MultiWii

Hello guys,

This is my first Multicopter and I've been trying to lift up my quad for a few days now, but for some reason I can't get full throttle with my 4 turnigy 750kv motors.

I've already calibrated all my 4 ESC's separately, set up the mid points of my Tx to 1500, and the range from 1090 to 1950 or so.

The motors are arming at the same time, everything looks calibrated and good to fly, but I just can't have enough throttle to lift, it seems that when the quad is about to lift, the throttle becomes insane and goes up and down by it self. Actually it "jumped" a little bit, but that's all.

I was wondering, maybe I should change de PID settings of the board? I've read somewhere that vibrations can cause this kind of problem (i've already tried to isolate the vibrations of the quad to the board), but I don't think this is my case.

I recorded a video to show you what is happenning:
https://www.youtube.com/watch?v=UOCw9AfILc4

PID adjustments aren't the problem. How did you cal the ESCs? Try doing each one individually off the throttle chan on the rx. What battery are you using? Are there excessive vibrations?


Mike

mike105105 wrote:PID adjustments aren't the problem. How did you cal the ESCs? Try doing each one individually off the throttle chan on the rx. What battery are you using? Are there excessive vibrations?


Mike

I've calibrated it one by one connecting the ESC's to the 3 channel of the Rx: full throttle > Plug on the battery > beeps > throttle down > beep beeep > throttle up again to see its reaction with throttle (I'm not sure if I should throttle up again, but otherwise it wouldn't work => constant beeps when battery is plugged on. At this last step, the motors actually rotate and looks syncronized with the throttle amount).

My battery: http://www.hobbyking.com/hobbyking/stor ... duct=16588

There's some vibration of course, but I wouldn't say it is excessive... I've tried to isolate these vibrations with a foam under the FC anyway.

I had the very same problem. For me the I value of the PID controller was set to aggressively. Setting that to 0 solved all my problems.

Hello guys,

So actually the problem was the excessive frame vibration, causing typical signal errors.

To solve this problem, I've uncommented the lowest lpf in config.h code:



I'm going to balance the props and hopefully I won't have these problems anymore.

Now I'm facing a new problem: can't fly higher than 1-2 meters... I'm going to write a new post for this though.

Video of my flight test: http://youtu.be/AuySZhwnmeo

Was that at full throttle? If yes then either or both:
1- the vibrations are still way too much.
2- the battery/motor/prop combination does not create enough lift (power).

For fun I just simulated a heavily vibrating aircraft. I was able to tune it back to a decent flight by drastically lowering the P-gain (and also I and D accordingly)

BirdShredder wrote:For fun I just simulated a heavily vibrating aircraft. I was able to tune it back to a decent flight by drastically lowering the P-gain (and also I and D accordingly)

Cool!

I'll try to tune my PID settings again and see what happens =]

Another question:
On the video of my flight test, the battery is fully charged with 10,6V (It is a 3S LiFePo battery pack). But after only 4 minutes, the quad won't lift anymore and it seems the battery is running out of charge. This is weird cause this battery should give something around 10 minutes of flight time according to ecalc.

Anyway, when I measure the the battery voltage it is around 9,9V and LiFePo batteries pack has 9,9V nominal power (3S). It shouldn't be out of battery at this time should it?

Do you think I should buy a 4S battery?

No I don't think you should buy a 4S. I still suspect it is a combination with an aggressive I value and vibrations/imbalance. Maybe your motors don't point up perfectly or one of your ESCs is a bit crappy. (I noticed a significant clock drift between my ESCs)

victorhugommf wrote:

BirdShredder wrote:For fun I just simulated a heavily vibrating aircraft. I was able to tune it back to a decent flight by drastically lowering the P-gain (and also I and D accordingly)

Cool!

I'll try to tune my PID settings again and see what happens =]

Another question:
On the video of my flight test, the battery is fully charged with 10,6V (It is a 3S LiFePo battery pack). But after only 4 minutes, the quad won't lift anymore and it seems the battery is running out of charge. This is weird cause this battery should give something around 10 minutes of flight time according to ecalc.

Anyway, when I measure the the battery voltage it is around 9,9V and LiFePo batteries pack has 9,9V nominal power (3S). It shouldn't be out of battery at this time should it?

Do you think I should buy a 4S battery?

What is the Discharge 'C' rating of that LiFePo battery pack??? For running motors on a multi-rotor it needs to be at least 25C or more. Link to the exact battery would be helpful.

Typically we use LiPo batteries which have a higher Voltage per cell (4.2V per cell a full charge) and high C ratings. The 3S LiPo's I use for 200-250 size quads are 40C discharge rated and full charge of 12.6V.
If anything I think you should buy a battery intended for the high discharge rates needed for multi-rotors.

To be honest you only use LiFe batteries for transmitters and video receivers as they self discharge slowly and don't suffer the same issues as LiPo if they are damaged.

A 450 quad will tolerate 20c batteries smaller quads such as a 250 will need higher discharge rates, the 40c listed above sounds reasonable (running 90c just to be sure)

@kilby. Could you explain why smaller quads need higher discharge rates? I can't imagine that is true. Are you perhaps confused with capacity? Smaller batteries will need higher C values to supply the same current.

The maximum CURRENT discharge is the battery capacity times the C rating.
So, a typical larger quad will carry a larger capacity battery whereas a smaller quad uses a smaller capacity battery due to weight restriction.
Examples:
Mine 200mm quad runs a 1000mAh battery. It has four motors with a max current draw of 10A so 40A draw from a 1Ah battery requires a 40C rating.
A 450mm size quad can carry a 4000mAh battery, If each motor draws 20A the total is 80A. So 80A/4Ah = 20C rating which is half that of my 200mm quad even though the total current draw is double..

Yes. That is exactly what I'm saying. A smaller quad with the same battery as a larger quad will need a lower C-rating.

Hello guys!

I've solved the problem changing the battery from a LiFePo pack to a standard LiPo battery. Now my quad has power enough to lift and now I'm trying to tune my PID settings.

Propellers and motors are still generating too much vibration, I'm waiting to get my propellers balancer that I've bought from internet to see if balacing them will help.

Ok, that LiFePo was the problem as we suspected.

What is the Gryo_LPF define set to in config.h? This will help with minor vibrations.
What does the Sensor graphs in MWconfig.exe look like with the motors/props running? Do some searches to find what this graph looks like with other people's setups.