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Barometer what to expect

Discussion in 'General Discussion' started by Hughie, Dec 31, 2014.

  1. Hughie

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    Am I expecting too much from my barameter.

    P2 is a month old and I have just fitted an IOSD mini.

    I have notice that the height reads a bit high ( e.g. 6-8m) at ground level, so today before I flew I monitored it for 10 minutes on the ground, with engines off.

    When I first switch everything on it seems to show 0m and then quite soon shows 0/1m - fair enough.

    After 2 mins height shows 3m
    After 3 mins height shows 4m
    After 4 mins height shows 5m
    After 7 mins height shows 6m
    After 9 mins height shows 7m

    What I should have done at that point was probably cycle the power and see if it went back to zero and see if the drift had slowed, but it was cold and my battery was already at 94% and I hadnt turned a prop yet. So I flew until it autolanded :)

    Can anyone comment on how the barameter should fluctuate? Should I expect to have to let it sit around to normalise before I fly or is it broken ?

    Thanks.
     
  2. paulajayne

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    Ambient pressure is changing and baseline reading it taken at power up.
     
  3. ianwood

    ianwood Taco Wrangler
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    Atmospheric pressure does not change that quickly unless you're in a storm. The barometer will be influenced by the movement of air around the drone (i.e. starting the motors), humidity, temperature, voltage changes, etc. The phantom's barometer is not a precision instrument.
     
  4. Hughie

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    I had considered that one possibiliy would be that the quad had been sitting at 20 C and then was taken out to a field at 1 C, and the change in temperature meant that the pressure reading would drift as the transducer and electronics changed temperature.

    Not sure about this anymore, as a decrease in temperature should lead to a decrease in pressure, not an increase.

    Having accuracy and precision is one thing, but having perpetual drift (apparent anyway) is another.
     
  5. PsychopathRC

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    I was concerned about this in past because I thought such inaccuracy might cause the Phantom to slam into ground, should it have to land itself. It might think it's higher then it actually is. However I was assured that it wouldn't do that and that the figure we see on our display is more of an estimate for us to follow. So it's nothing to really 'worry' about unless you want to fly close to stuff like the ground x)
     
  6. Buckaye

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    One of my first crashes was trying to use the ground station too close to the ground and the rather inaccurate estimate of height from the barometer.

    I now program nothing lower than 50 ft to avoid that.

    I've noticed my barometer saying I'm at 0 ft on the ground and then 5 ft on the ground when I land the next time...

    It's fair to say that I wouldn't trust the barometer for anything more than an estimate...
     
  7. macheung

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    Barometers are not that accurate in an absolute sense. Aircraft pilots adjust their barometers as they approach the airport for landing and the adjustments can be hundreds of feet, though granted that is after moving hundreds of miles after hours of time. The phantom barons are good when it comes to instantaneous readout of change (going up or down) hence it can hold a hover. However, we can expect it to be off by at least a few feet after 15-20 minutes of flight.

    Use it as an accurate guide to let you know whether you are climbing or descending. But only as an appropriate guide for true AGL altitude.
     
  8. iswimmer

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    Most of the upward drift in altitude occurs as it warms up after starting up. If you want to eliminate most of the drift in the reading you can let it warm up for a few minutes and then turn it off and back on - that resets it to zero. Then when you fly it will be much more accurate.
     
  9. Hughie

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    I did try that.
    I left it for 20 minutes (during this time it was drifting at 1m /minute)
    Then reset it, and it adjusted back to zero and then it still drifted, at 0.5m / minute.

    I guess that is just how it is.
     
  10. Hughie

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    Pilots don't adjust their baromoteric altimeters because of drift though. They adjust them because the QNH of the airport they want to land at is different to when they set it last. These are different things.
     
  11. SkaffenAmtiskaw

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    when it comes to barometric pressure,

    1 Hpa = 30Ft altitude change. so the difference between 1013, and 1014 is 30 ft. 1 mtre = 3 ft. so yes, general changing weather will be noticed by the Phantom over just a minute as the change is small. this is Pressure altitude.

    Temperature, 1 deg Celsius = 120 Ft variation in Altitude. so over the period of 10 mins, a temperature change of just 1 degree will result in a significant barometer altitude readout. this is Density altitude.
     
  12. PsychopathRC

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    Took the words outta my mouth ;)
     
  13. ianwood

    ianwood Taco Wrangler
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    As stated before, unless you're flying in advance of an approaching weather front, atmospheric pressure will not change enough over a period of 15 minutes for your Phantom to exhibit a noticeable reaction.
     
  14. N017RW

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    IMU is used to establish/maintian stable flight, agreed?

    The barometer is just another degree of freedom to accomplish this.

    Component quality notwithstanding, it was never intended to provide accurate, repeatable altitude measurements but rather to aid in maintaining stability. It is not possible for such a low cost device to perform in the way many expect or were led to believe by DJI. This has been proven here anecdotaly.

    Higher cost units have better components and software to help minimize the errors associated with these devices.

    It seems DJI has tried to leverage it's inclusion to provide some data to make the experience more enjoyable but has caused many to doubt the proper operation of their multi-rotor.
     
  15. paulajayne

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    Good first post.
     
  16. Pmcdn

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    I don't believe there is a barometer in these quads. I believe altitude is determined by the GPS. Pilots are required to adjust their altimeters before takeoff as well as when they approach their landing destination airport due to the fact that the barometric pressure is constantly changing.

    It's because of the nature of the GPS altitude function why you sometimes see an altitude of a few feet (or minus a few feet) while it's sitting on the ground.

    These quads also do not have radar, as indicated in some literature. The orientation of the onscreen triangular icon (which way it's pointing) is provided by the onboard compass and the distance rings are measured by the current location of the quad in relation to the established home point provided by the GPS, not radar FYI.
     
  17. Hughie

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    Based on what evidence? The resolution of GPS altitude is no where near good enough for doing what these machines do.

    Yes and what makes you think this is a diificult thing for a microcontroller to do when the quad is initialising on the ground. Typically it would use something like this
    http://www.adafruit.com/datasheets/BMP0 ... ly2008.pdf
    and use whatever the output is when on the ground as the offset.
     
  18. N017RW

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    There is a barometer on board.

    http://wiki.dji.com/en/index.php/IMU
     
  19. neilp1

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    Agreed on the great post. Brilliant use of the brain weapon to help educate us more about these things