Re: [Paparazzi-devel] Incorrect Proportional Pitch Deflection, Correct Derivative Pitch Deflection

[Felix Ruess]

Hi Chris,

the attitude estimation algorithm itself (ahrs int_cmpl_quat uses quaternions for representing rotations internally) has no problems with gimbal lock.

But we also always calculate the "other" attitude representations (in that case from the quaternion). So we always have the attitude as quaternion, euler angles and rotation matrix representations. E.g. to display the attitude in euler angles, (displaying quaternion values is not very intuitive;-) and to be able to use other parts that don't take quaternions as input yet.

This is why you can use the normal fixed wing stabilization/guidance which only takes euler angles as input.

So it's not a "quaternion method taking Euler angles as input", it's just that for fixed wings there is no control written using the quaternions representation directly.

That is what I was trying to say in the previous mail.

If you start to convert the attitude representation from a quaternion to euler angles, you will get discontinuities at +-180deg (+-90deg for pitch), that is just a property of euler angles representations and nothing you can do about it.

The only "real" solution to properly avoid this problem is to not use euler angles (except for things like displaying it for the user) and write the control using quaternions as well.

Cheers, Felix

On Sat, Feb 18, 2012 at 10:55 AM, Chris Wozny <address@hidden> wrote:

All,

After tirelessly analyzing the data from some "flights" today, I've
determined that some (if not all) of the issue I'm having is coming
from gimbal lock. At such a high angle of attack, I'm seeing phi and
psi values rolling over from positive 180 to -180 degrees and vice
versa very rapidly. My understanding was that complementary
quaternions "solved" the gimbal lock problem. From the commit logs, it
looks like Felix and Christophe both put a lot of work into this and
wanted to see if either of you saw gimbal locks at all? I know Felix
mentioned that any quaternion method taking Euler angles as input can
run into these singularities, but I just wanted to double check since
the almighty wikipedia says the quaternions are an alternate to Euler
angle solutions and are not intertwined. At this point, I have a
feeling that the only solution to this problem would be to mount the
Lisa board (with IMU on it) in a perpendicular fashion to the
fixed-wing VTOL aircrafts' chord line.

Best,
Chris

On Fri, Feb 17, 2012 at 5:42 PM, Chris Wozny <address@hidden> wrote:
> All,
>
> I've run into a peculiar situation with my PD gains for pitch on my
> elevons in AUTO1. I'm using Lisa/M 1.0 on the dev branch. The
> deflection I'm receiving to counteract the motion of the aircraft in
> pitch is correct (i.e. PITCH_DGAIN is correct.) However, the
> proportional gain is the opposite of what it should be doing i.e. when
> I'm pitched lower than my set point, the elevons pitch further down
> instead of trying to pitch the aircraft back up. The obvious
> correction to me would be to flip the sign on the PITCH_PGAIN, but the
> range on the numbers is only negative and zero, so I don't know how
> that would work. Does anyone know how to flip the effect of the
> PITCH_PGAIN? The pitch and roll commands in MANUAL work properly so I
> don't think it could be a reversal of the servos that would fix it
> (especially since the PITCH_DGAIN works properly.) Finally, the servos
> do not hold their commanded position in AUTO1 and overshoot when going
> from the commanded pitch/roll back to neutral. I've included the logs
> of the flights, the airframe configuration, and even the video of the
> (attempted) flight which the logs correspond to. Has anyone seen this
> type of behavior from their aircraft?
>
> Video of (Attempted) Flight
> http://www.youtube.com/watch?v=i9y_Nh8QQYY
>
> Video of Commanded Input in AUTO1
> http://www.youtube.com/watch?v=uDeERiqehjU
>
> Airframe Configuration
> http://ece.arizona.edu/~cwozny/mini_vertigo_az_high_aoa.xml
>
> Flight Data/Log
> http://ece.arizona.edu/~cwozny/flight_03_filmed.data
> http://ece.arizona.edu/~cwozny/flight_03_filmed.log
>
> I hope everyone has a good weekend!
>
> Cheers,
> Chris

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