PNI TCM 2.6 - Manual (Page 19)

PNI Sensor Corporation DOC#1009269 r11
TCM2.5 & TCM2.6 User Manual Page 19 of 49
Interpreting the Calibration Score
The TCM2.5/2.6 provides feedback on calibration through the calibration score, which has
the follow
ing format: “...HnVnMn.n....”
The first two numbers in the calibration score, HnVn, respectively describe the quality of the
calibration for the horizontal component and vertical component of the host system’s local
mag
netic field. Higher numbers reflect higher quality. The highest possible score is a “9”.
The factors that contribute to a good score for Hn and Vn are as follows:
a good, magnetically quiet location was chosen for the user calibration procedure.
the magnetic environment is stable; there are no large sources of changing fields.
the calibration data points included changes in system heading and inclination to
allow for proper measurement of the horizontal and vertical field vectors.
there are no significant soft-iron distortion effects.
The last number in the score, Mn.n, describes the magnitude of local field. Larger numbers
denote strong local fields. Small local fields are preferable, since less correction will be
necessary, and they utilize less of the magnetometers’ dynamic range. A magnitude score
greater than 30 indicates strong magnetic fields at the TCM2.5/2.6 location; you should
consider alternative mounting locations. Any score less than 10 is very good.
Note: The calibration score values mostly provide a qualitative estimation. For example, a
good score would be H9V9Mn.n. A poor score as anything less than H9V9Mn.n. A poor V
score generally indicates that you need to tilt more during the calibration. A poor H score
indicates you did not turn two full circles or that you turned too quickly.
Soft Iron Effects
The TCM2.5/2.6 can calibrate for hard iron effects, or local fields that can be modeled as
static fields such as those created by permanent magnets. Hard iron distortions are
significant in most systems. There is another class of iron effects, soft iron, that are an
amplification of magnetic fields created by highly permeable materials, such as ferrous
metals. The TCM2.5/2.6 does not compensate for soft iron effects. Soft iron effects,
however, are generally far weaker than hard iron effects in most systems, and can be more
readily defeated by choosing a suitable location to mount your compass module.
In some systems, however, it may be difficult to avoid large masses of ferrous metal that
may create non-trivial soft iron effects, such as an armor plate in a tracked vehicle. In these
instances, try to locate the module as far away from the ferrous metals as possible. Soft
iron effects decrease with distance by an inverse square relation so even modest
separation can be effective.
Other Limitations
As discussed, the TCM2.5/2.6 models local disturbances as a static magnetic vector
contribution to the earth’s field. Any local fields, which are not static, will create errors. You
cannot calibrate for anomalies that are not fixed with respect to the compass. For example,
you may know that the TCM2.5/2.6 will be used in close proximity to other vehicles. You
cannot calibrate for the effects of these other vehicles, as they will be moving with respect
to the TCM2.5/2.6. This is a limitation universal to all compasses. Consider, therefore, the
TCM2.5/2.6’s position relative to any potential sources of field that will not be static:
magnetic cargo or payloads that may be placed in close proximity, fans or other electrical
equipment that may be turned on and off, and so on.