Hi all, I have had this problem which is driving me mad. its happened twice now.
Note I posted this on APM but had no response.
The thing I am confused about as you will see below and from log. The DesRoll, Roll, DesPitch, Pitch, DesYaw, Yaw and Des Alt, alt all correspond. From this I can only conclude the flight controller intended the copter to behave the way it did.
I hope the links and so on work as below is copy and paste from AMP:-
thanks.
After re-building our H8 and re-tuning it with autotune. (previous post Autotune Crash) I had a few good flights. The rig was flying very nicely, autotue had done an excellent job at tuning the PIDs.
However, we went out on a job today and decided to to use this rig as it was best suited and is / was flying very well.
After a few flights I got lazy and desired to do a RTL. The gig flew back very nicely and started to land. At this point I jumped over to to Loiter as it missed the home point and was going to land in a pond.
As soon as I switched to Loiter the rig started to yaw clockwise and descend. I reacted by applying ccw yaw and waking the throttle full up.
I got no response, the hely just continued to yaw CW and decent into the pond, Unfortunately the camera was not recording at this point so no sound captured.
I have looked at the log several times and my understanding of it is:-
vibrations are low.
https://www.dropbox.com/s/uczdnodfzrdp7u9/Vibration.JPG?dl=0
I bereave the vibrations are low typically between 0.4 and -0.4.
VCC Voltage:-
Still fluctuating too much but there is also power from the rail and USB.
The point when all went wrong is exactly when I switched from RTL to Loiter.
Tracking, looking at DesRoll vs Roll it was racking very well up until the point there it switcher to Loiter around 19.5 mins into the log. The then starts drifting ever so slightly.
https://www.dropbox.com/s/qfvy0di1qi5va0b/RollvsDroll.JPG?dl=0
The point where is diverse off is when the Helli is lifter out of the water.
Yaw:-
You can see the Yaw at the switch over point. is basically starts spinning CW.
https://www.dropbox.com/s/3nbzr998sayaa7n/Yaw1.JPG?dl=0
Now something funny here. I'm not sure how accurate the timing stamp is but looking at the RCIN4 it does look as though I instruct a yaw at around 177478,1502 and the heli starts yawing at 177530,282.81 a little after. This is odd as as I remember it I commanded a CCW yaw to counter-act the CW yaw after it started yawing!!!
Also the DesYaw is matching the Yaw! this is also strange.
Altitude:-
At 178938,1548 you can see I hit the throttle (Altitude) up to stop the heli from descending into the water.
However you can see the altitude droops NOT fast but relatively slowly and is matching DAlt. So it os clear that the NAV is wanting the hely to decent. Question is why????
https://www.dropbox.com/s/071dvgg6o4ciez1/Alt.JPG?dl=0
As for the battery, i'm not sure which data in the log to look at. but they were at 73% when I checked after the crash..
PLEASE could somebody have a look and cast some light into this.
Pixhawk Log:-
https://www.dropbox.com/s/vxbrtca58eh0036/31.BIN?dl=0
Tlog:-
https://www.dropbox.com/s/hc2gsa24mkku2w6/2015-12-16%2011-53-53.tlog?dl=0
Thanks
Anthony
Replies
A good new year to you all.
Now a little challenge. I have replicated the above issue and after watching back video and logs have solved it.
I post a little video for you to watch. i have overlaid the motor outputs and at some points the inputs.
I will not say what the cause is as yet. I will post the answer later, to give people a chance to see if they can work it out.
You don't need the logs to suss this one out. Just watch the video.
I am posting this like this as it (well I bereave) is interesting and could help others if they see behavior like this.
well here it it:-https://youtu.be/OQc5vUWWvOQ
I will post the cause of this odd behavior later. Lets see if anybody can work it out.
OK I see a few people have watched the video.
So, because the body is enclosed there is no air flow internally. The ESC start over-heating and drop the power to the motors.
