Impossible to stop motors due to magnetic interferences

[MustangPhil]

I was about to complain about the new FF6 software on the Anafi, but the root cause was different:

Today, I wanted to try the new software upgrade.

Did my check-list including the Anafi and command magnetometer calibration in an area free of any magnetic interference (in the middle of a muddy road in the French countryside).

Took-off with a hand launch on this dirt road took pictures and movies (see below the road, trees and on the left the wooden table). It flew perfectly well !

Then had this stupid idea to land on a large wooden table under the tree...

The Anafi was Ok until it touched the table, then it bounced erratically on the table refused to stop the motors. Tried to take-off again to gain altitude and land elsewhere but the Anafi did not respond to my orders.
I ran on it and touched the ‘off’ button on the battery : no luck, still running... hit a prop with my fingers (protected by a glove due to cold weather) then it stopped...

Then I made the following pics and measurements with my hand compass and found on the table ... 360° magnetic interference in the exact place where I tried to land...

Hope it will help you, and will add the ‘check interferences in the take-off / landing area / and during calibration dance’... blame it on me and this F...ing wooden table with a metallic frame !

 

[pmh1nic]

Glad you and the Anafi survived the encounter. What do you think cause the frame to become magnetized?

 

[MustangPhil]

I don’t think the metallic parts of the table ‘became magnetic’. Any metallic part, made for example of soft iron or any ferromagnetic material, is magnetic when exposed to the natural magnetic field of the earth.
This phenomenon is particularly true for ‘long parts’ such as frames like the one on this table, but also the metallic bars that are embedded in concrete.

The worst case for drones are often the flights around bridges. A bridge is made of concrete, metal, and often contains electric wires that ‘cross some water’ at this particular point... so it is known to be a magnetic field ‘mess’.
In another post, I also showed the hood or the roof of a car should not be used as a take-offf or landing spot. You should avoid also calibrating your drone inside a car (even if it is rainy/cold/windy outside...)

This post is just about the fact I was not suspicious enough about this wooden table with wooden benches... Beware magnetic perturbations, they are all over the place !!

 

[MustangPhil]

A previous test I caught on video about car roofs and hoods :

Car roofs and hoods are not drone’s landing strips

Just my little video of an analysis of the magnetic interferences of a car roof and hood : Conclusion : more than 50° variations all over the place, so avoid taking-off and landing on the roof and never calibrate the drone in or near the car ! Chances for fly-aways are around 99%.

parrotpilots.com

... and after a crash on my home-made BBQ (supported by 3 long metallic bars) I stuck paper ‘arrows’ that point to the ‘magnetic north’ on the landing area... I built not only a BBQ but also a giant magnet !!

 

[pmh1nic]

I don’t think the metallic parts of the table ‘became magnetic’. Any metallic part, made for example of soft iron or any ferromagnetic material, is magnetic when exposed to the natural magnetic field of the earth.
This phenomenon is particularly true for ‘long parts’ such as frames like the one on this table, but also the metallic bars that are embedded in concrete.

That statement seems to be contradictory. On the one hand you say you don't think the metallic parts "became magnetic" but then state "any ferromagnetic material, IS [becomes] magnetic when exposed to the natural magnetic field of the earth". If it wasn't magnetic prior to the exposure to a magnetic field it becomes magnetic when that exposure occurs. I think you might be addressing the question of whether or not it maintains that magnetism when removed from the magnetic field.

 

[MustangPhil]

Your remark is particularly interesting and as I am not a specialized in magnetism, I tried to understand the phenomenon involved.

I have just phoned a company specialized in de-magnetisation of industrial parts and spent 30’ with an engineer that gave me partial answers On the subject :

- The earth has a magnetic field of less than 1 gauss (0,25 Gauss to 0,65 Gauss that corresponds to 25 to 65 micro-Tesla ... or 0,0025 to 0,0065 Tesla. Let’s say 50 microTesla.
- as a comparison, a strong magnet made of neodymium is around 1 Tesla or 10.000 Gauss or 1.000.000 microTesla.
- He first said that a metallic bar could not be magnetized by only the earth magnetic field.
- After I explained him that drones where able (with inboard sensors, magnetometers) to measure values around 50 microTesla or 0,05 T, I asked him ‘what values could be commonly found on metallic parts ?’. He said values of 10 to 20 Gauss were relatively common (so that’s 1.000 micro Tesla if I’m not wrong).
- when they do de-magnetization of parts, they can’t garanty they will obtain less than 5 Gauss due to remanent magnetism.

On a metallic long bar, magnetization may be induced by the manufacturing process that involves repetitive chocs and frictions that ‘re-arrange’ the electrons and the molecular arrangement of the metallic material and induced an intrinsic magnetization.

He proposed me to test the bars with a thin cotton wire where a paper-clip is attached, they consider it is the beginning of ‘magnetization’.

I am afraid that drones that deal with the weak strength of earth field are much more sensitive to this, so I’ll test it also with my smartphone magnetometer (there’s an app that gives values of all your sensors, including magnetometers, accelerometers...)

What I found with a simple hand compass just lets me think we should be careful of ‘hidden’ magnetism. Now I’d like to measure what is ‘Ok’ and what is not for our drones...