Looking for Trouble ??

[Dronason]

@Fifty7 For this kind of flight my advice is switch to ATTI before motorStart and leave it there. That's what I'd do. Whatever you do, DON'T RTH and disable Smart RTH so that the A/C doesn't initiate RTH on it's own.

It's not just distance to the ferrous material that matters. It's also the location of the A/C relative to the ferrous material and the direction of the geomagnetic flux. It's not just magnitude. The ferrous material causes a distortion of the geomagnetic flux. An analogy is trying to cross a smooth river in a straight line. A constant thrust angle will get you from one side to the other in a straight line. Throw in a few large boulders, the flow becomes distorted and the thrust angle depends on your location in the stream. But, it's not just the proximity to the boulders.

Here is a flight where it wasn't just distance.The altitude was 30 meters when a geomagnetic distortion was encountered.
View attachment 63358

The direction computed from the magnetometers became different from what the gyro was saying and a compass error was declared (not shown is the switch to ATTI)
View attachment 63359

This all happened in a location very close to the previous path of the P3 with no adverse effects
View attachment 63360

Many, if not most, of the compass error flyaways occur because the gyros have said one thing and the magnetometers have said another. But, the problem can be that the FC's idea of heading can now be incorrect and an RTH will cause the A/C to fly in the wrong direction.

On the question concerning the use of the compass in ATTI mode I agree with @Vertigo,,sort of. In the first place the magnetometers aren't the primary source of heading info. After motorStart it's mostly the gyros. But, the real question is, without GPS info what would any kind of heading (gyros or magnetometers) info be used for. The only situation where heading info is used in ATTI mode is in an RTH in which case the P3 will fly the gyros, not the magnetometers

Fully agree with you, apart at the end where I think that what is called "Gyroscope" in the Go app is not to give direction, it is to give angular rate of change (°/sec). When viewing the sensor, the values are normally all zero, they change when you do a rotation and go back to zero when the rotation is done. There would be way to integrate that speed of rotation to get the absolute rotation, but this would be inaccurate. This Gyroscope is similar to this sensor LSM330DLC - iNEMO inertial module: 3D accelerometer and 3D gyroscope - STMicroelectronics by example. This is to help the flight controller in stabilizing the P3. It is not giving you the absolute direction but only how fast it is rotating.
The GPS is able to give direction if the speed is sufficient but not when it is hovering.

 

[BudWalker]

Fully agree with you, apart at the end where I think that what is called "Gyroscope" in the Go app is not to give direction, it is to give angular rate of change (°/sec). When viewing the sensor, the values are normally all zero, they change when you do a rotation and go back to zero when the rotation is done. There would be way to integrate that speed of rotation to get the absolute rotation, but this would be inaccurate. This Gyroscope is similar to this sensor LSM330DLC - iNEMO inertial module: 3D accelerometer and 3D gyroscope - STMicroelectronics by example. This is to help the flight controller in stabilizing the P3. It is not giving you the absolute direction but only how fast it is rotating.
The GPS is able to give direction if the speed is sufficient but not when it is hovering.

When I say "fly the gyros" it's just a short way to say that the gyros provide input to the FC which then computes the heading by integrating and summing those inputs. It's actually a whole lot more complicated than that.

You might be interested in the totalGyroZ field in the .csv that DatCon produces. It's the integration and summation of the gyroZ value. By looking at totalGyroZ when the P3 is absolutely stationary totalGyroZ can be used to determine the drift rate of gyroZ.

Not sure what the reference to the Go App is about. The red triangle you see comes from Yaw which, in turn, is the FC's value for heading.

 

[Fifty7]

It's actually a whole lot more complicated than that.

That's probably the only part of the last couple of posts that I understand [emoji4]. Atti mode is not a problem I can do that. I hardly ever use RTH and I would not be that far from my takeoff point in this case so I would have no reason use it here. I will disable smart RTH just in case and will always be within LOS so I won't need the screen to determine orientation. Thanks everyone for all the advice. Will let you know how it turns out.



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[Dronason]

When I say "fly the gyros" it's just a short way to say that the gyros provide input to the FC which then computes the heading by integrating and summing those inputs. It's actually a whole lot more complicated than that.

You might be interested in the totalGyroZ field in the .csv that DatCon produces. It's the integration and summation of the gyroZ value. By looking at totalGyroZ when the P3 is absolutely stationary totalGyroZ can be used to determine the drift rate of gyroZ.

