PNI TCM - Manual (Page 19)

PNI Sensor Corporation DOC#1014688 r06
TCM User Manual – July 2013 Page 14
Other Considerations
The TCM measures the total magnetic field within its vicinity, which is a
combination of Earth’s magnetic field and local magnetic sources and distortions.
While the TCM’s calibration algorithms can compensate for local static magnetic
sources, it is not possible to compensate for dynamic changes in the magnetic field.
Consequently, it is recommended to keep the TCM away from dynamic magnetic
fields. If this is not possible, then take measurements only when the state of the
magnetic field is known. For example, if an electric motor is nearby take
measurements only when the motor is off. Alternatively, different sets of magnetic
calibration coefficients can be generated in advance for various states and then called
when appropriate. Using the prior example, generate and use one set of coefficients
for when the motor is off and another set for when the motor is on.
The main objective of a magnetic user calibration is to compensate for hard-iron and soft-
iron distortions to the magnetic field caused by components within the user’s host system.
To that end, the TCM needs to be mounted within the host system and the entire host system
needs to be moved as a single unit during a user calibration. The TCM allows the user to
perform a calibration only in a 2D plane or with limited tilt, but provides the greatest
accuracy if the user can rotate through 360° of heading and at least ±45°of tilt.
The following subsections provide instructions for performing a magnetic calibration of a
TCM system. Several calibration mode options exist, as summarized in Table 5-1. To meet
the accuracy specification, the number of samples should be the “Minimum Recommended”
value, or greater. Calibration may be performed using Studio or using the PNI binary
protocol, and up to 8 sets of magnetic calibration coefficients may be saved. The
recommended calibration patterns described in the following sub-sections provide a good
distribution of sample points. Also, PNI recommends the location of the TCM remain fairly
constant while only the orientation is changed.
Table 5-1: Magnetic Calibration Mode Summary
Calibration
Mode
Accuracy
Tilt Range
during Cal
Number of Samples
Minimum
Recommend
Allowable
Range
Full Range 0.3° rms >±45° 12 10 – 18
2D Calibration <2° <±5° 12 10 – 18
Limited Tilt
Range <2° over 2x tilt
range
±5° to ±45° 12
10 – 18
Hard Iron Only
Restores prior
accuracy
>±3° 6
4 - 18