Severe Oscillations During Autonomous Missions (8m/s) – CUAV X7+ & Hobbywing X6 SE

[rdtta_lap]

Hello everyone,

I am reaching out for assistance with an ongoing oscillation issue on my quadcopter. The drone flies perfectly in manual modes, but it becomes unstable during autonomous missions.

Here are the details of my setup and the specific behavior I am observing:

Hardware Setup

• Frame Type: Quadcopter

• Flight Controller: CUAV X7+

• Motors: Hobbywing X6 SE

• Propellers: 24-inch foldable

• Total Takeoff Weight (TOW): 5555g (5.55 kg)

Issue Description

When flying manually, the drone is incredibly stable. There are no noticeable twitches, vibrations, or oscillations.

However, the problem occurs when I put the drone into an autonomous mission. Specifically, when cruising at an altitude of 50 meters and a speed of 8 m/s, the drone flies normally for a certain distance before it suddenly begins to oscillate heavily. During these oscillations, the motors make significantly more noise, indicating they are working hard to compensate for the instability.

Troubleshooting Steps Taken

I suspected this was a tuning issue at higher speeds, so I made multiple attempts to retune the aircraft. I progressively reduced the Proportional (P) and Integral (I) gains for both the Roll and Pitch axes. Unfortunately, these adjustments did not resolve or mitigate the continuous oscillations during missions.

Request for Assistance

Has anyone experienced a similar issue with this specific propulsion system or weight class at higher speeds? I would greatly appreciate any guidance on which parameters I should investigate next, or if this might be related to aerodynamics, harmonic vibrations, or a specific navigation controller setting rather than basic PID tuning.

I have attached my flight logs (.bin) for review. Thank you in advance for your time and expertise!

00000065.BIN

RAW ENABLED LOG.

00000063.BIN

MISSION FLIGHT

[MaxBuzz (Maxim Buzdalov)]

I wouldn’t say that this is a perfect flight (a segment from your AltHold flight):

Flying missions is not the cause. This machine flies just as badly in manual modes too. In fact, this episode is worse than what was recorded in missions: here, the deviation in both roll and pitch reach 8 degrees, whereas in the mission flight it was just about 6.

You did not enable PID logging (LOG_BITMASK is the old default, 176126), so I cannot say much about the PID contributions. Set LOG_BITMASK to 180222 (aka enable PID logging) for more usable logs.

What I see from RCOU entries is that this machine hits MOT_SPIN_MIN aka the minimum configured motor output in flight, when all these incidents happen.

By the looks of it, it happens during braking (slowing down by a countering attitude input), and the magnitude is probably coupled with the wind direction. Additionally, the output gets noisier towards the throttle-down, which may indicate that there is some sort of resonance/PID noise activation first, which then makes it easy to hit MOT_SPIN_MIN and destabilizes the machine.

Misconfigured throttle-based notch may be a contributing factor to the noise problem. The multi-source bit is not set, which means there is only one notch based on the average throttle, which in quadcopters is incapable of covering anything. The inability of a single notch to cover multiple noise sources becomes worse as the motor outputs become different, and even if in plain hover that notch may be silencing out the propeller noise, it can no longer do it during aggressive moves - which you may actually hear as audible noise getting worse.

Here, you absolutely need multi-source. Depending on the characteristics of your machine, you may be better off with FFT-based filtering if you don’t have ESC telemetry available.

Yet another possible contributing factor is a MOT_THST_EXPO value of 0.76, which may also contribute to poor performance at low throttle. Such radically high values should only come from thrust stand measurements, in my opinion.

Hope this helps a bit.

[dkemxr (Dave)]

Agree with @MaxBuzz . The Rate oscillation in this log is bad. How do you have the Flight Controller mounted? The vibe levels are suspiciously low.

[rdtta_lap]

@MaxBuzz Thank you for the detailed analysis.

I will change the LOG_BITMASK to 180222, perform another test flight, and share the new data logs.

Regarding MOT_THST_EXPO, the value is set to 0.76 because I am using propellers larger than 20 inches. Standard guidelines recommend 0.75 or higher for props of this size.

For the motor settings, my motors begin rotating at 5%, so I have MOT_SPIN_ARM set to 0.05 and MOT_SPIN_MIN set to 0.08.

I have one question about the harmonic notch filter: Should I enable the multi-source option now and test it during this next flight?

@dkemxr

[MaxBuzz (Maxim Buzdalov)]

@rdtta_lap

Should I enable the multi-source option now and test it during this next flight

Surely yes.

[xfacta (Shawn)]

I recommend this, same as you had but with lower frequency, lower attenuation, no extra harmonics or double notch, plus Multi-source.

For this result:

Try reducing the thrust expo to 0.72 for 22inch props or 0.69 for 18inch props.
There was quite some instability during descent at one point.

[dkemxr (Dave)]

Did you try initially without that isolation mount? Suggest you remove it.

[mike_tk (Mike)]

This motors have CAN RPM feedback (but with nuance), it’s much better then throttle notch for filtering.
Also it looks to me that this motors have linearized throttle, so expo values can be much lower.

[rdtta_lap]

I have applied the recommended parameter changes, updated the LOG_BITMASK, and completed a new test flight.

The raw log file is attached below. I am still noticing some slight oscillations during the flight. Could you kindly review this new data and let me know what you see?

Mission

00000069.BIN

Raw

00000067.BIN

[rdtta_lap]

I have implemented the parameter changes and suggestions discussed above.

During troubleshooting, I also discovered that loose battery mounts were generating significant mechanical vibration. As a temporary fix, I added foam to ensure the batteries fit securely.

After securing the batteries, I completed the following steps:

1. Applied the notch filters.

2. Ran a full AutoTune.

3. Manually fine-tuned the P, I, and D terms.

Following the tuning, I put the drone through autonomous missions at 50m, 70m, and 120m altitudes. Visually, I no longer see any oscillations, and my initial review of the logs shows no suspicious data.

I have attached the latest logs below. Could you please review them to see if you spot any hidden issues I might have missed, and advise if any further parameter changes are necessary?

00000071.BIN

[dkemxr (Dave)]

@rdtta_lap

3. Manually fine-tuned the P, I, and D terms.

Why?

I don’t think you will get a good tune with that FC mount. Rate output looks messy. Re-enable batch logging so the notch filter can be reviewed and make another flight that is not an Auto Mission. Just some hovering and some pitch/roll/yaw events.