Random yaw

[gtranquilla]

My 800mm Hex flies straight and true in still air in GPS and Attitude mode using the WooKong M. But when there is a slight breeze, it seems to yaw quite a bit without RC stick input. Is this normal? Seems my WKM on a previous F550 did not do this... Will yaw gain adjustment help? I need to make sure my forward motion aerial videos are not yawing during the flight. Here is a sample showing multiple attempts to straighten out the yaw during a flight from tee of to green.

 

[maxwelltub]

Ya, don't know the cause but I have had the same issues. Sometimes it's a little bit, at worst it rotated 180 degrees! Acts like a motor failure, which it might be, but it corresponds to wind for sure. Don't have an iosd so not sure what the issue is yet.

 

[gtranquilla]

So far....my iOSD MK2 does not show any significant discrepancies between motor currents/voltages during these flights. COG is at MR airframe dead center on the horizontal axes. CCW yaw rotation occurs when the wind is coming in from the left or port side. Need to check now to see if the yaw rotation remains the same, i.e., ccw when changing course 180 degrees.

Ya, don't know the cause but I have had the same issues. Sometimes it's a little bit, at worst it rotated 180 degrees! Acts like a motor failure, which it might be, but it corresponds to wind for sure. Don't have an iosd so not sure what the issue is yet.

 

[maxwelltub]

Ya so far I've just been able to fly out of it, but it scares me not knowing what's going on with the thing.

 

[gtranquilla]

The yaw issue is mentioned in DJI's FAQ section and here is what they have to say about that! Totally useless information...... View attachment 13582

 

[gtranquilla]

Here is an iOSD flight datagraph download of the port and starboard motors on my HEX. I was test flying it in a crosswind late today with the wind coming in from the port side. Most of the motors show rpms that are closely aligned. But I noticed a lot of variance in the rpms between the port and starboard motors.....still trying to determine what it all means...... Also to counter the random YAW to clockwise I had to adjust TX YAW trim quite a bit to the left. In any case I decided to swap the left and right motors and will test fly again tomorrow if there is wind..... View attachment 13583

 

[maxwelltub]

Because I dont have an Iosd can you tell me what we are looking at here. Are these two colors two motors or all CW and CCW motors?

 

[PeterLester]

DJI's iOSD provides a lot of raw data but makes no attempt to explain it...... The lime green trace represents the rpm of the port or starboard motor and the forest green trace represents the rpm of the starboard motor during a hover in GPS mode with a breeze coming in from the port side. Note also that the rpm scale does not seem to make sense to me since these motors are 470k and batteries are 6S such that full speed should work out to about 10,584 rpm and the hover volts is closer to 50% kv.....Regarding cw versus ccw, alternate side motors on a HEX will rotate in opposite directions obviously.... I think that the port motor was rotating clockwise and starboard ccw. Any difference between these two in a hover would therefore result in the MR slowly rotating cw or ccw. In my case the MR was rotating clockwise slowly...... a lot of RC Xmitter yaw trim to the left was able to counteract that but as the breeze was inconsistent, so was the YAW affect.

Because I dont have an Iosd can you tell me what we are looking at here. Are these two colors two motors or all CW and CCW motors?

 

[maxwelltub]

Assuming the x axis is time, look at 95 seconds. Looks like that might be your problem spot. And then a few other little blips after 130. Not that that answers anything really but it might be a start.

 

[gtranquilla]

I investigated the WKM tendency to YAW further today by swapping the port and starboard motors and replacing two props then flew in a light crosswind coming in from the port side. ..... Results remain identical in that the Hex tends to YAW ccw. So I adjusted the RC TX yaw trim 8 points to the left and that resolved the issue to a significant degree but not totally. So the issue is not the motors which leaves me with the following possibilities.. 1) ESC issue but unlikely 2) MR COG issue but unlikely 3) Magnetic compass calibration and/or angle of declination adjustment 4) YAW tuning adjustment although I changed that several times before 5) WKM MC or IMU internal design and/or firmware issue. Based on the following statement received from Quadrocopter USA I am starting to think that the MikroKopter system is better at resolving this issue....... View attachment 13606 I also sent in a bug report to DJI..... Hopefully they will get around to resolving this issue in order to remain technically competitive with MK and others.

