I think this is incorrect, copper is widely used for EM shielding. A (grounded) film between the PDB and the controller should reduce magnetic interference to the compass coming from PDB/ESCs. Any ferrous material would work though afaik.
That makes sense.. ..my board is about 2" away, on top a PDB and miles away from all conceivable EMI/EMF sources: motors >12", ESCs >6", GoPro [does emits lots of EMF]-FPVTX-3DRadio all >6"; yet Randy's estimate of my interference is about 60%, after doing a compassmot calibration...
So what is the source? My theory is the motors themselves.. I use the bell type with strong magnets and large stators, which, while > 1' away (via extended CF booms) the four when rev up probably create a strong plate shape magnetic field in between, driving the mag off, orientation.
In any case, putting the mag on a total of >6.5" pole and GPS about 4" halfway,(on top of the theoretical plate shape magnetic field) had significantly reduced the interference to less than 2%, after a re-run of the compassmot!
I now have no doubt that having an external mag on a pole solves the problem.. my GPS locks in in an instance as well and often, to as many as 13 sats...
..recalibrating my rates (angular, loiter etc. which is now increased without instability) and compass offsets, declination, loiter is now rock solid, and haven't had any fly-aways, that had caused me one crash... except with the latest commit where the stops seems to drift quite a bit (wish this is user-parameter-toggle definable.. bet soft, medium and hard stops for loiter and waypoints, to eliminate the now pervasive drifts, also compounded by windy conditions.. w/c is a separate issue)..
Hope this helps.
Do you have some details on the external mag option?
Sounds very interesting.
I use an HMC5883L from sparkfun w/c sits on top of a pole.. (wiki for configuring an external mag) I'll let the attached picture speak for itself..
Another thing, (although you probably know this already but just to make sure ;-), when you do the re-calibration, if you do have a GoPro, make sure to turn it on (FPV and record) as it does makes some difference on the calibrated offsets. That said, further note that (works for me best) to re-calibrate, both declination and compass, auto-learn are turned on, then fly the quad on alt-hold (few 4-5m off the ground), clock wise stopping (pausing .5 sec @ stop) every 15 deg till 360 full circle then likewise, counter clockwise.. Then turned off the dec. and compass auto-lean and calibration is set (for life! ;-).
Thank you for the clarification and picture.
Looks quite impressive.
Will keep this in mind.
Problem is that a ferrous material, can itself become magnetised. Park the copter for a long time in a static magnetic field, and the plate will be causing the problem, not stopping it. It can also be magnetised by physical deformation (knocking it etc..). The way that things that are magnetic will become magnetised, is why aircraft and ships have sometimes to be demagnetised, and their compasses have to be 'swung' to find the errors.
A _closed_ layer of a conductor, can form a Faraday shield, preventing any AC electrical field from passing.
Conductive shielding that is grounded, can be again used to effectively 'short out' AC fields, but won't stop a static field as you would get from (say) a battery wire.
The aircraft GPS units, used in 'full size' craft, have magnetic compasses in them, because GPS can't give a direction when stationary, or an accurate direction, till reasonable distances are moved. However most are quite smart, and calibrate themselves from the GPS bearing, once speed is above a few mph, and remember this.
The only material you could use that would give magnetic shielding, is mu-metal. This though should be heat treated after cutting and bending.
Correct R J Hamlett, please see my post
This may be a daft question, but why do you bother with a magnetic heading when using a gps or inertial, nav as they can navigate with true bearings and not unreliable magnetic ones?
GPS gives you the heading when the craft is in forward motion. When stationary, it won't know the heading.
And gyros drift in time and need to get corrected. It means they need reference which comes from the magnetic compass...
Interesting but I wonder if that is entirely true, as if you are navigating from lat/long to lat/long, then being stationary is irrelevant as the track is a simple calculation and the heading can be determined after a miniscule movement i.e. 1mm.
In the fullsize world we use GPS or inertial Nav, magnetic headings are an afterthought for ATC benefit really. After all the helis I have flown don't bother with magnetic as all autopilot calcs are done in true, even when in a stationary hover. They just use the true north (which doesn't change) as a reference not magnetic north as that changes all the time and is too easily affected by external influences.
I can definitely see that if the heli was totally stationary i.e. no change in lat/long at all then in theory it could start to rotate around its hover axis, but the inertial sensors should pick that up. And no heli is ever totally stationary in the hover!
Just a thought!