MR60

### Max mag field length--Why A Max?

Why a maximum value of 550? I understand intuitively why there would be a minimum value.

We typically see less than 6% mag field interference, but always get nailed by the max mag field length. Values are typically 680 to 790.

The integrated compass/mag is on a long multicopter with more than 8" space to the battery and motors. Running a compass around the mag shows little to no interference.

Mag calibration (only the external enabled) usually brings up values like -120 -50 80 ish.

What is "Max mag field length" and why the max?  If we exceed the max, what behaviors might that propagate?

Example:

Test: Compass = GOOD - mag_field interference within limits (4.20%)

Max mag field length (686.01) > recommended (550.00)

#### Replies

• The way I see it is that mag sensor is needed for drone to know the absolute direction at any time in flight. The problem is that mag sensor is susceptible to outside interference and therefore is not reliable.

A low drift gyroscope could be used for the same purpose without the disadvantage of being susceptible from outside interference. The best would be a fiber-optic gyroscope but they are too big, too heavy and probably too expensive for quadcopter design. MEMS gyroscopes have more drift but depending on flight time could be within limits.

Forrest Frantz said:

Low drift gyro???

• MR60

Low drift gyro???

• We had issues with mag field as well. Eventually the decision was to use low drift gyro.

• MR60

- MaxMagField = maximum of mf(index)

- where index = 0 to length, where length = the number of observations from the flight

- mf = sqrt(mx^2 + my^2 + mz^2) or the 3-D distance (magnitude) of all three components

- mx, my, & mz which are "Mag Field Strength" or "Body fixed magnetic field (mgauss)"

... and look something like

The yellow line is mf and the other lines are mx, my, & mz

So why would a strong mag signal be bad?  If the mag signal was being influenced by a ferrous metal, yes, big might be bad, but if not???

Jon said:

I also have problems in this area. Yesterday I did the normal compass dance and was quite pleased with offsets at -11, 2, -35. Flew my quad and put the log through Mission Planner's Auto Analysis with the result Compass = good mag_field within limits 13.73%, max mag field length 679.10 which is higher than the recommended 550.00.

I'm not sure what I can do to bring the max mag field length closer to the recommended values.

During my search to better my understanding I found at :https://github.com/ArduPilot/ardupilot/blob/master/Tools/LogAnalyze...

 '# check for mag field length change, and length outside of recommended range
 if "MAG" in logdata.channels:
 percentDiffThresholdWARN = 0.25
 percentDiffThresholdFAIL = 0.35
 minMagFieldThreshold = 120.0

maxMagFieldThreshold = 550.0'

But couldn't find info on how to reduce the max mag field length.

• I also have problems in this area. Yesterday I did the normal compass dance and was quite pleased with offsets at -11, 2, -35. Flew my quad and put the log through Mission Planner's Auto Analysis with the result Compass = good mag_field within limits 13.73%, max mag field length 679.10 which is higher than the recommended 550.00.

I'm not sure what I can do to bring the max mag field length closer to the recommended values.

During my search to better my understanding I found at :https://github.com/ArduPilot/ardupilot/blob/master/Tools/LogAnalyze...

 '# check for mag field length change, and length outside of recommended range
 if "MAG" in logdata.channels:
 percentDiffThresholdWARN = 0.25
 percentDiffThresholdFAIL = 0.35
 minMagFieldThreshold = 120.0

maxMagFieldThreshold = 550.0'

But couldn't find info on how to reduce the max mag field length.

ArduPilot/ardupilot
ArduPlane, ArduCopter, ArduRover source. Contribute to ArduPilot/ardupilot development by creating an account on GitHub.
• MR60

Same issue here. With latest 3.4, and latest mission planner auto log analyzer, I also get very good & low magnetic interferences but extremely high max magnetic field length, above the recommended value. My compass is external (internal disabled) and at least 10 cm away from any electronics.

Is it a mission planner false indication or else what can explain such high values when nothing electrical is near the external mag ?

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