7" quad - quad angle randomly drifts in acro despite zero stick input

I assume you are running BL_Heli. What settings are you using?

I would suggest:
Motor Timing: 25
Demag to High

Forgive the slightly off topic, but I saw this and had a good laugh. And saw a couple more posts about crashing and disarming to land, etc.

I’ve been flying mostly 3" copters for the past few years now (I know most of you know that, heh). Even using them for mapping, photogrammetry, 3d models, etc. Working out great. (Just need lidars to miniaturize now)

I just got back from camping in the mountains for a couple weeks. Lost my video one time and cut the throttle in mid air, over a valley. Copter had to have fallen hundreds of feet down. Maybe even hundreds of meters. Found it (using signal level as a guide). Took off and kept flying.
Huge benefit of the smaller copters! Especially w/ the battery on the bottom. They just fall flat and level down onto the battery. I’ve had similar crashes a bunch of times w/out any damage.

-edit to mention, I took off before GPS lock on this particular flight, which is why I couldn’t have it fly itself home… Something I almost never do. And the one time I do fly w/out gps signal, of course I had an issue

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Yep, I’m running BLHeli32. This is the settings:

That sounds quite amazing; what does your setup look and (and what frames do you use)? What are your tricks for getting around interference on the compass and GPS? Also curious, why cut throttle instead of using RTL? :slight_smile:

There is a chance that this may be caused by ESC setup being a little sensitive. I would do a test to see if motor timing reduces the severity of the problem. Ramp up power can also play a part if you are getting down into desync problem range.

Oscar Lang also makes suggestions to use higher PWM Frequencies but I have not played with that myself.

I’ve been using some flexRC frames… Skydiver and komori. Both modified… I cut away some extra decorative things he has on them. And I add some extra carbon fiber to the bottoms of the arms to stiffen them.

Interference was a challenge… But really just everything you should do anyway. Twist wires (except for the ones that shouldn’t be :slight_smile: ). Keep signal wires away from power. Added capacitors to both the battery input at the ESC, and at the 5v regulator on the FC. Copper shielding tape under the GPS. It’s not perfect. I require 10 min to get GPS lock under the best conditions. But it’s always fine once I take off.

And because of that 10 minute GPS wait… I sometimes fly w/out gps lock. Which is what I was doing when I lost my video and cut the throttle to cause a very fast landing. (edited my above post to mention that).

Started a thread about it long ago here: MicroArduCopter, 3" props, Omnibus Nano, Success!
But… Not even sure it’s worth reading anymore, so much has changed… Andy got involved, and really improved things, but all of that info is spread out in a bunch of threads in the copter 4.0 section.

Here’s a pic… This one is 2.4ghz video, just to show a microcopter w/ 2.4 :). But I usually fly 5.8. Sometimes I hang extra cameras off them when I use them for real work. Usually little runcam split type, but I have carried bigger action cams on these tiny copters when I needed.


Thanks for asking about it! (But I’ll try to not take your thread OT anymore.)

-quick edit to mention, this one has 18650 batteries, and I get about 18 minutes flight time, but can’t punch it and go fast. I usually use some smaller lipo’s and get between 6-7 min flights… And can fly over 100kph w/ those.

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Sure, I’ll try motor timing 25 first.

I’ve read that I should lower the ramp-up power if I’m at risk of desync… is that the direction you would recommend? I’ve played with this setting before and if I go much lower (say down to 15%), I can see the the slow-ramp when I arm the quad.

I’ve stayed at lower PWM frequencies because I believe it had caused some de-sync or motor error issues in an older quad with 48Khz, but I can try 48Khz as well.

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Thanks for sharing! That discussion is like a 1k posts of hidden gold. I’m enjoying reading through it so far. Also, omg why didn’t I think of copper tape.

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I can’t offer much more advice over what Oscar Liang gives here. I have done a lot of work getting reliable ESC operation on a 10 inch racer but a few things have evolved a little sinse then.

But it is worth checking to see if there is an ESC setting that is contributing to the problem.

I got distracted the last week and got to test this today with 0.06 spin_min, 150Hz gyro_filter, 70Hz flt_D.

Though it didn’t help with the grind/oscillation, I learning something from playing with the ESC settings. It seems increasing my PWM frequency from 24Hz to 48Hz reduces how hot my motors feel after testing for the grind. Dropping motor timing a bit (23 -> 21) also may help with the heat a bit though that was not tested enough to say for sure.

