AHRS400 Series User’s Manual
Page 10 Doc# 7430-0004-01 Rev. A
The AHRS Kalman filter is not enabled in scaled sensor mode. Therefore,
the rate sensor bias will change slightly due to large changes in temperature
and time. If the unit is changed from angle to scaled mode, the last
estimated rate sensor bias values are used upon entering scaled mode.
3.4.3 Angle Mode
In angle mode, the DMU acts as a complete attitude and heading reference
system and outputs the stabilized pitch, roll, and yaw angles along with the
angular rate, acceleration, and magnetic field information. The angular rate,
acceleration, and magnetic field values are calculated as described in the
scaled sensor mode.
The DMU analog outputs are enabled in this mode, including stabilized
pitch, roll, and yaw angles.
The Kalman filter operates in angle mode to track the rate sensor bias and
calculate the stabilized roll, pitch, and yaw angles.
In angle mode, the DMU uses the angular rate sensors to integrate over your
rotational motion and find the actual pitch, roll, and yaw angles. The DMU
uses the accelerometers to correct for rate sensor drift in the vertical angles
(pitch and roll); the DMU uses the magnetometers to correct for rate sensor
drift in the yaw angle. This is the modern equivalent of an analog vertical
gyro that used a plumb bob in a feedback loop to keep the gyro axis
stabilized to vertical. The DMU takes advantage of the rate gyros’
sensitivity to quick motions to maintain an accurate orientation when
accelerations would otherwise throw off the accelerometers measurement of
the DMU orientation relative to gravity; the DMU then uses the
accelerometers to provide long term stability to keep the rate gyro drift in
check.
The AHRS400CA and AHRS400CB use a sophisticated Kalman filter
algorithm to track the bias in the rate sensors. This allows the DMU to use
a very low effective weighting on the accelerometers when the DMU is
moved. This makes the DMU very accurate in dynamic maneuvers. Unlike
other Crossbow DMU systems, the user does not need to set an erection
rate.
The AHRS outputs the stabilized pitch, roll and yaw angles in the digital
data packet in angle mode. To convert the digital data to angle, use the
following relation:
angle = data*(SCALE)/2
15
where angle is the actual angle in degrees (pitch, roll or yaw), data is the
signed integer data output in the data packet, and SCALE is a constant.
SCALE = 180° for roll, pitch and yaw.
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