Please select Into the mobile phone version | Continue to access the computer ver.
Explain Compass and IMU purpose ...
3819 35 2015-7-6
Uploading and Loding Picture ...(0/1)
o(^-^)o
w1der
Second Officer

Sweden
Offline

Hey gang ...

Can some one explain to me what the COMPASS and IMU actually does for the Inspire 1 and what would happen (worst case scenario) if these calibrations wasn´t done right.

PLEASE ... if your are only second guessing or if you only read what some one else was second guessing ... do not answer!
I want answers from the ones that actually knows what they are talking about and can explain it using facts ...
2015-7-6
Use props
w1der
Second Officer

Sweden
Offline

No one !?

[irony]
But we are sure that the "fly aways" are because of bad compass and or IMU calibration !?
[/irony]

OK ... I will try my luck getting this explanation on a regular RC forum where people build this stuff from scratch on their own ...

Peace out !
2015-7-7
Use props
Farnk666
Captain
Flight distance : 1711394 ft
Australia
Offline

Compass determines direction, AC orientation and heading - IMU determines / senses attitude in 3 dimensions
Both supply data to the flight controller (along with GPS) which enables it to balance thrust from each motor/esc combination for the desired flight mode (Hover / Turn / Directional flight/etc) and know which direction it is facing or travelling.

Therefore, sensor calibration within appropriate limits of both IMU and Compass is needed for the craft to fly consistently and in a controlled manner.
If the IMU data is inaccurate, the craft will not be able to balance thrust and be stable in flight nor hold level.
Some of the immediate flyaways upon takeoff are definitely IMU failures or calibration problems.
2015-7-7
Use props
Kalle
New

Germany
Offline


Here is a good declaration about compass

http://www.phantompilots.com/thr ... plete-primer.32829/

and IMU



2015-7-7
Use props
Kalle
New

Germany
Offline


It is particularly important that when it comes to a new place on the grid to a compass error, you must perform a new compass calibration under any circumstances at this location, even if thereafter the compass error no longer appears and everything looks normal. Because the reason for the previous compass error was certainly a source of interference near metal in concrete, bridges, railings, starting from the car roof, and so on !

If you re-calibrated before every flight, there is a danger that you already calibrated interference of the new environment and this could lead in the worst case to a FlyAway.

So when occurring compass error, most seek a place nearby and then on again the copter and stand at no more existing compass error.
2015-7-7
Use props
w1der
Second Officer

Sweden
Offline

Thanks for the Link Kalle ...
Some valuable info about when and not to do a compass calibration there!

I get that the IMU calibration is important for the bird to be able to maintain a level hover.
And a bad IMU setting could create a "tilt" causing the bird to drift/"fly away".
On the cheap "toy quads" you have extra buttons to trim this manually ... so ... this makes perfect sense to me!
If IMU was bad and the bird started tilting ... I should be able to correct this with stick input (like I do on the "toy quad") ... but for some reason this doesn´t work with the reported fly aways that we see ...
Or is this just because of lack of experience of the pilots?

And ...
I still don´t understand how a bad compass calibration could cause a "fly away".
Even if the compass was bad ... as long as the IMU is good ... and you let go of the sticks ... the I1 should just hold it´s position ... or am I missing something here?
"Forward" stick input always makes the Inspire go where it´s nose is pointing no matter what compass reading you have!?  



2015-7-8
Use props
Kalle
New

Germany
Offline

The compass is very sensitive to electromagnetic interference like an metall table or other magnet field, which can cause abnormal compass data leading to poor flight performance or even flight failure.





                                                                               
www.crashreport.net

CrashReport - Investigation of malfunctions

                                                                       

2015-7-8
Use props
w1der
Second Officer

Sweden
Offline

Kalle Posted at 2015-7-8 16:42
The compass is very sensitive to electromagnetic interference like an metall table or other magnet f ...

This is just making me more confused!
According to the info in your previous link ...
As long as you do compass calibration far away from metallic object your fine ... so taking of from the "man hole" should not be a problem really ... !?

So compass and GPS is in correlation here (it´s the GPS that uses the compass?) ... and if you have a bad compass calibration ... switching to atti mode would (in theory) solve the problem ?
2015-7-8
Use props
Kalle
New

Germany
Offline

Yes ... in many cases it is helpful to switch in atti mode and I think this is the first and the best way that can you go in this situation. If the copter have contact again with atti, please remember that your copter than go with the wind, but the good thing is .... you have the control again.

