DMU380ZA Series
User’s Manual
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Doc# 7430-3810 Rev.02
Page 19
estimate the X, Y and Z gyro rate bias. The AHRS380ZA adaptively tunes the EKF
feedback gains in order to best balance the bias estimation and attitude correction with
distortion free performance during dynamics when the object is accelerating either
linearly (speed changes) or centripetally (false gravity forces from turns). Because
centripetal and other dynamic accelerations are often associated with yaw rate, the
AHRS380ZA maintains a low-passed filtered yaw rate signal and compares it to the
turnSwitch threshold field (user adjustable). When the user platform (with the
AHRS380ZA attached) exceeds the turnSwitch threshold yaw rate, the AHRS380ZA
lowers the feedback gains from the accelerometers to allow the attitude estimate to coast
through the dynamic situation with primary reliance on angular rate sensors. This
situation is indicated by the softwareStatus
turnSwitch status flag. Using the turn switch
maintains better attitude accuracy during short-term dynamic situations, but care must be
taken to ensure that the duty cycle of the turn switch generally stays below 10% during
the vehicle mission. A high turn switch duty cycle does not allow the system to apply
enough rate sensor bias correction and could allow the attitude estimate to become
unstable.
As described in 3.3 VG380ZA theory of operation, the AHRS380ZA algorithm also has
two major phases of operation. The first phase of operation is the high-gain initialization
phase. During the initialization phase, the AHRS380ZA is expected to be stationary or
quasi-static so
the EKF weights the accelerometer gravity reference and Earth’s magnetic
field reference heavily in order to rapidly estimate the X, Y, and Z rate sensor bias, and
the initial attitude and heading of the AHRS380ZA. The initialization phase lasts
approximately 60 seconds, and the initialization phase can be monitored in the
softwareStatus BIT transmitted by default in each measurement packet. After the
initialization phase, the AHRS380ZA operates with lower levels of feedback (also
referred to as EKF gain) from the accelerometers and magnetometers to continuously
estimate and correct for roll, pitch, and heading (yaw) errors, as well as to estimate X, Y,
and Z rate sensor bias.
The AHRS380ZA digital data is output over the UART port at a selectable fixed rate
(100, 50, 25, 20, 10,
5 or 2 Hz) or on as requested basis using the GP, ‘Get Packet’
command. The AHRS400 supports the same scaled sensor and angle mode packet format
of the VG380ZA. The AHRS380ZA
defaults to the ‘A1’ Angle Packet which outputs the
roll angle, pitch angle, yaw angle, and digital IMU data. In the AHRS380ZA, the
‘A1’
packet contains accurate magnetometer readings. See Sections 6 and 7 of the manual for
full packet descriptions. All data is also available on the SPI output port registers. Please
refer to section 5 for a complete description of the SPI port functionality.
IMPORTANT
For proper operation, the AHRS380ZA relies on magnetic field readings from its internal
3-axis magnetometer. The AHRS380ZA must be installed correctly and calibrated for
hard-iron and soft iron effects to avoid any system performance degradation. See section
3.4.1 for information and tips regarding installation and calibration.
3.4.1
AHRS380ZA Magnetometer Calibration and Alignment
The AHRS380ZA uses magnetic sensors to compute heading. Ideally, the magnetic
sensors would measure only the earth's magnetic field to compute the heading angle. In
the real world, however, residual magnetism in your system will add to the magnetic field
measured by the AHRS380ZA. This extra magnetic field will create errors in the heading
measurement if they are not accounted for. These extra magnetic fields are called hard
iron magnetic fields. In addition, magnetic material can change the direction of the
magnetic field as a function of the input magnetic field. This dependence of the local
