In this episode of INAV Blackbox Analysis, we have something quite common again: everything works well in Acro or Angle mode, but as soon as the pilot enabled PosHold, problems start to appear.
When two weeks ago I said that you should only 8 channels when using FrSky SBUS I meant it and you just had to take my word for it. Today a small proof how Betaflight 3.5 feedforward component looks on the Blackbox with 16 channels and 8 channels. Of course, in both cases, RC smoothing type FILTERING was chosen.
Sometimes you win, sometimes you lose. This time I’ve lost. Not by much, but always. During testing of my DIY LoRa RC link, radio caught a glitch during a flip few meters above a ground. It was not even a failsafe situation. Link recovered a few milliseconds later, but it was too late and quadcopter crashed into the ground.
During a roll, while being behind a tree, RX antenna got hidden behind a carbon fuselage and both antennas were at 90 deg. That was enough.
Damage is not severe, nothing I can not 3D print in one evening. It’s more like a discredited honor or something.
The glitch was so short that is was not even recorded in blackbox log. RSSI was fine, no locked rcData. Quadcopter just kept 90deg attitude for too long.
There is a slight chance it was not faulted in software or hardware. Maybe there was a strong rouge TX polluting the aether. Why? I’ve caught a failsafe on a different quad (2.4GHz FrSky link) while being only a few meters away and a friend caught a failsafe on a TBS Crossfire. So maybe it’s not entirely my fault after all.
A week ago I wrote that HC-SR04 ultrasonic rangefinder is crap and it is still true. Only around 1m useful range and very jumpy behavior above are at least problematic…
There is a better solution. US-100 ultrasonic rangefinder. Comparing to HC-SR04, it has at least twice the useful range. My preliminary tests gave solid readout over concrete up to 2 meters and something. Let’s call it 2 meters. But what more important, US-100 was reporting out-of-range state instead of some random values when being out of range! Huge step forward comparing to HC-SR04.
Unfortunately, better does not mean good. US-100 also has some problems.
Readouts becomes “jumpy” at the edge of useful range and they are far far away from declared. But OK, declared always means “perfect conditions”. Still, trust me on this, comparing to this, HC-SR04 is crap…
Only few days ago I mentioned that I started to reintroduce sonar support to INAV. When DigitalEntity told me that popular HC-SR04 is crap I did not belived him. I expected that “official” 4m range on a noise-machine aka quadcopter is unrealistic, but data I recorded today clearly shows: HC-SR04 is crap and is absolutely not suited for quadcopters and probably other UAVs. Why? Because it goes nuts and starts to pick background noise instead of surface.
Let’s take a look here:
Bottom trace shows pseudo Signal-to-Noise-Ratio (SNR) while top one shows raw HC-SR04 altitude and INAV position estimator altitude (GPS, barometer and accelerometer combined). As you can see, it’s not that bad. There is a correlation between both of altitudes and SNR is pretty low.
Same thing is happening here. Quadcopter descended to land, both altitudes went down to finally meet at
zero. SNR is low too. Nice.
But, very bad things starts to happen as soon as altitude crosses 1.5m. Not only SNR goes up, but HC-SR04 starts to report completely unreliable data! If it was reporting out-of-range state, it would be fine. But no, it keeps reporting something between 1 and 2 meters while real altitude is much higher. Total crap.
Conclusion is simple: HC-SR04 can be used on multirotors but only on very low altitudes. Something like 0.75m over concrete. Or 0.5m above short grass. Or even less over long grass. Definitely not good enough for terrain following flight mode.
Next week I will test US-100 ultrasonic rangefinder. It is supposed to give much better results. I hope so…
Is it worth to soft mount flight controller? Are gyro pads I recommended here better than rubber standoffs? I feel that they are better, but I do not have any solid data to prove it. So I can only tell, that I find them better and that is all. But, I can answer the question is it worth to soft mount flight controller at all. The answer is YES and here is a proof…
The problem of twitching motors
After I updated my 5" racer to stronger motors (EMAX RS2205S 2300KV over RS2205 2300KV) I noticed something very very irritating: twitching motors. When the throttle was in more less middle position or after rapid throttle change, twitching from motors was very audible. Quad was flyable, but performance was very low. I had to reduce yaw P gain by more than 50% not to see those twitches in FPV footage!
Blackbox log revealed the truth: from time to time, the strong vibration was getting into yaw gyro traces. I also localized the source of those twitches: stronger motors with more torque and higher radial acceleration were able to shake the whole machine when the frequency was close to the resonance frequency of the frame.
I’m not completely sure why, but I’ve been pushing this topic away for quite a long time now. But it’s finally time to present it in this blog too. So, here we go.
Something like 3 month ago I’ve started to record and publish a video series about basics of gyroscope data processing in modern flight controllers (Betaflight, INAV, Cleanflight). It started as a tutorial how to setup notch filters in INAV, but ended up as a much bigger thing. Series consist of 4 episodes where I use Blackbox logs to show gyroscope signal noise and how to fight with it. Over next few days I will be posting links to those videos here, but if you eager to see them sooner, just use this link.
In Episode 1 I talk about:
- Gyroscope noise sources
- How unfiltered, raw, gyroscope signal looks like
- How noise sources manifests in gyro traces
- How filtered gyroscope signal looks like
Ah yes, I’m running a YouTube channel too, feel free to subscribe 🙂