Optical Flow intstability

PX4 Autopilot
1

Hi all, I have enabled optical flow and range finder (downward facing distance sensor) on my drone and disabled the GNSS (EKF2_GPS_CTRL = 0), but I get a strange behavior and the drone seems to be “swinging”. I am attaching a log and a video showing this, do you have any takes on why that is?

I guess that the altitude instability can be fixed with some PID tuning, but I am not sure about the horizontal instability - any ideas on that?

Thanks,

~ Nick

2

Hi Nick
I haven’t looked at your log but some starting points for you.

  1. Have you correctly set the relative position of the optical flow sensor (EKF2_OF_POS_X/Y/Z)? If not set correctly, the angular-velocity correction will be faulty which will degrade the OF quality and degrade overall performance.
  2. Have you done some investigation if the delay is set correctly? You can start logging in a bench-setup and move the system abruptly, then compare the OF and IMU data.
  3. 1./2. often improve the oscillations massively. Further you can increaese EKF2_OF_N_MIN to trust the sensor a bit less and decrease MPC_XY_P to make the controller slightly less aggressive.

Hope this helps, Marco

3

Thanks for the tip. Will try it out and let you know

~ Nick

4

Hi @haumarco, I did this bench test (quite short as you see) and tried turning the drone along the z axis abruptly. This is the result I get and I interpret it like there is no delay between OF and IMU - does it make sense?

image
image2064×1236 41.7 KB

~ Nick

5

yes, possible. you can check sensor_combined, which is the imu topic but at a much higher rate. you can also compare the timestamps with a python script or similar. also make sure to do the other steps i mentioned