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IMU Help

Discussion in 'General Discussion' started by Steve Spain, Apr 16, 2016.

  1. Steve Spain

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    Ok, this is an IMU question I have not seen addressed in the forum. My last IMU calibration was done without the camera, so I installed it and recalibrated.

    I used a magnetometer app on my phone to find the lowest reading in a large room devoid of magnetic/motorized objects. (40 microTessla was my low reading). So, indoors, with a brand new out of the box gimbal and lens, after recalibrating, I get a compass reading of 1275-1300 :(

    So I moved outside, (50 microTessla is my low reading here) and I am getting calibration in range, with compass reading of 1475-1500.

    So here is(are) my question(s): Should IMU calibration always take place outdoors? Does calibration utilize satellite GPS information?

    Thanks,

    Steve
     
  2. Blue Bird

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    No you do not have to worry about gps sensors for IMU calibrations. How ever it would be for just calibrating the compus itself. IMU calibration does effect the gimbal during the calibration process as it helps center and level the camera.
    Just be sure your on a completely level surface before starting the calibration and do not touch or disturb the bird until complete. After that, then you can calibrate the compus out side free of magnetic interference such as metal and power lines etc. You should be good to go then.
    Good Luck!,, BB ;)
     
    #2 Blue Bird, Apr 16, 2016
    Last edited: Apr 16, 2016
  3. Steve Spain

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    Thank you. When I calibrate the IMU and get an IMU compass reading outside the range, like the 1250 I was getting, is the solution to just keep moving locations until you get readings in spec? Recalibrating in the same location seemed to make no difference.

    Put simply, if you calibrate the IMU and any of the readings are out of range, and the AC/Gimbal integrity is good, is there a solution other than changing locations? Or does it mean your AC needs service?
     
  4. The Editor

    The Editor Moderator
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    OK - let's start afresh......
    The Compass and IMU are two completely different things (although they appear on the same page in the app with regards to mod values).
    The IMU is a solid state mems gyro unit which measures the aircraft's movements in three dimensions using a series of very sensitive Accelerometers and gyroscopes. This is distinct and different from the internal magnetometer or compass module.
    The IMU can be calibrated near to metal objects - it isn't fussy in that regard since it isn't influenced by magnetic flux. The important thing is the IMU is calibrated (preferably indoors to avoid wind vibrations) on a totally level and vibration free surface.
    The compass on the other hand should never be calibrated indoors - irrespective of what some app says.
    Only ever calibrate the compass outside of buildings since these are full of metal (pipes, beams, radiators, metal studs, wiring etc etc).
    This is why you are getting correctly calibrated values outdoors and skewed ones indoors (even though the gimmick app says otherwise).

    Calibrate your IMU first on a cold machine - that is one that has sat on the table for half hour or so to totally cool down the IMU which is internal to the flight controller. Turn the Inspire on (with gimbal in place) and as soon as the aircraft has initialised and the gimbal has done its startup sequence immediately go into the IMU calibration screen and run the calibration.
    Once this is done, then go outside and calibrate your compass.
    Check your mod values afterwards and you should be good to go.
    Bear in mind that if you then take your Inspire indoors and turn it on and check your mod values again the compass will likely be outside the range of 1400-1600. This is because the building structure is now affecting the compass. Once you return outside the values will normalise.
     
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  5. Alastair

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    Couple of points....when you do the IMU callibration make sure the AC hasn't been on for at least 20 minutes. This process can be done at home, inside. Metal does not affect the IMU calibration. Call me anal but I use a sprit level. I use folded newspaper as a shim under the legs, to make sure it's perfectly level. Then, I turn on the the RC and bring up the calibration screen. Only then do I turn the AC. When it boots up. I start the calibration. Do not touch the AC or cause any vibration during this process. The IMU values appear in the MOD box on the "acceleration" line. A proper IMU calibration should read between 0.99 - 1.01. When you do the compass calibration, make sure there are no cars or anything metal around. Also, stay away from cement structures, as some contain rebar. A good compass reading will appear in the MOD box on the "compass" line. A reading between 1400 - 1600 is considered safe. No further IMU calibration is needed unless the bird receives a hard bump, or there is a firmware update performed. As far as the compass is concerned, others on this forum have noted before, once you have good reading it's not necessary to do one every location, unless you've driven several hundred miles. Just make sure you're compass values are within range. As "the editor" has mentioned before, if you do a compass calibration every time, one of those times there will some buried metal. The compass will calibrate and the values will look normal, until you take off. Then, without the influence of the metal, the values will change resulting in a possible crash. Safe flying. Whew! Sorry to be long winded.
     
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  6. Alastair

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    Haha great minds think alike.
     
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  7. Steve Spain

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    Many thanks to you all for shedding light on this. I'd been going through the paces without a good understanding of the principles. Guess I've been really lucky so far, and hopefully your insights will keep me flying straight!
     
  8. Blue Bird

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    By the way Ed, I installed the new LB fan over the weekend and finally got a perfect IMU calibration. If you recall that thread, I had a broken fin on the cooling fan. All is perfect now and Blue Bird is flying like a dream.:cool:
     
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  9. RaptorMan

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    E, great write up and very helpful.

    I just completed my calibrations after firmware update -- IMU looks good with MOD values of 0.00 and 1.00 and MOD value for compass after calibration outside was bouncing between 1485 and 1520 so that looks good to. No problem with sticks either.


    Brian
     
  10. Bruce Cuz

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