DMU380ZA Series
User’s Manual
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Doc# 7430-3810 Rev. 02
Page 18
comStatus
10
0 = nominal, 1 = No External GPS Comm
Status
softwareStatus
11
0 = nominal, 1 = Algorithm Initialization or High Gain
Status
sensorStatus
12
0 = nominal, 1 = Sensor Over Range
Status
Reserved
13:15
N/A
The VG380ZA also allows a user to configure the Status byte within the BIT message.
To configure the word, select the BIT Configuration tab from the Unit Configuration
menu. The dialog box allows selection of which status types to enable (hardware,
software, sensor, and comm). Like the IMU380ZA, MEMSIC recommends for the vast
majority of users, that the default Status byte for the VG380ZA is sufficient. For users,
who wish to have additional visibility to when the VG380ZA EFK algorithm estimates
that the VG380ZA is turning about its Z or Yaw axis, the softwareStatus bit can be
configured to go high during a turn. In other words, the turnSwitch will turn on the
softwareStatus bit. In the VG380ZA, the turnSwitch is by default set at 10.0 deg/sec
about the z-axis.
AHRS380ZA Theory of Operation
3.4
The AHRS380ZA supports all of the features and operating modes of the IMU380ZA
and VG380ZA, and it includes an additional internal 3-axis magnetometer and associated
software running on the processor, for the computation of dynamic heading, as well as
dynamic roll and pitch. The product name, AHRS380ZA, stands for Attitude Heading
Reference System 380, and it is indicative of the attitude and heading reference
functionality that the AHRS380ZA replicates by providing dynamic heading, roll, and
pitch measurements, in addition to the VG and IMU data. The dynamic heading
measurement is stabilized using the 3-axis magnetometer as a magnetic north reference.
As in the VG380ZA, the dynamic roll and pitch measurements are stabilized using the
accelerometers as a long-term gravity reference. Unlike the AHRS400 and earlier
MEMSIC AHRS Series products, the AHRS380ZA can be configured to turn on and off
the magnetic reference for user defined periods of time (see Section 8 Advanced
Commands). In addition, the AHRS380ZA can accept external GPS data (refer to the
INS380ZA section for details) for improved performance.
At a fixed 200Hz rate, the AHRS380ZA continuously maintains the digital IMU data as
well as the dynamic roll, pitch, and heading. As shown in Figure 4, after the Sensor
Calibration Block, the IMU data is passed to the Integration to Orientation block. The
Integration to Orientation block integrates body frame sensed angular rate to orientation
at a fixed 200 times per second within all of the DMU380ZA Series products. 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.
As also shown in the software block diagram, the Integration to Orientation block
receives drift corrections from the Extended Kalman Filter or Drift Correction Module. In
general, rate sensors and accelerometers suffer from bias drift, misalignment errors,
acceleration errors (g-sensitivity), nonlinearity (square terms), and scale factor errors.
The largest error in the orientation propagation is associated with the rate sensor bias
terms. The Extended Kalman Filter (EKF) module provides an on-the-fly calibration for
drift errors, including the rate sensor bias, by providing corrections to the Integration to
Orientation block and a characterization of the gyro bias state. In the AHRS380ZA, the
internally computed gravity reference vector and the distortion corrected magnetic field
vector provide an attitude and a heading reference measurement for the EKF when the
AHRS380ZA is in quasi-static motion to correct roll, pitch, and heading angle drift and to