 

[maxwelltub]

What firmware are you using btw. I had this problem in a major way yesterday. First part of the day was flying inside with zero problem, Second half was outside and about 50-100 and it flew like ****. I am at 5.22, but am going to down grade to 5.16 today. I also switched the Yaw remote gain adjustment to my transmitter so hopefully I can dial it in from there.

 

[maxwelltub]

Jack pot. When looking up other tuning issues I stumbled across this potential clue.

"I have done extensive research on Z axis center of mass. The lower you have it then the more inertial response from lateral movements. The more inertia you have the the less can be the gyro sensitivity gain because it will cause oscillations that have to be compensated for in the PID Algorithm. The higher the gain the earlier the movement is seen and the better the stabilisation works. As was said, arm length does not contribute to stabilisation because stabilisation is a function of the computer. The response from the Gyro is mostly effected by the prop. design and it's weight. the timing of the ESC, KV. factor of the motor and the pid algorithm. The more global inertia you have in the system the worse will be the stabilisation. Generally the bigger the model the worse the problem gets. Bad oscillations in the descent are caused by too lower rpm which causes partial blade stall. This also can happen with large yaw movements when some of the motors slow down."

http://www.multirotorforums.com/showthread.php?2096-WKM-in-the-wind-Your-experiences-What-could-be-improved/page2

 

[gtranquilla]

I am using FW 5.20..... Worked with RC remote gain specific to YAW tuning with no noticeable improvement. Will try replacing the GPS support shaft from the medium to the highest/longest to see what happens. It seems as if the concept of dead reckoning is not fully applied with respect to YAW control to the DJI WKM controller or else there are errors coming from the IMU........ http://en.wikipedia.org/wiki/Inertial_navigation_system

What firmware are you using btw. I had this problem in a major way yesterday. First part of the day was flying inside with zero problem, Second half was outside and about 50-100 and it flew like ****. I am at 5.22, but am going to down grade to 5.16 today. I also switched the Yaw remote gain adjustment to my transmitter so hopefully I can dial it in from there.

 

[maxwelltub]

So I switched to 5.16, increased my basic gains and lowered my Alt gains. Took it out for a flight today and seemed pretty good. Although there wasnt much wind. There where a few light gust but not enough to know if the problem is truly solved.

 

[gtranquilla]

I relocated my ESCs so that the extra length of 3 wires to the motors did not have to be wrapped around the motor arms...... could create inductance to affect the magnetic compass.... seems to improve the situation somewhat but wind was so light for good testing. Will try raising the GPS/compass higher and fine tuning the angle of declination and maybe redo the compass calibration.... I am using FW 5.2 and have no complaints on more than 200 successful flights.

 

[kloner]

what size props is on it and how heavy is it? say 960mm across?

 

[maxwelltub]

Well for me its a 800mm with 15inch props. I had a s800 and switched almost everything to a carbon core frame. Although the problem I have been having is more recent, and I think it was a gain issue but I won't know until I fly in wind again.

 

[gtranquilla]

My Hex is 800mm similar to S800 but flat configuration with 15 x 5.5 carbon fiber props and weighs 12 lbs AUW designed from eCalc for a maximum flight time of 15 minutes using parallel 6S - 5000 maH batteries.

what size props is on it and how heavy is it? say 960mm across?

 

[kloner]

it has to be some sort of deficiency in the motor or the prop balance. the controller can only do what it has control of doing. if a motor or prop doesn't speed and slow right, it'll get like this. bearings going bad from flying unbalanced props is a likely source. heat is a quick easy way to feel the problem after landing. most likely every other motor will be hotter. but one will probably really shine. a small ir termo is easiest to watch that.

 

[gtranquilla]

Kloner, Thanks you for that valuable information. It suggests that the DJI WKM controller is capable of eliminating random yaw in hover and forward flight provided the motors and props are 100%..... Will buy an IR thermo asap.....

it has to be some sort of deficiency in the motor or the prop balance. the controller can only do what it has control of doing. if a motor or prop doesn't speed and slow right, it'll get like this. bearings going bad from flying unbalanced props is a likely source. heat is a quick easy way to feel the problem after landing. most likely every other motor will be hotter. but one will probably really shine. a small ir termo is easiest to watch that.