I have been flying a 900mm size Y6 with a 4s power system for some time.  Initially I had mounted the Pixhawk on a plate set on 30mm standoffs from the top plate of the frame while the PDB is in between the top and bottom plates and the battery is on the bottom of the frame.  This gave me really good performance in terms of the compass.  I ran a compass_mot test and the interference was below 30%. 

Now I have upgraded to a 6s setup and am now seeing this compass variance warning during flight and the copter goes crazy in any GPS flight mode, although flying in stabilize is really good.  I have since run a compass_mot test and the interference is above 60%... 

Im not sure how much further I can move the Pixhawk away from the power system, so can I just turn off the internal compass?  I know that in AC 3.3 you can set compass_use2 to 0 for determining heading, does this disable the internal compass?  

Any recommendations are much appreciated.

Chris  

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What did you change exactly?
Going from 4S to 6s should have decreased your current draw, and therefore compass interference.
I have flown with internal compass desactivated, but I can keep it with no issue on a setup where battery is quite close to the pixhawk.

 Looks like you've got good separation, I've got less separation on my 6s between the pixhawk+powersystem and external and I get 2-6% in compassmot. How high are your internal's offsets? Probably up there, I'd just disable the internal.

Setting compass_use2 to 0 did not change the outcome of the compass_mot test but if the test is only measuring the internal compass that makes sense.  I will try to fly it today with the internal compass disabled and see what happens.

In reference to Manu_B's question, I have changed the whole propulsion system (motors,props,esc,battery) 

Derek - my offsets are:

COMPASS_OFS_X -53
COMPASS_OFS_Y -35
COMPASS_OFS_Z 97
COMPASS_OFS2_X -160
COMPASS_OFS2_Y 386
COMPASS_OFS2_Z -411

Just came back from the field testing this setup with the internal compass disabled.  All was good!  

After further review I found something interesting... I was reading the compass mot test graph wrong, the interference levels were actually good (below 30%).  (I mixed current and interference up)  

So now I am confused as to why I am having compass issues.  Is the internal unit just bad?  I can also try using a second externally mounted ublox unit to regain redundancy... 

Although disabling the internal compass worked, I was determined to figure this out.  I tried moving the Pixhawk up and away from the PDB as far as I could as well as trying a 4s battery and nothing seemed to make a difference.  So, I took a Pixhawk unit off another helicopter I have and tested that one on this rig.  No internal compass issues, no compass variance, good performance.  

So, I guess the internal compass of that Pixhawk unit was faulty...  

A few things:

-The 6s battery could be physically bigger, or the higher voltage could be causing more EMI than before.  Moving the Pixhawk away can help but most likely you want to get the external compass away from the battery as far as possible.  Note that this is only half of the solution, you also want to make sure the COMPASS WIRES are twisted up tight and run them as far away as possible from the battery or its power leads.  Twist all the wires on your copter for best performance.

-Secondly, I have had this issue when upgrading from 3.2 to 3.3.  To solve it, instead of disabling the internal compass, try loading Arduplane or other firmware onto the board, and then flash back to Arducopter 3.3.2.  This has solved the inconsistent compass error for me when all other attempts failed.

Just my 2 cents.

Form the picture you posted the external compass wires run very close to the XT60 battery connector.  You could try re-routing those wires. Also, the External Compass could be moved to the front of the frame. Right now it's bang above the battery.

That is interesting to hear about the firmware causing and fixing the issue.  As I said before, I replaced the pixhawk unit itself and the compass problem went away.  I have since sent the suspect Pixhawk back to be replaced.  Maybe if I encounter this issue again I will try flashing different firmware. 

Thanks for the tip.  The problem however, was with the internal compass only.  Since that picture was taken I have rerouted some wires and cleaned things up a bit though.  With a new Pixhawk unit, there are no compass issues and I have been flying the rig regularly now with no issues. 

Thanks

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