Quadrotor Throttle Imbalance on 2 of 4 Motors

Good Morning,

I am seeing in straight forward flight that 2 of 4 motors have a throttle imbalance, meaning one is high and one is low (sometimes momentarily clipped by lower throttle limit), and they seem to average to the other 2 motors. I did not notice this on the field, only in the logs. Its the same 2 motors in multiple flights in multiple directions (so I think wind is not the issue). The frame and setup are symmetrical.

The desired roll, yaw, and pitch rates vs actual are not perfect as I am flying at a steep 60 degree angle, but they do not seem extraordinary.

I have no idea what is causing this imbalance and how to correct it. I am getting new motors soon so I may try that out but its a lot of effort relative to the conviction I have that the motors are the issue.

Any thoughts?

log file (ignore the flight paths that are not straight; the non-straight paths understandably have lots of clipping): https://paste.c-net.org/MooseStalk

i guess first easy thing i could try is swapping the motors. if it looks different then its at least partly a motor issue.

One interesting thing I just noticed is that flying backward (+60 deg in the logs) vs forward (-60 deg in the logs) improves the balance. This could imply a CG / aero issue.

a screenshot from the logs of flying forward, then backwards:

Hello @lordvon

Check if you don’t have motors misaligned and if your ESCs are properly calibrated.

will do. that may explain it; here is the screenshot from hover, which shows a slight imbalance for the same motors:

Should all throttle values be spot-on in hover or does this look normal?

Double-check the things I pointed to you.

But from this screenshot, your imbalance doesn’t seem the worst I have ever saw. A small ammount of imbalance is always present, if I remember well, there is a threshold internaly to ardupilot that triggers imbalance messages.

After a hover flight, check if your motors 1 and 2 are not abnormaly hot. If they aren’t that hot and your yaw manouverability is not being affected by it, I think you can live with this.

by alignment do you mean motor spin direction? that has been triple++ checked and they are also tilted 6 deg circumferentially, which ensures that they are not slightly pointed in the wrong direction (as can be the case without tilt).

i always feel motors after every landing, and they have only ever been lukewarm at most, never hot.

No, I saw about being perpendicular to ground, few degrees wrong can lead to some yaw imbalance.

That’s good, so maybe this little imbalance will not bother you a lot.

esc calibration made almost no difference. its an x-quad with motors with 2 and 3 having seemingly normal (~1400) throttle and 1 being high (~1600) and 4 being low (clipping at ~1100).

The biggest problem could be pitch imbalance (1 and 3 higher as a pair than 2 and 4) or vehicle-frame yaw imbalance (1 and 2 higher as a pair than 3 and 4). what can i look at as a measure of the actual corrective moment (e.g. is the autopilot trying to make a roll/yaw or pitching moment) being attempted by the autopilot?

I’m sorry I am not the best person to answer your questions… maybe @xfacta can help you more with this.

seems like rout, yout, pout are the parameters i was looking for. seems if these have a mean value around 0, then the quad will be in an minimal-effort steady state