If this did not help switch to RTH that is your last chance to save your copter.



www.crashreport.net
CrashReport - Investigation of malfunctions
2015-7-8
Use props
Farnk666
Captain
Flight distance : 1711394 ft
Australia
Offline

I don't believe that a bad compass cal would cause a fly away by itself - but could possibly create issues when the pilot applied control inputs.
I would assume that good IMU but bad Compass cals should see the UAV able to hover, but be erratic in flight.

A bad IMU would see drift happening immediately on launch - the degree of which could see the UAV head off by itself at speed, depending on how far out of range the data is.
This could potentially create a situation where the maximum stick inputs would not be enough to compensate for the drift - leading to a 'fly away' situation.
The other significant factor will be pilot panic / response as the UAV enters a uncontrolled state - this will be why DJI asks for the flight logs when people report issues - these will contain all the raw data from the onboard sensors and control inputs.

Having said all that - there would be some functionality in the FC firmware to catch this sort of thing and report errors.  
2015-7-8
Use props
w1der
Second Officer

Sweden
Offline

Kalle Posted at 2015-7-8 17:22
Yes ... in many cases it is helpful to switch in atti mode and I think this is the first and the bes ...

RTH uses the GPS to find it´s way home ... so that wont work with a bad compass I guess ...  
I don´t like guessing games!
I was looking for answers with "facts" explaining this so that I could understand this better ...

Thank´s for the links though ... I appreciate it!


2015-7-8
Use props
Kalle
New

Germany
Offline

@Farnk666

First .... here a lot off inspire 1 flyways without pilot errors, you can see the stick moves

http://www.crashreport.net/forum ... e-1-flyaway-videos/
2015-7-8
Use props
Kalle
New

Germany
Offline

The compass information into account in the FC in a sort of averaging routine and correct so that the long-term stability of the gyros, which have long-term drift. This correction is carried out continuously, but is particularly noticeable in a rotation of the copter noticeable because change here the compass readings.

Provides the compass comical values, which can lead to a crash or Flyaway because the FC determined implausible position values.

I hope you anderstand my bad english speaking ... writing

www.crashreport.net
CrashReport - Investigation of malfunctions
2015-7-8
Use props
jon
lvl.4

United Kingdom
Offline

Farnk666 Posted at 2015-7-8 17:38
I don't believe that a bad compass cal would cause a fly away by itself - but could possibly create  ...

Good post, very logical. Especially the bit about whether control stick input would always be enough to correct IMU 'error'.

Often its reported that the controls appear reversed, I don't think they are, I think it's just that neutral sticks = xxx mph rather than stationary, and that messes with the pilots mind, the few seconds available to quantify the scenario, and make the necessary inputs to gain control are often not enough, and the bird contacts 'something' before the learning curve has even begun.
PS. That flyaway vid in Germany is interesting, the RTH appears to take a long time to get control of things. You can hear him initiate it early on, but it continues to fly away at speed.  The couple of times I've deliberately initiated RTH it has begun the process of climbing to SA and returning pretty much immediately.

I too don't get the compass cal thing. I understand that calibrating it close to metalic, or magnetic things would be bad, as the calibration would be corrupted, but, if the calibration was done in a safe place (previously) and you just chose to take off from the cast iron drain cover, the saved calibration should work just fine, as soon as the bird is clear of the metal item interfering with it.  Again, I don't get it..
2015-7-8
Use props
Skywolf007
Second Officer
Flight distance : 2758875 ft
Slovakia
Offline

Farnk666 Posted at 2015-7-8 17:38
I don't believe that a bad compass cal would cause a fly away by itself - but could possibly create  ...


don't only think about it in a simple way...
IMU + Compass + GPS is working together. Its a very complicated system and the bird is reading all the information at once, and trying to average it.
The IMU main advantage is his disadvantage too. The IMU is constantly calculating changes, and adding them to previous recorded position. So basically it has one fix point and as soon as you are moving it will calculate the direction, speed etc parameters and adds it to a previously recorded position, creating a new position, speed heading etc..
These calculations are made multiple times/second.
Now about compass  error and flyaway - as soon as the compass gets an error or is not calibrated well the IMU will have bad data to work with, so it cannot produce  the correct information for the motors, therefore the bird will make a fly-away.

I'm not a DJI technician so I cant know how often does the IMU compares data to GPS or how the GPS/IMU data is connected, but I guess the refresh rate is in second/s. That's why if you get a compass error the bird flies away but it will stop the drift after second.
2015-7-8
Use props
Kalle
New

Germany
Offline

I have my inspire calibrated once and then never again. I have start from bridges and in near from magnetic fields alway without problems. I think you have only problems if you calibrate on this unsafe places.

