0+ Cortex processor.



    -John

  7. #127
    Join Date
    19th February 2017
    Location
    The world, probably
    Posts
    532
    Quote Originally Posted by UhClem View Post
    While that is good enough to keep the small angle approximation error below 1%, there is still some error. I have no idea what the limitations of this algorithm are as I haven't found any description of its derivation.
    As far as I know it basically assumes that the "arc" of the angle is the same length as the opposite side of the right triangle, so the length of the arc (also known as the angle in radians) is equal to the sine of the angle. As the name would suggest, it's good for small angles but not large ones- 0.4% error at 9 degrees or 1.6% at 18 (unless my math is wrong, which is totally possible).

    In other news I was able to get a bit done on the ArdIU code this weekend- more specifics when I have a few minutes. Basically, I have the basic groundwork set up for datalogging, including a handy template function that should be able to write any datatype to the SD. I still have a while to go, but I'm making progress!

    <shamelessadvertising>Speaking of progress, there are still 3 unclaimed beta boards...</shamelessadvertising>

    NAR #104043

    crmrc.org

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