Dangerous divergence: FIXED and understood

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Re: Dangerous divergence issue: problem ISSOLATED

Postby skew » Sat Apr 13, 2013 2:53 pm

Angel, I was going try switching to a + config to see if that changed it. Between the weather and work there has not been much time to play with my AQ. I will try and switch mine this week and see if its better for me also. Are you using the default CTRL_FACT_ values on the +? Anything above CTRL_FACT_ROLL .4 CTRL_FACT_PITCH .4 on my X will cause mine to freak out and crash if pushed to hard. I changed just about every pid it didn't change the behavior only changing ctrl_fact for me finally made mine flyable and cheaper to fly. At .4 I can bang the controls with out much worry unless its in very strong wind. I should be able to tell on mine by switching to a + if it helps me.
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Re: Dangerous divergence issue: problem ISOLATED

Postby afernan » Mon Apr 15, 2013 11:12 am

I´ve dome more testing in "+" configuration but this time I´ve used larger propellers (12x4.5). The behaviour was even better than with 10x4.5. PID´s were full deffaults, as before.

Incredible stable, smooth and the more important: none divergence at all (I really pushed hard the stick). Really a pleasure to fligh now.

So, I insist in the conclussions of my previous post: There is a difference between "+" and "x" configuration wrt overshooting, divergence or oscillations (using exactly same hardware and PIDs).



Angel
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Re: Dangerous divergence issue: problem ISOLATED

Postby LPR » Mon Apr 15, 2013 12:53 pm

Angel

I really like your persistence on finding a solution this aggressive control issue. With my octo I can change settings and have this behavior but with default settings except for tilt rate D at 6000 I can not get the octo to be unstable. Increasing tilt rate D or increasing the maximum tilt angle by adjusting navi speed PM and OM to PM=30 and OM=40 also causes the same issue in Alt/Pos Hold but not in a mission.

Larry
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Re: Dangerous divergence issue: problem ISOLATED

Postby afernan » Wed Apr 24, 2013 11:22 am

Issue FIXED (and understood)

I´m very happy to say that after months of analysis, testing and crashes I´ve understood this issue and so, it permited me to get a fix. Detalled explanation is inside the document attached with analysis details, plots and test demonstrations of fixing.

A short abstract is below:

The problem occurs due to a lack or loose of tilt control after reaching the CTR_MAX = 300pwm in a hard manoeuvre.

This is directly link to the mixing table used. When using 50% based table, the problem is increased since we need higher control levels, and so we reach the ctrl_max=300pwm earlier.

The issue has been solved doing two things (for my particular copter: light quad in X mode that showed big divergence issue):

1.- Using a mix table 80% based (instead of 50% from wiki)
2.- Increasing the CTRL_MAX = 400 (default =300)


Now I have a wonderful and fantastic machine that I can fly soft or hard without any divergence issue.

Hope this help and maybe can be added some sentence on wiki.

Angel
Attachments
analisis.zip
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Re: Dangerous divergence: FIXED and understood

Postby LPR » Wed Apr 24, 2013 1:14 pm

Angel

That makes perfect sense to me. If your copter is light weight and your using big props having a higher percentage of the available motor control PWM change for attitude is good.
The large props will give strong yaw control. You could try reducing yaw to 80% and increasing the attitude control to 70%.
A more balanced motor mix could give more consistent control over a variety of weights.
Both of the quad I built are flying in the x mode with a 50, 50 mix and do not have a problem but both have small props and weigh more.

Larry
Last edited by LPR on Wed Apr 24, 2013 1:26 pm, edited 1 time in total.
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Re: Dangerous divergence: FIXED and understood

Postby afernan » Wed Apr 24, 2013 1:24 pm

@Larry:
The bottom line is: in order to know if your copter is close or far from having this kinda problems, plot a MOT_PITCH/ROLL after a "hard" flight and check the values. If peaks are close or limited to 300pwm, then you should increase the mix table tilt percent (from 50% to 60-70%) and increase the CTRL_MAX a bit (to 400 for instance) in order to be safe in hard maneuvres.
Angel
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Re: Dangerous divergence: FIXED and understood

Postby LPR » Wed Apr 24, 2013 1:30 pm

Angel

Yaw and pitch and roll PWM changes are added together half of the time so yaw can contribute to reaching max PWM.
Using some of the PWM change that is not needed for yaw control would likely give a more consistent result over a variety of quad weights.
If a quad had long arms and small props shifting the motor mix the opposite way would give good yaw control and still give good attitude control.

Larry
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Re: Dangerous divergence: FIXED and understood

Postby skew » Wed Apr 24, 2013 3:14 pm

afernan wrote:@Larry:
The bottom line is: in order to know if your copter is close or far from having this kinda problems, plot a MOT_PITCH/ROLL after a "hard" flight and check the values. If peaks are close or limited to 300pwm


Angel, What you propose makes sense. I went back to some of my old logs and MOT_PITCH/ROLL was hitting 300pwm ALOT!! I was using the 50% X Mix table. I looked at some recent logs I switched to the 100% mix table about the time r100 came out a didn't think much of it because I was only doing mission and not flying manual mode. But my MOT_Pitch/Roll is avg around 225-250 max. I was noticing last weekend that it was very stable in strong wind but I never thought much about it until now. I am going to try your settings and change my ctrl fact pitch/roll back to the .05 default.
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Re: Dangerous divergence: FIXED and understood

Postby afernan » Wed Apr 24, 2013 3:32 pm

@skew

Forget to say that rest of PID´s are all back to deffaults. In particular: ctrl_fact_P/R = 0.05 and , TLT_RTE_D, CTRL_TLT_ANG_IM and CTRL_TLT_ANG_P. All problems are solved now.
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Re: Dangerous divergence: FIXD and understood

Postby LPR » Wed Apr 24, 2013 3:40 pm

Angel

A formula for adjusting the motor mix that would include the copter's weight, motor to motor distance and prop size would be a good thing.

Larry
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