Avoiding the rebar effect

[cheddarman]

From the DJI P3 manual:

Calibrate the compass before each flight. Launch DJI Pilot app and follow the on-screen instructions to calibrate the compass.

When to Recalibrate
1.When compass data is abnormal and the Aircraft Status Indicator is blinking green and yellow.
2.When flying in a new location or in a location that is different from the most recent flight.
3.When the mechanical or physical structure of the Phantom 3 Advanced has been changed.
4.When severe drifting occurs in flight, i.e. Phantom 3 Advanced does not fly in straight line.

Follow the book and you'll have fewer problems!!

 

[Meta4]

Here's a blurb from dronesense.com, which jibes with everything I've been saying:
================ Start quote =================
You should always calibrate before an aircraft’s first flight to feed accurate positioning data into the flight system.
Additionally, you should calibrate if you were flying in Texas and now want to fly in California as there will be much declination.

Your blurb is junk.
Whoever wrote it doesn't understand what compass calibration does at all (they think it has something to do with your geographic location !)
If someone hasn't understood the most basic aspects of compass calibration, their information cannot be relied on at all.

I'd suggest reading the first post in the link I posted in #16

 

[Meta4]

From the DJI P3 manual:
Calibrate the compass before each flight. Launch DJI Pilot app and follow the on-screen instructions to calibrate the compass.

Follow the book and you'll have fewer problems!!

Unfortunately DJI has confused everyone with contradictory and incorrect information in their manuals regarding compass calibration.
There is no need to recalibrate your Phantom before each flight and no need to do it after travelling any distance.
Understanding what compass calibration actually does will make your flying safer than calibrating every flight.
Read the first post in this link for accurate compass information:

A short explanation of compass function, calibration and errors.

BACKGROUND The aircraft compass is often misunderstood, both in terms of function and purpose, but it is important to understand what the compass measures, why, and what happens to that information. At the most basic level the purpose of the compass is to measure the earth’s magnetic field in...

mavicpilots.com

 

[Rickster565]

Thanks for your input Harleydude. Based on what you said about your experiences taking off from concrete, I'll try the same spot again on my concrete driveway. I was sure that rebar (or WWF) in the concrete was the cause my P3P's refusal to allow me to start the motors. Perhaps it was another issue that caused the no-start, and I was just thorough enough to identify corrrectly. Since then I have been launching from my neighbor's driveway (about 50 feet way) ....but it is asphalt.
I'll report back next week.

The non-starting of the motors taking off from a concrete surface puzzles me as most of my flights are from concrete surfaces of all types due to the Construction Filming I do....I have taken off from concrete parking structures with tons of precast slabs with tons of rebar and not once have ever experienced a non-start condition? I'm wondering if your non-start might be due to possibly an intermittent electrical issue with your bird?

As Harley Dude elaborated I also generally take off aggressively.....but obviously not going to make a difference with a no-start condition. I'll be following this one for sure.

 

[Harleydude]

Your blurb is junk.

I don't suppose you feel like proving that assertion with any that "evidence" nonsense???

Whoever wrote it doesn't understand what compass calibration does at all (they think it has something to do with your geographic location !)

Ummmmm...yeah...it kind of does. Magnetic North varies depending on your location on the planet. But you seem to know a lot about it. I look forward to being schooled.

If someone hasn't understood the most basic aspects of compass calibration, their information cannot be relied on at all.

True. So I guess that rules you out. Next.

I'd suggest reading the first post in the link I posted in #16

Naaaaa....I know how an electronic compass works...as do the nice folks at dronesense.com. This is affirmed via my life experience of shooting all over the state without recalibrating my compass for months.

But hey...you seem to be an expert. So please school the class.

This should be phun AND entertaining.

D

 

[Deleted member 62848]

Uuummmm touchy subject,,I always take off from concrete aswell,never any issues so yeh how motors not start has me stumped like #24,I find that hard to believe myself,something else would have caused that

 

[Meta4]

Ummmmm...yeah...it kind of does. Magnetic North varies depending on your location on the planet. But you seem to know a lot about it.
I look forward to being schooled.

Then read the link I pointed you toward twice already.
I'm not going to waste time and effort going over what's very well known and explained in that link.

If you think compass calibration is somehow related to geographic location, you don't understand anything of what compass calibration does.
And without that very basic information, everything else you believe about the DJI compass is incorrect and you are just spreading more misinformation.
Don't bother responding until you've read it.

 

[Deleted member 62848]

I read it can I reply,,I still dont fully understand what the earth does with its magnetic forces and solar flares and changing of earth's orbit but I think just easy to say we get stuck in ways of doin stuff.
Man if the drone does funny toilet bowl issues I would do it but otherwise it's ok to leave it

 

[REL]

I'm starting to regret my original post Harleydude and Meta4 ....please don't get into a pissing contest about this. You are both welcome to your positions (and I appreciate the input from both of you) please .... just agree to disagree and lets move on.
I'm going to see if I can duplicate my non-start issue tomorrow. If I can't ..... then I must have
mis-identified the cause (in my case) as being selecting a take off point on my concrete driveway ..... which may have rebar ....but likely has has WWF (welded wire fabric).
Happy Father's Day .... lets toast to that

 

[Meta4]

I'm starting to regret my original post

What's happened here is just highlighting the misinformation that's common whenever compass calibration is discussed.
DJI has contributed to this with confusing, contradictory and incorrect information in their manuals.
But the science is well understood and has been for several years.
It's an issue of facts, not opinions and it's frustrating to have to deal with long-debunked myths again and again.

