Un-Commanded Loss of Altitude!!!

[DLien]

Hi, I need a bit of help.

I have the WKM installed on a RCT long arm Hex frame. I am using MT 2814-770 motors and Turnigy Plush 30 amp esc’s to turn APC 12 x 3.8 props. I have a photo ship one gear and gimbal installed with a Gopro onboard bring the total flight weight with batteries to 4120 grams.

Recently, I have been experiencing an un-commanded loss of altitude about 2.5 minutes into every flight. This can happen from level, slow flight, in either atti or GPS mode , regardless of the wind conditions. The rig will descend rapidly without warning but I would not say that it is falling. The descent can be stopped by increasing the throttle to 80% or 90% provided that you are prepared to add the throttle needed. You need about 30 feet to catch the rig and stop the descent. If no action is taken, the decent would continue to the ground. Once the flight is stabilized, the WKM starts flashing 2 or 3 white lights (IMU data is lost). If you hover for about 5 seconds the white light flashes will stop. There are no white flashes before the decent begins and I am nearly convinced that the IMU data loss is a result of the problem and not the cause.

The problem is not related to the firmware of the controller, I have tried 2 different ones. I have also re-checked the balance of each prop and know that they are dead on. The issue seems to happen regardless of the direction flight. I am get flight times of about 11 minutes on 2, 5000mah 4s batteries (run in parallel making 10,000 mah’s) depending on how much maneuvering is done. The ESC’s come down warm but uniformly warm, meaning that they are not too hot to touch for more than 5 or 10 seconds. The motors are a bit warmer than ambient temperature and are all about the same temperature.

I suspect the ESC’s because of the heat, thinking that I am pulling too much power through them. eCalc on the other hand supports a usage rate of about 7 amps at hover with a max draw of about 21 amps per motor. Holco’s motor tests http://www.rcgroups.com/forums/showpost.php?p=17387701&postcount=1 show higher usage rates, but still within the 30 amp max of the ESC.

Is there a way to test the performance of an ESC? I am also looking for other (inexpensive) ideas to help me find the cause of the problem. Does it make sense to invest in a set of smaller props to see if problem happens with cooler ESC’s? Should I replace the 30 amp ESC’s with 40 amp units instead?

Thanks...

 

[Electro 2]

I'd guess this is an FC issue, unrelated to ESC/motor/prop. It's probably commanding a drop, the question is why? 2 different firmware revisions tends to point towards a hardware related issue, since both were on the same hardware. That said, I have no idea as to why. The loss of IMU data alert may be a sign, though. Defective IMU, loose IMU mounting, bad IMU cable, unseated connectors ?? Dunno. The 2.5 minute thing is puzzling too.

 

[rolle]

take the kopter indoors and look at the IMU (right side of it). Inside the IMU there is a LED blinking 9 times (you won't see this outside when the sun is shining).
if it s blinking 9 times green, everything seems to be okay with wkm. but i guess it is blinking red at the third position?
or may be at an other? do you use an antiflash when connecting the batterie? if so, please remove it!! dji won't ork properly with that! look at the IMU blinking again after remove the antiflash.

 

[araines2750]

Typically, the white flashes indicate too much vibration or magnetic interference of the IMU.
This is why they had to redesign the DJI S800.... arm & frame flex.....carrying vibrations to the IMU.
The typical arm extension concept, while seemingly a good idea, often creates a structurally deficient airframe resulting in unacceptable vibrations to the IMU.
Several of my customers are a result of an arm extension attempt gone sour.
I hope you can get better results than those I know who have tried.
Andrew

 

[Benjamin Kenobi]

Hi mate. Is the IMU mounted with air flowing over it? The small hole on the front can suffer from static pressure build up. I've covered mine with some foam to dampen it a bit. White flashes can either be vibration or a mismatch between IMU/GPS. Have you calibrated your GPS compass? Location settings in software correct?

 

[Electro 2]

What is an "Antiflash" No such device that I'm aware of here in the US.

 

[gtranquilla]

There could be several issues at play here...... In my case the IMU baro sensor was being spoofed by the nearby props.... especially when flying forward...the IMU was originally mounted in the top plate.... I bought the DJI-Innov S800 anti-vibe kit and relocated the IMU down below on the IMU plate on the landing gear and most issues were resolved. Also I had a MR wobble problem traced back to undersized motors/props...... swapped out both and chose the Turnigy Multistar 420KV motors and 15 inch x 4.7 carbon fiber props and the wobbles disappeared. see my signature below for some hardware details..... View attachment 12769View attachment 12770View attachment 12771

 

[DLien]

To close the loop on this, I found it to be related to vibrations that I could not find the source of. I also tested buffering the barometer sensor and it had no impact. I think there is some sort of weird harmonic occurrence that happens in flight. Perhaps more space between the plates would prevent the problem. The plates on the S800 are also fairly close together as well if I am not mistaken.

I resolved my problem by moving the IMU to the landing gear to. I have the Photoship One mkTR gear and gimbal which does a real good job of eliminating vibration. I used some VHB foam tape and a zip to attach it front side of the gear leg between the roll servo mounts. The test flight was absolutely perfect. I plan to keep it there unless there is some reason that it needs to be moved to a better location.

Thanks for the help and the ideas to check out...

 

[Benjamin Kenobi]

Great! The S800 anti-vibe kit works in much the same way. The IMU is just really sensitive to high frequency and low frequency and if you look at it. But hey ho.