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IMU Values

Discussion in 'Inspire 1 Help' started by Kipp, Jun 5, 2015.

  1. Kipp

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    After my Inspire 1 crash last week I am back up and running. I am now putting together a 1 or 2 page laminated reference guide with everything from a pre-flight checklist to battery indicator information. I've searched the Forum and the one thing I can't find is a solid indication of what all the IMU values should be. If this is posted somewhere let me know and moderator please delete this thread.

    My understanding of this is the gyroscope and acceleration should be +/- 1. The compass should be +/- 300 with the compass mod being around 1500.

    Is this correct?

    Any other ideas of what would be good in a neat laminated quick reference checklist?

    I will upload and post my finished reference guide if anyone wants it once it's done.
     
  2. amkorp

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    with a sucessful IMU calibration and the aircraft on level surface the value you should be aware of is the compass mod. I believe the acceptable range is 1400-1600.
     
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  3. The Editor

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    Gyroscope (undisturbed) should be 0.00
    Accelerometer -0.99 - 1.01

    Compass (once calibrated separately as an IMU calibration does not have anything to do with compass) 1400-1600 (Ideally as close to 1500 as possible)
     
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  4. sirnikolas

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    *takes a screen shot*
     
  5. damoncooper

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    Actually (I do learn things from time to time), the compass mod values are highly locale specific I have learned. The closer you are the equator the smaller the norm mod values are. A guy in Mali has norm mid compass values of 300, the folks here in Boston, 1200-ish is normal etc.

    What's important is you know what's "normal" for your area and eyeball that value before takeoff and understand whether or not you are taking off from an area of high magnetic field interference.
     
    #5 damoncooper, Jun 6, 2015
    Last edited: Jun 7, 2015
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  6. John Alan

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    Ahh... learned something o_O ... now I (maybe) understand WTF the compass mod number does... :cool:
    It's an internal variable likely based on GPS position to store an degree offset value based on where you are...
    http://en.wikipedia.org/wiki/Magnetic_declination

    Edit - Although on further review it may not be that simple... :oops:
    The 300 in Mali is not matching up with the maps in the linked article...
    I'm 1+ hour SW of Chicago with a slightly negative declination of about 3 degrees and see 1500-ish on my bird
    Boston is about negative 15 degrees (on the map) and your reporting 1200-ish
    I'm taking a WAG and thinking Zero degrees offset must be about 1638.3 or so...
    (based on a 0 to 32767 scale with zero point at 16383)
    Now if northern California (plus 15 degrees) is seeing around 2000 normally... then there is something to my WAG speculation and guessing...
    Otherwise I'm totally wrong in my line of thinking here and should be ignored... :p
     
    #6 John Alan, Jun 7, 2015
    Last edited: Jun 7, 2015
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  7. damoncooper

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    Yeah it's not magnetic declination. That's a whole other ball of wax.

    The mod values are just variances perceived by the sensors but the size of the compass mod value is dependent on locale/latitude.

    It's not a huge deal except it's worth noting that "my" normal mod value and "your" normal mod value may be quite different depending on where you are on the globe.

    What's important is for you to know what a "normal" range for your locale is and beware of calibrating your compass in an area where there are strong magnetic fields, large steel structures, etc.

    This free iOS app is handy (in addition to the compass mod value) in helping detect whether you are standing in an area of strong magnetic fields:

    Local Magnetic Field Check (optionally check before takeoff but ALWAYS before calibrating compass):
    https://appsto.re/us/o9xCE

    To use it, fire it up in an open area known to NOT have strong magnetic fields, ferrous material or large metal structures and note the general value range. Ignore the small constantly changing fluctuations. This is usually in the 30-50 range.

    Next, go find a large metal object (ie the engine hood of your car), ferrous materials or a magnet and get near it. Note the new high value.

    Before calibrating the compass, check to be sure the value is in the "low" range. If not, DO NOT calibrate there. Seek another open area for compass calibration. Doing so risks your aircraft and safety of people on the ground!

    Before each flight, check the compass mod value to verify its within normal range AND optionally check the app as a sanity check. Higher than normal is fine for takeoff but app values near the high range should give you pause before flying in that location.

    I have noticed that many a crash report these days begins with "I had just calibrated my compass....".

    Understand that when you calibrate anything on your aircraft you are teaching it what's "normal". Doing that incorrectly (even a little) can have devastating results.

    Kinda like raising kids.

    But I digress....

    Also be aware that the compass calibration procedure says to "rotate the aircraft" around ITS axis. It doesn't say to rotate YOURSELF around YOUR axis.

    It's a small point but the manual, the in-app diagram, and many seasoned pilots agree that rotating the aircraft with IT at the center of axis is what is prescribed for best results.

    Many a YouTube video, including at least one video by DJI themselves with a "demo dude" show rotating while calibrating. That's not what the manual says to do, for the record. It says to "rotate the aircraft" with a diagram showing the aircraft as the center of axis. It may not be a big deal or it might be a big deal I'm not really sure but it's worth noting. It also may be that some folks are not being careful while calibrating the compass and as they hold the aircraft out in front of them rotating around, the aircraft is passing over a metal object or magnetic field that is impacting the calibration unbeknownst to them.

    Be careful when calibrating is the bottom line.

    ImageUploadedByTapatalk1433677915.723846.jpg
     
    #7 damoncooper, Jun 7, 2015
    Last edited: Jun 7, 2015
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  8. Jason Schiffner

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    Hey guys. I made a very thorough checklist that covers everything. Plus it has checkboxes that light up. Once everything has been checked, then it tells you its good to fly. It's a pretty good checklist that alot of people are using now. http://www.dronefrontier.com/inspire1checklist/
     
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  9. sanderx

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    Jason, I think your pre-flight checklist is very useful but please stop cross posting it as you have posted this now on 5 threads covering this list.
     
  10. Jason Schiffner

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    Sure, no problem. I think I put it everyone that it would make sense now, so I will definitely do as you say and stop posting it. ;-)