PNI SeaTRAX - Manual (Page 18)

PNI Sensor Corporation DOC#1018154 r02
SeaTRAX User Manual Page 13
Other Considerations
The SeaTRAX measures the total magnetic field within its vicinity, which is a
combination of Earth’s magnetic field and local magnetic sources and distortions.
While the SeaTRAX’s calibration algorithm can compensate for local static magnetic
sources, it is not possible to compensate for dynamic changes in the magnetic field.
Consequently, it is recommended to keep the SeaTRAX away from dynamic
magnetic fields. If this is not possible, then take measurements only when the state of
the magnetic field is known. For example, if an electric motor is nearby take
measurements only when the motor is off. Alternatively, different sets of magnetic
calibration coefficients can be generated in advance for various states and then called
when appropriate. Using the prior example, generate and use one set of coefficients
for when the motor is off and another set for when the motor is on.
The main objective of a magnetic user calibration is to compensate for hard-iron and soft-
iron distortions to the magnetic field caused by components within the user’s host system.
To that end, the SeaTRAX needs to be mounted within the host system and the entire host
system needs to be moved as a single unit during a user calibration. The SeaTRAX allows
the user to perform a calibration only in a 2D plane or with limited tilt, but provides the
greatest accuracy if the user can rotate through 360° of heading and at least ±45°of tilt.
The following subsections provide instructions for performing a magnetic calibration of a
SeaTRAX system. Several calibration mode options exist, as summarized in Table 5-1. To
meet the accuracy specification, the number of samples should be the “Minimum
Recommended” value, or greater. Calibration may be performed using Studio or using the
PNI binary protocol, and up to 8 sets of magnetic calibration coefficients may be saved. The
recommended calibration patterns described in the following subsections provide a good
distribution of sample points. Also, PNI recommends the location of the SeaTRAX remain
fairly constant while only the orientation is changed.
Table 5-1: Magnetic Calibration Mode Summary
Calibration
Mode
Static Accuracy
Tilt Range
during Cal
Minimum
Recommended
# of Samples
Allowable #
of Samples
Full-Range 0.3° rms >±45° 12 10 – 32
2D Calibration <2° <±5° 12 10
– 32
Limited-Tilt
<2° over 2x tilt
range
±5° to ±45° 12 10 – 32
Hard-Iron-Only
Restores prior
accuracy
>±3° 6 4 - 32