PNI TCM 5 - Manual (Page 22)

PNI Sensor Corporation Doc #1007537 r12
TCM3 & TCM5 User Manual- Sept 2011 Page 22
4.2 User Calibration
All compasses can perform well in a controlled environment, where the ambient magnetic field consists
solely of the earth’s field. In most practical applications, however, an electronic compass module will be
mounted in a host system such as a vehicle that can contain large sources of local magnetic fields:
ferrous metal chassis, transformer cores, electrical currents, and permanent magnets in electric motors.
By performing the user calibration procedure, you allow the TCM to identify the major sources of these
local magnetic anomalies and subsequently cancel out their effects when measuring the earth’s magnetic
field for computing compass headings. When you perform the user calibration procedure, the TCM takes
a series of magnetic field measurements. It analyzes these total field measurements in order to identify
the components that are created by the earth’s field, which is the desired signal, from those components
that are generated by the local environment, which we wish to subtract out.
The end goal of the procedure for the TCM is to have an accurate measurement of the static three-
dimensional magnetic field vector generated by its host system at its mounting location. This vector is
subsequently subtracted out of run- time field measurement to yield the resultant earth’s field vector.
One major benefit from the TCM ’s triaxial magnetometer/triaxial accelerometer system configuration is its
ability to compensate for distortion effects in all orientations throughout its usable tilt range. As we have
mentioned, a compass must measure the local field vector generated by the host system at its current
position within the system in order to accurately calibrate. Because the TCM ’s magnetometer is strapped-
down, or fixed with respect to its host system, this local field vector does not change as the host system’s
attitude changes, allowing the TCM to accurately compensate in all pitch and roll orientations. Gimbaled
fluxgates, for instance, are unable to provide accurate calibration in non-level orientations because its
magnetometers, being gimbaled, change position with respect to the host system as attitude changes.
This presents a different local distortion field than that measured during calibration.
Key Points

The minimum points the unit can use for a successful calibration is 12.

The unit will need to be rotated through at least 180 degrees in the horizontal plane including at
least 1 positive and 1 negative Pitch and Roll movement.

Tilt as much as possible during the calibration. This allows the compass to take full advantage of
the 3-axis magnetometer.

You are trying to get an even sampling of the magnetic field over as many headings and tilts as
possible, including upside down if possible.

Pay attention to the coverage percentage. The lower the percentage the less accurate the com-
pass.