PX4Flow and Failsafe enabled: no local position

Dear PX4 Experts,

hopefully you can help me, guide me to overcome my issues with the PX4Flow (1.31). First some information about my drone:

  • basically it is a Holybro QAV250
  • GPS is disabled
  • Pixhawk 4 mini is mounted in the center of the drone
  • PXFlow is enabled and mounted downwards in the center of the drone
  • TFmini is enabled and configured for range finder

PX4-Autopilot: 1.12dev
PX4Flow: latest from github
QGroundControl: Daily from June, 13th 2021

For PX4-Autopilot and PX4Flow I tried already latest releases. But no change in behaviour.

If I open the MAVLink Inspector in QGroundControl I get for DISTANCE_SENSOR meaningful values from the TFmini.

If the Drone is on the groubd I get for OPTICAL_FLOW_RAD only zeros except for distance (I guess that is the distance form the Sonar), temperature and the both time values. All other values are zero. I am aware of the quality. From the PX4flow code, quality must be > 0, otherwise you will get only zero.

That leads me to two questiouns:

  • Why do I not get meaningful values if the drone is on the ground? I calibrated the lens to about 2m.

  • If I take the drone and move it by hand up than following I notice:

  1. About 40cm distance from ground suddenly I get values. Quality is not good. Arround 100-130
  2. If I move up heigher quality becomes better and better
  3. Arround 90cm quality becomes a value > 200

Would be great if one of you experts can explain me a bit more why I see taht behaviour and what I can do. Because I think that this behaviour is the reason why I can’t takeoff. If I send the takeoff message the drone starts to takeoff and imediatly lands with the message:

Failsafe enabled: no local position

Which is true, as long my understanding is correct that the local position in my case is calculated from the PX4Flow and TFmini.

Btw: My goal is to use the drone with ROS2 in Offboard mode. I run already the ROS2 example from PX4 (of course I changed the height from 5m to 1.5m, because of my calibration), but still the same behaviour: No local position.

I am aware of some other, similar threads in this forum. Unfortunatly they could not help me to understand my PX4flow issue.

Thanks for your help!