7" quad strange pitch behaviour after autotune

ArduCopter Copter 4.3
1

Hello,

I have a question wrt odd flight behaviour on a 7’ quad after autotune.

I am in the process of tuning a 7 inch recon - 7 quad, which is going very well so far.

Mostly I am following the tuning guide in the doc section and the 7’ standard build thread by @andyp1per which is an excellent thread to follow, many thanks for the guidance!

I have progressed to the point where I ran autotune on yaw, then autotune on all 3 axes.
Quad seems to fly nicely on the basis of ‘feel’ so that looked good.

Next I wanted to check if RTL is working, and it is, but flight behaviour was strange.
The quad flies to the home point, but while doing so pitches quite suddenly down, then recovers.
It repeats this several times on the way back to home, then lands.
So in essence the test was succesful, but it is quite worrying to watch.
Pitch down angles are in order of 60 degs…

I tried another flight, flying forward / back, port / starboard, up / down with only stick commands.
In flight modes loiter, alth and stab. The issue seems to also occur in all these modes, mostly while flying backward and releasing stick, And when flying ‘down’. But there are other occasions where the pitch down does not happen…

So I looked at the logs, it seems to me that when the pitch down occurs, the quad is not following the commanded pitch signal well.
If the commanded pitch is increasing, the quad pitches down. I checked signs in the tuning parameters and they look ok to me…

So now I am a bit puzzled, and would like to request advice.

Is this a form of ‘rotorwash’ instability? (that was next on Andy’s test).
Are the autotuned pid values somehow unreasonable?
Did I configure something wrong?

I have quite a bit experience with rc and uav helicopters, but I am new to arducopter, so it’s quite possible I didn’t get some obvious thing right.

All help very much appreciated!

Boris

parameter file included using “upload” button
2023_03_29_before_rtl_propwash_vibration_test.param (19.7 KB)

log file included using wetransfer link

2

This is weird - it looks like a very large pitch spike on the IMUs. Can you retest with 4.3.6? Its possible this has been addressed, but I’m not sure. It’s not propwash - the tune looks good until you get these spikes. It looks more like an IMU driver issue.

3

After that problem is solved you will want to visit the Motor Ranges. 2 motors are bouncing off of MOT_SPIN_MIN. With the very high thrust/weight you have on that craft this will need to be set as low as possible respecting stable spin.
MOT_THST_HOVER.05 Yee haw!

4

I will retest with 4.3.6 but weather is not helping at the moment.
Will report back after rain ( and wind ) slacks off a bit.

On the driver issue, I would think that an issue like that would have shown up during the earlier flights using the same fw version as well. And I cant remember seeing anything like that. I can check my logs, and see if anything similar shows up.

Thanks for the help!

5

Hi Dave,

I think I will dive into the motor range issue after retest on 4.3.6. it may well be the reason behind this flight behavior.

The reason mot_thst_hover is so low is because I started off with the standard tuning / first flight docs on the website, and used default mot_spin_min etc… which worked fine initially.

I discovered Andy’s " 7 inch standard build " thread about halfway during my build and testing… And mainly followed a mix of both sets of info after that. As a consequence, my mot_spin_min is quite high (default) and mot_thst_hover quite low (after hover test). Added together they are very similar to the value I see in other 7’ builds.

There is nothing special about the quad I am trying to tune (standard recon 7 frame, t-motor motors, 3 bladed 7’ hq props, 3700mah 6s lipo).

So perhaps if 2 motors are prevented from going below a quite high / default “mot_spin_min”, then a sudden/unexpected (pitch) response occurs… which could show up quite clearly after a autotune and when flying down / doing a ‘brake’ maneuver.

Anyways sounds like a theory worth testing, will report back if I have some results…

Thanks for the help!

6

Definitely upgrade to latest stable, 4.3.2 in that log could have had an issue with timing and loops in attitude control that manifested as small dips in pitch or roll.

Otherwise I would say there’s something going on with motors, maybe ESCs, more likely motor 2 and 3. Maybe wires shorting to CF frame?

Also definitely set
MOT_SPIN_MIN,0.12
and I strongly advise connecting to Initial Parameters in MissionPlanner and plug in your battery chemistry and cell count then accept everything it offers in that regard.
Also set
BATT_FS_CRT_ACT,1
BATT_FS_LOW_ACT,2 or 3
It is a common mistake to think you dont need these while testing → when in fact this is the most crucial time.

Let’s see the next .bin log when you get a chance to fly

7

Hello,

I did the upgrade to 4.3.6, left all other settings as before to isolate the test.

Today finally no more rain, but still fairly strong wind ( approx 15 knots where I fly).
Went out to fly and test if the strange pitch behaviour has disappeared.
And first impression is that it did, so that’s good.

Like before I flew roughly half, then full stick forward / aft, sideways & up / down.
In stab, alth and loiter, behaviour did not return.

If anyone is interested, logfile included in the same way as before

If I look at the logs, they look much better, tracking is ok as far as I can tell.

I will definitely follow the other advice ( adjust motor ranges, go through initial parameters and set batt_fs properly ). But the main issue seems to be resolved already.

Many thanks for the help!

Boris