Compass error demonstration

[Slim.slamma]

Keep your fingers on the sticks and scratch auto takeoff
Compass error is like return home for first time

Should be part of testing
Like how to come out of a VRS

 

[Deleted member 94047]

Keep your fingers on the sticks and scratch auto takeoff
Compass error is like return home for first time

Should be part of testing
Like how to come out of a VRS

From what I have understood, you cannot recover from a compass error through stick command. The problem is that your stick commands will be interpreted erroneously by the aircraft. Switching to ATTI mode (if available) would be the way out, and even then you have to have the time and the presence of mind to realize what is going on and react.

Also, the more I think about it, hand launching seems to be the best option to prevent this scenario. I just have to add "remove watch" to my preflight check list.

 

[Slim.slamma]

you cannot recover from a compass error through stick command.

Whatever direction the AC shoots off in pull back the opposite the direction. Maintaining control is just that simple.
Release the sticks and she will fly in that very same direction again. It’s up to the pilot to maintain control or even better yet know to switch into Atti mode

Another reason some take off in Atti mode not GPS
Hand launch it’s fool proof tho

 

[Slim.slamma]

If you knowingly take off with a compass error
Fly straight up full throttle 100” then release the stick and she will fly in one direction with no stick input
Now pull back on the stick and start to gain some control it becomes a back and forth motion. Eventually u get the AC close again.

Bridges will trip a compass error. So be quick with stick input
It’s all practice

 

[Deleted member 94047]

I am not sure if that would work. Sar explained it well here

If, for example, when the aircraft was facing south but the IMU yaw value was north, the aircraft started to drift south due to a north wind, the FC will attempt to bring it back north. But since the IMU thinks the aircraft is facing north, it will do that by applying forward thrust (negative pitch). Since the aircraft is really facing south, negative pitch will actually push it further south. The FC will respond with more negative pitch, leading to uncontrolled flight to the south.

The aircraft will enter into an uncontrollable state trying to correct itself; the more it tries, the more it gets out of control. I wouldn't bet on it that in such a state, the aircraft will even respond to pilot input. In fact, the many cases we have read here attest to the fact that aircraft becomes completely unresponsive to any stick command.

 

[Slim.slamma]

Those same people in a Vortex Ring State would probably say the same

Only forward can do the trick tho not any input

 

[Paul2660]

This post brings up a question for me from a recent flight. MP2, getting ready to fly from a remote spot in Arkansas, take off was on solid rock, limestone. On power up, I received the warning "magnetic interference", which surprised me since I have taken off from this spot many times before, without a problem. It's solid rock, no metal near by. MP2 with Smart controller, latest firmware on both.

The Magnetic interference message was in the main screen, which I call the about screen in DJIGo4.

I powered off, picked up the landing pad, (hoodman pad), double checked the ground, powered off drone, powered back on, and still had the error.

I then did a compass calibration, error cleared and I was given a clear screen in djigo4. No errors.

Did three flights, no issues, at all took off and landed from the same spot. Controls were spot on for all three flights.

So my question, is did a compass calibration clear a magnetic interference error? And what would have given the error in the first place.

The Smart controller has a compass also, as you can calibrate the smart controller just like the drone. But in this case I only calibrated the MP2. The drone had been in my car all day during the drive to the location and stored I will also store this drone in my car most of the time, not near any car speaker usually. But if the compass has been corrupted by being placed near a speaker (like a woofer with a larger magnet) wouldn't you need a demagnetizing device to fix this, not a simple compass calibration?

The only other metal object within 6 feet of me, was my tripod and camera (Arca tripod head all alloy but does have some metal screws) tripod is carbon fiber, and it was well out of close contact to the drone. I have not flown the MP2 since that day, if there are any logs on the Smart controller that might assist I can try to locate them, but so far I have attempted to view any logs from the SC.

I fully understand the importance of not flying with a Magnetic interference error, but again was surprised that a compass calibration would clear this.

Paul C

 

[offtheback]

With compass errors arising from magnetic interference one of the major causes of problems, I thought it might be useful to show a controlled demonstration of how that works.

To recap, when the aircraft is powered up the compass (3-axis magnetometer) reading is used to initialized the IMU yaw value, after which the rate gyros are the primary source of rotation data that are used to update the IMU yaw value. The compass data are only used for low-gain (gradual) corrections for drift and bias in the rate gyro data. As a result, it's important that the IMU yaw is correctly initialized for the FC to know the direction that the aircraft is facing. If the yaw value is substantially incorrect then the FC will apply inappropriate corrections to try to hold position.

In this test, shown in the graph below, I used a small, rare-earth magnet to reverse the magnetic field at the M2P compass (it has only one), so that the compass measured the aircraft heading as north when it was really facing south. The magnet was just a few inches from the aircraft. On power up, the IMU yaw was incorrectly initialized at -46 seconds as approximately north, as expected. At -38 seconds I removed the magnet and, with the spurious magnetic field removed, the compass correctly changed to south. That would normally happen naturally as the aircraft took off and left the (small) area of magnetic interference.

