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Crash due to compass?

Discussion in 'Phantom 2 Vision Discussion' started by stefan_fvt, Feb 18, 2014.

  1. stefan_fvt

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    Hi,
    I had my first crash today, it was a terrifying experience! I an attempt to get nice straight down photos for terrain mapping and producing orthophotos i removed the original camera and replace it with an small Ixus (135 g). See pictures.

    As always I wait for the fast green lights to come on before lifting off. But this time as I hadn't the camera plugged in I couldn't see the number of locked satellites etc. I took of 2 m up in the sky and noticed the Phantom not holding the position so good. It was slowly moving around. I thought it might be due to inbalance of the center of gravity of the camera. So I adjusted it slightly while the Phantom was airborne and then flew up to about 50 m.

    Then I flew some stretches back and forth letting the Ixus take pictures with the intervalometer. However when I was about to return home the Phantom started behaving strangely and all of the sudden it started going in some kind of big circle and I was loosing control. To make this story shorter I will jump forward. The phantom crashed in vegetation (reed) and seems OK.

    I started to think this could be compass related after searching the forum. You can se from the picture that the compass unit was slightly higher than original location. Also I striped the wire from the camera next to the compass. After the crash I hooked up the Phantom to DJI assistant and could read the Compass Mod value to about 2300. As I understand this is quite high.

    I moved down the compass to it's original location, did a compass calibration and Basic Cali and got the value down to 1730. I tried moving the camera cable close to and even next to the compass while connected to the assistant but the value only change slightly (5 units). If I put the camera next to the compass the value goes up to like 3500 and this would of course be an issue. But in the location the Ixus camera was placed during the flight it doesn't affect the mod value.

    So my question are:
    Could the compass mod value of 2300 alone have caused the crash?
    Isn't it strange that it all of a sudden started behaving like this? The battery was still well over 50 %.
    Should I be afraid of flying the Phantom again and what precautions could I talk not letting it happen again. Of course I shouldn't be flying if it spins in circles and maybe even calibrate the compass every flight.

    Hope someone with experience could shed some lights on this.
     

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  2. Pull_Up

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    If you ever replace the compass, move the compass, or add or remove any hardware from the aircraft it's always a really good idea to do a compass calibration so that it can take into account any effects these things may have. The circling you spoke about is indicative of the IMU getting conflicting information from the compass. Whether that's due to moving it, putting the cable nearby or the camera, who knows.

    Number one lesson is if you notice anything out of the ordinary with your aircraft never assume it's wind, or balance, or any other factor. Land straight away and trouble shoot on the ground.

    You can calibrate every flight if you want, as long as you are sure you are doing it in an area that is suitable. I would certainly calibrate after any changes (additions or removals) to things attached to the aircraft. I would also, irrespective of whether you calibrate every flight or not (I don't, for example, if nothing's changed) do a one minute test hover, 10 ft off the ground in front of you, at the beginning of every new flying session - you will notice any difference in your aircraft's usual performance in that time.
     
  3. iDrone

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    As I'm sure we all know, the compass is an extremely sensitive sensor. Not sure how the compass got moved, perhaps by the crash? Point being, once mounted & calibrated at the factory you want to avoid letting any magnetic, electro-magnetic objects, or ferrous metals anywhere near it; this especially includes tools (screwdrivers, hex keys/drivers, etc), hence the reason it's mounted at the bottom of the leg, way below the P2V and as unobscured from the aforementioned no-no's as possible.

    Slide the compass back to its original position. DO NOT use a screwdriver unless it's been demagnetized. Remove the zip tied camera cable & mount it as far away from the compass as possible (not only does it have a metal plug, it carries DC power and modulated data... a potential source of EM force you really don't need to take a chance on). Temporarily remove the camera.

    Connect to Vision Assistant and check your compass & run an Advanced IMU Calibration if you haven't already. If your compass values are whack, you may have to do Colin's Magnet Hand Wave to restore normal Mod values (see his video archived elsewhere).

    Go to a compass-frienly clearing and do a Compass Dance. Take it up for a 10-20' hover, if still circling over a known good takeoff & flight area you might want to replace the compass. These are available as a spare parts item for under $20 USD, extremely cheap insurance.

    If it seems to fly stable again, land and reattach your camera; run another hover test. If not so stable, land and try another Compass Dance.

    Good Luck,
    iDrone :ugeek:
     
  4. jwuman

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    i calibrate everytime i fly, i usually fly in different locations when i go out, is there any drawback to doing this? i have not had any issues doing this. I did have one fly away but i believe that was due to me calibrating on my tail gate of my truck , then moving the craft a few feet away onto the ground, plus props were out of bal. and the wind was 17 mph...anyway,,,is it wrong to calibrate compass before every flight....?
     
  5. iDrone

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    Not necessarily wrong... probably unnecessary and possibly risky. The compass sensor needs to be calibrated along with the IMU's tilt-sensor so it knows "Where on earth am I?" esp when the aircraft is tilting or turning. Once the system is calibrated it shouldn't need another calibration until you change physical locations by roughly a few hundred miles or the aircraft has been modified or damaged.

    One of Vision's "tell's" is that it starts circling when hovering. It does this when there's a conflict in direction between GPS & compass. Most Vision's will hold station within a 1-2ft radius. Circling 3ft or more from a point increasing in radius suggests there's a conflict, time to land & troubleshoot... often a Compass Dance will resolve the conflict.

    Where you do the Compass Dance is important. Open field way from anything magnetic, empty pockets of keys, phones, etc, stand 100ft away from high-tension lines putting out EM fields, 10ft or more away from cars, metal sheds, etc, even your TX/Controller. Anything with a magnetic field nearby that the compass can pickup or that saturates it will throw its calibration off.

    How do you tell if you need to calibrate the compass? If Vision thru its NAV beacons, or Vision Assistant don't tell you first, the consensus is to do a hover test at 10-20ft for a minute before flying any higher & farther. If it holds station, have a nice flight! If it starts circling in increasing diameters, time to land & troubleshoot & try a Compass Dance.

    As for moving Vision after power-up, Vision needs to be stationary on its LZ so it can: #1 Calibrate its IMU, #2 Test compass to confirm nominal sensor values, and #3 Scan the sky and set a GPS HOME Lock in case you need to RTH. Power-up your Vision on a tilted surface, metal bench, chair, table, or tailgate would probably not fall under recommended practices.

    I hope this helps a little,
    iDrone :ugeek:
     
  6. jwuman

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    Thanks Idrone, i have noticed from time to time a circular hover, the last flight i had was flawless and the gps position hold was flawless, I will start not calibrating from this point on and continue to use the assistant SW to check calibration values in between flights...yea the fly away pre flight i described was me being to confident with the machine and just failing to think about what i wasnt doing properly and recognize it,,knew i shouldnt have been doing the calib. on my tailgate, knew i shouldnt have moved it,,,just too anxious to fly and learned a very valuble lesson, plus crash and recovery yeilded zero damage,,,,so i was lucky since the flyaway went due east crashing into dune grass,,,north and south of that was the drink,,,,