Not sure what the reference to the Go App is about. The red triangle you see comes from Yaw which, in turn, is the FC's value for heading.

I will have a look to this totalGyroZ field. That's interesting but currently as I updated GO, I cannot see my .txt flight record with CsvView, only the .DAT and it is a pain to work with them.

 

[BudWalker]

I will have a look to this totalGyroZ field. That's interesting but currently as I updated GO, I cannot see my .txt flight record with CsvView, only the .DAT and it is a pain to work with them.

Yeah, I know the .DAT can be inconvenient; I've done a lot of it Unfortunately, totalGyroZ exists only in the .csv created by DatCon. This is because totalGyroZ is derived from gyro info which doesn't exist in the .txt.

 

[WetDog]

No, if you're taking off with a compass error a lot of different things can happen, so bring it down as soon as possible. Try taking off from a different location first, like in front of the house. If you are still getting a compass error then you may have a bad compass. Compass errors have nothing to do with calibrating the compass. It usually means there's too much metal around but not always. Let us know if you do get a clean start up somewhere else.

Here is the problem with 'compass error on takeoff': If the error is because there is too much ferrous material around so that the compass is getting all wigged out and then you pop up in ATTI mode (which is fine) and then the effect of the metal goes away (remember it does that with the square of the distance so the effect falls off rapidly) then you are fine.

If you actually have an internal problem and you take off, bad things can happen quickly. You can't tell from the start.

I suppose if you have flown in a specific place, always get the error and otherwise the craft flies fine, then there is minimal risk. What I would not do is ignore the warning in a new area, after any sort of crash or 'upgrade'.

 

[BudWalker]

Here is the problem with 'compass error on takeoff': If the error is because there is too much ferrous material around so that the compass is getting all wigged out and then you pop up in ATTI mode (which is fine) and then the effect of the metal goes away (remember it does that with the square of the distance so the effect falls off rapidly) then you are fine.
...
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Actually, this can also be a problem. The P3 gets it's heading info from a combination of sensors; mostly the gyros, then the accelerometers. Shortly after motorStart the Flight Controller initializes the Yaw value based on the compass magnetometers but after that it is info coming from the gyros and accelerometers that determine Yaw. The magnetometers have only a small effect on the value of Yaw.

If the P3 launches from a man hole cover that initial Yaw value will be incorrect because of the geomagnetic distortion and it's effect on the magnetometers. When the P3 clears the effects of the man hole cover the compass/magnetometers become correct, BUT the Yaw is being determined by the gyros which hasn't changed.

Here's an example of where that happened

The P3 was launched from a concrete driveway that contains rebar. magYaw is a diagnostic value calculated from the magnetometers. magYaw and Yaw (the value that the P3 uses for heading) are the same incorrect value until the P3 launches at time 7.4. Then magYaw (blue line) changes to -162, the correct value, BUT Yaw stays at the old, incorrect, value of -27. You can also see the effect on the magnetometers (red line) as the P3 leaves the effects of the rebar. This is the reason that a COMPASS_ERROR_LARGE is declared (denoted by the blue background).

Later, at time 71 the COMPASS_ERROR_LARGE had been replaced with a YAW_ERROR_LARGE

The pilot initiates RTH shown off-blue background but the P3 still has incorrect Yaw info. The P3 then towards what it thinks is home. It was 2 minutes before the Yaw was finally corrected.

 

[Vertigo]

But, the real question is, without GPS info what would any kind of heading (gyros or magnetometers) info be used for. The only situation where heading info is used in ATTI mode is in an RTH in which case the P3 will fly the gyros, not the magnetometers

the magnetometer can be used to maintain a constant heading, which is not only useful when flying GPS or RTH, but for instance, just to keep it stationary or fly along a straight line. Wind and other effects will cause the quad to yaw. Gyro's will register this and the quad can counter it, but over time the heading will drift. Just fly a racing quad, keep it hovering in the wind. Over time it will slowly yaw in some random direction. Its not like you cant control it, but it does require some corrections now and then.

So if you have good compass data, its not a bad idea to actually use it, even without GPS. I think the real question is why the Phantom uses known bad compass data, if that is what it does.

 

[BudWalker]

...
So if you have good compass data, its not a bad idea to actually use it, even without GPS. I think the real question is why the Phantom uses known bad compass data, if that is what it does.

