Yaw Instability, External Compass Calibration Failure & Loiter “Toilet-Bowl”

ArduCopter Copter 4.5
1

Hello ArduPilot community,

I’m running ArduCopter V4.5.5 on a prototype X8 octocopter and am having some issues with yaw, compass and GPS loiter that are stumping me. I was able to get pitch/roll tuned in acceptably, but the yaw issues are preventing me from doing more extensive test flights (stabilize mode) and the GPS loiter issues are preventing me from testing the autonomous functions.

I am hoping someone can point me in the right direction to solve these issues. I am happy to provide additional data or log files as needed!

1. Yaw Stability

  • Sluggish yaw response
  • Occasional sudden yaw “jerks” of ~60°
  • Tried tuning yaw PID, marginal improvements in yaw-hold
  • Tried calibrating compasses, but had issues with external compass… see below

2. External Compass (IST8310) Calibration

  • Calibrated compasses
    • Internal compass calibrates okay, external never calibrates (gets about 67% done and never improves, even with “relaxed” fitness)
    • Swapped internal ↔ external compass priorities
    • Mags show some interference from motors running… not sure how much is too much, and I am hesitant to do the “reverse the props and run up the motors” trick to measure the interference per the online documentation

3. Loiter “Toilet-Bowl” Spiral

  • Behavior:
    • Immediately after climb-out (~3 ft AGL), copter enters catastrophic toilet-bowl spiral
      • It’s not a fast spiral but it is the tell-tail toilet-bowl effect and has caused some soft crashes already
      • I recover by quickly flipping back into stabilize mode, but even after doing that there seems to be some weird drift and lean that is unexpected
  • Logs:
    • Compass FAIL (mag_field change 831.35% >550 µT) and intermittent GPS_GLITCH

Other

  • Vibe analysis shows elevated Z-axis peaks on a new FC mount plate I am working with as compared to some other flight tests… but the toilet-bowl syndrome at least was an issue before with stiffer mounting
  • I am still working on the FC mount plate design, but wanted to mention it incase that is a root-cause for any of the above issues.

Hardware & Key Params

Component Details
Frame Custom X8, 18x6.5 foldable props
FC Holybro Pixhawk 6c Mini
ESCs T-Motor 8-in-1, endpoints calibrated
Motors MAD 5005 IPE 280kv
Batteries 2x parallel 6s10ah Li-Ion (within current spec)
Firmware ArduCopter V4.5.5 (142aece2)

DSC00405
DSC004051920×1280 708 KB

2

Send a log so we can understand what’s happening.

By your description it does sound like magnetic interference. An internal compass, in my experience, is not gonna help you with a drone of that size.

Pay attention to the wiring of that external compass. It should be as far as possible from power lines. If you’re not being able to calibrate the compass, it’s a huge sign that something is wrong with the hardware.

3

BIN file, update to the latest firmware.

4

Here is a .bin file of a flight where a yaw twitch happens near the end of the flight (last 15-20seconds). I have also included an onboard video clip of the yaw twitch for reference.

5

Have you complete a magfit flight? Refining Calibration Parameters using a Flight Log — Copter documentation
Don’t use the flight controller internal compass.
I had those props and adapters and they are a source of major vibration.
Calibrate the compass without a battery plugged in just the usb into the flight controller, then do the in flight mag fit compass calibrate for you params.

I am no pro just some tips that I have picked up.

6

You should try the magfit calibration with the external compass. Don’t use the internal compass.

7

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Update to the new version firmware…

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How are you powering your flight controller? Looks like your board voltage has tons of noise in it.

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8

Powering from the current sensor that came with the Pixhawk 6c mini (PM02 v3 it looks like): Pixhawk 6C Mini – Holybro Store

9

Thanks, I will give this a try

10

Try these for a start

COMPASS_USE,0
COMPASS_OFS2_X,-78.9942
COMPASS_OFS2_Y,218.89482
COMPASS_OFS2_Z,52.638126
COMPASS_DIA2_X,1
COMPASS_DIA2_Y,1
COMPASS_DIA2_Z,1
COMPASS_ODI2_X,0
COMPASS_ODI2_Y,0
COMPASS_ODI2_Z,0
COMPASS_MOT2_X,-0.3817207
COMPASS_MOT2_Y,-0.8003372
COMPASS_MOT2_Z,-0.23543927
COMPASS_SCALE2,1
COMPASS_ORIENT2,0
COMPASS_MOTCT,2

Those pitch and roll P and I terms should be equal - try lowering the P term values to equal the I term values.

Use these battery settings:

BATT_ARM_VOLT,19.10
BATT_CRT_VOLT,18.00
BATT_LOW_VOLT,18.60
MOT_BAT_VOLT_MAX,24.60
MOT_BAT_VOLT_MIN,16.80

and make sure your batteries always go back in exactly the same position and orientation, if you remove them for charging. LiIon batteries will affect your compass calibration a lot.

1 Like
11

Is that specific to Li-Ion or do all lithium-based batteries impact compass calibration? Why is that?

12

Innormal RCIN
Innormal RCIN1705×522 55.6 KB

Looking at the logs, there is an interference signal on the rudder that is RCIN4 that does not look like what you intended. The signal is very short, about 0.3 seconds, and we believe it is a maneuver that is unlikely to have been intended by the pilot. It is also in the same timeframe as the sudden movement you described. What is the product of the controller and receiver you are using?

13

Yes, it’s their internal structure and chemistry that makes strong magnetic fields in particular directions.
LiPos are not so bad, much less sensitive. Still best to be as far away from the compass as practical.

14

Those yaw spikes are probably me banging on the stick testing the yaw response. It’s slower than desired. For large coaxial aircraft like this, do I need to cant/twist the motors in? Right now they are all planar.

I am using the Pocket TX and ELRS 2.4 system

15

Why would the internal compass calibrate but the external not calibrate? And even then, why would I see such bad yaw performance and errors if the internal compass calibrated?

I will try the inflight calibration routines recommended above.

16

I attempted to calibrate the internal + external compasses via a long USB cable, no batteries attached. The internal compass wouldn’t calibrate unless I set fitness to “Relaxed”, and I couldn’t get the external compass to calibrate at all; the progress bar would get to 99% then jump back to 0%. I even changed COMPASS_OFFS_MAX from default 1800 all the way to 3000 and still no luck.

Do I have a bad compass/GPS module?
Could the carbon frame + aluminum mount + carbon mast that the GPS unit sits on be an issue?