DMU380ZA Series
User’s Manual
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Doc# 7430-3810 Rev. 02
Page 20
magnetic field on input direction is called the soft iron effect. The AHRS380ZA can
actually measure any constant magnetic field that is associated with your system and
correct for it. The AHRS380ZA 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 AHRS380ZA. It cannot help for time varying fields, or fields
created by parts that move with respect to the AHRS380ZA. Because time varying fields
cannot be compensated, selection of a proper installation location is important.
During the calibration procedure, the AHRS380ZA makes a series of measurements
while the user system is being turned through a complete 360 degree circle. A 360 degree
rotation gives the AHRS380ZA visibility to hard and soft iron distortion in the horizontal
plane. Using NAV-VIEW, a user can see the hard and soft iron effects by selecting the
Misalignment option on the Configuration Menu, and viewing the magnetic circle during
the calibration.
The AHRS380ZA uses these measurements to model the hard iron and soft iron
environment in your system, and store these as calibration constants in the EEPROM.
The status of the AHRS380ZA magnetometer calibration is indicated by the
softwareError
dataError
magAlignOutOfBounds
error flag available in the ‘T0’
packet. The current release of this software does not currently implement this feature
however. In future releases, this functionality will be restored. The user can access the
hardIron and softIronScaleRatio calibration data as configuration fields in NAV-VIEW,
or by using the communication protocol over UART or SPI. Also, the softwareError bit
of the masterFail byte within the BIT word is transmitted in every measurement packet.
When the AHRS380ZA has not been properly calibrated, this softwareError bit will be
set to fail (high). The current release of this software does not currently implement this
feature however. In future releases, this functionality will be restored.
In order for the AHRS380ZA calibration to work properly, the AHRS380ZA must be
installed in your system prior to calibration. If you perform the calibration process with
the AHRS380ZA by itself, you will only correct for the magnetism in the AHRS380ZA
itself. If you then install the AHRS380ZA in a vehicle (for instance), and the vehicle is
magnetic, you will still see errors arising from the magnetism of the vehicle. The
AHRS380ZA must be calibrated after installation and prior to use of the system
The AHRS380ZA also provides a command interface for initiating the hard iron / soft
iron calibration without the using NAV-VIEW. The user can send a
‘WC’ command
to
initiate the calibration, and then rotate the user system through 360 degrees.
The ‘WC’
command has two options
–
auto-termination and manual termination. With, auto-
termination, the AHRS380ZA tracks the yaw movement and after 380 degrees of rotation
returns the ca
libration complete response, ‘CD’.
The auto-termination sequence can
falsely terminate if the 360 degree rotation is not completed within 2
minutes of the ‘WC’
command initiation.
Manual termination requires the user to send a second ‘WC’
command with the termination code in the payload. Manual termination is a good option
when the user system moves very slowly (e.g., large marine vessel) and completing the
360 degree rotation may require more than two minutes.
The calibration
complete, ‘CD’, command response message contains the X and Y hard
iron bias, as well as the soft iron ratio and soft iron phase angle. This information can be
interpreted to give an indication of the quality of the calibration. See the section Hard
Iron/Soft Iron Overview in Appendix A: Installation and Operation of NAV-VIEW for
more information on the hard iron bias and soft iron ratio. Section 7 has programming
details for the ‘WC’ and ‘CD’ commands.
WARNING