Toliet Bowl GPS hover fix, fixed mine, now rock steady

Hi everyone, here is an interesting thing I have had my phantom for many months now and putting up with toilet bowl effect, I complained sent many emails and looked on the web when I suddenly realized my Compass was 90 deg out in other words it was facing the direction of flight no across it. Now I assembled this straight out of the box and it is.obviously mounted on the wrong side of the leg ,so I unmounted both legs swapped the compass over remounted the legs and test flew and bingo rock steadi.
It had been glaring me in the face for so many months and I didn't realize it. Johnno.
 
bumper said:
denodan said:
Anyone having trouble with drifting in GPS, or toilet bowl effect? Well out of the box my was bad, and looking around the internet, it seems there is a simple fix.


Is this fix from DJI? Or is it an experiment posted on a forum? (Not saying such research isn't often accurate.)

Reason I ask is the fix doesn't make sense to me*. When you calibrate the compass, as indicated by the green light going out, you have just adjusted for the local conditions including inclination and declination. If the compass won't calibrate, you can use the "magnet trick", as shown on one of DJI's videos, to "degauss" the flux sensor.

*Consider for a moment that the Phantom, when hovering, only knows what direction it is pointing based on the compass. So if rotating the compass X number of degrees, that only alters what direction the Phantom thinks it's pointing by that same number of degrees. That would have no effect on toilet bowl effect. Toilet bowl effect is doubtless caused by the normal GPS dithering inaccuracy, which is typically on the order of several meters.

bumper
Actually, If the compass N doesn't match the direction the GPS thinks is north, you will get this toilet bowl effect. I would have thought that compass cal took care of this, but if not, then the procedure recommended should help. I once accidentally mounted my compass 180 degrees off (put it on the wrong leg) and that was not good - when the GPS detected I drifted a particular direction, say north, it would compensate by moving the craft in the opposite direction as indicated by the compass. Since the compass was backwards, the Phantom accelerated in the direction of the initial drift rather than correcting the drift. So if the compass is off angle then when you drift N, it will compensate by moving it S according to the compass but actually go S-SE (for example). That eastward movement will be detected and it will try to compensate by moving what it thinks is W, but will actually move W-SW and so forth. Hence you get toilet bowl.
 
This is a very confusing thread! Sorry to comment, but the compass is mounted properly from the factory and does not need any further physical adjustment! If you complete the compass dance and IMU calibration as per DJI documentation, you should be good to go!
 
sloflyer said:
The originator of this thread, denodan, is correct. Page 29 of the Naza manual shows rotation of the (combined gps/compass ) module to correct TBE. http://team-blacksheep.freshdesk.com/helpdesk/attachments/12145075

But it was also pointed out in this thread that this issues was corrected with a March 2013 firmware update, so that now the PV2 reconciles true north vs magnetic north based on gps location, adjusting automatically for local magnetic declination.

Can anyone confirm this is correct?

I am in Alaska and the local mag declination is 17° 49' EAST. I have some TBE counterclockwise after take off when hovering, but it seems to improve a little after flying around, however the left drift on flying forward doesn't seem to improve, it is about a 5% drift angle, maybe less. Compass and advanced calibrations have not resolved this. No history of crashes or prop damage. Some posts talked about rotating the compass on the leg, but my pv2 doesn't seem to allow that, the compass is flat and up against the flat open side of the leg, only the clamp side is round.

Also, is the iPhone display compass heading showing magnetic north or true north? Knowing this for sure would help me figure out if my PV2 is confused about north. My search has turned up some conflicting answers to this question. Mine seems to show true north.

Thanks guys.
 
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I am in Alaska and the local mag declination is 17° 49' EAST. I have some TBE counterclockwise after take off when hovering, but it seems to improve a little after flying around, however the left drift on flying forward doesn't seem to improve, it is about a 5% drift angle, maybe less. Compass and advanced calibrations have not resolved this.

