First major incident: Need help with diagnosis

htotheunit

htotheunit

Member
Hello,

Today I was flying along with my hexa at a steady pace, when all of a sudden it began to rotate as though it had lost a rotor. Knowing the ground below was clear I began to slowly reduce the throttle to bring it down without causing too much damage.

Nothing appeared to happen when I reduced throttle then all of a sudden the aircraft rolled to the left steeply before rapidly and uncontrollably descending (definitely over the Wookong's maximum angle) it slammed into the ground damaging 3 motors (2 are salvageable, but i'd rather not risk it, they're full of mud), numerous props, cinestar gimbal and 3 arms.

I don't estimate the repair cost as too much (luckily) but as this hexacopter is an expensive piece of kit I have spent hours looking into possible causes. I have came up with the following:

1. Fault with Wookong or a speed controller on rotor number 5 causing it to power on full, causing the anti-clockwise rotation leading to the Wookong over compensating/getting confused. (Evidence is the sound of the video, and the purple line on the graph showing rotor 5 speed) however I have tested all speed controllers (albeit not in flight due to the airframe being knackered) and there appears to be no issue.

2. Engine malfunction causing slow rotating prop. I don't personally think this could cause the rapid descent seen, I have lost a prop before and managed to bring it down with no incident (replaced speed controller after this, problem fixed) Again, all engines have been tested (except for the knackered ones)

3. Some kind of issue/malfunction through interference or faulty IMU/compass data. I have had a couple of unexplained yellow flashing lights from the Wookong in the past leading to the craft leveling off until compass data was back, but I put this down to interference. All the cables to and from the IMU, PMU, Flight controller, GPS etc are correct, undamaged and besides the past couple of flights yellow light incidents have been flown may many times without incident.

Here's the video with the motor data from IOSD for rotor 4 (Yellow) 5 (purple) and 6 (green)

I didn't include the other rotors as they appeared to be functioning correctly, Motor 2 did have noticeably low throttle during the crash however I believe this was down to the FC not knowing what to do.

http://youtu.be/c2mgyTtCid4 (Full speed, no graphics)

http://www.youtube.com/watch?v=HAC1TTFd6-U (audio with motor power graph, the timing of the graph was estimated using the altitude stat, audio and video length, you can hear motor number 5 faulting first followed by rotor number 2)

Here's my spec:

Vulcan 1200 Hexa pro
Tiger MT3515 motors
Simon K flashed Maytec 40A opto speed controllers
Futaba super 8FG
DJI Wookong M running version 5.26 (latest version)

Gain settings:
Basic: Pitch: 270% Roll: 270% Yaw: 150% Vertical: 150%
Attitude: Pitch: 100% Roll: 100%

Any little bit of help that you can give, or theories you may have it would be greatly appreciated.

I'm stumped!
 
Last edited by a moderator:

htotheunit

htotheunit

Member
Sent this over to DJI for their help deciphering the data, Hope they get back to me....

"Hello,



I was flying my hexacopter equipped with a DJI wookong M flight controller in a straight line when motor number 5 suddenly and inexplicably accelerated to full power, sending the multicopter into a counter clockwise spin. I attempted to land the aircraft by slowly reducing the throttle thinking I had lost a propeller or motor, but the aircraft at that point entered a steep (way beyond Wooong M's limits) descent where it crashed.

I have IOSD fitted to my aircraft and noticed a couple of strange things regarding mode switching whereby the wookong switched from GPS mode into ATTI even though the GPS signal was above 7 satellites, and there was no input from the mode switch channel. I have also flown the aircraft in ATTI mode with no issue with control.


Is there any chance you could take a more expert look at my flight data? As you can see the moment the multirotor starts behaving strangely is at 96, while the actual impact occurs around 103.


I have also compiled two videos shot from the aircraft, one showing the crash video, and the other with a timed line across the data from engines 4,5 and 6.

Obviously I appreciate you may be very busy, however I have looked into every possible aspect of this incident from Propellers falling off to radio contact being lost, multirotor set up and I haven't found anything to point toward what happened. Being a responsible aerial photographer I need to fully investigate this incident in order to prevent this from occurring again. This particular platform has been performing well for a year and was last taken apart for a full inspection two weeks ago.


Video of incident - http://www.youtube.com/watch?<wbr>v=c2mgyTtCid4&feature=youtu.be
Video of incident with graph of M4,5 and 6 - http://www.youtube.com/watch?<wbr>v=HAC1TTFd6-U


Any help figuring out what happened would be greatly appreciated.


