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Purpose of Compass Calibration
6421 30 2015-5-6
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roy
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I was asked what was the purpose of the compass calibration and my response was two fold. One was to help prevent fly-a-way's as the compass properly orientates the quad for proper direction.

Quadcopter starts to set orientation to face certain direction. It needs a sensor that gives different readings when it rotates horizontally. This feature is achieved by using magnetometer. Magnetometer is simply an electronic compass, it gives readings in X-Y-Z directions, but what we focus here is the Z-Axis assuming quadcopter is horizontal; some math is required to get the absolution magnetic field direction if quadcopter is not horizontal in case we look for high accuracy. Other sensors provide other functions

So the compass plays a very important roll in keeping the quad orientated in its directiom. if its not correct and the GPS gives the quad a direction  to travel and orientation (north) is not correct then you can get a fly-a-way....
2015-5-6
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crash1sttime
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Were you asking a question or giving us a complete breakdown of the compass and its purpose ?
2015-5-6
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homer
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If the Quad has 3 or more Gps Satellites and it is moving horizontal wouldn't it know which way it is traveling?  The compass would be a backup for the GPS and the only way to know which way it is pointing while hovering. So my thought is the compass only needs calibration when it gives you a notice. I have been doing the dance every time I move about 5 miles from the last flight.
2015-5-6
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crash1sttime
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homer@paddocks. Posted at 2015-5-7 00:34
If the Quad has 3 or more Gps Satellites and it is moving horizontal wouldn't it know which way it i ...

Thats a bit like saying if your car satnav has 3 sats and your blindfolded you would know which way you to point the car to follow the route

As a driver you know that if the arrow on the satnav is pointing north and your car is facing south that you need to turn the car around and point it in the direction that the satnav is indicating

like the OP says the compass lets the quad know which way is north so it can follow a course,
2015-5-6
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acenothing
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homer@paddocks. Posted at 2015-5-7 00:34
If the Quad has 3 or more Gps Satellites and it is moving horizontal wouldn't it know which way it i ...

This can not be right.  If 3 or less the compass may know only it traveled in a direction.  So if you travel north 100 yds then slide sideways without rotating the bird say 200 yds it know nothing.  

I think if you have 6 or more sats the compass is baggage.  With less than 6 it only knows which way is north.  Other than that is baggage.
2015-5-6
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roy
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While all GPS receivers can tell where you are, entry-level models have no way of ascertaining which direction you are facing. So you have to start moving and walk a few steps at a good pace before it can accurately point you towards your destination. This is why mid-range and high-end units have an electronic compass — to show the direction you should travel even while standing still.
This can be found in any GPS operation information. Compass Needed !!!And so back to the Magnetic Compass.

GPS manufacturers recognise that whilst a differential compass is fine most of the time, users still like the security of a magnetic compass. The traditional magnetic compass has been fitted with electronic sensors and bundled into some of the GPS devices. This is what is meant when a GPS is advertised with an electronic compass. It costs a little more but allows you to take bearings when stationary or out of sight of the satellites.


Hope this helps


Roy


2015-5-6
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homer
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At this point I am going stick with the Instructions, Calibrate at ever new flight locations.
Quick Start Manuel, Calibrate before every flight.
2015-5-7
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droneflyers.com
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acenothing Posted at 2015-5-7 07:20
This can not be right.  If 3 or less the compass may know only it traveled in a direction.  So if  ...

Not baggage in any case, IMHO.
It is simply one additional input - one of MANY - all of which are weighed by the Flight Controller - to make decisions.
2015-5-7
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danrok
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The compass provides the heading direction.  Without it the flight controller would not know which direction the drone is facing.

That cannot be done accurately and reliably using GPS only, especially if the drone is hovering stationary.
2015-5-8
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roy
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Here is what DJI has to say about starting up after a repair....
We have repaired your Phantom 2 Vision +. The repair technician has repaired your unit by replacing the Vision + Camera. The firmware has been updated to version 3.12, the Phantom was test flown and is fully operational.

To regain control of the Phantom if fail safe is activated, flip the transmitter switch from GPS to ATTI then back to GPS.
Make sure to calibrate the compass in every new flight location. The compass is very sensitive to electromagnetic interference, which can cause abnormal compass data leading to poor flight performance or even flight failure.

Do not calibrate your compass where there is a chance of strong magnetic interference, such as magnetite, parking structures, and steel reinforcements underground. Do not carry ferromagnetic materials with you during calibration such as keys or cellular phones. Do not calibrate beside massive metal objects. Due to the effects of the shipping process your IMU inside the flight controller will require an advanced IMU calibration through the assistant software.

Due to the effects of the shipping process  recalibrate the IMU.... I'm wonder what effects. When we travel by car with our unit on a short or long trip do we need to be concerned. Something to check out now
2015-5-8
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acenothing
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danrok Posted at 2015-5-8 18:00
The compass provides the heading direction.  Without it the flight controller would not know which d ...

