Terrain Following under 4 feet (1.3m) crash into Ground

ArduCopter Copter 4.0
1

Dear Friends,
I got a new phenom today and im thinking i got one parameter wrong but i dont find out which one.
I put the whole mission planner generally to “feet” because when I plan a mission with the Auto Way-point tool “Survey” i can only adjust the hight in full meters (1,2,3 m etc…) If I want to fly in e.g. 1.3m hight i have to modify the altitude by hand. So i switch the whole MP to feet.
Also if you see my Logs than you see that the Rngfnd Altitude change very quickly in a Range of ± 20 cm. The Reason for that is that im using two RngFnds. One Laser and one Radar.

Coming to the Point: When you see this Log:

you see that the aircraft nearly perfect followed the terrain. The Altitude I put to 4 feet (1.3m)

The next Log :

I put the Altitude to 3feet (1m) and unfortunately i crashed

I tried this several Times, always the Same, 4feet perfect, 3 feet too close to the Ground as if the Aircraft is using the Baro…

Has anyone got an Idea why?
Thanks alot in Advance

2

Today, after some testing with my Testcopter I found the conclusion.
On the Testcopter I also put several Lidars facing downwards and he hold the altitude perfekt (± 10cm in 1m altitude with 5m/sec)
The only different to my big Workcopter is, besides from the Size, that I didnt build an PX4Flow in.
So, I deactivate the Optica flow and all was fine.

3

Hello. please tell me how you combined the two rangefinders (lidar and sonar) to maintain altitude. Used arduino?

4

hi,
no, I actually just added RNGFND1 and RNGFND2 both looking downwards ( RNGFND1_ORIENT, RNGFND2_ORIENT both 25)

To be honest I dont know how ap is dealing with this 2 Information.

Maybe someone here knows

5

Thanks. I will hope that someone will explain