BiCopter Oscillations

[jihlein]

I think most of us have seen the BiCopter oscillate in rearward flight. I also suspected that if we reversed the motor rotations, the BiCopter would oscillate in forward flight. User @Kevin_Erik proved this to be true. But this left us no closer to a fix.

So while bench testing some other software changes the other day, I discovered by accident that on the bench, with the props off, I could get the oscillations to occur simply by giving an aft pitch command and having the servos tilt back. Learned two things here:

1)The oscillations are not caused by some aerodynamic side effect of the BiCopter shape since the copter wasn’t moving thru the air
2)Just the torque of the motors alone is enough to present the oscillation, or at least present the conditions that cause it

I’ve had a hunch that the yaw moment induced by the differential roll cmd was to blame for this, because that yaw moment changes directions whether the motors are tilted forward or aft. After some thought, I decided to try setting the roll D term to zero, since the D term is susceptible to noise and can amplify it.

To make a long story short, no more oscillations when the motors tilt aft! Now unfortunately the weather here has been unflyable, so this hasn’t been flight tested yet, but I’m hopeful.

The two attached videos show this. The first has the autotuned Roll D value, and as you can see, it oscillates when the motors tilt aft. The oscillation pulls some pretty good current, my current limited power supply is limiting and you can hear the low voltage warning. The second has Roll D set to zero, and there’s no oscillation regardless of motor position.

I should note that since they are controlled by servos, I’ve already been flying with pitch D and yaw D set to zero.

Can anyone else replicate this experiment with similar results? These test cases were run with the dRonin firmware after an autotune. It would also be interesting to see if the same results can be had with BetaFlight.

Video with Autotuned Roll D:

Video with Roll D set to zero:

[Kevin_Erik]

@Jihlein

I’ve the FPV canopy installed and the latest firmware updated. Will try to get an Autotune and field test without D-Term. That is assuming weather is permitting as its been rainy as of late. This build has sOat installed too so my results should be similar to yours. If all goes well, i plan on testing 5x3x4 Dal props and 6x2x4 Dal props for a proper comparison.

Note: Basic Camera and TX installed, can record but only via external DVR. (poor quality)

[Kevin_Erik]

Here is the video of today’s “flight” (not the autotune) with the FPV canopy (Mute your sound as i cant lower it in YouTube): https://www.youtube.com/watch?v=Hl3aS3eD5OA&feature=youtu.be

There are a few things to be aware of…
*Flight weight with 1300 mAh Lipo’s was 464 grams.
*Starting Tune Roll 10/5, Pitch 15/10, Yaw 10/5. While this didn’t oscillate during the pre-tune, it was hard to control. (Sloppy)
*Autotune results appear as if Pitch and possible Yaw were too weak going into Autotune. Please see the attached images for details.
*Copter flown without D-term. Picture shows otherwise but i changed the values before saving.
*Bicopter flown in Horizon mode as i forgot to add the rate mode(s) your using.
*It appears as if the copters level is off despite fully tuning it before hand. (leans left constantly) Either Horizon mode is having issues or during the pre-arming the surface the gyro calibrated on wasn’t level.
*DAL Props 5x3x4 (blunt tips) were used.
*On the second battery (not shown) i flew using only FPV.
*sOat installed.

Cheers!

kevin

Attachments:

1. 

   Overview.jpg
   

2. 

   Roll.jpg
   

3. 

   Pitch.jpg
   

4. 

   Yaw.jpg
   

5. 

   Damp-and-Sensitivity.jpg
   

6. 

   Stab-Tab.jpg
   

[jihlein]

Well for what it’s worth, my flight attempts with all PID D terms set to zero was a disaster, it won’t even lift off with getting the bad shakes.

Still something here I’m missing……

[Kevin_Erik]

If you recall i did quite a few flights with Betaflight before switching to dRonin. There should be log files on the forums and possibly on my home PC. You could use Betaflights log viewer to see if there are any error commonalities between the flight systems.

[Kevin_Erik]

Here is one with a Betaflight Log: https://rcexplorer.se/forums/topic/kevins-bi-copter-thread/page/3/#post-59742

[Kevin_Erik]

Question, the Motor update rates are signifigantly higher than the Servo rates. Even with the HV Servos I’m using, at most I can get 330 Hz @ 7.2 Volts, while the Motors / ESC’s are running D-Shot 600 (26.7us).

Note: 330Hz or 3030us, which leaves PWM still three times faster. dShot is something on like 100 times faster depending on the protocal chosen.

Could the differance in rates be causing be Roll / Yaw to fall out of sync? I may try to run PWM motor rates that are nearest the Servo’s to see if this makes a differance at all. Maybe even a harmonic of the Motor frequency would help.

[jihlein]

I’ve thought this too, and it’s worth a try.

But why only in one direction only?

If I get out this afternoon, I’ll put it on my list of things to try.

[Kevin_Erik]

Directional change may exaggerate the desync. Assuming this is the case.

[Kevin_Erik]

Tomorrow i will try SyncPWM with the following base PID’s for the auto tune.

Roll 10 / 5
Pitch 20 / 15
Yaw 20 / 15

Note: Same 5″ DAL props and will attempt to fly (wishful thinking) using the flight mode Stabilized-1 (rate / Att or Horizon / rate)

[Kevin_Erik]

Ok, after some reading and messing around, i was able to get PWM working at 330 Hz. This includes performing the ESC calibration.

Will now go try to Autotune / fly this, then post the results.

Side note: Another option we have not looked at is disabling the ESC braking.

Kevin

[Kevin_Erik]

Pre-Auto Tune settings..

Roll 10/5
Pitch “Default”
Yaw 20 / 15

Here are the results from the Autotune, while the ESC’s are set to 1-Shot 330 Hz . (Servos set at 330 Hz) Please note that I Zero’d out the Yaw D-Term after the Autotune.

Attachments:

1. 

   Overview-1.jpg
   

2. 

   Roll-1.jpg
   

3. 

   Pitch-1.jpg
   

4. 

   Yaw-1.jpg
   

5. 

   Damp-and-Sens.jpg
   

6. 

   Stab-Tab-Result.jpg
   

7. 

   Output-Tab.jpg
   

[Kevin_Erik]

First flight Oscillated constantly no matter the flight mode or maneuver. Guessing that Either my Pitch / Roll D-Terms are at fault or the Yaw P and I were too strong. Will re-tune and try again.

Video (Mute your sound): https://youtu.be/DeSL1wqYckM

Side Note: Apparently my ESC has been off the whole time. Didn’t notice a difference in flight with 1-Shot (330 Hz) Was smooth and responsive despite the shakes. Maybe at some point i will have to consider turning this on to see what if any difference there is.

Attachments:

1. 

   Shown-as-flown.jpg
   

[Kevin_Erik]

Trying to re-tune using the following values:

Roll 9/9
Pitch 15/15
Yaw 10/10

Will stay on 1-Shot (330 Hz) for another try just to see if a better tune and no D Terms makes the difference.

[Kevin_Erik]

Will have to postpone till tomorrow as the winds have really picked up. 20 + km gusts

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