PNI SeaTRAX - Manual (Page 23)

PNI Sensor Corporation DOC#1018154 r02
SeaTRAX User Manual Page 18
gravitational vector, which in turn will result in an inaccurate heading reading. For this
reason, the SeaTRAX should be stationary when taking a measurement.
As previously mentioned, PNI calibrates the accelerometer in its factory prior to shipment.
But over time the bias and offset of the accelerometer will drift. For this reason PNI
recommends the accelerometer be recalibrated every 6 to 12 months. The user may return
the SeaTRAX to PNI for accelerometer calibration, or the user may perform a user
accelerometer calibration. The remainder of this section covers the user accelerometer
calibration.
5.2.1 Accelerometer-Only Calibration
The requirements for a good user accelerometer calibration differ significantly from the
requirements for a good magnetic calibration. Specifically, a good accelerometer
calibration involves the SeaTRAX experiencing a wide range of pitch and roll values,
preferably seeing both ±180° of pitch and ±180° of roll. Also, it is necessary for the
SeaTRAX to be very still during an accelerometer calibration. If possible, PNI
recommends using a fixture to hold the device during calibration, although resting the
SeaTRAX on a hard surface normally is sufficient.
The accelerometer either can be calibrated while mounted in the host system or it may be
removed and calibrated outside the system. The advantage of performing the calibration
while mounted in the host system is the user does not need to remove the SeaTRAX from
the system, which can be burdensome, and a simultaneous Mag-and-Accel Calibration
may be appropriate. The advantage of performing the calibration outside of the system is
it may be much simpler to obtain the desired range of pitch and roll.
Note: While the SeaTRAX is shown removed from the host system, the Accelerometer
Calibration may be performed with the SeaTRAX mounted in the host system.
Figure 5-2 shows the two basic starting positions for the recommended 18-point
calibration pattern. Starting with the SeaTRAX as shown on the left in Note: While the
SeaTRAX is shown removed from the host system, the Accelerometer Calibration may
be performed with the SeaTRAX mounted in the host system.
Figure 5-2, rotate the device about its z axis such that it sits on each of its 4 edges, taking
one calibration sample on each edge. Then place the SeaTRAX flat on the surface and
take a calibration sample, then flip it over (roll it 180°) and take another sample. Next,
starting with the SeaTRAX as shown on the right, take a calibration point with it being
vertical (0°). Now tilt the SeaTRAX back 45° and take another calibration point (+45°),
then tilt the device forward 45° and take another calibration point (-45°). Repeat this 3-
point calibration process for the SeaTRAX with it resting on each of its 4 corners. Note
that it is possible to perform an Accelerometer Calibration with as few as 12 sample