Roll angular rate anomaly

[W1CMC]

I’m having problems with a Holybro S500, Pixhawk 6X flight controller, PX4 v1.15.4. What I see appears to be a high oscillation on the roll angular rate, although whether that is caused by a faulty sensor or induced by some kind of feedback in the control loops I have no idea. Probably related: the motors get really hot and the battery life is severely reduced. Some plots from the logs:

Roll angular rate:

Actuator controls:

Does anyone have any idea what could be causing this or a suggestion for how to troubleshoot it?

73 de W1CMC

[JulianOes]

Hard to help you without a link to the actual log. And maybe some pictures of your frame and how the autopilot is mounted would help as well.

Otherwise, all I can say is that “yes that looks like bad oscillations”.

[W1CMC]

Good point. Here are the flight logs:

Flight Review

Flight Review

I was just standing in my front yard. I did try to stimulate the PIDs a little in one of the flights.

[W1CMC]

The flight controller that exhibits the roll angular rate oscillations is a Holybro Pixhawk 6X, an upgrade from the 6C the drone came with. I put the 6C back on the drone and the problem went away. Here are two flight logs recorded with the 6C:

Flight Review

Flight Review

Since the same set of ESCs was used in all flights of both 6X and 6C, it seems the problem must be something about the 6X.

I have been flying with the default parameters for the Holybro S500 airframe, no PID tuning or anything.

[W1CMC]

It really looks like a faulty gyroscope. This is the 6C flight controller:

Here is the corresponding plot for the 6X:

Note that the vertical scale on the 6C is +/- 50 deg/s and on the 6X it is +/- 300 deg/s. Also the X axis rotation rate really looks like it is oscillating.

[JulianOes]

Do you have a log of the “faulty gyro” in flight?

The two logs you shared look fine.

[W1CMC]

I have posted four logs in total, two from the 6X with what I suspect is a faulty gyro

Flight Review

Flight Review

and two from the 6C which is flying well (and, importantly, the motors do not overheat).

Flight Review

Flight Review

Which two logs are you referring to?

[JulianOes]

Ok, thanks.

Looking at the 3 gyros, they all seem to be showing the same signal with high oscillations. That makes me believe it’s not a hardware issue but a filter and controller tuning one.

The frequency is around 50 Hz looking at the FFT.

Could you share a picture of how the 6X is mounted? Any chance something is loose in roll?

[W1CMC]

I mount the flight controllers with Velcro tape; the pictures show the 6C mounted on the drone, the tape on the bottom of the 6X and the two flight controllers side by side.

[JulianOes]

I don’t think velcro like this is a good mounting option, as Imagine it could be quite loose, especially if one of the cables pulls on it a bit.

I would recommend double sided tape, and make sure the whole width (or corners) are covered, and not just a strip in the middle like in your velcro picture.

You want to make sure the Pixhawks are properly secured to the frame.

I’d try it out and check if the roll oscillations improve.

[W1CMC]

I’m happy to try, but puzzled as to why exactly the same attachment mechanism works well for the 6C and creates oscillations in the 6X? Is double sided tape really much more secure than Velcro tape rated for 10 pounds (4.5 kgf)?

Unfortunately, I have to postpone experiments until after June 7th when our competition ends.

[JulianOes]

You tell me. Wiggle it with your hands. I’m not sure what the sort of connection is. For one thing, the 6X is taller and has more mass, so that would have been my guess.

I’m not worried about it falling off (which is what you hint at with the velcro rating) but rather how it wobbles in roll, which is your issue.