I have a 3" build (build log) that I want to tune better.  It's currently on the BF 4.2.5 default tune with RPM filters enabled.  I took it out yesterday and did some UAV Tech-style flights, with lots of rolls, flips, full-throttle punches, propwash-inducing manuevers, etc.  At one point the quad did a very strange thing.  I was doing a quick back flip, and near the point where the quad was fully inverted, it just spuriously rolled over 180 degrees and was upright again facing the opposite direction.  (My very first Rubik's Cube!  )  I can be sloppy on the sticks, but I was pretty sure I didn't cause that roll.  Today I looked at the Blackbox log.  Sure enough, the flight controller had done something very strange.  Here's a screenshot:

   

As you can see, the roll setpoint jumps instantaneously from zero to 800+ degrees/second--a move I'm not physically capable of making.  Then it settles back to zero along a suspiciously nice exponentially-decaying curve.

Here's another suspicious thing.  The feedforward at that point stays flat as a board at zero.  Feedforward is supposed to respond to quick stick movements, so we should see it move.  It looks as though whatever glitch occurred was seen by the PID loop but not by the feedforward algorithm.

It seems like this is either a Betaflight 4.2.5 bug or an electrical glitch in the FC.  What do you think?  The quad currently has a 470µF capacitor, which should be plenty for a 4S 3" quad.

did you decrease the motor idle speed?
first part of your description sounds like a half desync. i would look for idlespeed motortiming and startuppower/rampup.

could also be signal issue from the receiver


uavtech would have some pid presets to start if you didnt choose one already :-)

(09-Jun-2021, 07:30 PM)hugnosed_bat Wrote: did you decrease the motor idle speed?
first part of your description sounds like a half desync. i would look for idlespeed motortiming and startuppower/rampup.

could also be signal issue from the receiver


uavtech would have some pid presets to start if you didnt choose one already :-)

Thanks for the reply, hugnosed_bat!  I did not decrease the motor idle speed.  Everything is at the defaults for BF 4.2.5, except that I enabled RPM filters.

I looked at the motor traces in Blackbox Explorer, and none of the motors are saturating at 100% as I would expect if it were a desync.  So I don't think that's the problem.  Also, remember that the actual roll setpoint (i.e., what the FC thinks it is being told to do) is jumping instantly from zero to 820 deg/sec.  If it were a desync, the setpoint wouldn't be affected.  Incidentally, that 820 deg/sec is interesting because my roll rates are set to 800 deg/sec maximum.

Some kind of receiver glitch could explain the sudden change in setpoint.  It's Crossfire and I've range-tested it to all corners of the field with the link quality remaining excellent everywhere.  But there could be some electrical noise between the receiver and the FC.  The receiver is mounted directly underneath the 4-in-1 ESC.  That's a noisy spot electrically, but I can't see where else I could put it.

About UAV Tech's PID presets: the gyro_scaled noise profiles on this quad are quite poor, and I want to address that problem before I mess with the PIDs.  I doubt I'll be able to relax the filtering very much from the defaults.  I'm going to bring that up in a separate thread.

I finally figured out what happened.  I had crash_recovery set to ON on this quad.  The quick flip caused the firmware to think the quad had crashed, so it went into self-leveling mode for a half-second.

i didnt know that feature, but that doesnt motivate to try it out xD does truly work most of the time?

shouldnt the fc recognize the rc rates? how does betaflightbdetect the crash ?

(10-Jun-2021, 10:38 PM)hugnosed_bat Wrote: i didnt know that feature, but that doesnt motivate to try it out xD does truly work most of the time?

shouldnt the fc recognize the rc rates? how does betaflightbdetect the crash ?

It can be a useful feature if you're flying race-style in a place with many obstacles.  I almost always fly that little quad around the yard at home, and there are lots of trees and bushes and ghost branches to fly into.  The crash_recovery feature sometimes keeps the quad in the air when it would otherwise end up on the ground.  In normal smooth flight it does nothing at all, and I had forgotten it was enabled.

There are several parameters that can be set in the CLI to adjust when it decides that the quad is crashing.  There's almost no documentation for them, but there are a couple of YouTube videos about the feature.  I guess the defaults are fine for racing, but not so fine if you're doing flips and rolls.

I'll probably leave it turned on around home, but I'll definitely turn it off next time I fly any freestyle with the quad.