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Vibration Induced Noise - How much is normal?

Posted by Christopher Lansdale on September 2, 2015 at 11:02pm

Hi all!

I'm creating an autopilot for my FT Guineapig (https://www.youtube.com/watch?v=NLoNXE-8eAk), but I'm getting a LOT of IMU noise. It can reach an amplitude of around 22 degrees error. I'm using the Adafruit 10DOF breakout board's LSM303 magnetometer and accelerometer for orientation, as well as their AHRS library for Arduino. While I have laser balanced my props to 1/2" over 4 feet distance, I still get a LOT of noise on the sensor while the motors are spinning, seemingly from vibration.

From your experience, is this normal and has to be dealt with software filtering, or is it a problem with the plane?

Thanks,

Chris

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Replies

  • Phillip Schmidt September 2, 2015 at 11:26pm

    Oh, my... where to start... That is a really crappy AHRS library for anything that isn't standing still with no vibrations.  

    It does not use the gyro at all, which is the most important sensor for a flying device.

    It will fail horribly as it deviates from level as well due to magnet field inclination.  There is no magnetic declination compensation either, which can be an significant source of heading error in many parts of the world.

    I'm not a fan of an AHRS that outputs in YPR degrees, but that's not the end of the world.  

    This is programmed to be very, very slow computationally.  Way too many trig functions and floating point divides.

    Your aircraft might be part of the problem, but It would be a miracle if anything did not crash when using that AHRS.  It is intended for things that sit on your desk, not flying things.

    • Christopher Lansdale > Phillip Schmidt September 2, 2015 at 11:53pm

      Thanks! Any libraries you would suggest? I've stayed away from others so far because most I've found have been for other sensors, and I would rather not modify them. I'm still new to the software side of things.

      Thanks so much!

      Chris

      • Phillip Schmidt > Christopher Lansdale September 3, 2015 at 1:24am

          I wrote my own as an educational exercise, but my code is not for those sensors.  It would not be hard to modify it for them, but i really don't have time at the moment.

          I would look at the Madgewick&Mayhony IMU or freeSpace IMU.

          Another thing to look at is the sensor library you use.  Many are designed to be operator friendly, but run rather slowly.  I wrote my own to save processor time.  When flying, speed is life, both physically and in control systems.  

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