It's unfortunate that this thread has been derailed by misinformation that keeps users from understanding the facts about their drone's compass..

I'm going to see if I can duplicate my non-start issue tomorrow. If I can't ..... then I must have
mis-identified the cause (in my case) as being selecting a take off point on my concrete driveway ..... which may have rebar ....but likely has has WWF (welded wire fabric).

There's no need to doubt your initial observation.
If you can't duplicate it, that doesn't prove anything.
You can't tell what is under the concrete and a position difference of just an inch can make a big difference in the effect.

The fact remains, launching from reinforced concrete surfaces can cause serious flight issues.
I see cases every week on the forums.
Here's an example from earlier today: Mavic Air flew off, accelerated and smashed in to a wall at nearly 40mph!
It doesn't happen every time, but it happens often enough to be a good warning.

 

[Meta4]

I read it can I reply,,I still dont fully understand what the earth does with its magnetic forces and solar flares and changing of earth's orbit

You don't need to understand that.
None of it makes any difference you your drone flying.

 

[Harleydude]

Then read the link I pointed you toward twice already.
I'm not going to waste time and effort going over what's very well known and explained in that link.

If you think compass calibration is somehow related to geographic location, you don't understand anything of what compass calibration does.

I read the primer and am now ready to write my Masters thesis on compass calibration. Thank you. You might wanna boil that down a bit. That said...I will rephrase my position and soften my tone.

IT IS MY UNDERSTANDING that persistent (but not permanent) compass damage can occur when the drone's electronic compass is exposed to a magnetic field, like a magnet (this isn't covered in the primer). The only way to fix this is via a regimen with another magnet.

IT IS MY UNDERSTANDING that exposing the compass to ferrous metals creates TEMPORARY issues that will either correct themselves (if minor enough) or can be corrected by moving the drone to a non-ferrous area.

IT IS MY VAST EXPERIENCE that DJI products do NOT have to be calibrated before every flight. In fact, assuming one is cognizant of ferromagnetic sources, they can go months without needing recalibration. In addition, you can travel many miles without needing recalibration.

Per your article, I simply check that the drone icon in the app is pointing the same direction as the actual drone (have been doing this for years). If they don't match, this is the canary in the cave that something is amiss. The repair hierarchy:

* Move the drone. If that doesn't work...
* Reboot the drone. If that doesn't work...
* Calibrate the compass.

Did I get it right?

D

 

[sar104]

I read the primer and am now ready to write my Masters thesis on compass calibration. Thank you. You might wanna boil that down a bit. That said...I will rephrase my position and soften my tone.

IT IS MY UNDERSTANDING that persistent (but not permanent) compass damage can occur when the drone's electronic compass is exposed to a magnetic field, like a magnet (this isn't covered in the primer). The only way to fix this is via a regimen with another magnet.

IT IS MY UNDERSTANDING that exposing the compass to ferrous metals creates TEMPORARY issues that will either correct themselves (if minor enough) or can be corrected by moving the drone to a non-ferrous area.

IT IS MY VAST EXPERIENCE that DJI products do NOT have to be calibrated before every flight. In fact, assuming one is cognizant of ferromagnetic sources, they can go months without needing recalibration. In addition, you can travel many miles without needing recalibration.

Per your article, I simply check that the drone icon in the app is pointing the same direction as the actual drone (have been doing this for years). If they don't match, this is the canary in the cave that something is amiss. The repair hierarchy:

* Move the drone. If that doesn't work...
* Reboot the drone. If that doesn't work...
* Calibrate the compass.

Did I get it right?

D

You guys are arguing different points. Yes - the magnetic distortion from the rebar or other ferrous metal does go away as the aircraft ascends, and the compass will then return to reading correctly. But that's not the problem, as @Meta4 is trying to point out. The problem, as I explained in some depth in the post over on the Mavic forum, is that the compass is not the primary heading (yaw) diagnostic - it's the rate gyros that maintain the IMU yaw value. And the IMU yaw is initialized by the magnetic yaw value from the compass at power up, so if that is incorrect then the IMU yaw will be incorrect. After the initialization, the magnetic yaw is only used in flight to correct for slow drift in the rate gyro data - it is not used to fix sudden discrepancies.

So the fact that the compass yaw fixes itself at takeoff doesn't help, because the rate gyros don't detect rotation, and so the IMU yaw remains incorrect. That incorrect IMU yaw is what leads to uncontrolled flight and/or a switch to ATTI mode.

 

[Rickster565]

Uuummmm touchy subject,,I always take off from concrete aswell,never any issues so yeh how motors not start has me stumped like #24,I find that hard to believe myself,something else would have caused thatView attachment 119120

**** that popcorn looks good

I haven't re-calibrated my PH3 in over three years....Longer for the FC40?