Random crazy altitude behavior in AltHold

Hi All,

1600mm Flat Octo / 55lb

Really struggling to figure out what is going on here. Ever 5 startups or so, the aircraft goes crazy in Altitude hold. It rapidly goes up and down like 10 meters. My initial instinct means this is due to vibration, and while I do know there is quite a bit of vibration, VIBE shows the Z still appears to be below 30. 4 out of the 5 times after powering up it files perfectly though (same setup) which is the confusing part to me. The only thing that does worry me is VIBE.CLIP2 is showing crazy clipping. I am assuming this is due to high Z vibration on IMU2 (third IMU)? The attached log is one of the flights where it goes crazy on me.

One thing I think I have noticed about the flights that go crazy is as soon as I take off I get the warning:
Critical: NavEFK2: lane switch 1
Warning: EFK primary changed

EFK2 is set to use IMU0 and IMU2
Does this warning mean it is switching to using IMU2 for EFK? (which I think may explain the aircraft going crazy?)

Any ideas?

Log389 (Crazy altitude behavior): https://www.dropbox.com/s/jl8kuvtvn1entzr/00000389.BIN?dl=1

Log386 (Good altitude behavior): https://www.dropbox.com/s/ez8ll3ij7xwel3q/00000386.BIN?dl=1

according to your logs its because of your vibration
high z vibration will affect EKF measurement and you will altitude holding problem
FC is trying to change EKF to get a better measurement (its looking for less noise on another IMU)


The 266Hz accel peak is very high. On IMU3 which is the only one you are batch logging.

I noticed the high 266hz as well. Why when I graph the same thing as you do I get different values?:

My Clip0 are very low, Clip2 are very high. Also the Z axis vibration is completely different.

Edit: I am assuming you uploaded the wrong photo?

Totally clipping is not good , even if its low.
Graph the Clip0, Clip1 and Clip2 values which increase each time one of the accelerometers reaches its maximum limit (16G). Ideally these should be zero for the whole flight but low numbers (<100) are likely ok especially if they occur during hard landings. A regularly increasing number through the log indicates a serious vibration problem that should be fixed.

Sorry, I did actually post the wrong graph. OK, this is the right one. It’s interesting that the Vibe levels are not shown as extremely high yet there is a massive amount of clipping shown on one axis.

IMU3 would seem to be the culprit. Which one is this on the Cube Black? From the HW ID screen.

I have the same behaviour in althold, for me it is due to vibration.
I’m going to remount the FC on a different softmount.