Why the issues flying near metallic objects?

[Buel]

Hi all.
What exactly is it that affects flying near metallic objects - is it interference with the compass or more complicated, please? Would love to know.

 

[msinger]

is it interference with the compass

Yes. This issue is often seen when DJI drones are close to magnetic metal objects.

See more on that topic here:

DJI Mavic Compass Calibration Guide | Mavic Help

Mavic Help is a site for help with DJI Mavic drones. Find the latest fixes, how-tos, guides, accessories, modifications, and more.

www.mavichelp.com

 

[AeroJ]

I am not aware that flying near metal objects is a bad idea ! I mean I wouldn't let a massive metal water tank come between me and the craft, but that's more to do with the water than the metal ! I fly around big metal structures quite a lot, most recently a 200 ft all-metal radio tower and that didn't screw with my M4P at all ! Nor do the 30 odd tall metal lighting poles on the local sports / flying field, and sometimes I ascend right alongside those !

We should avoid large metal things being around when we are calibrating the compass, sure, but in flight ? Never had a problem myself.

I would expect things to get more flakey if I flew from outside it into a barn or warehouse made of metal panels and trusses but even that would have more to do with loss of sky view and therefore potentially GPS as well as GPS signals getting bounced around / ghosted by the metal panelling, but out in the open, generally speaking, I reckon that should be fine no ?

Also, let's not forget the huge numbers of people who fly successfully from within cars, almost completely enclosed metal cages !!

 

[msinger]

We should avoid large metal things being around when we are calibrating the compass, sure, but in flight ? Never had a problem myself.

Most times, problems arise when powering on the drone and/or taking off near magnetic metal objects. And those objects are often quite small -- like rebar in concrete sidewalks.

 

[slup]

It's very rare that just flying near ferro magnetic objects cause problems in flight... staying near them will over time cause a disturbance but it require you to have the drone in it's mag field longer than just flying by or circle around and the flight disturbance will not be a huge problem suddenly... it will slowly affect the drone until you get slower uncommanded movements horizontally.

These DJI photo drones can hold position against outside forces like the wind when you release the sticks... in order for that to work the drone needs to be able to know a position, either from the GPS or/and from the vision position sensor on the belly of the drone. From this it can detect when a drift occur & in which direction and with this info it knows how to return to the correct position.

The drone will return to the position (or apply counter thrust) by means of commanding the motors to spin up or down. In order to know which motors it needs to command it's crucial that the drone (or actually the IMU) knows in which direction the drone is pointing. This direction is established a few seconds after that the drone has been powered on with info from the compass that gets transferred to the IMU... after this "IMU initialization" by the compass it will be the IMU that handle the flight & the compass in it self becomes less important.

If the drone (or more exact the compass) is disturbed by magnetic interference during these first seconds after power on the IMU will inherit that disturbed direction... and by this it will not know in which direction the drone points in reality, it will have a so called yaw error.

So with this yaw error the IMU will command the wrong motors to prevent a drift as soon as it needs to hold position. This will very quickly cause a fast fly away either in a circular or a straight flight path (depending on how much the compass was deflected at power on).

To prevent the disastrous effect of a yaw error airborne, you should keep the drone far away from magnetic disturbance during the power on phase... & also then check on the app map so the drone points the same way there, relative houses or roads as it is pointing in reality. If the pointing direction doesn't match in the app versus reality... you must power the drone off, move to another take off spot & then power it on again (& by that re-initialize the IMU).

Flying in magnetic disturbance after a power on with a correct IMU initialization will not cause any odd flight behaviour immediately (this as the mag disturbance affect the compass which isn't used as the main direction sensor... it's the IMU that handle the flight). But flying in magnetic disturbance for a longer time will over time affect the IMU, this as the compass, gyro & other sensors will feed in adjustments to the IMU. If these adjustments aren't correct it will cause smaller uncommanded movements horizontally... & if the drone stays in the disturbance the adjustments made to the IMU will cause the same effect as a full "yaw error"... but it will take a very long time to grow to a full fly away effect.

 

[AeroJ]

Thank you @slup, that was most illuminating !
Should we find ourselves in that state of being in flight and experiencing the worsening symptoms you describe, is there anything we can do about that once it starts ?

I haven't started Monowheeling myself and the drone to flying locations yet, but when I do I will be very sure to keep the hugely numerous and powerful magnets in that EUC motor at least 10 meters away from where I start up and launch the drone !