I agree its only real value it "which way the" bird is facing.  Other than that the GPS is required.  The GPS will tell everything OTHER then the direction it is facing.

With that in mind, it seems silly to do the dance.  And yes I do the dance dispite the fact that the compass has nearly no value to the process.
2015-5-8
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Gerry1124
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acenothing Posted at 2015-5-9 01:20
I agree its only real value it "which way the" bird is facing.  Other than that the GPS is require ...

Without the compass calibration, you would never be able to fly a course lock.
2015-5-8
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acenothing
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Gerry1124 Posted at 2015-5-9 01:29
Without the compass calibration, you would never be able to fly a course lock.

Thanks for the additional item.  1. I have never used course lock  but if I do I will keep it in mind.  2. So, CL and which direction the bird is facing that's it?  Again seems like the dance is a bit much for those 2 things.
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Gerry1124
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acenothing Posted at 2015-5-9 01:45
Thanks for the additional item.  1. I have never used course lock  but if I do I will keep it in m ...

Say for instance you lose GPS lock,  less than 6 sats.  Your Phantom has CL and HL to depend on finding its way home when it goes automatically into ATTI mode.   Would you rather it not have the compass calibrated and just fly off in any direction, or would you want to flip to HL in ATTI mode and pull straight back on the right stick and have it return home?
2015-5-8
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acenothing
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Gerry1124 Posted at 2015-5-9 01:53
Say for instance you lose GPS lock,  less than 6 sats.  Your Phantom has CL and HL to depend on fi ...

I doubt that would work.  HL is a GPS REQUIRED operations.  The compass does not know where home is and likely the bird will stop and hold until you manually return it.  Hopefully you can see the bird or at least see the FPV to get it back.

Again if you fly 100 meters out, say north, and slide 100 meters sideways, facing north and moving east.  Then you lose GPS.  The compass does not know home is 135 degrees at about 120 meters.  It only knows north.

What am i missing?
2015-5-8
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homer
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roy@mcscomputer Posted at 2015-5-8 18:15
Here is what DJI has to say about starting up after a repair....
We have repaired your Phantom 2 Vis ...

I would think the shipping process could effect by, Other packages and transportation procedures, Conveyors, Scanners, X-ray?, Transported by Airfreight. I had no instruction on my repair about IMU, just the C-Dance.
2015-5-8
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parfitm
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acenothing Posted at 2015-5-9 02:17
I doubt that would work.  HL is a GPS REQUIRED operations.  The compass does not know where home i ...

What happens if the compass completely malfunctions? I need to fly in the far north, not far from the magnetic pole. In that location a compass is pretty much worthless. I've seen a compass just slowly spin.

Will the Phantom 2 still be under my control even without the compass, or without that directional input, will it fail to respond correctly to my control inputs?

I've heard that people have had trouble in these situations. But won't the P2 know which way is forward and backward in terms of its basic response to the control sticks?

Any ideas?
2015-5-8
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grangerfx
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I assume that a properly calibrated digital compass along with a good GPS fix and barometer combine to give the best position and motion estimation. You calibrate your compass when you move the start location because there are local magnetic anomalies which can cause several degrees of offset for magnetic north. Without an accurate compass heading, the drone may drift around more when trying to get back to its correct GPS position because it is not orientated as it expects.
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PanamonCreel
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grangerfx@gmail Posted at 2015-5-9 12:13
I assume that a properly calibrated digital compass along with a good GPS fix and barometer combine  ...

The compass calibration cannot compensate for horizontal earth magnetic field angle variations ( declination ) since it only sees the magnetic north and doesn't care/know from where it is coming from,   instead the declination is compensated for by saving offsets from the GPS data, which has true north,  but for that the Phantom has to fly around for a bit to obtain that offset since the GPS doesn't know the orientation if the unit is "stationary"  .  The main purpose of the whole compass calibration process is to compensate for fixed disturbances to the earth magnetic field that are caused by ferromagnetic and magnetic items that are installed on the Phantom.  Secondary reason is to adjust for local earth magnetic field strength variations, the vertical angle of the field ( inclination)  since it is a full 3D sensor and possible magnetizing offsets caused by having the phantom transported too close to a strong magnetic field.
As for flyaways being caused by a compass calibration not done as a root cause, I don't buy that due to the rather minor involvement in the control of the craft and to me the non human factor flyways all point to the flight controller which should have plausibility checks and error handling routines to prevent catastrophic failure from a inaccurate/failed minor sensor.
2015-5-10
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danrok
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Bear in mind that there is no compass as such, the sensor detects the direction of the Earth's magnetic field.

This is what that looks like:
2015-5-10
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JtrJr-Droner
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PanamonCreel Posted at 2015-5-10 21:44
The compass calibration cannot compensate for horizontal earth magnetic field angle variations ( d ...

