09-Jun-2021, 06:31 PM
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.
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.