This is very late, there are other threads about this as well, but the issue is true, and there is a lot of information and also wrong assumption, even in this thread. I know many old assumptions are old, but there is the same technology behind it and it conceals bits of information that is very interesting for a nerd like myself, and also good to understand in order to understand the operation of symmetrical craft's GPS-based flying.

I got my first drone just in the autumn 2016, a Phantom 1. Before that no real RC stuff (but have to mention my early 80's indoors 27 MHz car with servo steering by Asahi Corporation, I still got it and going to get it going one day). Since the autumn I've learned a lot, and I want to share the things I've learned. This thread is not the perfect place for it, but it's relevant, so I just type something here first.

Just yesterday I may have found out what an earth is the use for "GPS offset X Y Z relative to center of gravity" in Naza Assist. It is not for the GPS, it is not for the Naza-M, it is for the COMPASS!!! (This is not verified yet, but I'll try to verify it during this week). Later models incorporate the compass in the GPS puck, so it's easy to talk about the compass as the "GPS".

Assumption fix: "GPS doesn't need the compass when..." or "GPS only needs the compass when..." Wrong! GPS *needs the compass*, all the time, regardless of the control mode or hover/full speed. The only option to go without compass is to fly in ATTI mode (no GPS). So if you are losing your drone over TBE, switch the GPS off.

Why GPS flying needs the compass? A real world heading is needed for the "steering" towards wanted GPS location. Simple as that.
Why not using the GPS course (EDIT: not heading) when moving? Because it's too slow. Magnetometer gives the heading dozens of times per second (I don't know the implemented rate in Naza though, but the one magnetometer really used in Phantoms goes way beyond 100 Hz if needed), regardless of the speed / movement of the drone.

Assumption fix: "Compass calibration / the dance calibrates magnetic north..." Nope. By the calibration dance the min/max of X/Y/Z of the magnetometer is stored, so the readings can be offset/scaled properly to give consistent readings where ever the nose points. Too bad the Assistant does not calculate the heading for us, it would be interesting to see it live.
Plain dance has fixed hovering TBE for me. However:

Drift to left/right when flying: magnetic declination problem (found it on many threads). I live in Finland and on my location the declination is 10 degrees east. I have rotated the compass clockwise quite a bit and it's better now. Not flying straight always, but I got it flying even more to the left when first turning the compass counterclockwise.

I'm messy as always, but here is the chain of dependencies:
IMU -> magnetometer -> GPS
Going back:
1. GPS needs compass.
2. Using the magnetometer as a compass we need to know the attitude of the magnetometer. The magnetometer is 3-axis for reason: we don't get any true heading before the attitude is used to rotate the XYZ values.

Cheers.
 
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Yea. You have a good understanding.

However in your 5th paragraph you mentioned 'GPS heading is too slow'.

Actually GPS cannot provide Heading only Course. These are two very separate items.
 
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Actually GPS cannot provide Heading only Course. These are two very separate items.

I may be off with terminology, but back then when I examined my SirfStar III BT GPS module output NMEA sentences, I saw that while moving, the direction of movement (angle, bearing?) was reported by the GPS unit itself, once per second according to the update rate. That's what I meant. It doesn't of course relate to the the nose direction, so using it for real would be slightly like hit and miss. Doable, but tricky.

EDIT: Forgot to say THANKS! And that I will learn. It's easier to talk with correct terminology :)
 
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I tried to verify the purpose of the GPS offset X Y Z. I dialied in the compass (in the leg) offset, paying attention to the negative/positive values. The result: did not get off the ground, tipped on its nose. I admit I was easy on the thottle, but did my best to see how it begins the lift off. I didn't want to compensate by pulling back, because it could have gone airborne AND out of control. Did the lift off attempt a few times with different settings - even a Z parameter only, then gave up and sanded the blade edges back in shape (the gravel mixture on our front yard was moist and really dense). All offset parameters back to zero, one test flight - works ok. Now what the heck is the purpose of the offset?
 

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