7. Vibration, EKF and compass¶

Vibration¶

The accelerometers measure acceleration, which the autopilot needs to know which way is down. A damaged propeller, loose motor or stiff mounting adds shaking that the accelerometers also pick up. Too much of it and the position and altitude estimates degrade.

ArduPilot logs vibration levels in m/s² for each axis. Its own guidance treats 30 as worth looking at and 60 as bad.

Vibration comparison

Flight	Peak vibration	Longest run above 30 m/s² (pre-event)	Rule
nominal	15.7 m/s²	0 s	not triggered
vibration (damaged prop)	54.9 m/s²	12.5 s on z	triggered

vibration-persistent-pre-event triggers if any axis stays above 30 for 2 s or above 60 for 1 s. The damaged-prop flight also logged 283 accelerometer clipping events, where the sensor hit its maximum, so clipping-pre-event triggers too.

The EKF: the autopilot's best guess¶

ArduPilot fuses all its sensors in an Extended Kalman Filter (EKF). For each sensor it predicts what the next reading should be, then compares that with the actual reading. The gap, scaled by how big a gap it expects, is the test ratio:

\[ \text{ratio} = \frac{\text{innovation}^2}{\text{expected variance}} \quad\text{(reported so that } 1.0 = \text{at the limit).} \]

Below 1 means the sensor agrees with the filter. Above 1 means the EKF is starting to distrust it.

Compass interference¶

High motor currents create magnetic fields. If the compass sits too close to the power wires, its heading shifts whenever the motors work hard.

EKF compass ratio

In the compass-interference flight, the EKF compass ratio reached 2.07. Before the event it stayed above 1.0 for 24.5 s in one run. ekf-compass-innovation-pre-event needs above 1.0 for 2 s, so it's triggered.

Heading disagreement

When the heading is wrong, the drone flies in curving, growing circles ("toilet-bowling"). The autopilot steers for where it thinks it's facing, not where it actually is.

Why a 'compasses inconsistent' message after a crash proves little

After the motor-failure crash, the simulator (like real ArduPilot) printed "PreArm: Compasses inconsistent". A crash can bend a compass mount, so this message comes after the incident. AnaTele records it (compasses-inconsistent-reported) but gives it little weight.