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Old Apr 04, 2016, 03:06 PM
v22chap is offline
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Multi Wii used with servos

I need some advice ... I have a friend who has built a twin rotor helicopter that is using 4 servos .. he has used the X type quad setup and it is flying ... but it seems to wonder all over and does not seem to hang in one spot like his quad does with the same board.. It seems like the accelerometers are not working at all ?? I told him I suspect that the MHz that the board talks to an electric motor is maybe different than it would to a servo thus making a big delay in movement and they are actually working but to slow and end up over controlling making it move around more than it should .
Any one have any ideas for him ??
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Old Apr 04, 2016, 05:01 PM
Ran D. St. Clair is offline
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Quote:
Originally Posted by v22chap View Post
I need some advice ... I have a friend who has built a twin rotor helicopter that is using 4 servos .. he has used the X type quad setup and it is flying ... but it seems to wonder all over and does not seem to hang in one spot like his quad does with the same board.. It seems like the accelerometers are not working at all ?? I told him I suspect that the MHz that the board talks to an electric motor is maybe different than it would to a servo thus making a big delay in movement and they are actually working but to slow and end up over controlling making it move around more than it should .
Any one have any ideas for him ??
I don't know if he has done what he can to adjust the PID values, but if it is wandering all over it sounds like the P and I values in particular are too low. Be careful about cranking the PID values up too far too fast. Helicopter mechanics have a lot of control authority and a violent oscillation can develop pretty easily.

It is true that servos are slower to respond than a good ESC. The best small signal frequency response I have heard of from a servo was about 16hz with most being much lower than that. I don't know a fair equivalent for an ESC but if it could respond to PWM at 300hz, then two PWM points for a full cycle would make the frequency response 150hz. Of course that assumes that the motor can actually respond that fast which is not at all certain.

When I used the KK2 and OAV to fly a helicopter it hovered nicely. I could usually hold it within a 1 ft. radius of a fixed point in space while hovering with no great difficulty (assuming no wind and outdoors without re-circulation as within a closed room.)

Meaning no insult, but is your friend fully aware of the swash plate geometry and how it would relate to the programming of a quad type flight controller? Helicopter specific flight controllers would have the swash plate geometry as a number of options, but a quad type flight controller, or OAV for that matter, would require that you do some calculations to get the elevator and aileron volumes correct to each servo. Otherwise you will have unwanted interactions between the various control inputs and all kinds of bad things will happen.

Vibration is also a possible issue. The larger diameter of even a small helicopter rotor produces lower frequency vibrations as compared to those produced by the smaller propellers of a quad. This means you probably want to set the cut- off frequency for any low pass filter to a lower value. Of course this assumes the rotor head is balanced as good as possible. The typical low pass filter for a quad might be 120hz depending on the size of the quad. For a small 450 sized helicopter 21 hz might be reasonable. This applies to any accelerator output as well.
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Old Apr 04, 2016, 07:20 PM
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Thanks for the input RanD
I think he has done some PID changes ... but we are both more helicopter guys than quad ... so probably have not done it all right ..

Quote:
When I used the KK2 and OAV to fly a helicopter it hovered nicely. I could usually hold it within a 1 ft. radius of a fixed point in space while hovering with no great difficulty (assuming no wind and outdoors without re-circulation as within a closed room.)
This is the same condition I found too on my twin setup heli Vtol . I have talked with him about OAV kk2 stuff ... but finding the time to completely convert it all over just makes it almost impossible to even though I offered him a mini KK board of mine .

Quote:
Meaning no insult, but is your friend fully aware of the swash plate geometry and how it would relate to the programming of a quad type flight controller?
I believe he probably is as he is a Bell Textron helicopter engineer in Carrolton Texas .
His weaker point is understanding how FC accelerometers work in each application ... he did a simple little test to try and see if he could see one working and couldn't and is now convinced that his multi Wii accelerometers are not working at all , thus he has shelved the project because it is not as stable as he thinks it could/should be as it doesn't hold a hover like a quad with the same FC will .

Vibration might be a problem .. but this is only a 12 " center to center micro twin vtol and he is a 3 D capable pilot and I am sure he has it pretty well balance out .

Thanks .. I will pass this info on to him and try to push him on to the forum so that atleast he might do a pm talk with some of you .

I really would like to see him succeed with this little VTOL as he has already succeeded with a larger VTOL and brought it to market for us.
A micro version of it would benefit us all as I am pretty sure he would produce it if he could get it to stay more stable so that it would be tempting to a beginner VTOL'er to want and fly, which would lead them on to the larger more expensive version eventually .


He really needs to get on this forum and talk with you as I am not a college /or engineer educated person ... but I know doing it by proxy is not a good idea ... but thought I could start the start up dialog any how .
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Old Apr 04, 2016, 10:09 PM
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v22chap,

I was able to adapt the multiiwii for chopper use. Here is the thread for the development work I did with cass3825's help on the programing side.

Although the esc electronics may be faster, the inertial effects for a motor/prop make the motor response much slower than the servo response to the flight controller. For the chopper I had to add extra signal smoothing for the swash servos.

Without knowing more about your friends design and implementation of the wii controls it is hard to say what changes he needs to attain his flying goals.

If I had to guess I'd say your friend reduced P to minimize servo jitter (the normal tuning procedure) but the real problem was the "too fast" response of the servos that appears like servo jitter caused from too much P. With P turned down the stability won't be good. Instead turn up the gyro filtering and keep P higher where it can stabilize as needed.

In this thread are lots of details that I went through for my Wii Chopper.

Here is a video of the chopper showing the effect of gyro filtering on the swash servos.

Wii Chopper gyro filtering R&D.wmv (2 min 17 sec)


Video of the Wii Chopper flying.

Wii Chopper first real flight (1 min 47 sec)


Video of the Wii Chopper with autolevel enabled...very stable.

Wii Chopper autolevel test (0 min 48 sec)
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Old Apr 05, 2016, 05:57 AM
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Thanks for the input Lead , I will pass it on to him .
I think I have him convinced to join the forum and atleast start a PM with you guys ... as there is a wealth of knowledge and experience here that you can't find any where else
Once he joins and starts talking ,, he can then decide how much he tells you about his set up .. as I know he is a little guarded of it since I know he really wants to bring it to market like he did his bigger Vtol unit .

He may need to give a shout out to Alexinparis too .
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