 

[slup]

...Should we find ourselves in that state of being in flight and experiencing the worsening symptoms you describe, is there anything we can do about that once it starts ?

If you mean the symptoms that can occur after flying in magnetic disturbance for a longer time... fly away from the disturbance (which again will feed in adjustments... correct ones, to the IMU) or return & land for a fresh power cycle which re-initializes the IMU.

Most likely you will chose the later after being puzzled by the odd flight behaviour...

 

[16x9frame]

I frequently get compass errors requiring recalibration if I power on the drone too close to my car. I've found I need to be at least 30 feet away from it to avoid the problem.

 

[msinger]

I frequently get compass errors requiring recalibration if I power on the drone too close to my car

As noted in the guide I linked in post #2 above, the compass should not be calibrated in situations like this. Most times, the calibration prompt is a warning that your drone is near a magnetic metal source rather than actually needing to be calibrated.

FYI for those who are not aware of this scenario.

 

[16x9frame]

As noted in the guide I linked in post #2 above, the compass should not be calibrated in situations like this. Most times, the calibration prompt is a warning that your drone is near a magnetic metal source rather than actually needing to be calibrated.

FYI for those who are not aware of this scenario.

When it happens, I get the "compass calibration required' message and the drone will not allow takeoff until the calibration is done (horizontal and vertical 360 rotations).

 

[msinger]

When it happens, I get the "compass calibration required' message and the drone will not allow takeoff until the calibration is done (horizontal and vertical 360 rotations).

Yep, that's the message I'm referring to.

Whenever you see that message, you should first try moving your drone to another location. It rarely means the compass actually needs to be calibrated.

 

[Meta4]

I frequently get compass errors requiring recalibration if I power on the drone too close to my car. I've found I need to be at least 30 feet away from it to avoid the problem.

When it happens, I get the "compass calibration required' message and the drone will not allow takeoff until the calibration is done (horizontal and vertical 360 rotations).

Those messages are very badly worded and misleading.
Ignore the messages that seem to be telling you to recalibrate the compass.
The issue is that you are too close to a source of magnetic interference.
No amount of recalibrating the compass will fix that, because there is no problem with the compass.

The solution is to power off, move away from the problem the compass is warning you about and start again.

 

[16x9frame]

I rarely encounter this error now that I know to power on far enough away from cars or other large metal objects. That being said, it still does happen occasionally and when it does, it won't clear and allow takeoff unless I actually do the recalibration. A power cycle has never cleared it for me, it comes back even after moving to another location, and after the recal, it allows takeoff from that new location. I might just have a quirky model.

 

[Meta4]

A power cycle has never cleared it for me, it comes back even after moving to another location, and after the recal, it allows takeoff from that new location. I might just have a quirky model.

Which drone do you have?
Just to confuse everyone, DJI made the mavic 2 and one of the early Air series demand recalibration if you hadn't flown for 30 days or if you had moved some distance from where you last flew.
There was no physical reason that recalibration was required since compass calibration has nothing to do with time since last flight or distance from anywahere, but DJI forced it anyway.

 

[16x9frame]

Which drone do you have?
Just to confuse everyone, DJI made the mavic 2 and one of the early Air series demand recalibration if you hadn't flown for 30 days or if you had moved some distance from where you last flew.
There was no physical reason that recalibration was required since compass calibration has nothing to do with time since last flight or distance from anywahere, but DJI forced it anyway.

Mavic 3 Pro, and I'm always flying from new locations at great distances from one another.

 

[Meta4]

Mavic 3 Pro, and I'm always flying from new locations at great distances from one another.

The Mavic 3 pro doesn't care how far you travel.
If you get that message, it should let you fly if you switch off, move away from the problem that caused it and power up somewhere else.

There is one other DJI oddity that I've encountered with the Mavic 3 pro falsely telling me to recalibrate the compass.
I explained it here:

Litchi, calibrate compass, how ? Instructions please for a deaf person.

From what I have read it seems there may be verbal instructions issued but if there are I can not hear them, I am deaf. When I touch the message I do not see anything happen. I ended up having to switch to Go4 and calibrate it there, then switch back to Litchi for the flight. Can someone walk...

mavicpilots.com

 

[EricJT]

Lately I have been flying from my dock at the cottage.....lots of metal in the planking, so I start my M2P holding it at shoulder level, wait for the power up sequence and then place it on the dock, checking to make sure the RC drone direction is in agreement with the drone's actual compass heading.

If there is a disagreement between the two headings, never so far, I would power down and restart.