Compass; Earth Magnetic Field - Nokia NPL-4 Series Manual

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Compass

This chapter describes electronic compass function integration to baseband. Measure-
ment is based on magnetoresistive sensor and controlled with baseband ASICs, UPP and
UEME via MagIC ASIC.
The electronic compass will have two magnetometer channels for detecting x and y
direction components of earth magnetic field and it uses Honeywell's anisotropic mag-
netoresistive (AMR) magnetometer component HMC1052 (containing both x- and y-
axes). Both channel rely on the magnetoresistive effect and provide the required sensi-
tivity and linearity to measure the weak magnetic field of the earth.
Each measurement axis is configured as a 4-element Whetstone bridge converting the
magnetic field into differential output voltage. This sensor element is capable of sensing
fields in milligauss range. In order to achieve the measurement resolution, a current
pulse to run through the set/reset coil of the sensor element must frequently reset the
sensor. This means basicly compensation of linear offset.
The MagIC ASIC will interface the phone engine through the CBUS. The calculation of
the compass heading and the calibration of the magnetometer are carried out in the
phone engine.

Earth magnetic field

The magnetic field of the earth is the physical quantity to be evaluated by a compass.
The magnetic field strength on the earth varies with location and covers the range from
about 200 to 700 mGauss. Earth magnetic field is assumed to be like as generated by a
bar magnet (in the earth). The magnetic field lines point from the earth´s south pole to
its north pole. Exactly, 2-dimensional magnetometer measures earth magnetic horizon-
tal field component.
The field lines are perpendicular to the earth surface at the poles and parallel at the
equator. Thus, the earth field points downwards in the northern hemisphere and upwards
in the southern hemisphere.
An important fact is, that the magnetic poles do not coincide with the geographical
poles, which are defined by the earth´s axis of rotation. The angle between the magnetic
and the rotation axis is about 11.5°. As a consequence, the magnetic field lines do not
exactly point to geographic or "true" north.
Copyright © 2005 Nokia Corporation. All rights reserved.
Issue 2 05/05
System Module and User Interface
NPL-4/5/
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