Important is IMU and compass calibration after a new update, then no longer. This is my own experience.  


www.crashreport.net
CrashReport - Investigation of malfunctions
2015-7-8
Use props
PeteGould
Second Officer

United States
Offline

I've been offline for awhile so am just seeing this.

In simple terms, remember that GPS tells the bird where it IS, but not which direction it's FACING.  For the Inspire to remain predictably facing in a specific direction - whether in GPS OR ATTI MODE - it needs the compass.  And to fly level, it needs valid IMU data.  Without accurate and predictable compass input, air currents will cause unintended changes in heading/direction.  Presumably the flight logic computer will battle this issue and try to correct for it as it gets updated GPS input, but if you lose GPS and switch into Atti mode AND have bad compass data AND have bad IMU data, now you're in an unbelievably bad mess: the bad IMU data means the bird won't fly level, the fact that it's not flying level will cause both drift AND rotation, further exacerbated by whatever the air currents and eddies happen to be, the lack of valid compass data means the bird has no idea that it's rotating and won't do a thing about it, and the pilot suddenly has to compensate for all of it at the same time, quite possibly while too far away to clearly see the rotation or tilt clearly.  Only a genuinely amazing, HIGHLY experienced pilot would have a prayer of recovering from that, and I'm not even sure they could.  I suspect, without knowing, that a lot of poorly configured Inspires fly more-or-less okay as long as they have GPS input, because the computer keeps correcting for the other errors, but when GPS is lost, those other errors come to the fore and sandbag the pilot.  Conversely a well-tuned system, inclined to fly straight and level without help, can enter Atti mode unexpectedly and will remain stable.
2015-7-8
Use props
SimplePanda
First Officer
Flight distance : 1719062 ft
  • >>>
Canada
Offline

Kalle Posted at 2015-7-8 20:19
I have my inspire calibrated once and then never again. I have start from bridges and in near from m ...

Well...

For IMU calibration I'd agree. Calibrate the IMU indoors on a level surface with no movement or agitation after an update or a repair job. I also tend to calibrate the IMU when a new flying season starts / if an aircraft has been in storage for a while.

Compass I calibrate basically every flying session.

In either case, check your MOD values. You can 0.99-1.01 for the IMU and 1400-1600 for the compass. Outside those values and you want to be calibrating.
2015-7-8
Use props
w1der
Second Officer

Sweden
Offline

PeteGould Posted at 2015-7-8 22:17
I've been offline for awhile so am just seeing this.

In simple terms, remember that GPS tells the b ...

Hi Pete ... thanks for contributing to this discussion / lesson.

I am going to think out loud from here ...

As of now I don´t understand how my cheap "toy quad" is able to fly, without having to do advanced IMU calibration (I think it only needs to be "flat" when I start it up) or any sort of compass calibration and without having "fly aways".

But I will try to understand the differens between my cheap toy and my expensive one below ...

If I am understanding this right ... the IMU keeps things level ... the compass reading helps it from not turning around it´s own axis ... and the on board computer is using the GPS to hold the birds position.
All of these needs to work at the same time for the auto pilot to keep the bird on a steady hover when the sticks are centered and to keep a straight course forward (pushing the right stick up) without us having to compensate for any side wind.

With total loss of GPS and compass ... it should act just like my toy quad (I guess).
Fly aways happens when corrupt GPS readings or messed up compass readings reaches the on board computer and this starts to counteract for it (as it´s believing it´s getting correct information)!?

The GPS, the IMU and the compass is vital for the "Fail Safe" feature to work ...
(Failsafe being when the RC loses control over the bird.)

In my opinion there should be another failsafe with a "manual override" ... allowing the pilot to take control over the aircraft (shutting down compass and GPS) when the autopilot is messing things up ... making us able to fly it safely home in manual mode.
I am not sure if switching to atti mode is doing this or if the auto pilot still interacts with the bird in this mode?   

This is sooo confusing ... !

2015-7-8
Use props
w1der
Second Officer

Sweden
Offline

SimplePanda Posted at 2015-7-8 22:44
Well...

For IMU calibration I'd agree. Calibrate the IMU indoors on a level surface with no movem ...

Why do I need to check the values ... doesn´t the pilot app check them for me and alert me if they are bad?
If it doesn´t ... it should!
2015-7-8
Use props
Skywolf007
Second Officer
Flight distance : 2758875 ft
Slovakia
Offline

w1der Posted at 2015-7-8 23:37
Why do I need to check the values ... doesn´t the pilot app check them for me and alert me if the ...