In the example given in the post immediately preceding your post the bad compass data occurs before launch and a few seconds after launch. This is used to set Yaw before launch. It's only after launch that the error is detected. So the P3 has used the bad compass data before it could know it's bad. I.e., it isn't using "known bad compass data".

After detecting the compass error the P3 could then set it's Yaw based on the new compass data. But, all the P3 can know is the compass data changed when it shouldn't have (because the gyros are saying there hasn't been a change). It can't know that the new compass data is correct. Since the P3 knows the compass data could be bad it switches to ATTI mode where the compass data isn't used. At this point the P3 isn't "using known bad compass data". Instead, it's not using compass data that could be bad.

 

[Juergen B]

Did you ever complete your flight there? Would love to see the pics... Are these the concrete towers in Staten Island? If you ever fly there and want some company / spotter / P4 pilot company, DM to let me know...


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[Fifty7]

Did you ever complete your flight there? Would love to see the pics... Are these the concrete towers in Staten Island? If you ever fly there and want some company / spotter / P4 pilot company, DM to let me know...


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No I did not. My work and home schedules have not cooperated for me to get there. Funny part is I work on industrial loop which is less than a mile from there. Where do you fly on the island (or nearby) without any trouble. Parks are a no no. I recently went to the Conference House beach and flew across the water to NJ.


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[Juergen B]

No I did not. My work and home schedules have not cooperated for me to get there. Funny part is I work on industrial loop which is less than a mile from there. Where do you fly on the island (or nearby) without any trouble. Parks are a no no. I recently went to the Conference House beach and flew across the water to NJ.

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I'm from SI but now live on the NJ side, sometime fly out of parks

or from the shore near Perth Amboy and there are a range of parks and spots along the shore.


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[Fifty7]

Cool. I'm heading up to PA this weekend and how to find some good scenery


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[Juergen B]

Cool. I'm heading up to PA this weekend and how to find some good scenery


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Yeah, have fun and post some pics after! My friends and I sometimes ride Motorbikes along the Delaware river on RT 32 near Frenchtown and New Hope is wonderful scenery along there, taking my drone along there soon!


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[Fifty7]

Did you ever complete your flight there? Would love to see the pics... Are these the concrete towers in Staten Island? If you ever fly there and want some company / spotter / P4 pilot company, DM to let me know...


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So I still haven't gotten to the spot where I planned my takeoff but yesterday I took off from my office. Didn't think I'd get very far without interference because I'm in a congested industrial area but i made it to the tanks without a problem.

 

[Juergen B]

Great capture!

 

[Mydrone70]

Yeah, have fun and post some pics after! My friends and I sometimes ride Motorbikes along the Delaware river on RT 32 near Frenchtown and New Hope is wonderful scenery along there, taking my drone along there soon!


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that's a great place to shoot some scenery used to live up that way

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[John Locke]

Hey Fifty7, if those tanks are going to be demolished for a new mall, that's a great opportunity to do a time phased video. With Litchi, setup a flight path that you fly over and over, identical flights through the next year as the mall is being built. After the mall is done, edit short sequential clips from each flights into single video that shows BEFORE footage in the beginning of the video, and as you fly around, the end of the video shows AFTER footage, showing the finished mall. Time phased videos are intriguing. Yes, it's a lot of work, but it will be epic in the end. Here's an example below of a "day to night" time phased video using Litchi. In your case it would be month to month time phasing. I wish I lived in the East so I could do a "Season to Season" time phased video with tree color, complete with snow.

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[Paul A Byers]

I've read a lot about compass issues when taking off or calibrating around metal objects. I want to fly in the area pictured below to get some pictures of the Gas tanks before they are demolished (to make room for a shopping mall. I know they are made of concrete & metal so my question is, am I looking for compass trouble or should i be ok as long as I'm not taking off from them? I would be taking off from about where the picture was taken so range is not an issue. I see videos of people chasing cruise ships so i want to think this should be fine but don't want to assume anything.

View attachment 63247

I needed advice calibrating near water and someone shared the post below with me. You may find it helpful:

http://www.phantompilots.com/threads/compass-calibration-a-complete-primer.32829

 

[Out & Back]

57, I've flown over them a few times. Never a problem. I launched from the Lowes parking lot early on a Sunday morning and flew over the tanks, out to the landfill and back along the Kill Van Kull.