I have been working on the construction of a rather large X8 drone which has an AUW of 36Kg. The test payload in this scenario has been 20lbs.
Details of configuration:
T-Motor U11 120kv motors in an X8 coaxial configuration.
T-Motor 26x8.5 props
T-Motor 80A Flame ESCs
Pixhack flight controller with firmware 3.3.2
Mission Planner: 1.3.37
4x 22Ah 6s battery packs. These are being used in two series pairs to run the motors at 50V
The 50V power is going through a UBEC in order to power the flight controller with 12V, as 50V was rated too high for the PM sensor. I instead use telemetry via the transmitter/receiver to monitor system voltages.
This past weekend the weather was good enough to go out and do some flying. We tried to auto tune the system in order to improve its performance. But this has resulted in a lot of oscillations. After conducting an auto tune, the system seems perfectly fine. I then land the aircraft with the auto tune channel still high, and disarm in order to save the PID values. Then when I go to fly again, the system oscillates aggressively - even though the values were not doing this prior to landing and confirming the saving.
This problem may be echoed in this forum thread:
Some people blame firmware 3.2 and indicate that problems went away after reverting to 3.1. However, I am on firmware 3.3.2.
I do not know if these problems continued in 3.3, or if there is anything related to the problems they are describing.
Because the auto tune takes a very long time for this particular machine (almost 10 minutes per axis), I end up doing each axis at a time while auto tuning. For reference, I have provided links to videos and DataFlash files from each step taken. All DataFlash files can be downloaded here:
I first started off performing the auto tune on just the roll axis. Video of this can be seen here:
The dataflash file from this flight is titled ‘2016-03-20 - 01 Auto tune Roll Axis’.
After saving the new PID values for the roll, I went to perform the auto tune for the pitch. However, when I took it into the air, the system had extreme oscillations on the roll axis. Even though the PID values were fine at the end of the roll auto tune - as before landing these values are active so that the user can test them to see if they like it. However, once the controller has been cycled and the PID settings saved, heavy oscillations occurred. You can see this behavior in this video:
Dataflash file - “2016-03-20 - 02 Oscillations Roll Axis”
The wiki indicates that if I am seeing vibration, to try lowering the ‘RC_FEEL’ value in the software to a value of .25
As the default is 50, I first tried a value of 25. This change had no effect, so I tried .25
This too had no effect, and so I reset it to 50. I then took a screen shot of the PID values for the roll axis, reset them to default, and then moved on to trying the pitch axis.
The pitch axis went the same. Video can be seen here:
DataFlash file - ‘2016-03-20 - 03 Auto Tune Pitch’
As you will see at the end of the video, after landing and saving the PID values, the system shows extreme oscillations on the pitch axis. Again the PID values are documented, and then reset to default in order to proceed with the next axis.
We then did the yaw axis. Video is here:
Dataflash file is titled ‘2016-03-20 - 04 Auto Tune Yaw’
Once again, at the end of the video you will see the resulting PID’s after landing and saving results in extreme oscillations.
To see the PID values each axis auto tuned with, check out the screen grabs in this folder:
Probably my biggest concern when flying the machine is the fairly rapid loss in altitude when changing position in the roll or pitch directions. When in ‘altitude hold’ mode, the system goes down on an angle whenever I attempt to fly it around the field. And so we tried to adjust the ‘Altitude Hold’ gains manually. We moved the default of 1.0 to 1.2, to 1.5, and finally 2.0. I could not see any improvement, but we unfortunately ran out of batteries by this point. It did feel that landing the aircraft became more challenging as the value increased, but it is hard to know for certain.
Video of these trials can be seen here:
DataFlash file is called: ‘2016-03-20 - 05 Altitude Hold Adjustments’
My next thought is to assign the ‘P’ value of each axis’ PID value to a channel 6 knob on the transmitter in order to manually adjust the gains during flight in order to try to dial out these oscillations. This may also prove to be a good way to try and improve ‘altitude hold’ performance. I see a lot of people saying poor altitude hold performance is often due to vibration issues on the flight controller. However, the values in the dataflash logs (except when the oscillations occur) seem to be pretty good I think. I think vibration issues are to blame for aircraft which raise or lower altitude on their own without any input whatsoever. In other words, the system simply rises or lowers while the throttle is at mid, without even trying to fly the aircraft in any particular direction. I think in our case, it is just not giving enough signal to the motors to maintain the altitude.
Currently the weather here is not very good for flying, and so I thought I would share my efforts thus far in case someone may have some insights or advice on what steps I might try in order to improve the handling of this aircraft.