PNI CompassPoint Prime - Manual (Page 30)

PNI Sensor Corporation DOC# 1014177 r08
Prime User Manual – July 2011 Page 24
6.1 Magnetic Calibration Overview
The main objective of a magnetic field calibration is to compensate for distortions to the
magnetic field caused by the host system. To that end, the Prime needs to be mounted within
the host system and the entire system must be moved as a single unit during calibration.
6.1.1 Hard-Iron and Soft-Iron Effects
Hard-iron distortions are caused by permanent magnets and magnetized steel or iron
objects within close proximity to the sensors. This type of distortion remains constant
and in a fixed location relative to the sensors for all heading orientations. Hard-iron
distortions add a constant magnitude field component along each axis of sensor output.
Soft-iron distortions are the result of interactions between the Earth’s magnetic field and
any magnetically “soft” material within close proximity to the sensors. In technical
terms, soft materials have a high permeability. The permeability of a given material is a
measure of how well it serves as a path for magnetic lines of force, relative to air, which
has an assigned permeability of one. Unlike hard-iron distortion, soft-iron distortion
changes as the host system’s orientation changes, making it more difficult to compensate.
The Prime features both hard and soft-iron correction. For more information, see PNI’s
website for the white paper “Local Magnetic Distortion Effects on 3-Axis Compassing”.
6.1.2 Magnetic Calibration Limitations
The Prime measures the total magnetic field within its vicinity, and this is a combination
of Earth’s magnetic field and local magnetic sources. The Prime can compensate for
local static magnetic sources. However, a magnetic source which is not static (such as a
motor which turns on/off) can create errors, and it is not possible to compensate for such
a dynamic nature. In such cases, moving the Prime away from dynamic magnetic fields
is recommended, or taking measurements only when the state of the magnetic field is
known (ex. only take measurements when a nearby motor is turned off).
6.2 Field Calibration Procedures
The following sub-sections provide instructions for obtaining calibration points when
performing either magnetic sensor or accelerometer field calibrations.
Before proceeding with a calibration, the Prime should be properly installed in the host
system, as discussed in Section 4, and the software should be properly configured with
respect to the mounting orientation, Endianness, magnetic vs true north, etc.