DMU380SA Series
User’s Manual
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Doc# 7430-0026 Rev. 01
Page 32
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 4.3 VG380SA theory of operation, the AHRS380SA algorithm also has
two major phases of operation. The first phase of operation is the high-gain initialization
phase. During the initialization phase, the AHRS380SA 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 AHRS380SA. 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 AHRS380SA 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 AHRS380SA digital data is output over the RS-232 serial link 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 VG380SA. The AHRS380SA
defaults to the ‘A1’ Angle Packet which outputs the
roll angle, pitch angle, yaw angle, and digital IMU data. In the AHRS380SA, the
‘A1’
packets contain accurate magnetometer readings. See Sections 6 and 7 of the manual for
full packet descriptions.
IMPORTANT
For proper operation, the AHRS380SA relies on magnetic field readings from its internal
3-axis magnetometer. The AHRS380SA must be installed correctly and calibrated for
hard-iron and soft iron effects to avoid any system performance degradation. See section
4.4.1 for information and tips regarding installation and calibration.
4.4.1
AHRS380SA Magnetometer Calibration and Alignment
The AHRS380SA 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 AHRS380SA. 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
magnetic field on input direction is called the soft iron effect. The AHRS380SA can
actually measure any constant magnetic field that is associated with your system and
correct for it. The AHRS380SA can also make a correction for some soft iron effects.
The process of measuring these non-ideal effects and correcting for them is called hard
iron and soft iron calibration. This calibration will help correct for magnetic fields that
are fixed with respect to the AHRS380SA. It cannot help for time varying fields, or fields
created by parts that move with respect to the AHRS380SA. Because time varying fields
cannot be compensated, selection of a proper installation location is important.