I can ASSURE you if this was a DJI FC the rig would have been lost. The Dji FC including the A2 simply would not know what to do.
From the flight log posted earlier you can see the Ardupilot firmware is aware there is a problem. It tries to compensate as hard as it can to fix the yaw.
It Cant because there is simply not enough power. It also realities there is clearly a problem to LANDS as best as it can.
I have had this with DJI A2 and it just goes mad. indeed it smashed a £35k rig into the ground from 100m up.
Just another reason the Ardupilot is AWESOME.
Check out RCOU.Ch1..8 before trouble, then about line 178377 the trouble starts, and you can clearly see motor on Ch3 go idle.
It seems that you had an malfunction on at least one motor, and keeping yaw is important in the mode AP was in (Loiter). What you see is pretty usual when the AP tries cope with the yaw , that is (due to malfunction)unusually "coupled" to the thrust.
Can this thing fly with two motors dead ?
Basically, if one motor stops working(or have no propeller) - to counter the power loss, the AP needs to remove equally much thrust from the motors spinning the other way - also, have a bit inefficient attitude control. - So - it seems it could not fly with this much loss of lift.
For what it's worth - if you switched to stabilize, you would be able to climb (and would have a constant yaw, depending on how much throttle you gave) - judging by the low sinkrate , the octo was just barely underpowered for this scenario, and a somewhat good pilot should be perfectly able to fly it back in stabilize (with a yaw).
Screenshot from 2015-12-20 20-09-19.png
Still, I bereave you have spotted an issue. Interesting the ESC's UBEC on Ch7 is used as a backup power for the FC. i wonder if something odd happened causing the UBEC to shut down, this would kill the ESC.
There is a drop from 5.17 to 5 V around that point. the average Vcc voltage is around 5.17 then it climes to 5.7V around the time the rig fails.
I see throughout the flight ChOut 1,2,7 and 8 are much higher then the others. These are all CW blades. When the rig droops it is spinning CW. With all the CW blades high the copter should if anything spin CCW.
So there MUST be a failure in one of the CCW motors / ESCs.
The part of the log that is relay frowning me is the DESYaw vs Yaw and DAlt vs ALT whilst in the final Loiter mode. . The values mach indicating to me the AP is intending for the Yaw to spin CW and the copter to decrease in altitude as it did.
It this AP saying "THERE IS A PROBLEM I AM LANDING NO MATTER WHAT"???
Or am I misunderstanding the log?
as I see it, the reason behind reduced DAlt is not to land, but from being forced to reduce throttle to prevent (more) yaw.
Ah that makes sense.
also, forgot to mention; if you look how different throttle PWM is for this machine during flight (4 motors always runnig at higher PWM than the others) - it tells me this is not a very well built/balanced/tuned machine, further reducing chances of succesfull flight with failure.
Andre, thanks for your reply. Please note this in an H frame not a X frame. The COG is dead center.
Also during the flight the rig had to sit at an angle most of the time as it was fighting 45mph wind. From memory when hovering it sat about 20 deg roll to the starboard.
Funny thing is the wind died down to almost 10mph from 1/2 way back in RTL and "landing"
I have flown this rig in 70mph wind and it never batted an eyelid. It held in solid for 20 mins.
I see your point now, so if one motor fails the AP will drop the apposing force motor.
I have flown the gig with 4 out of 8 motors disconnected, essentially like a 4X and it did manage to hover and maneuver, all be it sluggish.
I see now that Ch1 goes VERY high and Ch3 drops to idle. So does this indicate motor 1 fails and the AP then dropped motor 3 to compensate for yaw?
It seams odd that it drops Ch 3 rather then ch 5 which would counteract motor the loss of motor 1.
I now also see motor 7 going high (ch7 trying to go high) now that makes more seance now. If motor 7 fails it droops ch3 to compensate.
Thank you again Andre you have cased some light for me.
So I think I should have a coffee and map out the chout of each motor and get a better grasp of the goings on.
i will post back soon.