Trimming your Radio with a Multiwii multirotor

[teslahed]

Many people find that multiwii quads need trimming or else they will drift (back and to the left, say).

The issue occurs because the multiwii code assumes your radio centres on values of 1500. But if you connect your multiwii board up to your PC with the radio receiver connected and transmitter on you can look at the actual values the board is seeing.

A typical example might be;

1517 on pitch
1512 on Roll
1515 on yaw

The average value is 1514.7 (call it 1515).

You could use trim or subtrim to take all the values down to 1500 but this is not the best way to do things. This is because both trim and sub trim work by just virtually holding your sticks over to one side a little. So if you trim 20 clicks of left on trim or subtrim you';ll have 20 clicks less left travel in total and 20 clicks more right travel in total. You control response is no longer perfectly symmetrical and your quad will feel a bit odd.

Better is to use the minimum possible amount of sub trim to get all the values centred on the average value of 1515.

If you look in the multiwii code;

Code Select Expand

/**************************************************************************************/
  /***********************     motor, servo and other presets     ***********************/
  /**************************************************************************************/
    /* motors will not spin when the throttle command is in low position
       this is an alternative method to stop immediately the motors */
    //#define MOTOR_STOP

    /* some radios have not a neutral point centered on 1500. can be changed here */
    #define MIDRC 1500

#define MIDRC 1500 is the value you need to alter to #define MIDRC 1515

I use sub trim on Pitch and Yaw to get those as close to 1515 as possible.

Then i would use the multiwii code and alter "#define MIDRC 1500" to "#define MIDRC 1515"

This should get rid of all the drift you are experiencing and result in a quadcopter that just sits level, even in gyro only mode, if you put it level to start with.

[Riggsbie]

Nice one !

Very well explained......

[strachg]

Great bit of info teslahead thanks, the problem i have is take aileron for instance if i centre it it is 1502 i move the stick to the right and back to centre and it is now 1498 then move stick again and back its now 1501. Happens with all channels, calibrated my radio (spektrum DX8) Still the same, i guess its the radio is just not accurate enough, Now who has stock of Frsky Taranis.

Graham

[teslahed]

That';s a common issue with radios Graham, most radios will be like this to some degree. A few points of difference isn';t a disaster but multiwii does have a setting to allow for this;

Code Select Expand

/**************************************************************************************/
  /***********************                  TX-related         **************************/
  /**************************************************************************************/

    /* introduce a deadband around the stick center
       Must be greater than zero, comment if you dont want a deadband on roll, pitch and yaw */
    #define DEADBAND 4

#define DEADBAND 4 is the variable you want to comment in and alter. Usually about 4 or 6 is enough to do the trick.

[strachg]

Thanks teslahead will  Check that out

Sent from my GT-N7000 using Tapatalk 4

[zYgoat]

Fantastic info thank you!

[Toledodave]

Is it safe to pull averages from MP and set according to in sketch?

[Toledodave]

Teslahed, do you happen to know about megapirates MIDRC setting?  My sketch reads MIDRC 500. When my radio limits in Mission Planner are like 998-2006. Doesn';t make sense to me. Does that mean my sketch is assuming my limit';s are 1-1000? Im figuring the sketch should say MIDRC 1496?

[teslahed]

Sorry dave, i am not sure how it works with megapirate.

[Toledodave]

No problem. Thank you for the reply.

[jimbo385]

Hi teslahead,

Thanks for pointing me here. I';m not sure if I';m doing things right though. This is what I have done so far.

  Props off!
  Sub  Trims set to 0
  Stick Trims set to 0
  Travel Adjusts set to 100%

In Multiwii, I';ve connected to the quad.
Pushed Throttle up to mid stick and taken the following screen print.




As you can see the motors are all over the place!

  Front Left is    1299
  Front Right is  1289
  Rear Left is     1411
  Rear Right is   1789

I can only assume that this is because I had some sub trim set on Aileron & Elevator which I have taken out. Next I will calibrate the motors again and get another reading.

So, after recalibrating my ESC';s I get this at midstick.




Now we have

  Front Left is    1376
  Front Right is  1498
  Rear Left is     1396
  Rear Right is   1514

Does this sound about right?

As far as Pitch, Roll & Yaw are concerned, they have the following settings;

  Pitch  1501
  Roll    1478
  Yaw   1498

In the Config.h file, I have set the MIDRC to 1492 and then use sub-trim on my tx to get all 3 to as close as possible to this value.

Is this correct?
Must admit that I';m getting a little confused! 

[jimbo385]

I managed to sort this out which is cool!

I also understand the deadband issue suffered by most radios as mine has this issue too.

I';m gonna introduce this the next time I re-flash.

Cheers