How to measure the speed from within without GPS, pitot-tube or fan

Discussion in 'Miscellaneous' started by MoTheG, Apr 7, 2015.

  1. MoTheG

    MoTheG Airman

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    I drew a segment of the lens in front of the sensor line:
    Linse.png
    There are e.g. triangular pits in front of every sensor for those sectors that are supposed to be blind spots.
    The refractive index should be low.
    ( prior art )
     
  2. MoTheG

    MoTheG Airman

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    That is not diameter but of cause circumference.
     
  3. MoTheG

    MoTheG Airman

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    Method to attain a correction vector for acceleration based estimate of attitude and velocity.
    To continuously estimate the attitude and velocity based on 3 axis turn rate and acceleration sensors, it is necessary to separate / distinguish gravity acceleration from change of movement acceleration based on current measurements and the estimated attitude and speed. This can not be done for any prolonged period without additional data or plausibility checks.
    With all estimates being correct the gravitational acceleration should always point down in a terrestrial reference system.
    To check this, the estimated gravity vector is transformed to the terrestrial reference system and integrated over time until the next correction.
    At the end of the chosen correction interval the resulting vector is compared to the expected vector, the difference is the error that now may be used to correct the current estimate. The attitude is easily corrected because its length does not exist, for the velocity the interval duration must be taken into account.
    The big problem with all this is that "gravity vector is transformed to the terrestrial reference system" as well as the separation of measured / sensed acceleration into gravity and change of motion both prerequisite knowledge of speed. It may happen that you get into a confirming circle that will lead to mathematically sound yet wrong estimates.
    To make this work I would always use a 3D magnetic compass, it costs little extra and raises precision, even though it does not provide one of two and a half attitude angles (attitude consists of one unity vector and one angle or two and a half angles) at any one time it will provide that extra absolute heading information. Today they are easy to come by. Types for under 1mT field strength are: Bosch BMM150, NXP MAG3110, MEMSIC MMC246xMT and there are certainly more.
    In the context of a fixed wing aircraft it can be assumed that the yaw is limited. Large yaw angles or yaw for prolonged duration may thus be used to check plausibility, but that is going into more complex modeling that requires the control inputs.
    a_{mov}=a_{sens}-g=a_{tran}+a_{rot}(v)
    v=\dot{\omega}r=\frac{a_{rot}}{\dot{\omega}}
     
  4. Flybyknight22

    Flybyknight22 Top Gun

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    “Absolutely” Ian!
    And on that note,..
    It’s time for a taste of freedom break BB56!

    60FF6905-EAFA-499A-8D77-7F823D94FF0F.jpeg
     
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