Thank you,
Harry"
 

htotheunit

htotheunit

Member
I noticed this interesting phenomenon on the flight data I took from IOSD. In normal flight, the output from the WKM correlates roughly with the counter rotating rotor, except when the pilot yaw's which (I assume) uses all 6 rotors in unison. Now if we take a look at rotors 1 and 4, rotors 2 and 5 and finally 3 and 6 we see that when rotor 5 increases RPM (the moment something went wrong) rotor 2 opposite rotor 5 loses a lot of power. The other rotors appear to be working ok.

Can anybody tell if this has happened to them, because I expect that the FC should have countered this movement by spinning the opposite rotor equally?
 


P

Pjtyros

Member
Hi, I have had exactly the same type of crash on three occasions, the craft motors esc's exactly the same as yours. After the first crash I changed the radio system, Graupner to Futaba (was going to change anyway). I re did all settings in the wkm. Etc. the second crash again was similar, after which I swapped the wkm for a new one. On the third occasion, only 5 or 10 flights later the same happened, I managed on the second and third time to get it on the ground with only broken props. At around the same time I was building a craft for a friend, again exactly the same setup, on first test hover the same happened, landed with no damage. I deduced that it was an esc problem so I removed the Maytec esc's and replaced with Hobbywing, now some 20 to 30 flights later no problems. I built another craft for a customer and that had a Maytec catch fire in the air. So I will never use Maytec products again.
Cheers.
Paul
ps. Where are you?
 

htotheunit

htotheunit

Member
Pjtyros said:
Hi, I have had exactly the same type of crash on three occasions, the craft motors esc's exactly the same as yours. After the first crash I changed the radio system, Graupner to Futaba (was going to change anyway). I re did all settings in the wkm. Etc. the second crash again was similar, after which I swapped the wkm for a new one....
Click to expand...

Hello!

Thank you for the information, During your incidents, did it show up on iosd data? I assume that the data from the iosd was what the wookong itself was telling the esc's rather than what the actual motors were doing? I'll have to look into that too, as if it is actual esc data, then I can't discount them!

Did yours make a similar screeching noise before hand? I guess if it was the esc's I'm pretty unlucky for two to go at the same time, also very odd that the two motors at full speed were the ones that hit the ground first?!

I'm based in Tamworth near Birmingham, collected all the new parts I need to fix it today so it won't take long to be back in the air, I just hope DJI get back to me so I can see if they found anything. I'll certainly look into new ESC's and probably swap them over! What's the refresh rate on the hobbywings? Are they Simon K flashed, and where did you get them from?

Thanks again,
Harry
 

P

Pjtyros

Member
I had no onboard video sound recording so hard to tell about the noise, I now have OSD but not then. The Hobbywing ESC's have been around since the year dot and are the most reliable I have used. No not Simon K flashed but when flying the craft for AP they are sound. Just do a google search as they tend to go out of stock quickly, try Quadcopters UK or Heliguy or Giant Shark. Cheers
Paul
 

S

skquad

Member
Bared on the examples from pjtyros's experiences I would say the esc is overheating and shutting down.

Sent from my GT-I9505 using Tapatalk
 

S

sk8brd

Member
it's the simon k esc and certain tiger motors. several have had instances like this lately all on simon k maytech's with tiger motors. put on some plush's and should be all good or non simon k maytech's and test test test. personally a lot of guy's have good results on plush's-maybe not considered the best but are reliable. many heavy lift cinestars are using those esc's without fault and there cheap. check hobby king for em.
 


dazzab

dazzab

Member
I haven't looked at your video but if you heard a 'screeching' sound I'd bet your ESCs are losing sync with the motors. I know with the Pixhawk and ESCs flashed with SimonK software using larger motors this has been an issue as well. Given others here seem to think it's ESCs as well I think that would be a good place for you to start.
 

