Hear my sad story. We assembled by ourselves a very huge hexacopter with pretty much the following setup:
- Pixhawk 2,1 running arducopter 3.4.6
- Here + GNSS external compass
- Two Multistar 6S 16.000 mAh connected in serie to have 12 S
- An attopilot current monitor 180A
- Range Finder Lite 40m
- etc… (if additional information may help, just ask me)
We flew before this little monster in Stabilize, AltHold, and Loiter. Our Loiter was beautiful, after running an Autotune in three axes. I’ve never built a Multirrotor so stable. We even run a Failsafe test, with tank full of water discharging all the time, and everything goes well.
But it was the time to test the dron in AUTO. We configure a very simple mission with four waypoints:
- Takeoff 3m
- WP1 to WP2: speed of 3 m/s and altitude of 3m and start spraying
- WP2 to WP3: goes from 3m to 4m and stop spraying
- WP3 to WP4: speed of 3m/s and altitude of 4m and start spraying
- RTL not spraying
Everything goes well the whole mission. Just some ERROR COMPASS VARIANCE (my nightmare), but the copter never goes crazy. When the copter swicth to RTL, it increase the altitude until 10 m (I forgot the fact that by default the copter has a predefined altitude when starting the RTL). After that, the copter reach the point when it was supposed to start the descend and… guess what? it just disconnected in the air !! So that was the thing. We suffered a fall from 10 m. Not to much damage, so we were lucky. Just a couple of carbon fiber rod broken.
We are not so expert exploring logs, so I have attached both logs, dataflash and telemetry, to see if someone can see something weird.
The only think that I can see is a sudden increase in the vibration on z axis. My hypothesis is that because the tank was still with water, when the descend started, the water wobbles, the copter tried to be stable, the vibration increased, and that breaks some connection and accident…
But to be honest, I am not fully convinced with this hypothesis. Anyone, after checking the logs, can see anything we are missing?
Log you’re attached reports 3.5.2 version
Please share .bin log that is stored on SD card.
Here is the link to download the .bin log.
Hope you guys can give us some light about this accident
I thought it might be due to the hover value logic on landing where half the hover value of throttle indicates landed state, but it appears that the throttle values went to max right before the crash. It looks like the voltage increased nearly 10 volts at the end of the flight as well. (Weird)
Maybe it was as silly as a bad planning? Here I am sharing the waypoints mission:
As you can see, it is just a TAKEOFF, four WAYPOINTS+ DO SERVO command to switch on and off the pump, and finishing with a RTL command. This time I did not use the Survey Grid utility from Mission Planner, but place each waypoint manually. For the RTL command, I set the altitud to 0, as it is done automatically when configuring a mission using the Survey grid utility.
Maybe it was something related to bad planning? Notice that I was using a Range Finder for altitude measurement.
It looks more like a power failure of some sort.
From waypoint #4 your current starts to drop off to zero then suddenly shoots back up to a flat line at about 140A.
Then just before the end starts to fluctuate again.
The tell tale is the abrupt end in the log at altitude just after your Vcc dives to zero.
Using such high amperage I would be looking very closely at your power supply and distribution.
I didn’t have a look at your log, but I would agree with Mike and my advice is: stay away from Multistar batteries! I had one bad crash and one almost crash, the high grass saved me, with them.
They are known to not cope well with a high current, even with medium current and are nowhere with the C rating on them (10C is actually 3C), so I switched to high quality batteries with min 30C rating, and never looked back.
Just my 2c
I have had good experiences with 8ah 10c 4s multistar batteries on my 4.5kg hexacopter - 2 wired in parallel giving suggested 160amps. I have certainly loaded these to over 130 amps without issue. I did have issues however with the 180a attopilot. It would report vastly varying voltage over a flight causing battery failsafe events. Rectified this by switching to Mauch hall effect power sensor instead. I would definitely recommend this to your Agricultural copter build as it provides a completely accurate (if configured with supplied calibration values) power monitoring solution upto 200amps.
I don’t think this is your batteries.
@rrr6399 was correct with his analysis.
At the end of the flight we see the throttle output go to maximum and the aircraft start to fall. The other strange thing is at the same time both the voltage and current go up. The only way I can think of this happening is if you loose the ground connection to your current/voltage sensor.
Your Vcc drops down and shuts down the pixhawk.
This is definatly a problem with your power supply. I suspect that your power distribution can not handle the current and that is why you are seeing a bunch of strange readings from the voltage/current sensor and then your power to the pixhawk dies.
What current sensor are you using?
Can you supply pictures of your power distribution?
I, likewise, have had very good results with MultiStar batteries. On one heli that I fly a 10S system on I have a 10A 4S MultiStar in series with two 5000 6S 30C Spyders (in parallel). The heli has a takeoff weight of 6.4kg with this battery configuration. The MultiStar keeps up with the paralled’d 30C 6S batteries with no problem on 30 minute flights and it has about 110 cycles on it now.