Isn't compass used in AltHold mode so what made the copter yaw?

ArduCopter Copter 4.1
#1

Hi guys,

I hope you are doing well.
I am flying and tuning a new H frame quadcopter - big one with 32 props.
When doing manual tune in AltHold, I figured out some strange behaviours. The drone did yawing without my input in the yaw channel. When analysing log, I realize there was some “noisy” or “abnormal” mangetic intensity in x and y axis. Right after the magnetic changed, the yaw rate changed, so I had to correct it by yaw stick (my input)

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According to this discussion, the compass input may be used for correcting the gyro drift Does Alt Hold require a compass? - #6 by Rob_Lefebvre

Is it the case for my situation?
If this is not the reason, how can I minize this yaw-auto-rotating problem in the AltHold mode?

This is the log in case you need to do additional analysing: https://drive.google.com/file/d/1YHCh7_DiTQccqknALtl7TLHwANnjtdaH/view?usp=sharing

Thanks,
Vu.

#2

Let me explain a little more about the time stamp:
At 1:
I observed the drone yaw leff - but it did not shown here!!!

At 2:
I started correcting it by applying input to yaw channel making the drone yaw right

At 3:
The drone came back to my desired yaw angle

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#3

It looks like the compass is responsive to throttle (current) to me:

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#4

That’s correct!
The external compass is mounted close to power wires - I know I need to choose a better position in loiter or poshold mode later on.

#5

Even with normal throttle, yaw rate is still there without pilot input, yaw rate changed right after taking off successfully.

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#6

Even in AltHold, the controller commanded copter to yaw

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I am setting parameters of compass:
. COMPASS_LEARN : 2 - EKF Learning
. EK3_MAG_CAL : 0 - When flying

#7

Hi guys, it would be much appreciated if someone can confirm where this yaw effect came from.
Thanks.

#8

Set these back to default and try the Magfit utility in MAVExplorer to calibrate the compass and set the compass motor offsets. Page down to the MAVExplorer section.
Magfit