At zero seconds I started the motors to generate a txt log file and establish which IMU was active. The txt OSD_yaw value is identical to the IMU1 yaw value, confirming IMU1 was active.

Note that although the compass and IMU yaw values disagree by around 180°, the IMU yaw value is held steady - that's because the rate gyros haven't detected any rotation. The aircraft reports a compass error but doesn't attempt to resolve the problem. At 33 seconds I rotated the aircraft to face north, as shown correctly by the compass. The rate gyros also detect the 180° rotation and, as a result change the yaw value by that amount to approximately south, even though it's now facing north.


View attachment 73554

As you might expect, this kind of discrepancy would cause serious problems in any of the P modes, such as P-GPS. If, for example, when the aircraft was facing south but the IMU yaw value was north, the aircraft started to drift south due to a north wind, the FC will attempt to bring it back north. But since the IMU thinks the aircraft is facing north, it will do that by applying forward thrust (negative pitch). Since the aircraft is really facing south, negative pitch will actually push it further south. The FC will respond with more negative pitch, leading to uncontrolled flight to the south.

I'm going to repeat this test with an actual takeoff, but that requires a large flat field so that I can switch the aircraft to ATTI mode once it starts to fly off without worrying about obstacles. I'll post the results of that later. In case anyone else is tempted to try this, do not attempt unless you have ATTI mode programmed as one of the mode switch positions for when the flight goes bad.

A great explanation!Easily would have gotten 1st prize at the science fair?

 

[RadioFlyerMan]

The aircraft often won't show a compass error while on the ground if the magnetic field is within tolerance. The compass error occurs after takeoff. In this experiment I got the typical "move or calibrate" message after removing the magnet but, unlike the time/distance calibration request that the M2 gives, it did let me start the motors.

While stationary on the ground during boot-up, what inputs and algorithms are used to determine if the magnetic field is “within tolerance”? It seems that the net magnetic field (earth + anomaly) is all that could be detected.

 

[sar104]

This post brings up a question for me from a recent flight. MP2, getting ready to fly from a remote spot in Arkansas, take off was on solid rock, limestone. On power up, I received the warning "magnetic interference", which surprised me since I have taken off from this spot many times before, without a problem. It's solid rock, no metal near by. MP2 with Smart controller, latest firmware on both.

The Magnetic interference message was in the main screen, which I call the about screen in DJIGo4.

I powered off, picked up the landing pad, (hoodman pad), double checked the ground, powered off drone, powered back on, and still had the error.

I then did a compass calibration, error cleared and I was given a clear screen in djigo4. No errors.

Did three flights, no issues, at all took off and landed from the same spot. Controls were spot on for all three flights.

So my question, is did a compass calibration clear a magnetic interference error? And what would have given the error in the first place.

The Smart controller has a compass also, as you can calibrate the smart controller just like the drone. But in this case I only calibrated the MP2. The drone had been in my car all day during the drive to the location and stored I will also store this drone in my car most of the time, not near any car speaker usually. But if the compass has been corrupted by being placed near a speaker (like a woofer with a larger magnet) wouldn't you need a demagnetizing device to fix this, not a simple compass calibration?

The only other metal object within 6 feet of me, was my tripod and camera (Arca tripod head all alloy but does have some metal screws) tripod is carbon fiber, and it was well out of close contact to the drone. I have not flown the MP2 since that day, if there are any logs on the Smart controller that might assist I can try to locate them, but so far I have attempted to view any logs from the SC.

I fully understand the importance of not flying with a Magnetic interference error, but again was surprised that a compass calibration would clear this.

Paul C

That error was most likely just a request for calibration that the M2P firmware requires every 30 days or if it has moved more than 50 km from the last flight, rather than actual magnetic interference. That request is, unfortunately, worded just like the actual magnetic interference warning. You can tell the difference in two ways. Firstly, look at the sensors»compass interference level, which will be low if there isn't actually any interference. Secondly, if it is the recalibration request then the motors will not start, whereas with actual magnetic interference they will start. You can also pull the mobile device DAT file for that power up cycle and the event stream will show exactly what was happening.

 

[sar104]

While stationary on the ground during boot-up, what inputs and algorithms are used to determine if the magnetic field is “within tolerance”? It seems that the net magnetic field (earth + anomaly) is all that could be detected.

I don't know exactly, but there are a couple of things it could be checking. Since it knows its location (via GPS) and has a global magnetic field model in the firmware, it knows approximately the strength and inclination of the earth's magnetic field expected at that location. If the measured field differs too much in either strength or inclination that expectation then it might flag that as interference.