It does notify you when you need an IMU calibration.
Its safer to check values, so you know.

On the other hand: Computer is not almighty... If you do a bad IMU calibration, the app will show correct values but the bird will not fly properly.

That's why you need a leveled surface, and no metal or electrical interference around  the I1 during IMU calibration. If you don't have a leveled surface, you move the bird shortly before/during calibration, or you have some interference around, the computer will calculate wrong IMU data(accelerometer, magnetometer barometer will provide the IMU with bad data). At the end you will see an IMU with good values, but badly set up. The bird will drift, fly away etc...

That's why a correct calibration is required..
2015-7-8
Use props
PeteGould
Second Officer

United States
Offline

w1der Posted at 2015-7-8 23:35
This is sooo confusing ... !

It's sort of like the difference between flying the Piper Cherokee I trained on and flying an Airbus A320.  The Piper literally uses cables wrapped around the steering column (a/k/a the yoke) and the foot pedals which directly drive the flight control surfaces, and has nothing aboard to make the flight path comfortable ("better get busy on those pedals," my flight instructor used to tell me, "if you don't want to be wearing your passenger's lunch.").  The A320 by contrast is all computer controlled, with very sophisticated systems to make the flight safer and more comfortable.  But that means there's also a lot more to go wrong.

Or look at the F35 fighter jet, which is SO computer-dependent that it's not even especially stable in the air UNLESS all those sophisticated computer systems are working together to control it, in which case it can do some simply amazing things.  But if all the computer systems fail in an F35, the pilot might as well eject, because I don't think a human can fly that thing without computer-assist.

The Inspire is a finely tuned instrument that is intended to work as a cohesive whole in the pursuit of capturing stable video from an inherently unstable vantage point.  That it does what it does with the apparent stability it exhibits in recorded material is remarkable.  But it does it at the expense of adding a good deal of complexity.
2015-7-8
Use props
Kalle
New

Germany
Offline

If the copter is flying in one direction, the rate is established with the magnetic compass. At the same time is expected by GPS, in which direction it flies.

Hovering the copter, and is driven away by the wind a little to the west, then he wants to compensate for the east. If the magn. Compass but not exactly right, he does not fly to the east, but for example, to southeast.

Naza says, "Hey now I have to compensate to the north to get back on the Hoverpoint back". But she flies again "wrong" ... etc.
The effect of this appearance, which is the Naza the copter in circular orbits flies - also called Toilettbowl.

So fundamental problem: The Earth's magnetic field is not always the same, I mean it is not always directed to north, depending on the location, the few degrees to the left or right to be "disturbed".

www.crashreport.net
CrashReport - Investigation of malfunctions
2015-7-8
Use props
mtnmaddman
Second Officer

United States
Offline

Are you able to control the toilet bowl and land? has any one experienced this?
2015-7-8
Use props
jimhare
Captain
Flight distance : 239035 ft
Australia
Offline

w1der Posted at 2015-7-8 17:05
This is just making me more confused!
According to the info in your previous link ...
As long as ...

I have a bizarre theory that isn't backed up by anything concrete but I'm going to throw it out there for the discussion.

If the compass/IMU are out could the flyaways be caused by an endless positioning loop?

In other words, does the Inspire suddenly think, "Hey, I'm supposed to be two feet to the left, better fix that"  and is trying to correct itself?  

Since it would continuously see itself as offset in space the result would be a full speed fly away as it tried to catch up but never can.

Completely off base?  
2015-7-8
Use props
Farnk666
Captain
Flight distance : 1711394 ft
Australia
Offline

jimhare Posted at 2015-7-9 06:10
I have a bizarre theory that isn't backed up by anything concrete but I'm going to throw it out th ...

I don't think you're off base Jim.
Something along those lines is happening (in my opinion) in some cases of flyaways.
I'm probably more thinking of IMU data issues rather than compass, but as was said above, all of this data goes into the FC algorithms so all or just one could be the root of a flight issue.

Probably goes somewhere to explaining the huge size of the flight logs kept internally on the Inspire - must be huge amount of data being processed in real time!

Ain't technology grand!
2015-7-8
Use props
w1der
Second Officer

Sweden
Offline

PeteGould Posted at 2015-7-9 01:53
It's sort of like the difference between flying the Piper Cherokee I trained on and flying an Airbu ...

I get what you are saying and you are making an excellent point!

How ever there are no "cables" on my cheap toy quad ... it´s actually quite sofisticated allthough it has no compass and GPS module and most of all ... it lets me be in control and thanks to that ... no fly aways.

