DMU380SA Series
User’s Manual
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Doc# 7430-0026 Rev.01
Page 35
Table 12.
AHRS380SA Series Advanced Settings
Setting
Default
Comments
Baud Rate
38,400
baud
9600, 19200, 57600 also available
Packet
Type
A1
S0, S1, A2, N0, N1 also available
Packet
Rate
25 Hz
This setting sets the rate at which selected Packet Type, packets are output. If polled
mode is desired, then select Quiet. If Quiet is selected, the VG380SA will only send
measurement packets in response to GP commands.
Orientation
See
Fig. 12
To configure the axis orientation, select the desired measurement for each axes: NAV-
VIEW will show the corresponding image of the AHRS380SA, so it easy to visualize the
mode of operation. See section 8.4 Orientation Field settings for the twenty four possible
orientation settings. The default setting points the connector AFT.
Filter
Settings
20 Hz
accels
20 Hz
rates
The low pass filters are set to a default of 20Hz for the accelerometers, and 20Hz for the
angular rate sensors. There is one filter setting for all three angular rate sensors. There
is one filter setting for all three accelerometer sensors. The reason for filtering the
accelerometers is that in many installations, the vibration level can be high, and it can
prove helpful to filter accelerometers.
Freely
Integrate
OFF
The Freely Integrate setting allows a user to turn the AHRS380SA
into a ‘free gyro’.
In
free gyro mode, the roll, pitch and yaw are computed exclusively from angular rate with
no kalman filter based corrections of roll, pitch, or yaw. When turned on, there is no
coupling of acceleration based signals into the roll and pitch or magnetometer based
signals to the yaw. As a result, the roll, pitch, and yaw outputs will drift roughly linearly
with time due to sensor bias. For best performance, the Freely Integrate mode should be
used after the algorithm has initialized. This allows the Kalman Filter to estimate the roll
and pitch rate sensor bias prior to entering the free gyro mode.
Upon exiting the ‘free
gyro’ mode
(OFF), one of two behaviors will occur
(1) If the AHRS380SA has been in freely integrate mode for less than sixty
seconds, the algorithm will resume operation at normal gain settings
(2) If the AHRS380SA has been in freely integrate mode for greater than sixty
seconds, the algorithm will force a reset and reinitialize with high gains
automatically.
Use Mags
ON
The Use Mags setting allows users to turn on and off the magnetometer feedback
for yaw/heading stabilization. The default setting is ON for the AHRS380SA. When
Use Mags is turned ON, the AHRS380SA uses the magnetic field sensor readings
to stabilize the drift in yaw, and it slaves the yaw to the compass reading provided
from the magnetic field sensor readings. When Use Mags is turned OFF, the
heading (yaw) angle measurement of the AHRS380SA will drift and freely integrate.
In effect, this setting converts an AHRS380SA into the functionality of the
VG380SA. However, unlike a VG380SA this can be done on a selectable basis and
changed in real time during a mission. The reason for this setting is to give the user
an ability to turn off the magnetometer stabilization when severe magnetic distortion
may be occurring. This setting is desirable when the user system temporarily
moves in close proximity to a large ferrous object. When the Use Mags switch is
turned from OFF to ON, the AHRS380SA will reinitialize the yaw/heading angle
with the compass reading provided from the magnetic field sensor readings.
Restart On
Over
Range
OFF
This setting forces an algorithm reset when a sensor over range occurs i.e., a rotational
rate on any of the three axes exceeds the maximum range. The default setting is OFF for
the AHRS380SA. Algorithm reset returns the AHRS380SA to a high gain state, where
the AHRS380SA rapidly estimates the gyro bias and uses the accelerometer feedback
heavily. This setting is recommended when the source of over-range is likely to be
sustained and potentially much greater than the rate sensor operating limit. Large and
sustained angular rate over-ranges result in unrecoverable errors in roll and pitch
outputs. An unrecoverable error is one where the EKF can not stabilize the resulting roll
and pitch reading. If the over-ranges are expected to be of short duration (<1 sec) and a
modest percentage over the maximum operating range, it is recommended that the
