PNI Sensor Corporation
DOC#1014688 r09.2
TCM User Manual
Page 19
which in turn will result in an inaccurate heading reading. For this reason, the TCM 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 TCM 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 TCM 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 TCM to be very
still during an accelerometer calibration. If possible, PNI recommends using a fixture to
hold the device during calibration, although resting the TCM 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 TCM 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.
Figure 5-2 shows the two basic starting positions for the recommended 18-point calibration
pattern. Starting with the TCM as shown on the left in 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 TCM flat on the surface and take a calibration sample, then flip it over (roll
it 180°) and take another sample. Next, starting with the TCM as shown on the right, take
a calibration point with it being vertical (0°). Now tilt the TCM 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 TCM 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 points, although it generally is more difficult to obtain a good calibration with just
12 sample points. Also, the maximum number of calibration points is 18.