AHRS400 Series User’s Manual
Doc# 7430-0004-01 Rev. A
Page 19
4 AHRS Operating Tips
4.1
Mounting the AHRS
The AHRS should be mounted as close to the center of gravity (CG) of your
system as possible. This will minimize any “lever effect.” If it is not
mounted at the center of gravity, then rotations around the center of gravity
will cause the DMU accelerometers to measure an acceleration proportional
to the product of the angular rate squared and the distance between the
DMU and the CG.
The DMU will measure rotations around the axes of its sensors. The DMU
sensors are aligned with the DMU case. The sides of the DMU case are
used as reference surfaces for aligning the DMU sensor axes with your
system. You should align the DMU case as closely as possible with the
axes you define in your system. Errors in alignment will contribute directly
to errors in measured acceleration and rotation relative to your system axes.
The DMU should be isolated from vibration if possible. Vibration will
make the accelerometer readings noisy and can, therefore, affect the angle
calculations. In addition, if the magnitude of the vibration exceeds the
range of the accelerometer, the accelerometer output can saturate. This can
cause errors in the accelerometer output.
The AHRS400 Series should be isolated from magnetic material as much as
possible. Magnetic material will distort the magnetic field near the AHRS,
which will greatly affect its accuracy as a heading sensor. Because the
DMU is using Earth's weak magnetic field to measure heading, even small
amounts of magnetic material near the sensor can have large effects on the
heading measurement.
"Bad" materials include anything that will stick to a magnet: iron, carbon
steel, some stainless steels, nickel and cobalt. Use a magnet to test
materials that will be near the AHRS. If you discover something near the
DMU that is magnetic, attempt to replace it with something made from a
non-magnetic material. If you cannot change the material, move it as far as
possible from the DMU. Even small things, such as screws and washers,
can have a negative effect on the AHRS performance if they are close.
AHRS can correct for the effect of these magnetic fields by using hard and
soft iron calibration routine as explained in Appendix C.
"Good" materials include brass, plastic, titanium, wood, and some stainless
steels. Again, if in doubt, try to stick a magnet on the material. If the
magnet doesn't stick, you are using a good material.
DO NOT try to stick a magnet to the AHRS. We have removed as much
magnetic material as possible from the unit, but we could not make the unit
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