PH2.1 + SBF GPS = uncontrolled flyaway

This is a fresh rebuild of a 1000 size Octo.
Previously flying with a Pixhawk 1, Septentrio AsteRx-m in SBF mode, working up to RTK testing.
Ground tested RTK to +/-6mm
Have used Firmware 3.5rc1 and 3.4.5

Fitted Pixhawk2.1 and first tests resulted in the Octo heading off in random directions with no response to RC stick input. Retrieved and saved by switching back to Alt Hold or Stabilise.

Fitted 3DR uBlox6 GPS module, recalibrating compasses once again
(I am really building muscles dancing around the back paddock with an 8kg Octo)
getting toilet bowling, reading the forum, working out the combination of Main compass #, internal and external, a recalibrating compasses a few more times (aching muscles now) loiter was very locked in, even in wind gusts to 3m/s.

Changed GPS cable from 3DR unit to AsteRx-m (they use the same plug) with no other changes except the SBF parameters, and yet another F#@n compass calibration, and the Octo takes off in random directions again.

Mission Planner reports a 3D Fix, as you can see in the logs, and the GPS location does not move around.
So I am at a loss as to the reason this is happening with the PH2,1 and not the PH1.

If this can’t be resolved the Pixhawk 2.1 is going to have to be shelved, which is a pity as I like the hardware and also a PITA. More bloody rewiring.

Flight log and AsteRx-m log attached.
Any suggestions welcome, anyone?

2017-02-21 09-12-11.bin (3.4 MB)
Septentrio Log

You changed your LOG_BITMASK to 894 so lots of useful information is missing like vibration, accels and gyro logs making it hard to diagnose.
Your BARO’s look good.
Your first magnatometer had a bit of trouble in the X field and the Z field - see in the graphs below how it moves drastically differently to the other 2. Y was ok so I didn’t include it. That’s weird. What’s it near? I’d disable it probably as the other 2 seem ok.

Other then that I can’t tell much. You will have to replicate it again with logging enabled. The default LOG_BITMASK in Copter is 176126.

Thanks, Grant.

@gmorph thanks for reminding me, I forgot I had reduced the logging as I had previously had ALL+Fast IMU on and had hoped it was overtaxing the CPU so reduced it to minimal for some testing.

I will conduct another flight with the logging wound up again.

The PH2.1/FW3.5rc1 seems overly sensitive to any sort of compass variance so I was running just the external compass in the 3DR GPS sitting on a 100mm stand above the stack.
The copter was Loitering very well with the 3DR GPS, no toilet bowling or yaw deviations.
Only the GPS cable was removed from the 3DR GPS and plugged into the AsteRx-m (they are plug compatible).

The concerning thing is that the copter will not respond to RC input when in takes off, only after switching back to a manual mode such as Stab or Alt Hold.

I am flying a survey mission today so it will be in a day or 2 before I get the next log up.

@mboland Log while disarmed please! That makes looking at this stuff much easier!

Don’t do the Fast IMU logging. It smashes your SDCard and is unnecessary in most cases. Plus it makes log analysis difficult as the log is so much larger and full of IMU logging. We can always add that back in if we see a problem but I doubt we will.

OK - this is important. In order to power the Compass in the external 3DR GPS module it gets its power from the GPS power cable only - there is no power going over the red cable in the 4 wire compass cable - in fact you can remove it if you need to. In other words if you just removed the GPS cable from the 3DR GPS Module (the original square module I’m talking here) then the compass in that module would also become un-powered and not function correctly.

Thanks, Grant.

@mboland How are you mounting the SBF board and are you using it’s compass? I needed a different rotation number when I mounted it, rather then reusing the same one as the 3DR gps used.

I will set log while disarmed
The AsteRx-m is mounted in a box beside the PH2.1 but I am not using the compass as there was just too much inconsistency with both the PH2.1 and AsteRx-m compasses.

Thanks for the heads up.
I will be sure to just do max logging without the Fast IMU.
And I completely overlooked the the powering of the 3DR compass from the GPS power.
Just checked and it had defaulted to 2 internal compasses, obviously the PH2.1’s as the AsteRx-m compass is not connected, although 1 of the PH2.1’s compasses has always had high offsets.
I will run power to the 3DR GPS and run the tests again.
I also have a preference for using a compass that is as far from the electrics as possible.

Build picture of AsteRx-m installation

This was all about external compasses and an intermittently faulty I2C breakout which caused the external compass to sometimes not be seen at boot.
This caused a shift in compass alocation but not the compass offsets, so the compass would be using the wrong offsets.

So the apparent flyaway was just a really really big toilet bowl effect which I wasn’t game to let go for too long.

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