S

sk8brd

Member
htotheunit-there are some guys i came across with maytech non simon k esc's on tiger motor's and i "believe" been ok. There was a special way to calibrate the simonk versions i heard talk of using 110% throttle rather then the normal 0-100% to calibrate them-this is not confirmed just what i read before. personally- with all the incidents lately i would skip them if your on lower kv tiger's. As i mentioned earlier i know of plenty of people on low kv tiger's with turnigy/hobbywing esc's that have been doing fine. most of the cinestar heavy lift crowd if not on mikrocopter use the turnigy plush's without fail. you can research it if you'd like but for me i'm going with the plush's.
 

gtranquilla

gtranquilla

RadioActive
You also mention Turnigy...... are your talking about the Multistar ESCs? I ordered up 8 of the Turnigy Multistar 45 Amp Multi-rotor Brushless ESC 2-6S..... in part because I have had such great success with the 30A versions with smaller motors, i.e., 130 flights to date with no issues. Ofcourse I would have chosen 40A if they were available..... Planning to use 8 x MN4014 - 400KV Tiger Motors if all goes according to plan.


sk8brd said:
htotheunit-there are some guys i came across with maytech non simon k esc's on tiger motor's and i "believe" been ok. There was a special way to calibrate the simonk versions i heard talk of using 110% throttle rather then the normal 0-100% to calibrate them-this is not confirmed just what i read before. personally- with all the incidents lately i would skip them if your on lower kv tiger's. As i mentioned earlier i know of plenty of people on low kv tiger's with turnigy/hobbywing esc's that have been doing fine. most of the cinestar heavy lift crowd if not on mikrocopter use the turnigy plush's without fail. you can research it if you'd like but for me i'm going with the plush's.
Click to expand...
 

S

sk8brd

Member
gtranquilla- i personally was talking about turnigy plush's but have heard that the multistar's are good as well. i think the esc problem with the lower kv motors comes from the simonk firmware that people use to mod their esc's to make them react faster to throttle input and fly smoother. in general most people i come across love their simonk flashed esc's performance wise but people have reported problems with simonk on low kv tiger motors. on higher kv motors used on smaller multi rotor's like 450 sized people have been very happy with the updated simonk firmware.

i don't know too much about esc's i'm sure there's others could chime in with more info but the problem seems to be with only certain simonk flashed esc's. maytech sells two versions of the same esc i believe-one version with simonk and one with out. i would stick to non simonk flashed esc's imo so the multistar's would be fine as they don't come from the factory with simonk pre loaded.
 

T

Tahoe Ed

Active Member
Eventually the E600 escs from DJI will be out. They were designed to work with lower KV motors..Unfortunately that will not help you now.
 


R

R_Lefebvre

Arducopter Developer
sk8brd said:
gtranquilla- i personally was talking about turnigy plush's but have heard that the multistar's are good as well. i think the esc problem with the lower kv motors comes from the simonk firmware that people use to mod their esc's to make them react faster to throttle input and fly smoother. in general most people i come across love their simonk flashed esc's performance wise but people have reported problems with simonk on low kv tiger motors. on higher kv motors used on smaller multi rotor's like 450 sized people have been very happy with the updated simonk firmware.

i don't know too much about esc's i'm sure there's others could chime in with more info but the problem seems to be with only certain simonk flashed esc's. maytech sells two versions of the same esc i believe-one version with simonk and one with out. i would stick to non simonk flashed esc's imo so the multistar's would be fine as they don't come from the factory with simonk pre loaded.
Click to expand...

This problem is not restricted to DJI controllers, nor any specific brand of ESC. We are seeing it too with Arducopter. It's a simple fact that the latest flight controller algorithms, are pushing the ESC's very hard. This, combined with large props and low KV motors, is exposing this problem with the SimonK firmware. There are fixes... the simplest of which is "don't use SimonK firmware". Others are more complicated, and involve modifying the settings of the SimonK firmware. I have heard this is possible, I don't have any details.
 

S

Scott Drysdale

Mixmaster
I agree with R_Lefebvre....
Also with this all have something to do with the fact that a lot of ESCs still have a combination of P and N - FETs.... whereas the more rugged ones have 100% N - FETs that can handle the higher speed + pulse rates without overheating?
 

R

R_Lefebvre

Arducopter Developer
Interesting idea, but I'm skeptical. I'd love to see a test of the theory. It would be important if this was true.

I think it's simply a case that the SimonK firmware passes through the throttle setting immediately, with absolutely no filtration or delay. And so when you open up the throttle suddenly, it opens up the throttle very fast. From there, 1 of 2 things could be happening. The motor accelerates faster than the ESC can keep track of (it is sensorless, and is really only guessing at position). So, say it opens up, and it accelerates so fast that it actually passes the point where commutation should happen... probably bad. The second thing, is that when it opens up the throttle suddenly, there's a huge surge in current which does something else nasty inside the ESC.
 

J

jeey

Member
The sound is the same that i had when my esc had timing issue, you should bench test the motors with some abusive throttle when you swap your esc and you will avoid this.

Hobbywing platinum esc is a good choice
 

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