PNI TCM - Manual (Page 17)

PNI Sensor Corporation DOC#1014688 r06
TCM User Manual – July 2013 Page 12
5 User Calibration
The magnetic sensors in the TCM are calibrated at PNI’s factory in a magnetically controlled
environment. However sources of magnetic distortion positioned near the TCM in the user’s
system will distort Earth’s magnetic field and should be compensated for in the host system with
a user calibration. Examples of such sources include ferrous metals and alloys (ex. iron, nickel,
steel, etc.), batteries, audio speakers, current-carrying wires, and electric motors. Compensation
is accomplished by mounting the TCM in the host system and performing a user calibration. It is
expected the sources of magnetic distortion remain fixed relative to the TCM’s position within
the host system. By performing a calibration, the TCM identifies the local sources of magnetic
distortion and negates their effects from the overall reading to provide an accurate heading.
As with the magnetic sensor, the accelerometer in the TCM is calibrated at PNI’s factory. But
the accelerometer will gradually change over time, and the user either will need to periodically
perform a user accelerometer calibration or return the unit to PNI for recalibration. As a general
rule-of-thumb, the accelerometer should be recalibrated every 6 to 12 months. Unlike a
magnetic calibration, the accelerometer may be calibrated outside the host system.
Accelerometer calibration is more sensitive to noise or hand jitter than magnetic calibration,
especially for subsequent use at high tilt angles. Because of this, ideally a stabilized fixture
would be used for accelerometer calibration, although resting the unit against a stable surface
often is sufficient.
Key Points:
Magnetic calibration:
o Requires incorporating the TCM into the host system to compensate for magnetic
sourcing and distorting components with the user’s system.
o Allows for 4 different methods of calibration. Full-Range Calibration provides
the highest heading accuracy, while 2D and Limited-Tilt Calibration support a
limited range of motion during calibration. Hard-Iron-Only Calibration updates
just the hard-iron coefficients with a relatively easy procedure.
Accelerometer calibration requires rotating the TCM through a full sphere of coverage,
but the TCM does not need to be incorporated into the user’s system during calibration.
If the TCM will experience different states during operation, such as operating with a
nearby shutter sometimes closed and sometimes open, or operating over a broad
temperature range, then different sets of calibration coefficients can be saved for the
various states. Up to 8 magnetic calibration coefficient sets and 3 accelerometer
calibration coefficient sets can be saved.