Another Tricopter stability problem

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hwurzburg
Posts: 75
Joined: Sun Jan 01, 2012 3:28 pm

Another Tricopter stability problem

Post by hwurzburg »

I am running CRIUS Multiwii in Tricopter with RCTimer 2830 750KV motors using 11x4.7 props...

I have had nothing but problems trying to get it stable in acro or level mode...the motors and props are balanced...
Initially I had the controller mounted on a double layer of soft foam tape....I improved things a lot when I hard mounted it...

its stable now with 10hZ LPF, and very high D settings (50!) in normal mode...but this seems extreme...but level mode is only okay , it needs to be more stable since it still will wobble a bit on its own and with control inputs(P=4,I=0.010)...

Observations:
holding on the ground and running it up with default PIDS I see the motor outputs jumping wildly in the Config util and in the air I see the YAW servo violently twitching although the copter is stable in YAW..when I see it twitch I hear the other motors surge and I get a bobble (I am looking for glass smooth corrections)

with very high pitch and roll D values (50) and the 10hz LPF, things get acceptably quiet in normal mode...but I still can hear motors surging occasionally (leading to minor twitching and altitude change) in level mode no matter what PI values I seem to use....

and if I change to the optimum prop for this motor (12x6), then it comes unglued again...if I could run those props I would get more power and longer duration...not mandatory, but its symptomatic of something wrong in my setup..

I have a very fast servo on the YAW mechanism...I am thinking that YAW corrections are creating low frequency shakes due to swinging the motor mass and prop around above the plane of the other motors..this is made worse by the heavier 12" props.....I have not tried anything but default YAW PID values, and tomorrow I will try adding D term values in hopes of slowing the YAW servo moverments...if that works then I will try lowering the D on the axis, etc.

I am after very smooth and stable performance for HD video...I don't need anything other than a slow docile stable machine...I don't mind the 10hz LPF...but I think there is something basically wrong with the setup and I should be able to get good LEVEL performance also...

fyi, things I have checked:
-YAW mechanism is slop free
-Props and motor were high-point balanced
-Controller is now mounted to body with velco only (a major improvement)

any ideas? with this much work I put into it so far (two weeks :o ), I would have expected a smooth as glass machine by now...I have HD video envy of others flying the MultiWii in a tricopter :lol:

noobee
Posts: 66
Joined: Fri Feb 25, 2011 2:57 pm

Re: Another Tricopter stability problem

Post by noobee »

just curious:
what is your esc?
is it smoothly in gyro-only mode?

hwurzburg
Posts: 75
Joined: Sun Jan 01, 2012 3:28 pm

Re: Another Tricopter stability problem

Post by hwurzburg »

ESCs are Plush 18A
in above discussion, I refer to gyro only mode as "normal" mode...

noobee
Posts: 66
Joined: Fri Feb 25, 2011 2:57 pm

Re: Another Tricopter stability problem

Post by noobee »

is it crius lite or se (with the bma180)?
are you using prop savers? (maybe try without if you are)

sorry i don't have any good suggestions, but as you discovered, securing the acc down to the frame is important vs using a foam tape. mine is the SE, it is well behaved in normal mode, but does have a bit of wobble on level mode with all-stock settings (also plush 18a but with the infamous cf2822 and 8x4.5epp on prop savers. i have not tried to tune it out yet.

another option to try is to update the esc firmware, which i'm looking into experimenting right now :)

http://www.rcgroups.com/forums/showthread.php?t=1513678

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