Re: [Paparazzi-devel] Calibrate the IMU/GYRO

[Felix Ruess]

Hi Heinrich,

Bias is not the issue.... first of all the AHRS aligner estimates the
initial bias and secondly with filters like int_cmpl_quat the gyro
bias is explicitly estimated while it is running.
But you get less than optimal behavior if you have a bad
"calibration", meaning if the scale (sensitivity) of the gyros is
wrong. And since most people don't have a turntable to calibrate that
you have to rely on the default sensitivity values of the datasheet.
In case of the IMU3000 (Aspirin1.5) they seem to fairly good, but
apparently not so good in the ITG3200.

Would be good if someone could get some proper data on this.

Cheers, Felix

On Sat, Jun 2, 2012 at 9:56 AM, Prof. Dr.-Ing. Heinrich Warmers
<address@hidden> wrote:
> Hi,
> we found that the DCM  is very  sensitive to the bias value of the rate
> sensors.
> We use a analogue IMU  with  the 10 Bit ADC  of the Tiny.
> If the bias was in the range of  one LSB the DCM fails or need a long time
> for the correction by the accelerations signals (1-2 minutes).
> This is a wind up problem of a PI feedback control system.
> We solve the problems by automatically  correct the bias for each power on.
> Therefore we mounted a  bubble tube at the aircraft and set it in the flight
> position before power on.
> This works very well.
> You can also chain the gain and time constant  in the correction part.
> You can also write a  bias correction as part of the flight plane.
>
> Heinrich
>
>
> Felix Ruess schrieb:
>
> Hi,
>
> The gyro sensitivity indeed seems to be the cause of this.
> I also did a quick test with Aspirin v1.5 (IMU3000 gyros) and I didn't
> observe any drift. So there the defaults (datasheet) gyro sensitivity
> seems to be correct.
>
> But testing with Aspirin v2.1 (MPU6000) I saw significant drift, so it
> seems the datasheet gyro sens is not really correct there.
> Gautier tested with Umarim (ITG3200 gyros) and the gyro sens seems to
> be not quite correct there either....
> Maybe this has to do with the voltage? The gyro sensitivity in the
> datasheet is only specified for 2.5V, but we are running them at 3.3V.
>
> Does anyone have a turntable to do a gyro calibration with either a
> ITG3200 (Umarim or Aspirin 1.0-1.3) or MPU6000 (Aspirin 2) to compare?
>
> Cheers, Felix
>
> On Fri, Jun 1, 2012 at 10:11 AM, Gautier Hattenberger
> <address@hidden> wrote:
>
>
> Hi,
>
> I did some test with the float_dcm filter and I observed this:
> - when doing full roll, the attitude takes some time to recover correctly
> (some degrees of error after 360° of roll)
> - when turning the plane upside down and going back to normal changing
> direction of rotation (+180°, then -180° on roll axis), there is almost no
> error
> As Felix said, this should not append as the rotation matrix should not have
> singularity. At least for normal flight (no roll or loop) it works well,
> even at high bank angle.
> A possible issue is a bad gyro sensitivity calibration (and it's most likely
> the case of everybody since we use datasheet values). The result is an
> angular error when integrating the gyros. If you turn your plane in one
> direction and then go back to initial position turning in the opposite
> direction, the error is compensated. But if you do a full turn, it doesn't
> and you end up with a final angle too small or too big. Then it slowly
> converge to the correct angle thanks to the accelerometers.
>
> Gautier
>
>
> On 31/05/2012 13:17, Felix Ruess wrote:
>
>
> Hi,
>
> While the rotation matrix representation of float_dcm should have no
> problems dealing with an inverted IMU, I have never much used or
> checked this algorithm.
> If you have problems with float_dcm you may want to use int_cmpl_quat
> instead.
> See http://paparazzi.enac.fr/wiki/Subsystem/ahrs
>
> Cheers, Felix
>
> On Sat, May 26, 2012 at 3:44 AM, wang yao<address@hidden>  wrote:
>
>
> hi i use the weasel.xml and have that problem
>
> have you got the problem i mentioned?
>
>
>
> when the plane is inverted and turn to upright again,  the PFD screen is
> hard to>recover to its original position.
>
>
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