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droney

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yaw axis lock not working during takeoff

by droney » Thu Oct 08, 2015 10:59 am

Hey guys (and gals), I could use some help with takeoffs on my brainfpv based quad (GPS and external compass). I am attempting to launch the craft in 'leveling' mode (attitude for roll and pitch, axis lock for yaw). I have noticed the flight controller is compensating for yaw errors depending on the heading of the craft at launch time. If the craft is placed heading north, no yaw compensation experienced and all motors spins roughly at the same rate. If the craft is rotated towards the west a bit, the two CW motors spin up faster in attempt to bring the craft back to a northerly heading. Conversely, if the craft is rotated towards the east, the two CCW motors spin up faster. I can also rotate the craft while the motors are spinning and hear/see the active compensation at work, with minimal and equal rpms on all motors only when the craft is heading north. The further away from a northerly heading I start with, the more compensation I see.

AttitudeDesired.yaw is zero, so there is no outside influence such as yaw transmitter trim. Power cycles and re-arming also has no affect. Vibration analysis is very clean. I also do not believe there is any wiring/component related issues given the results of the heading experiments above. Switching to an internal mag has no affect. Attempts to disable the GPS I get an 'unknown' error code. I am also using the 9/28/2015 firmware release, but see the same issue in the 7/20/2015 release. I have not gone back to earlier releases. I also do not know if this is limited to brainfpv or not.

Results are easy to reproduce.... watch the props spin at low throttle in the three different starting orientations I mentioned in the first paragraph. Can someone please confirm or refute these findings with one of these two firmware releases? All other suggestions welcome. Thanks!
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icee

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Re: yaw axis lock not working during takeoff

by icee » Thu Oct 08, 2015 11:12 am

If you arm and lift off facing east, does it try and yaw significantly to north?

Note AxisLock does not try to attain a given heading, but just tries to push the rate integral neutral when there's no significant command.

Most of our user base flying attitude modes is using axislock for yaw (including me). Things don't snap north. On the other hand, when the craft is on the ground with moderate throttle settings, very, very small perturbations in sensor biases can cause motors to spin up or spin down, because the integral will try and fight them. So I could see a bad magnetometer calibration doing this if the craft remains on the ground (but not in flight).
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droney

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Re: yaw axis lock not working during takeoff

by droney » Thu Oct 08, 2015 11:26 am

icee wrote:If you arm and lift off facing east, does it try and yaw significantly to north?

Note AxisLock does not try to attain a given heading, but just tries to push the rate integral neutral when there's no significant command.

Most of our user base flying attitude modes is using axislock for yaw (including me). Things don't snap north. On the other hand, when the craft is on the ground with moderate throttle settings, very, very small perturbations in sensor biases can cause motors to spin up or spin down, because the integral will try and fight them. So I could see a bad magnetometer calibration doing this if the craft remains on the ground (but not in flight).



I have not been able to get it in the air since this issue started. Because the compensation is so extreme, the craft too unstable to launch (it's basically trying to lift off on two motors). I have re-calibrated several times, so I don't believe that is the cause.
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pug398

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Re: yaw axis lock not working during takeoff

by pug398 » Thu Oct 08, 2015 10:50 pm

I have it on CC3D, Sparky1&2 and Quanton with no yaw issue. I doubt there is an issue with Brain firmware. Set it up from scratch again erasing the settings partition. Don't import any settings. I have seen remnants cause a problem with attitude. If you still have trouble or even before go to attitude page>temperature compensation and see what coefficient is for z gyro after calibration. See if way higher than x,y.
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droney

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Re: yaw axis lock not working during takeoff

by droney » Tue Oct 13, 2015 4:35 pm

pug398 wrote:I have it on CC3D, Sparky1&2 and Quanton with no yaw issue. I doubt there is an issue with Brain firmware. Set it up from scratch again erasing the settings partition. Don't import any settings. I have seen remnants cause a problem with attitude. If you still have trouble or even before go to attitude page>temperature compensation and see what coefficient is for z gyro after calibration. See if way higher than x,y.



After clearing all settings and performing a setup from scratch, the six point calibration is now failing the accelerometer calibration. Upon further investigation, it appears the accelerometer and internal magnetometer are both unresponsive to movements but the gyros appear to be somewhat working. Accel (x,y,z)=0,0,-9.805 and mag (x,y,z)=0,0,0 always, regardless of manual movement. I found the SensorSettings->AccelScale and MagScale values were all set to zero. Settings AccelScale and MagScale to '1' enabled the accelerometer and internal mag.

Another observation, noting the board is properly oriented on the quad, is a positive x rotation results in a positive y gyro response. Similarly, a positive y rotation yields a positive x gyro response. Lastly a positive z rotation yields a negative z gyro response. Any thoughts on this gyro axis swap and z inversion?

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