Vtol rapid descent when lidar engages

ArduPlane VTOL Plane
1

hi. i have a VTOL running a benewake tfmini plus, latley i have run into some issues during RTL.
when on descent the aircraft comes down slowly, then at around 5-7m above the ground the aircraft basically just drops out of the sky. it will descend extremely rapidly and sometimes slam into the ground breaking landing gear.
anyone have any idea what could be causing this?
i chanhed the lider engage to be at 1.5m and then it happened at 1.5m. so i assume it is something to do with the lidar readings. the craft had flown well for about 4 days and maybe 8 flights before this happened again.
it has happened on 3 different aircraft running the same setup
attatched is a log from 2 of the aircraft with the issue, lidar issue logs - Google Drive

setup is:
cube orange+
arduplane v4.2.3
12s
benewake tfmini plus

*side note. during the RTL transition it overshoots by a fair distance, any solution to fix that?

2

ok, I looked at A.bin - what can you tell us about the flight at the end compared ti the recorded trace shown for the landing where the aircraft ‘bounces’ up after rangefinder engage?

look at the last minute or so using UAV Log Viewer

And just to ask the obvious question, you’ve followed the ardupilot page on this Lidar as appropriate?

https://ardupilot.org/copter/docs/common-benewake-tfmini-lidar.html

3

so when the “bouncing” starts is right around where the aircraft would essentially cut power to the motors and fall out of the sky. and i have confirmed it happens when the lidar kick in by changing the rngfnd engage alt to 5m and 1.5m and it fell at each respective height

yes setup was done following the appropriate lidar settings.

4

So as soon as the LIDAR takes over height measurement the drone considers itself to have landed (as its recording landing and disarming messages rather than just stopping recording like the power is cut - the brownout situation described in your facebook post)?

5

yes its like the aircraft thinks it has landed and it goes into the final spool down. but it happens before the aircraft is on the ground

6

Apologies for the late reply - I think you need to do some testing to see if you can replicate this in a safe way - I wondered if the duration of your flight was long enough to incorporate an atmosphric pressure difference from take off and that has offset your baro and there is some kind of interaction there. I don’t know the details though, you would need to ask a dev team member.

It could be a brown out that happens as soon as the lidar is switched on, but its not like the autopilot is resetting or switching off - its recording the ‘landing’ attempt’. Maybe its just the lidar that is resetting?

Whatever it is, I am sure its an expensive airframe and therefore I would look to put it on a rig so it can be suspended above the ground and you can reocrd data whilst it is in a safe environment - props off if you want to run the motors too, although you won’t get the current draw on the system.

7

We have the same problem. Periodically, the plane falls on its belly. that is, everything works, but occasionally it crashes