So I tried the last thing on my list: take the one of the approximate autotune results with 150/70/70 filter and drop them manually. So
Roll: 0.13/0.13/0.025 -> 0.1/0.1/0.02
Pitch: 0.14/0.14/0.028 -> 0.1/0.1/0.025
Yaw: 0.35/0.35/0 -> 0.25/0.25/0

This helped reduce that grinding/oscillation (and double checked it was with 150Hz GYRO, 70Hz FLT_D), and I can confirm this by testing previous PIDs. I should narrow down if it’s YAW or RP next, then I’ll go fly a bit.

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Thanks very much for sharing your PARAMs. I’m at a point where I can try comparing my PIDs against this and tweaking mine a bit. For example I’ve been keeping P/I locked together but maybe I should toy with increasing I.

I got a question I couldn’t find an answer through a few minutes of search… I found a setting “AHRS_GPS_GAIN” that is used to control how much GPS is used to estimate attitude and “AHRS_RP_P” that controls how fast accelerometers control attitude.

Is the former always disabled in acro and can one ‘tune’ the latter? When I fly in a straight line, I see small vibes that I just can’t seem to tune out with PIDs or filters.

AHRS_RP_P related to translating the pilots input, it won’t affect the tune. If the vibes are at either maximum or minimum throttle that is an indication that MOT_THST_EXPO is wrong

Hi Andy,

I didn’t want to spam your awesome thread on a LR 7" build with this. I flew my H743 build (Iflight 4-in-1 ESC and Xing 2806.5) with some of the scheduler/loop rate/gyro rate settings from your post along with BDSHOT enabled. I got it to feel pretty good but still found similar items that I had on my F405.

I found that I still need to go with a higher spin_min (at least 0.1), low mot_thst_expo (down to 0.1) to push performance (for my flying style) and improve propwash handling during descent. Higher most_thst_expo causes vibes at low throttle. I watched your acro flight and I noticed your throttle does tend to be above hover, but you did notice there’s heavy vibes in RTL when throttle drops. Does that seem similar to the behavior I’ve been seeing?

For this H743 build, I also found there’s some yaw offset injected when I bump throttle in acro; not sure why yet. The battery plug does come out the back near the compass/gps and I didn’t get a chance to check for compass interference like your did, but I wouldn’t expect compass/gps to do anything in acro mode. Maybe it’s interference from the IMU that I need to look into.

Also very neat that you have a setup to find frame resonance; I’ve also found from flying that I could benefit from a static notch around 168Hz.

Happy flying,
John

for the yaw issue, I may try disabling the 2nd IMU and only run on the MPU6000

Hi John, the RTL vibes are from frame resonance - I have lowered the static notch and will try it out when I get a chance.

I definitely recommend you tune against the MPU6000, it is far less noisy and gives a much better result.

mot_thst_expo at 0.1 is not right for those size props - something else must be wrong. Maybe the ESC is at fault.

Yaw offset sounds like noise - do you have a log? I found there was a big difference in the performance depending on how the FC was mounted.

I thought I’d give this a shot…

So this isn’t my motor (2806.5 Xing/Brotherhobby) or my prop (7035), but I pulled a thrust chart from a 2806.5 running 7056 props 6S and the ESC % vs thrust looks more polynomial than exponential starting at 10%.

Thrust expo is not actually an exponential it is a polynomial based on:
(corrected formulae)
Thrust = Thrust_Max * ( (1 - expo) * x + expo * x^2 )

In your case the expo looks low because your thrust is dropping off at higher rpm. This may be due to the propeller stalling or the motor staturating. I have not been able to do thrust measurments in a wind tunnel to work out how best to characterise this effect.

The way I handle this currently is to base the expo on the first 80% under the assumption that in normal flight the propeller will have more inflow and will not saturate as it does in static tests.

Thanks Leonard. And the thrust_max is the range of thrust from spin_min to spin_max (so if thrust chart is 42g @ 10%, then if I set spin_min to 0.1 then 0% in arducopter is 42g)?

If run the fit from 0.1 to 0.9 just to approximate this, I seem to end up with an expo of 0.3 (noting again, these are not my actual quad motors+prop… but I do have a set of the props they tested so maybe I can give this a try)

Correct.

That looks right. Keep in mind that at the upper end you may start to see propeller stall or deflection that artificially flattens the curve. If you did the fit using only the points to 50% you will probably find the expo is higher.

It is this trade-off that I have not made any solid decisions as to what is the best approach.