The Earth's Magnetic Field - part 2
NOTE: During the design and construction of this company’s CA-320 digital compass, we were surprised at how little we knew about the Earth’s magnetic field. This turns out to be important from an aircraft maintenance standpoint and therefore will be the subject of a multipart series.
This month we continue our study of magnetic fields and how aircraft structures and systems may affect them. We have previously learned that magnetic lines of flux
traveling through the air are uniform in strength and have a permeability value of
one. When these lines of flux encounter a medium in which the permeability is other
than one, the flux concentration changes and with this change linearity errors are
introduced.
The errors created by these disturbances can be grouped into two major types
- hard iron and soft iron errors. Hard iron errors arise from permanent magnets and
magnetized iron or steel within the sensing zone of the magnetic sensor. Some of
the characteristics of hard iron errors are as follows:
- Remain constant and in a fixed location relative to the compass for all heading
orientations.
- Add a constant magnitude field component along each axis of the sensor output.
- Are capable of disturbing a relatively large area depending upon the degree of
magnetization. Permanent magnets, such as found in pumps or speakers, can
upset the magnetic fi eld more than three feet or one meter away.
- If plotted in a circle, the errors appear as a shift in the origin of the circle and are
commonly referred to as one-cycle errors.
To compensate for hard iron errors, the circle offset must first be determined. This is
usually done by rotating the compass and platform in a circle and recording enough
points in the circle to determine the offset. Electronic systems can then store the
errors for a given heading and correct accordingly. For flux valve based systems,
these errors cannot be easily removed and therefore the offending materials must
be located and eliminated.
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