Re: [Paparazzi-devel] Vibration issues (?) on a quadrotor

[Christophe De Wagter]

Note that vibrations are also quite important for gyro's. On accelerometers the standard deviation is a good measure for the noise band. However, on gyro's it is not the noise band that is detrimental, but the fact that vibrations can change the bias (rapidly).

MEMS/Piezzo Gyroscopes (actively vibrating elements in x-direction that measure residual vibration in the y-direction since during a rotation due to inertia the active x vibration will lag and be visible in y) are by design always sensitive to vibrations (it's a vibrating element that senses vibrations), temperature (material properties) and g-forces (bending causes capacitive changes while the capacity is used to sense vibrations). 

To limit the effect of external vibrations the resonating frequency is quite high and different on each axis. (20 to 30 kHz) and many other tricks are used. So to asses the damage of external vibrations on gyro's you should check the average (bias) with and without external vibrations and look at the vibration spectrum. The biggest problem here is that we are looking at frequencies above the accelerometer measurement range. Anything above 15kHz will have "some" influence in "some" cases.

Mechanical damping therefore remains a concern. Good vibration damping is not easy either. Some simple rubber bobbins for instance can also increase the vibration amplitude. And the lighter the electronics, the harder it is to dampen it. Very little soft foam on the corners is our best trick so far. 

-Christophe 

On Fri, Feb 17, 2012 at 6:18 AM, Prof. Dr.-Ing. Heinrich Warmers <address@hidden> wrote:

Hi,
i think the Rate sensor signals are ok. Please replay the test if you hold you quadocpter in the hand and
use a throttle that the quadcopter is just hover. The Idea with a additional mass may be help.
What is the problem with the values? Is the system flying well and only the values are to noisy?

Heinrich

Loic Drumettaz schrieb:

Hi,

I've made some test with the quad fixed on a table. With 43% throttle, i have the following results:

GYROMETER
voie	min	max	moy	stdev
p	-15.1073	20.6046	0.99363	6.3917
q	-12.3935	11.5403	-0.69831	4.3985
r	-36.1035	48.0075	1.7297	16.4202
ACCELEROMETER
voie	min	max	moy	stdev
ax	-13.6587	10.9194	0.15068	4.3117
ay	-8.347	6.8557	-0.16339	2.8296
az	-15.4049	-2.4464	-9.7606	2.1168
MAGNETOMETER
voie	min	max	moy	stdev
mx	-0.94291	-0.7847	-0.86101	0.032067
my	0.092289	0.19825	0.14064	0.020364
mz	0.146	0.44435	0.27401	0.05397

I don't know if this test is relevant, since the landing gear does introduce additionnal vibrations. Nevertheless, i find it easier to compare different damping arrangements.

I'll try to balance the motor/propeller and the rubber screws on the IMU. Is it also efficient to add some mass on the IMU, in order to lower the natural frequency? I've checked that the motor and frame are well fixed and stiff enough.

Can anyone confirm my guess that the issues I had with RC climb mode comes from vibrations ? (see my previous message)

Thanks

Loïc

Hi,

a vibration level of 20% full scale of the rate sensor  is  normal.
The best way is to damp vibrations and to balance the propeller   together with  with the  motor rotors.
You can try rubber screws   like on this page: https://www.mikrocontroller.com/index.php?main_page=index&cPath=77&zenid=f3cc9c70489e257d4d4e04552f0e1343
If the damping is to soft the quadcopter will became unstable.
Is your motor mounting, your frame, wiring and the PCBs  fix?
Can you sense  the vibrations  with  your hands if you hold the quadcopter?

Heinrich Warmers

_______________________________________________
Paparazzi-devel mailing list
address@hidden
https://lists.nongnu.org/mailman/listinfo/paparazzi-devel
  

_______________________________________________
Paparazzi-devel mailing list
address@hidden
https://lists.nongnu.org/mailman/listinfo/paparazzi-devel