Subsystem/ahrs

AHRS subsystem

The AHRS subsystem specifies which attitude estimation filter you are using.

Currently possible AHRS subsystems are

• int_cmpl_quat
• int_cmpl_euler
• float_cmpl_rmat
• float_dcm

e.g. for the latest complementary filter:

File: conf/airframes/myplane.xml

<firmware name="fixedwing or rotorcraft"> ... <subsystem name="ahrs" type="int_cmpl_quat"/> </firmware>

Implementations

There is a test program ( sw/airborne/test/ahrs/compare_ahrs.py ) to compare different AHRS implementations on simple test cases.

Complementary Euler (fixed point)

Not recommended for fixedwings, as this filter doesn't compensate for centrifugal force when flying turns.

File: conf/airframes/myplane.xml

<firmware name="fixedwing or rotorcraft"> ... <subsystem name="ahrs" type="int_cmpl_euler"/> </firmware> <section name="MISC"> <define name="FACE_REINJ_1" value="1024"/> <!-- optional, defaults to 1024 --> </section>

Complementary Quaternion (fixed point)

If you are NOT using the dev branch, this is currently not recommended for fixedwings, as this filter doesn't compensate for centrifugal force when flying turns. In the dev branch this is fixed (but untested) and the needed AHRS_GRAVITY_UPDATE_COORDINATED_TURN already set by default for a fixedwing firmware.

Other flags of interest are:

• AHRS_PROPAGATE_LOW_PASS_RATES : apply a low pass filter on rotational velocity
• AHRS_MAG_UPDATE_YAW_ONLY : use mag to update yaw only

For the latest integer complementary quaternion filter (int_cmpl_quat):

File: conf/airframes/myplane.xml

<firmware name="rotorcraft"> ... <subsystem name="ahrs" type="int_cmpl_quat"/> </firmware> <section name="AHRS" prefix="AHRS_"> <define name="H_X" value=" 0.51562740288882"/> <define name="H_Y" value="-0.05707735220832"/> <define name="H_Z" value=" 0.85490967783446"/> </section>

Also see the Local Magnetic Field section.

Complementary Rotation Matrix (floating point)

If you are NOT using the dev branch, this is currently not recommended for fixedwings, as this filter doesn't compensate for centrifugal force when flying turns. In the dev branch this is fixed (but untested) and the needed AHRS_GRAVITY_UPDATE_COORDINATED_TURN already set by default for a fixedwing firmware.

Other flags of interest are:

• AHRS_PROPAGATE_LOW_PASS_RATES : apply a low pass filter on rotational velocity
• AHRS_MAG_UPDATE_YAW_ONLY : use mag to update yaw only

File: conf/airframes/myplane.xml

<firmware name="rotorcraft or fixedwing"> ... <subsystem name="ahrs" type="float_cmpl_rmat"> <define name="AHRS_PROPAGATE_RMAT"/> </subsystem> </firmware> <section name="AHRS" prefix="AHRS_"> <define name="H_X" value=" 0.51562740288882"/> <define name="H_Y" value="-0.05707735220832"/> <define name="H_Z" value=" 0.85490967783446"/> </section>

Also see the Local Magnetic Field section.

DCM (floating point)

Similar in principle to the float_cmpl_rmat filter (but less cleanly written).

Other flags of interest are:

• USE_MAGNETOMETER : use magnetometer to update yaw (untested ?)

File: conf/airframes/myplane.xml

<firmware name="rotorcraft or fixedwing"> ... <subsystem name="ahrs" type="float_dcm"/> </firmware>

Local Magnetic Field

To calculate the normalised local magnetic field insert the relevant location coordinates hereand calculate.

Screenshot of noaa page.

Copy the north(x),east(y),vertical(z) component values into scilab and execute "X/norm(X)".

Screenshot of scilab page.