I've been having this issue for about a year now, and as of yet, haven't found a solution.
To start with, here is a video displaying the wagging yaw issue:
https://www.youtube.com/watch?v=7amHcd-Ws3A
This yaw wagging only occurs with a top mounted yaw mechanism. I'm sure it's got everything to do with the mass of the yaw mechanism initially moving the tail in the opposite direction of where the controller wants it to go. Simply mounting the mechanism upside down 'solves' this issue since the mass of the mechanism helps move the tail in the direction the controller wants it to move.
I've tested this with Arducopter 2.9.1b and 3.1.2 firmware.
I've also swapped servos in order to reduce internal servo gear slop as much as possible.
When I used the KK2 board on this tricopter, it did not have this wagging issue.
The only downside to using a bottom mounted yaw mechanism is the much increased chance of prop strikes, especially when landing hard, or in rough terrain or braking for a landing. So I really hope this issue can be fixed or that someone can lead me in the right direction.
Want logs? Just tell me what you want logged and I'll let the tricopter wag for you.
Here's my previous request for help from last year: http://diydrones.com/forum/topics/arducopter-tricopter-firmware-oscillation-issues-with-top-mounted
Replies
A little surprised they're working so well for everybody, happy though.
TS, I modified my tri putting the blade closer to the hinge. At the same time I mounted the ESC on the bottom side of the hinge to give it a counterbalance. Having the ESC mass at the tips gives a bit more rotational momentum so that also helps to dampen the wagging. Maybe a little less agile but better than unstable.
/Matt
Tri increase your Rate_Yaw_I quite a bit and your Rate_Yaw_D by a little bit, that got rid of my wag. I'm using 0.038 and 0.010 respectively.
My Rate_Yaw_P is 0.280.
I tried increasing my D and I terms and that helped to some extent. But what really worked best was the lowering of the Stabilize Yaw P term as mentioned by Turdsurfer. I put it on my Channel 6 tuning knob and dialed it down from 3.5 to 1, and found the wobble was completely gone by 1.3. I've flown it a bit since then and have observed no unpleasant side effects.
Your mechanism is in a very unstable equilibrium and the servo doesn't have enough force. You have to make it in balance or simply use a much stronger and faster servo. Better servo is not the best way, so balance your yaw mechanism and your copter will rock.
The theory is, that your controller needs enough actuation power to achieve the desired performance. There is no way you can get the performance you want with unstable and underpowered setup. If your configuration is out of balance, you'll suffer a lot.
My servo is powerful enough, even more powerful than the original recommended servo.
Today I reduced the stabilize yaw P rate from 4.5 down to into the range of 1 to 2. In that range the wagging stops. Of course the tricopter now deviates from course more. I'll be testing it more in the coming days.
Hi, i have the exact same problem on my Tricopter.
And like you i've tried to tune it without success.
I'm running an emax ES08MD servo.
Did you succeed in trimming out the tail wag?
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