Moog Crossbow GNAV540 - Manual (Page 31)

GNAV540 User Manual
7430‐0808‐01 Rev. B Page 31
Setting Default Value Comments
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. Algorithm reset returns the unit to a high gain state, where the
ack unit rapidly estimates the gyroscope bias and uses the accelerometer feedb
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 cannot
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
restart on over range setting be turned off. Handling of an inertial rate sensor
over‐range is controlled using the restartOnOverRange switch. If this switch is
off, the system will flag the overRange status flag and continue to operate through
it. If this switch is on, the system will flag a masterFail error during an over‐range
condition and continue to operate with this flag until a quasi‐static condition is
met to allow for an algorithm restart.
The quasi‐static condition required is that the absolute value of each low‐pass
rate sensor falls below 3 deg/sec to begin initialization. The system will then
attempt a normal algorithm start.
Dynamic
Motion ON The default setting is ON. Turning off the dynamic motion setting results in a
higher gain state that uses the accelerometer feedback heavily. During periods of
d off time when there is known low dynamic acceleration, this switch can be turne
to allow the attitude estimate to quickly stabilize.
Turn Switch
threshold 0.5 deg/sec With respect to centripetal or false gravity forces from turning dynamics (or
coordinated turn), the unit monitors the yaw‐rate. If the yaw rate exceeds a given
TurnSwitch threshold, the feedback gains from the accelerometer signals for
attitude correction are reduced because they are likely corrupted.
NAV Function
The NAV Function supports all the features of the IMU, VG and AHRS functions. In addition, the NAV function
integrates the sensor information from an internal or external GPS receiver, and runs internal software on the DSP
processor for computing navigation and orientation information. In this function, the unit outputs GPS information
(Latitude, Longitude, and Altitude), inertial‐aided 3‐axis velocity information, as well as heading, roll, and pitch
measurements in addition to digital IMU data.
At a fixed 100Hz rate, the unit continuously maintains the following: digital IMU data the dynamic roll, pitch, and
heading data, as well as the navigation data. As shown in Figure 2 on page 23, after the Sensor Calibration block, the
IMU data is passed into an Integration to Orientation block. The Integration to Orientation block integrates body
frame sensed angular rate to orientation at a fixed 100 times per second. For improved accuracy and to avoid
singularities when dealing with the cosine rotation matrix, a quaternion formulation is used in the algorithm to
provide attitude propagation. Following the integration to orientation block, the body frame accelerometer signals
are rotated into the NED level frame and are integrated to velocity. At this point, the data is blended with GPS
, and ou position data tput as a complete navigation solution.
As shown in Figure 2 on page 23, the Integration to Orientation and the Integration to Velocity signal processing
blocks receive drift corrections from the Extended Kalman Filter (EKF) drift correction module. The drift correction