Motors Cutting in and Out in Loiter Mode

While flying my Iris, the motors began to cut in and out, but this only happened in loiter mode, not stabilize. The video I have attached shows the error occuring around 45 seconds, and continuing pretty much until it gets to the ground. You can hear the motors throttling up and down in loiter mode.

Video of the problem

The log file I am attaching is from a different flight that I do not have video of, but it was the same problem, it occured in loiter mode, again near the end of the flight. Thank you for any help you can provide.

Since this topic is about my iris can it please be moves to the iris support page?

[color=#00BF00][quote=“njfsu2”]Since this topic is about my iris can it please be moves to the iris support page?[/quote]
No. The Iris subforum is for hardware support. You are describing very likely a tuning problem which is a software support topic and for that reason belongs in the software support forum.

But it is a brand new iris. looking at the graphs myself (no experience) it seems the gps has an issue. Do what you have to, not asking for much just support for my brand new iris that i have not changed any software on. thanks again.

Again, the Iris forum is for HARDWARE questions. Inside the Iris is a Pixhawk flightcontroller and on that Pixhawk runs the APM:Copter SOFTWARE.
You can, of course, ask software questions in the hardware forums but chances are that you won’t get any answer then because the guys who support the software mostly read the software forums.

Looking at the video, did you change any tuning parameters? I had a behavior like this in a quad that was vastly mistuned and flying in gusty winds.

Sorry, thank you for your reply. Just hoping for some help. I am afraid to continue flying the IRIS until I have some resolve. Do not need a crash and or accident involving damage to other property.

No, every parameter is stock. Only thing I did is update firmware. It flies correctly most of the time, but this issue occurs occasionally.

As I’m facing similar issues, I did a lot of tests around this.

Here is in a nutshell how far I’ve gotten:

  • The altitude loss seems to be related to the barometer failing which is indicated by the BarAlt diverging dramatically from the GPSAlt.
  • My initial though was that wind (either from gusts or because of IRIS moving) could be causing this. So I placed IRIS on the ground and used a hairdryer (with cold air setting!!!) and blew air onto IRIS and into IRIS (vents and back) to see the effect on the barometer. Even if you funnel the air directly into the back and create an overpressure inside the hull, the perceived altitude by IRIS drops only by less than 2 meters and not as much as the 10-15 meter that I have been observing.
  • Next I noticed that the barometer failing was related to high vibrations along the z-axis, which in return was related to oscilation on the Channel Out (motors). See this post here for more details. What was causing the z-axis vibrations was the big mystery at this point.
  • Next I looked into the problem of the uBlox GPS touching the Pixhawk. The same problem that you are also facing. The results of these test can be found in this post. After isolating the Pixhawk from the GPS by raising the hull and thereby the GPS, the sporadic z-axis vibrations go away.

At this point it looks to me like the Pixhawk touching the GPS creates a feedback loop for vibrations, which leads to oscillations. With that we need to wait until 3DR comes up with a fix for this design mishap, unless you want to fix it yourself and see if it makes any difference.