I have said this before ...
The failsafe feature that is supposed to protect us from this kind of stuff is actually the problem causing a lot of crashes!

I guess I will just have to wait until the day when all of this happens to me ...
Switch to atti mode and see if I am able to compensate manually for any bad IMU readings I might have ... and fly it home safely trying not to panic ...  
2015-7-9
Use props
w1der
Second Officer

Sweden
Offline

What I have learned from this thread is ...

Compass calibration ... do that ONLY when absolutely necessary ... this to avoid the risk of getting a bad calibration.
If you do a compass calibration before every flight your actually adding the potential risk of getting a BAD calibration.
So ... if it was working just fine on your previous flight and the new scenario is not that far away ... don´t mess with it!

IMU calibration ...
As we don´t really get all the details about whats being changed in the software updates.
I will do a IMU calibration after every update and ONLY then ... (perhaps give it a couple of more tries to see if I can get my gimbal to be level with the horizon).
How ever I will NOT do a IMU calibration other then on a PERFECTLY level surface, witch is hard to find out doors/on scene ... so this will be done at an appropriate location.

GPS ...
Total loss of GPS ... not a big problem ... just fly in atti mode.
Faulty GPS readings ... probably causes the bird to start chasing ghosts ... switching to atti mode, praying to any god of your likings that this gives you control over the situation.

Kill switch stick input ...
This is remarkable, but not ones have I seen any one trying to kill the motors when they discover that the bird is going crazy on them.
So we actually don´t know if this will even work when you loose control.
2015-7-9
Use props
PeteGould
Second Officer

United States
Offline

w1der Posted at 2015-7-9 16:28
I will do a IMU calibration after every update and ONLY then ...

...or any time the aircraft has been subjected to any sort of physical shock, such as being dropped, even in its case.  Remember that some of the earliest flyaways were thought to be due to IMU decalibration as the result of physical shocks to the aircraft while being shipped.
2015-7-9
Use props
w1der
Second Officer

Sweden
Offline

PeteGould Posted at 2015-7-9 17:35
...or any time the aircraft has been subjected to any sort of physical shock, such as being dropped ...

Hjm ... In the event of a hard landing !?
2015-7-9
Use props
Kalle
New

Germany
Offline

Yes, very important after a hard landing !!!  
2015-7-9
Use props
ambambcopter
lvl.3
United Kingdom
Offline

For what it's worth I think "Jimhare" has a point about an endless positioning loop.
Forcing the brain to compute the correct answer with invalid data will produce garbage.
Also there is one totally overriding requirement that the machine needs to react correctly to the RC commands.
It MUST know what is and when it is LEVEL.
The brain is constantly adjusting motor power to any of and all of the motors every fraction of a second to just stay in the air.
Even in ATTI mode, Without the ability to stay LEVEL it would tumble to the ground as it is inherently unstable.
2015-7-9
Use props
PeteGould
Second Officer

United States
Offline

w1der Posted at 2015-7-9 17:48
Hjm ... In the event of a hard landing !?

As Kalle said - definitely after a hard landing.
2015-7-10
Use props
BeenThere
lvl.1

United States
Offline

PeteGould Posted at 2015-7-8 22:17
I've been offline for awhile so am just seeing this.

In simple terms, remember that GPS tells the b ...

I have understood that even in ATTI mode there were still a lot of "systems" at work to keep the craft stable and flying. But after reading your post here, thinking logically, if the craft behaves elegantly in ATTI mode, then that is verification of a good IMU and compass cal? Meaning without benefit of GPS, it sounds like it is "more" dependant on good calibrations.
2015-7-10
Use props
BeenThere
lvl.1

United States
Offline

PeteGould Posted at 2015-7-9 17:35
...or any time the aircraft has been subjected to any sort of physical shock, such as being dropped ...

My question of course would be what constitutes a shock? If you land with a bit of a touch at landing, say from one-half of an inch, versus bringing it down so smoothly that were you on board there would be no perception of touching earth? And also I wonder if small shocks are cumulative?
2015-7-10
Use props
ambambcopter
lvl.3
United Kingdom
Offline

I agree BeenThere, I believe the IMU calibration is critical regardless, even if in ATTI mode.
I think if the IMU gets corrupted, particularly as I stated before, the machines understanding of what is LEVEL.
The chances of catastrophe are high even in ATTI mode.
Regarding bouncing the craft.
Perhaps if the impact is strong enough to cause power contacts etc. to break momentarily, that could possibly generate a spike in the electrics.
And solid state electronics don't like spikes.
2015-7-11
Use props
Advanced
You need to log in before you can reply Login | Register now

Credit Rules