Small 360mm quad oscillation problems

Hi, I’ve got a little 360mm quad (A-type frame):

It’s got preinstalled t-motor 2205-2000kv motors with little 15a ESCs (except one that was DOA so I’ve temporarily replaced with an afro 20a) and it’s got loads of payload space so perfect for using for research/experiments. Except I can’t get it to fly properly - it oscillates badly on fast roll movements and so I can’t get it to autotune. I’ve used rate roll kp dial on ch6 to get it to as good a starting point as I can, and it flies pretty OK generally, except if you put large roll movements it starts to oscillate. Autotune in roll axis therefore doesn’t go very well. I know the vibrations are a little high - I’m trying different padding to try and improve it, but is there anything obvious you can see in the log that would be causing this?

Am I missing something? The guidelines for this forum section say to upload the bin logs, but it doesn’t allow you to do that…

Hmm should I be using H frame type instead of X? Would that explain oscillations?

Ok well through trial and error and reading Leonard’s excellent blog about tuning smaller quads ( I eventually solved it.

  • Start with much smaller pid values - half the defaults for roll&pitch
  • Increase p&r rate filters to 80hz
  • Increase MOT_SPIN_ARMED from 70 to 100
  • Set MOT_THST_BAT_MAX to 12.6, MOT_THST_BAT_MIN to 9.9
  • MOT_THST_EXPO to 0.65, MOT_THST_MAX to 0.95 (defaults I think)
  • Changed AUTOTUNE_MIN_D to 0.002
  • Mapped a spring switch to ch9 to motor emergency stop mode!
  • Changed THR_MID to 324 (and it’s only 3s so far, eek!)

Seems these smaller higher KV craft are far more sensitive to bad tuning than the bigger quads I’m used to. And to vibrations, it’s gone nuts on me several times due to clipping but I’m struggling to dampen it any further. But I’m flying now with good response and no oscillations, hurrah!

Oh, and I’ve also learnt no amount of autotune can save you from landing in a freshly laid dog turd. Clear your landing zones before you take off, folks!!

OK well talking to myself but will leave breadcrumbs in case anyone else stumbles along here with similar problem.

  • These smaller more agile quads are far more sensitive to tuning, so are more difficult to initially tune. Starting PIDs are key, as Leonard suggests in link from previous comment start with half or quarter of default PIDs (for P,I and D values).
  • THR_MID also very important to set as most of these smaller quads seems to be hugely overpowered (as in have a much higher than ‘normal’ 50% hover ratio).
  • They are very sensitive to wind, at least mine is with quite a large main body relative to size. Therefore much more difficult to autotune unless there is absolutely no wind.
  • I started with less aggressive autotune (0.1) then went to 0.75 then to 0.5. 0.5 produces higher PIDs which seem much more dialled in for me.

Now flying beautifully with no oscillations or clipping even at full throttle punch. Great job devs!

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Thanx for sharing your experience.
I have a high wobbel on the roll axis on both of my small copters (250er X and 340er Octo).
Thanx for reminding me on Leonards thread . i think i should read through again. :slight_smile:
Since i did not find an answer on the wiki: do i need to reboot the pix to get the new pid values active or can i change them during flight via MP?
I tried it this morning during flight but had no success improving the wobbel. But maybe that’s due to the fact that they are still too high (P around 0.025) but can’t recall the exact values.

May you please share your current PID settings as well to get an rough idea on the range i should move in as starting point.

Thank you!

Thanx to the hints here and in Leonards Thread i managed to eliminate the wobble completely on my 250size X quad.

Video from hoovering in the garden ->

Current setup is X1806 Motor, HQ5x4x4 Props, KISS 18A ESC, Dropix FC, Drotek PDB, FPV eqipment, 3DR radio v1, custom made CFK frame, SLS Xtron 1250mAh 3S accu. Overall weight 515g

THR_MID is currently at 320.
p&r rate filters to 40hz (will change to 80 later)
P&I value i need to check at home.
D value is at 0.001