Moog Crossbow GNAV540 - Manual (Page 29)

GNAV540 User Manual
7430‐0808‐01 Rev. B Page 29
AHRS Function
The Attitude Heading Reference System (AHRS) Function utilizes a 3‐axis magnetometer (internal or external) in
addition to the accelerometers and gyroscopes, as well as the associated software running on the DSP processor.
This enables the computation of dynamic heading, as well as dynamic roll and pitch. AHRS Function provides
dynamic heading, roll, and pitch measurements in addition to the VG Function and IMU Function data. The dynamic
heading measurement is stabilized using the 3‐axis magnetometer as a magnetic north reference. The dynamic roll
and pitch measurements are stabilized using the accelerometers as a long‐term gravity reference. The unit can be
configured to turn on and off the magnetic reference for user defined periods of time.
This function utilizes data from calibrated sensors, the gyroscopes, the accelerometers and a magnetometer
(internal or external). For details of the IMU functions, refer to n IMU Functio on page
functions, refer to
25. For details of the VG
. Vertical Gyroscope (VG) Function on page 26
In addition to the features described in the previous sections ( and ),
the AHRS algorithm has two major phases of operation. The first phase of operation is the high‐gain initialization
phase. During the initialization phase, the unit 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 unit. 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 unit 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
IMU Function Vertical Gyroscope (VG) Function
heading (yaw) errors, as well as to estimate X, Y, and Z rate sensor bias.
The AHRS Function provides the same scaled sensor and angle mode packets of the VG Function. The AHRS Function
defaults to the A1 Angle Packet which outputs the roll angle, pitch angle, yaw angle, and digital IMU data. In the
AHRS Function , the A0 and A1 packets contain accurate magnetometer readings. For more information, refer to
Chapter 9. Communicating with the GNAV540 Unit and Chapter 10. Programming Guidelines for packet descriptions.
NOTE: For proper operation, the unit relies on magnetic field readings from a 3‐axis magnetometer. The unit must
be installed correctly and calibrated for hard‐iron and soft iron effects to avoid any system performance
degradation. Refer to Chapter 4. Magnetometer Calibration and Alignment Guidelines for information about
magnetic calibration; review that section before using the AHRS Function.
NOTE: The GNAV540 unit and the external magnetometer (if used) must be mounted at least 24” away from large
ferrous objects and fluctuating magnetic fields. Failure to locate the unit in a clean magnetic environment will
affect the attitude solution.
AHRS Advanced Settings
In addition to the configurable baud rate, packet rate, axis orientation, and sensor low‐pass filter settings, the unit
provides additional advanced settings which are selectable for tailoring the unit to a specific application
requirements. The AHRS advanced settings are listed in Table 9 below:
Table 9 AHRS Series Advanced Settings
Setting Default Value Comments
Baud Rate 38400 9600, 19200, 57600 also available
Packet Type A1 S0, S1, S2, A0, A2, N0, N1 also available
Packet Rate 25 Hz This setting sets the rate at which selected Packet Type, packets are output. If
polled mode is desired, then select Quiet.