 

[sar104]

Whatever direction the AC shoots off in pull back the opposite the direction. Maintaining control is just that simple.
Release the sticks and she will fly in that very same direction again. It’s up to the pilot to maintain control or even better yet know to switch into Atti mode

Another reason some take off in Atti mode not GPS
Hand launch it’s fool proof tho

The aircraft will enter into an uncontrollable state trying to correct itself; the more it tries, the more it gets out of control. I wouldn't bet on it that in such a state, the aircraft will even respond to pilot input. In fact, the many cases we have read here attest to the fact that aircraft becomes completely unresponsive to any stick command.

The problem with P-GPS mode is that, unlike in ATTI mode, the FC is not interpreting elevator and aileron commands as purely pitch and roll instructions - it is interpreting them as instructions to fly forwards/backwards and right/left relative to the direction that the aircraft is pointing. It will start by applying simple pitch/roll but then correct as needed to make the aircraft fly a track that equals its assumed heading. So the problem is the same as positional holding - if you push the stick forwards and the aircraft applies negative pitch, but then detects (incorrectly) that it is moving backwards, then it will keep increasing negative pitch to try to compensate - another runaway positive feedback loop.

 

[RadioFlyerMan]

I don't know exactly, but there are a couple of things it could be checking. Since it knows its location (via GPS) and has a global magnetic field model in the firmware, it knows approximately the strength and inclination of the earth's magnetic field expected at that location. If the measured field differs too much in either strength or inclination that expectation then it might flag that as interference.

I think this whole topic and test is an excellent endeavor. And I like the idea of using an external handheld compass to detect a local ground level influence. It only takes a few seconds.

 

[sar104]

I think this whole topic and test is an excellent endeavor. And I like the idea of using an external handheld compass to detect a local ground level influence. It only takes a few seconds.

The compass is a useful tool in this situation, but the best test to ensure that there is not magnetic interference that will affect the flight is just to check that the aircraft orientation arrow on the map is actually pointing in the same direction, relative to north, as the aircraft on the ground. If those match then the IMU yaw has been correctly initialized.

 

[pmshop]

Sparks "auto correct" themselves when magnetic interference is removed.
IE compass calibration error shows when interference is nearby. But move the Spark and away from the interference and all turns green without recalibrating.

Can this happen with the Mavic Air?
I ask because in my experiance, the only way to clear the same error on the Mavic Air is to move location and actually perform a calibration or move and reboot the system.

 

[pmshop]

If you knowingly take off with a compass error

How can you do that with a Mavic Air?
Mine is grounded until I clear the error - motors will not start

 

[sar104]

Sparks "auto correct" themselves when magnetic interference is removed.
IE compass calibration error shows when interference is nearby. But move the Spark and away from the interference and all turns green without recalibrating.

Can this happen with the Mavic Air?
I ask because in my experiance, the only way to clear the same error on the Mavic Air is to move location and actually perform a calibration or move and reboot the system.

Lack of measured interference as shown in the app does not mean that the IMU yaw was correctly initialized. I've looked at a number of cases where Mavic Airs were launched from magnetically distorted locations and, in every case, even once it climbed out of the distorted field it did not correct the IMU yaw.

 

[sar104]

How can you do that with a Mavic Air?
Mine is grounded until I clear the error - motors will not start

There are two quite separate possible cases here. The IMU yaw can be incorrectly initialized without the FC detecting the magnetic interference, in which case the compass errors will not appear until it is flying. If it does detect magnetic interference then it will show an error on the ground. The MP and M2 firmware will still allow motor start in that situation, so I'm surprised that the MA will not. Are you sure that those magnetic interference messages are not just recalibration requests in disguise - as seen on the M2? A DAT file would show what is really happening.

 

[Slim.slamma]

The problem with P-GPS mode is that, unlike in ATTI mode, the FC is not interpreting elevator and aileron commands as purely pitch and roll instructions - it is interpreting them as instructions to fly forwards/backwards and right/left relative to the direction that the aircraft is pointing. It will start by applying simple pitch/roll but then correct as needed to make the aircraft fly a track that equals its assumed heading. So the problem is the same as positional holding - if you push the stick forwards and the aircraft applies negative pitch, but then detects (incorrectly) that it is moving backwards, then it will keep increasing negative pitch to try to compensate - another runaway positive feedback loop.

Ok my apologies, I don’t own a Mavic but I know how my phantom standard and i1 reacted in these exact same locations, pix below. With a compass error. I was totally wrong for flying in these areas but I clearly remember how my drones reacted after take off. I had to walk out of the block to keep visual. If she got far enough away without knowing the orientation of the drone it’s over.
The drone always shot forward in a east direction straight up the block.

The reason I joined the forum in January 2015

A few months back with my I2 in those exact same location take off was impossible. Now I get solid green. Go figure

4 Dji platforms all reacting

 

[Slim.slamma]

d, in every case, even once it climbed out of the distorted field it did not correct the IMU yaw

Bam that’s what I’m saying

That’s was the only way to correct. The Mavic series must be different