Moog Crossbow GNAV540 - Manual (Page 39)

7430‐0808‐01 Rev. B
GNAV540 User Manual Page 39
Chapter 4. Magnetometer Calibration and Alignment Guidelines
This cha ing and aligning a magnetometer with the GNAV540 unit. pter provides general guidelines for calibrat
• Compensation for Magnetic Fields, page 39
• Magnetometer Alignment Using NAV‐VIEW 2.2
•
Code
, page 39
Magnetometer Alignment Using , page 40
• Installation Guidelines, page 40
This section provides guidelines to calibrate and align the magnetometer. This information applies when
implementing AHRS Function or NAV Function (AHRS Function on page 29, NAV Function on page 31).
NOTE: For effective calibration results, the GNAV540 unit must be installed in the system during the alignment
process. This also applies to an external magnetometer. If calibrated outside of the system, the magnetism of
the system will not be measured. Without those values, the GNAV540 will not be able to compensate magnetic
readings: either heading readings will be incorrect or error messages will be displayed.
Compensation for Magnetic Fields
Magnetic sensors measure magnetic fields which are then used to compute heading. A correct heading angle is based
only on the earth’s magnetic field. However, the magnetic fields of the GNAV540 unit and the surroundings (such as
the system/vehicle in which the unit is installed) are also measured, which affect the magnetic reading. To
compensate for these magnetic fields and ensure accurate heading readings, the GNAV540 unit must be calibrated.
The GNAV540 unit compensates for the extra magnetic field(s) by taking a series of measurements and then
applying those measurements to a two‐dimensional algorithm. The algorithm is used to calculate the hard iron and
soft iron effects, the values of which are then stored as constants in the EEPROM of the unit. Those constants are
used for correcting the magnetic readings, ensuring accurate heading output. Hard iron effects are shifts in the
magnetic field from ferrous objects or other magnetic material in the proximity of the magnetic sensor. Soft iron
effects are the change of direction of a magnetic field; this change is caused by hard iron on the input direction. Hard
n magnetic fields are temporary: soft iron can be magnetized
ng operation.
iron magnetic fields are permanent whereas soft iro
and then demagnetized, or have varying effects duri
Follo in
w g is an overview of the calibration process:
1. Install the GNAV540 in the system in which it will be used
2. If an external magnetometer is used, install that device in the system
netic interference 3. Select an appropriate test location, free of mag
4. Run the calibration and alignment procedure
5. Set the calibration readings in the GNAV540
NOTE: Ensure the test location is appropriate for magnetic calibration. The calibration process provides corrections
for magnetic fields from fixed locations that are relative to the position of the unit. The calibration does not
compensate for time varying fields, or fields created by magnetic material that moves relative to the
GNAV540 unit.
Magnetometer Alignment Using NAV‐VIEW 2.2
During the calibration procedure, the unit makes a series of measurements while the user system is being turned
through a complete 360 degree circle. A 360 degree rotation gives the unit visibility to hard and soft iron distortion