PNI TCM - Manual (Page 24)

PNI Sensor Corporation DOC#1014688 r06
TCM User Manual – July 2013 Page 19
vector, 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.