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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    Full absolute Attitude and rate plus Speed without GPS or pitot tube or mathematical integration by
    3-axis hall sensor, 3-axis turn rate sensor, 3-axis acceleration sensor, 3-axis IR horizon sensor
    VvonInnen.png
    Separation of gravity acceleration and turning acceleration is based on attitude knowledge.
     

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  2. MoTheG

    MoTheG Airman

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    Example:
    the bank angle is -45°; the acceleration is towards the bottom of the aircraft (z-axis) and -1.41g; the turn rate around the x-axis is 0 rad/s (bank angle is fix); the turn rate around the y-axis is -0.3535534 rad/s; the turn rate around the z-axis is +0.3535534 rad/s.
    From this we can calculate that the turn is around the Z-axis at +0.5 rad/s and the turning acceleration is 1g ~= 10 m/s².
    Therefore the speed is 20 m/s (45 mph). It takes the aircraft 13s to complete a 260 m diameter. 41.4 m radius circle.
     
  3. MoTheG

    MoTheG Airman

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    x;y;z be a coordinate system aligned with the sensors with x pointing in the direction of flight.
    X;Y;Z is aligned with X pointing north and Z pointing up.

    The filtering of the calculated speed is even more important than the filtering of the measurements. Because multiple measurements are used the error is compounded. Due to the division the calculated speed may be way too high because the turn rate was near zero. The result may be judged by it's agreement with the linear acceleration as well as the amplitude based estimate of the SNR of both the acceleration and the turn rate signals.
    If either value is small the speed will be off. The higher the turn rate and the G-load the more precise the speed.

    IR horizon sensor and compass together provide more information than needed at all attitudes, this can be used for corrections. The Y-axis is provided by both the Compass and the IR-horizon sensor.
     
  4. MoTheG

    MoTheG Airman

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    Is this good to prevent stalls ?
    Yes, it is. With the same approach as for the speed, the wing load can be calculated. A stall is most likely to happen in too slow a turn. The too high turn rate for the load can be detected. If the load is high more thrust or pitch down can be added automatically before the plane goes out of control.
    Wload.gif
    This can be used to avoid losing a tail heavy plane.
     
  5. MoTheG

    MoTheG Airman

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    Search cloud: airplane gyro UAV velocity infrared http://wiki.paparazziuav.org/wiki/Sensors/IR

    This applies to fixed wing model aircraft. The speed is NOT only derived by use of a Integral \integration\accumulation only if the measurement is too poor does the speed have to be extrapolated.
     
  6. MoTheG

    MoTheG Airman

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    Does anyone know why not NIR sensors are used for horizon detection but thermopile\thermocouple sensors? The grass we usually fly over should be bright at the 800nm - 1µm that IR-Si-diodes like.
    Seems to be more about thermal emission than light reflection.
    PIR-Sensors out of movement detectors can not be use because they do no measure absolute brightness.
     
  7. MoTheG

    MoTheG Airman

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    This is how the above is different to a Kalman-Filter where the trust in the measurement is constant.
    Because the speed is the quotient\ratio between two measurements we are independent of their absolute magnitude. Because of this we may weight results with low measurements less when updating the state vector.
     
  8. lysle42

    lysle42 Ace Pilot

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    man there is a great little cell phone app that is real close to what our radar says 3% error if temp is set right only about 4 bucks fo app and free test. much easer and close enough for me. RCspeed
     
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  9. lysle42

    lysle42 Ace Pilot

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    RCspeedo on google play store
     
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  10. MoTheG

    MoTheG Airman

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    Yes, just totally different application. This is meant for autonomous operation or assistance. How is a phone going to bring the plane back if the RC does not work or going to prevent stall?
     
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  11. lysle42

    lysle42 Ace Pilot

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    it won't
     
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  12. Airforce101

    Airforce101 Top Gun

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    Hi Sheldon, say hello to Raj and Wolowitz for me, will you?

    ;)
     
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  13. MoTheG

    MoTheG Airman

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    I am Howard, not Sheldon. My mom said I looked good in those turtlenecks. And yes, engineering is a science if you are not just a technician.
     
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  14. bogusbandit56

    bogusbandit56 Top Gun

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    You lost me after the words, Full absolute.:(
     
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  15. MoTheG

    MoTheG Airman

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    estimation of speed from centripetal force and turn rate by known attitude and g-load for fixed wing (model, drone, UAV) airplane. "No GPS", "without GPS"
    for the mechanics see: http://www.dept.aoe.vt.edu/~lutze/AOE3104/turningflight.pdf
    The key idea that makes this practical is in post #7
     
  16. MoTheG

    MoTheG Airman

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    sigma_v.png
    As I mentioned above the absolute error \ variance depends largely on the turn rate. The up-side of that is, that you can expect good results for high turn rates, that is minimum time (as opposed to minimum radius) turns.
    The error for the centripetal (centrifugal) acceleration is simply the error of the attitude + the error of the acceleration sensor.
     
    Last edited: Apr 12, 2015
  17. MoTheG

    MoTheG Airman

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    That did not come out right. What I meant is:
    Because the speed is the quotient\ratio between two measurements the relative error of the speed is not a function of the speeds absolute magnitude. Therefore the speed can have little error even if it is low if the turn is tight enough.
     
  18. quorneng

    quorneng Ace Pilot

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    You can measure straight line speed pretty accurately with two people and two stop watches. ;)
     
  19. MoTheG

    MoTheG Airman

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    For the horizon sensor I have an idea that I do not know if it is practical \ workable:
    Instead of going by the relative brightness of detectors covering continues sectors one might use an encoding scheme.
    Using multiple detectors for the same sector one could place lenses in front of them in such a way that the sector is subdivided with each lens looking at multiple subsectors at the same time, but each detector looking at a different set of subsectors.
    Assuming we live on a barren flat glowing surface with a dark sky, one could use five detectors to place the horizon into one of 32 subsectors, the one that is changing the most and neither among the uniformly bright ones nor the dark ones.
    The encoding scheme could be overlapping single continues sectors per detector, non overlapping non continues multiple sectors per detector, gray code.
    Nowadays it is easier to use redundancy and processing than expensive detectors and calibration. Even with the detector on the other side of the aircraft blinded by the (setting) sun, one of the described devices should give a reading.
     
  20. MoTheG

    MoTheG Airman

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    Here is the most obvious way to do it with ten diodes and 5 comparators:
    Align five pairs of diodes along the horizon, have one pair look at the top and bottom half, one pair looking at quadrants, one at octants and so forth. Compare the pairs and take them as digital digits, done.
     
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