when this is 1 (which is the default), we will get the declination
automatically via the AP_Declination library

when it is 0 we will use the value configured by the user

explain the algorithm a bit more

This adds a large amount of noise robustness to the compass offset
learning algorithm, at a cost of 120 bytes of memory. The changes are
based on a long discussion with Bill Premerlani.

this seems to work much better than the old algorithm, converging
faster and more accurately. Even better, it has no linkage to DCM, so
no possibility of nasty feedback effects

This will set the declination based on lat/lon if the user has
not yet saved one to the EEPROM, OR if they have specified via the
config parameter that they want it to overwrite the declination every
3D fix.

Signed-off-by: Andrew Tridgell <tridge@samba.org>

there is no point in saving this, the value is only configurable at
compile time for now, and is always set

this ensures they are set if you have MAG_ENABLE==0

these allow you to control if the compass should be used for yaw and
if it should learn its offsets. This is useful for locking in compass
offsets once they are confirmed to be good, and for learning offsets
without using them in flights.

The default is to behave the same as previously, which is
COMPASS_LEARN=1 and COMPASS_USE=1

this leaves the previous heading values alone if we are at a pitch of
exactly 90 or -90, at which point we can't compute a meaningful
heading

This is a fix for an interesting bug when a DCM matrix reset was added to the ground start.  This bug only showed up if (A) a ground start were performed after an air start or due to use of the "Calibrate Gryo" action, (B) if the current orientation were sufficiently different from 0/0/0, and (C.) if the particular magnetometer had sufficiently large offsets.  Why did resetting the DCM matrix to 0/0/0 pitch/roll/yaw at ground start cause a bug?  The magnetometer offset nulling determines the proper offsets for the magnetometer by comparing the observed change in the magnetic field vector with the expected change due to rotation as calculated from the rotation in the DCM matrix.  This comparison is made at 10Hz, and then filtered with a weight based on the amount of rotation to estimate the offsets. Normally it would take considerable time at normal in-flight rotation rates for the offset estimate to converge. 

If a DCM matrix reset occurs when the offset nulling algorithm is up and running, the algorithm sees the DCM reset as a instantaneous rotation, however the magnetic field vector did not change at all.  Under certain conditions the algorithm would interpret this as indicating that the offset(s) should be very large.  Since the "rotation" could also have been large the filter weighting would be large and it was possible for a large erroneous estimate of the offset(s) to be made based on this single (bad) data point.

To fix this bug methods were added to the compass object to start and stop the offset nulling algorithm.  Further, when the algorithm is started, it is set up to get fresh samples.  The DCM matrix reset method now calls these new methods to stop the offset nulling before resetting the matrix, and resume after the matrix has been reset.

this also adds a healthy attribute, and error checking on all I2C
calls

Added a limit to the atan function to calc the heading. Just trying to avoid, bad values screwing up the DCM. - Jason

git-svn-id: https://arducopter.googlecode.com/svn/trunk@2777 f9c3cf11-9bcb-44bc-f272-b75c42450872

git-svn-id: https://arducopter.googlecode.com/svn/trunk@2699 f9c3cf11-9bcb-44bc-f272-b75c42450872

git-svn-id: https://arducopter.googlecode.com/svn/trunk@2221 f9c3cf11-9bcb-44bc-f272-b75c42450872

git-svn-id: https://arducopter.googlecode.com/svn/trunk@1698 f9c3cf11-9bcb-44bc-f272-b75c42450872

git-svn-id: https://arducopter.googlecode.com/svn/trunk@1677 f9c3cf11-9bcb-44bc-f272-b75c42450872

Most of the compass functionality is now abstracted in a base class, with the various sub-classes implementing just their unique functionality.

git-svn-id: https://arducopter.googlecode.com/svn/trunk@1647 f9c3cf11-9bcb-44bc-f272-b75c42450872