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Is cold IMU cali - good in overall?

Discussion in 'Pro/Adv Discussion' started by t2adze, Aug 22, 2016.

  1. t2adze

    Jan 3, 2016
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    Hey all

    I know that only thing cold imu calibration does, is remove annoyinf and long aircraft warming up message.
    But for the imu and ac itself, is it ok to do cold imu calobration?

    I usually have ac in a small room with ac on for an hour and after i do cali.
    But today when doing it, i thought, does a medal has a other side?

    Plz advice

    Sent from my SM-G900FD using PhantomPilots mobile app
    phantomi likes this.
  2. phantomi

    Jul 17, 2014
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    I have been thinking same thing. It is not the same to calibrate IMU at ambient temperature as to cold it before. Does it calibrate well "frozen"?
  3. fastfed

    Jul 5, 2016
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    well if there was a problem IMO, it would show almost right away, I believe after 5-10 minutes after craft is on, whether cold or hot calibration there would be difference
  4. Numone

    Jan 10, 2016
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    Surrey, England
    I did a cold IMU calibration after popping the AC minus a battery in the fridge at 4c for 20 mins. To date, all I have experienced are getting a "ready to fly" notification after a few seconds. That's a reduction of more than a minute from the previous startup wait. I do have to say, I was very worried about doing this, but so far, I've not looked back.
    Digdat0 likes this.
  5. Oso


    May 19, 2015
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    Sacramento CA
    The only potential downside that I have read here on the forum is that with the artificially reduced warm up times there may not be enough time for the barometer to stabilize. As such, you may see less accurate altitude estimates. Note - I don't know for sure if this has been confirmed or debunked. If I can find the thread(s), I'll update my post with the link. Cheers!

    EDIT: I have found several posts noting the same as I wrote above, but I haven't yet found the one thread in particular that I'm hoping to find. It wasn't that long ago, so it's odd that I can't find it so far.

    My P3P crashed, DJI not cooperating
    #5 Oso, Aug 22, 2016
    Last edited: Aug 23, 2016
    JoBe likes this.
  6. Dronason

    Aug 9, 2015
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    Normally the best would be to calibrate these sensors at the same temperature as they will be operating. So calibrating them at low temperature will not help from accuracy point of view. That said, these sensors have minimal drift over temperature.
    I would agree with you that speeding up the startup time (which is long, usually only for the first flight) can have some side effect. We could hope that b.e. barometer stabilization has its own timeout, but it is only guessing.
    I think we should remain reasonable. If you put the P3 in the freezer for hours and take it out just before calibration, it is wrong as the worst is temperature change during the calibration. Best is to put it in cool and calme room at least 30 minutes before doing the calibration.

    From a flight record .DAT file of my P3P (Midday in summer, maybe 26°C (79F) ambiant), it is possible to retrieve the IMU temperature using the CsvView program (CsvView - an app that visualizes .csv files).
    There is no noticeable drift or change of barometer value during that time, but as the value resolution is 1 meter it is expected.
    It starts at about 32°C (90 F) at -50 sec to finish at 64°C (147 F) at +53 sec. The start is function of the ambiant temperature.
    At that time = 0, temp was 49 °C (120 F).

    From a flight in mountain in the morning, temperature was lower, around 15°C (59F). There was no IMU calibration between the 2 flights.
    Start at 15°C (59 F) at -115 sec and finish at 64°C (147 F) at 60 sec.
    At that time = 0, temp was 47 °C (117 F).
    I don't know where was the exact time for the end of warming time. But probably the first lift was just after.

    What is noticeable is that whatever is the start temperature, the expected end of warm-up is at about 48°C (118F) and the operating temp is 64°C (147F) and remain constant during the flight. At my opinion, there is a heater in the IMU so it operate at a constant temperature.

    Probably the temperature is recorded when a calibration is done and it is used as basis to define the temperature to reach to declare that the calibrated value are ok and define end of warming time.

    Nice would have been to have a .DAT flight record from an IMU calibration (if it is at all possible).

    In winter, you will probably want to redo an IMU calibration at a lower temperature than in summer so you get almost same warm-up time.

    Big thanks to ferraript and BudWalker for their valuable work on these tools to be analyze to visualise flight records.
    neven likes this.
  7. Vertigo

    Jun 12, 2016
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    My understanding is that during the IMU calibration, the phantom creates a table with data points for the sensors at various temperatures. Cooling your phantom before doing the calibration just allows it to have a larger data range, starting at a lower point (which allows it to arm at a lower temperature).

    I doubt this has any negative influence, as the sensors will still be warmed up to normal temps, so if there is any negative effect at all, it will only apply to the first seconds or perhaps minute of flight (when you can fly rather than have to wait).