I have a custom quadcopter frame that is controlled by a Pixhawk. While testing the frame in auto flight, I have observed extensive pitch oscillations when the quad is in horizontal motion while descending as shown in the descent phase of the plot below (figure 1). I say horizontal motion since the problem exists when flying forward, sideways or backward while descending in auto mode.
These oscillations are not always present during this stage of a flight but come up randomly. During multiple flights through the same flight path, you can see from the plot below (figure 2) that these oscillations are not present for the descent part of this flight.
The system was tested with manual and auto tuning to correct this problem to no use. I have had excellent flights with the same system and same tuning values in all other stages of flight. Stabilize, Alt Hold and Loiter were also found to be excellent.
I have looked online but have not found any discussion on this topic. Any help would be greatly appreciated. A copy of the logs for this flight are attached.
If you are descending quickly, you might be running into https://en.wikipedia.org/wiki/Vortex_ring_state, where you are flying into your own downwash. So you could try lowering your maximum descent rate.
I am familiar with Vortex Ring State since I had to deal with it a few weeks ago but this didn't look like it was one. The max descent rate is set to the default of 1.5m/s. Also during the descent stage of the flight the system had a horizontal speed between 3-4 m/s and was descending at a rate oscillating between .2 m/s and 1.2 m/s (about 0.75 m/s in flights when no oscillations were present). Also some headwind between 5-15 m/s was present during this flight. Also vortex ring state as you mentioned, occurs when we are descending into our own prop wash which we weren't doing so during this flight. When descending vertically in auto, we didn't see any problem in these flights.