Hi Chirstophe,
i agree with yours comments.
We used the true airspeed sensor, an have be flown up to a wind of
15m/s.
The delta flow up to 28m/s. Is this a low speed?
We use the DCM rate of 25 Hz the same as the feedback loops.
On HD videos taken from a camera on the aircraft we were surprised
that the fly was verry smooth.
We saw no ozillations. We saw ozillations, when we used a low roll
angle 40° border.
After setting this value to 80°
the behavior was ok and the aircaft flow tunrs wiht a 20 m radius.
Fore the Implementation of the DCM code there are 2 main points.
1. Correction to hold the orthogonality of the DCM matrix in every
calculation step
2. Correction of the attitude also when flying circles
The run time of each step is less than 1.5 ms for floating point
calculation.
Most of the time is spend in calculation of the transcendental angle
functions.
This can be speed up by using tables as Paparazzi does for fix point
calculations.
Since the filter has a complementary filter structure, you can all
ways find situations in witch the filter fails.
But this is also true for full kalman filters.
My idea to have a correct attitude information is to use different
ranges of the accelerator sensors.
We have situations in which the 10 bit resulution of the adxl355 or
adxl354 is bad.
10 bit of a +-3g sensor give a pure angle error of +-0.3°.
This seems to be to high even for quadrocopters.
When we fly wiht poylon electric aricraft we have acellerations in the
range of 10g.
The problem can be solved by usenig 2 sensors (One full range 2g, and
one full range 16g).
Another point to high speed calculation for quadrocopters.
Most quadrocopters use the gyro signal direct (without building a
difference ) to get stability, called back stepping.
For this a high rate ( 50 Hz) is necessary.
The main point for high frequencies is to deal with Shannon theorem
even the bandwidth is to high.
The motors generate vibrations in the range of 200..2000 Hz. These
frequencies are mixed in the gyro signal.
I measured this signals has a amplitude up to 0.3 of the maximum rate
signal.
Another point is the calculation of the non linearity. Same algorithms
need very small delta angle to get the the fix point of the solution.
My own quadocopter fly with a feedback frequency of 20 Hz.
and a gyro signal rate of 100Hz.. Another club members use 50 Hz on a
AVR (quads- and three copters 20cm diameter), both without problems.
So i do not understand why many projects (ASTEC, Paparazzi, UAVP,
Openpilot etc.) deal with very high rates.
Regards
Heinrich
Christophe De Wagter schrieb:
About the discussions of AHRS filters:
DCM or quaternions or euler angles are
all fully equivalent notations for attitude and can be converted from
any notation to any other. The advantage of euler is its intuitive (for
small angles) interpretation, its disadvantage is the non-linearity at
90 deg pitch, the advantage of quaternions is the reduced memory size
(e.g. when used in a full kalman filter it is better to add 4 states
than 6/9 states). The DCM attitude notation is needed in all attitude
cases at least as an intermediate step to convert vectors and is used
in all cases (euler->DCM = [cos.cos sin.cos ....] and
quaternion->DCM= [q0*q1/2, q1*q3/2, ..].
Now indeed without ground-speed information extremely
large errors will occur. This can be provided with (slow and delayed)
GPS or approximated with an airspeed sensor. This secretly assumes that
the plane is pointing in the direction of motion and therefore is only
valid for fixedwing. All fixedwing attitude algorithms use this: that
is DIY'DCM based <subsystem name="ahrs" type="dcm_float"/>
as well as <subsystem name="ahrs" type="ic"/> as well as
textbook extended kalman filters. The difference only lies in filter
gains and computational power: e.g. the IC (integer complementary)
filter does the same as DCM but with 30 times less
computational effort for the autopilot (but still needs some fixedwing
integration update).
However: all the above mentioned filters do not have a 3D
velocity and change in velocity feedback making them not useful for
helicopters and give trouble when applying very long longitudinal
accelerations (large airspeeds). The 3D speed feedback is possible and
quite easy to implement but makes the filter much less stable, which is
why this is not applied as a default.
So Hector Garcia certainly has a point about the performance but
as dr Warmers says in practice it will work nicely on slow fixedwing
aircraft with not too much wind (much defined as in: constantly clearly
blown away from its normal trajectory with visible yaw and pitch
oscillations).
Sincerely
-Christophe
On Wed, Aug 10, 2011 at 8:29 AM, Christophe
De Wagter
<address@hidden>
wrote:
Work is on its way to include Magnetometer in order to get the
auto-takeoff right (as it does not work so well yet since the filter
needs a bit of time to get the heading right).
Please note that
http://www.sparkfun.com/products/10252 is
not compatible at this moment as the driver needs to be changed a bit
for the talk-through-i2c. It was already made and will hopefully be
added to the master soon.
-Christophe