Panamon,

That you for the through description. I'm trying to better understand the Phantom systems and how they interact with the real world. Your description was quite helpful to me.

Jerry
2015-5-10
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roy
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Just a note, the compass points me in the right direction (magnetic north)  which way to go, and the travel is by pilot or map (GPS) . It just gets me started with some orentation of direction until it can determine positioning throught GPS lock on my location. Kind of like jumping on the interstate and not knowing if your direction is north or south and you find you won't the wrong way in your directions. It takes time to get to an off ramp and turn around. Would have been better if I knew which way I wanted to go to start with.... Magnetic compas helps do that.
Simple analogy but useful.
2015-5-11
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acenothing
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roy@mcscomputer Posted at 2015-5-11 17:19
Just a note, the compass points me in the right direction (magnetic north)  which way to go, and the ...

I total agree the compass is part of the overall data for the pilot.  In the end for most of the stuff to work properly you need either the GPS to be working or visual on the bird, including FPV.

Using the car and roadway analogy.  For those of us with a handheld GPS we know the system does not need a compass NOR need to be orientated (unlike the P2).  It knows what direction you are going based on its sampling of GPS data ONLY.   Again I think the weight the group puts on the compass is clearly over=rated.  It is mainly baggage, and the dance is mainly to allow the time for the GPS to with set up.
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PanamonCreel
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acenothing Posted at 2015-5-12 00:25
I total agree the compass is part of the overall data for the pilot.  In the end for most of the s ...

I don't know about your handheld GPS but mine certainly has an electronic compass that requires calibration .  GPS only orientation works good if you're moving at some speed at an assumed orientation of the device but won't work so well if you're slow moving or stationary and/or change the orientation of the device away from the assumed one.
2015-5-11
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acenothing
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PanamonCreel Posted at 2015-5-12 05:43
I don't know about your handheld GPS but mine certainly has an electronic compass that requires ca ...

I think you are wrong.  The GPS know where you are on the map not which way you are pointed.  I you are at Lat. X Long. Y and move to Lat: +100 meters and Long Y.  It know you traveled north REGARDLESS of the compass. You then slide sideways left to LAT x+100, Long Y+100, It knows where you traveled west 100 meters.  It also knows HOME is about 144 meters southeast of it position.  It does not care about whis way it is facing.

I have had a couple of handheld GPS for backpacking and I NEVER EVER had to calibrate them as a compass.  The GPS handled all the work.  Again the DJI compass is more baggage than value.  If you have sight of the bird and know the orientation you do not need the compass.
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PanamonCreel
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acenothing Posted at 2015-5-12 06:24
I think you are wrong.  The GPS know where you are on the map not which way you are pointed.  I yo ...

The GPS just receives location coords which, dependent on the qty of sats and the GDOP,  will move around a certain amount even if the receiver is stationary and units without an integrated electronic magnetic compass will show the compass direction moving around, it will also not detect the true orientation of the unit since the GPS cannot determine this and it will just show which direction the receiver moved but not which way it was oriented while moving.  The Phantom needs to know how it is oriented in order to know which motors fire in order to move in the correct direction and that's what the electronic magnetic compass is for.  
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acenothing
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PanamonCreel Posted at 2015-5-12 10:18
The GPS just receives location coords which, dependent on the qty of sats and the GDOP,  will move ...

I agree orientation is the only thing the compass does for you.  The rest it I think is incorrect.  ONLY the GPS will tell you distances and direction of movement.  The compass is valuable only is you lose sight of the bird or you can not tell its orientation.  EVERYTHING else is done via the GPS.
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PanamonCreel
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acenothing Posted at 2015-5-13 03:16
I agree orientation is the only thing the compass does for you.  The rest it I think is incorrect. ...

Well the electronic compass is more important than just for losing sight of the Phantom.  It is an important component for the holding stability of the Phantom as well because again the GPS only knows it went from point A to point B but it does not know at what yaw rotation the Phantom was at while going there.  Ever did geocaching with your handheld GPS receiver? If you did then you would know how the GPS can send you on a wild goose chase while moving slowly  or while being stationary.
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acenothing
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PanamonCreel Posted at 2015-5-13 21:28
Well the electronic compass is more important than just for losing sight of the Phantom.  It is an ...

Geocaching is a great family activity.  My two girls and I did a lot of it in the late 1990's when GPS hand held were $500.  YIKES.  

Wild goose chases were common but I would like to think the tech has improve a lot in that late 20 years.
2015-5-13
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PanamonCreel
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acenothing Posted at 2015-5-14 02:17
Geocaching is a great family activity.  My two girls and I did a lot of it in the late 1990's when ...

yeah and that improved technology is called electronic compass
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roy
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Well said Panamon , do a search on all the new hand held GPS units Ace, you will find all the new better GPS units come now with magnetic compass... Technology... Getting directions right!
2015-5-17
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