Re: How do electric compasses work?

Greetings:

Reference:
Philips Semiconductor Application Note AN00022
Electronic Compass Design using KMZ51 and KMZ52
http://www.semiconduc tors.philips.com/catalog/

Electronic compasses such as the gyrocompass have been used for
more than 60 years and the GPS satellite navigation system receivers
have been available for consumers for the last 5 years. However,
recently developed magnetic field sensors in integrated circuit (IC)
technology have become available in low cost electronics packages.
The advantages of these devices is that they have no moving parts
and are easy to interface with microprocessors and other types of
navigation equipment and the sensors are not blocked by buildings
etc. as the GPS receivers are. Here in California a hand held digital
compass unit with one degree accuracy can be purchased for less than
$40 (about 27 Pounds Sterling). The price of these units will continue
to fall as competition increases.

The new IC magnetic field sensors use the magnetoresistive (MR) effect.
MR sensors make use of an electrical current carrying magnetic material
which can change its resistance in the presence of an external magnetic
field. The MR material used is called permalloy and contains 19% Iron
(Fe) and 81% Nickel (Ni). Using IC technology a thin permalloy film
is deposited on a nonmagnetic insulating substrate (chip) and 4 MR sensors
are etched in the film in the form of a square, one sensor on each side.
In electronics we call this a bridge circuit. A bridge circuit is a very
sensitive method to measure electrical resistance and changes in resistance.
In the Philips Semiconductors KMZ52 MR chip, two miniature electromagnets
are also integrated on the IC to compensate and calibrate the MR sensors
for magnetic field distortions caused by the metals found within automobiles,
airplanes ships etc.

The building blocks of an electronic compass consist of a two dimensional
MR field sensor, a signal condition unit (SCU), a direction determination
unit (DDU) and a display. The MR sensors are placed in a plane 90 degrees
from each other so that when one sensor is facing in a north - south
direction it will have a maximum signal and the second sensor in an
east-west direction will have a minimum signal. As the compass changes
orientation the SCU amplifies the weak MR currents, compensates for
temperature effects and delivers large signals to the DDU. The DDU
derives the azimuth information from the amplified MR sensor signals
and determines the accuracy needed for the display. The display unit can
provide simple 8 point compass headings (N, NE, E, SE etc.) and provide
an alphanumeric display or light LEDs placed around a compass display. In more
complex DDUs and displays, the compass heading can be displayed to one
degree accuracy by using signal processing and computational corrections.
Also, The compass can display directions relative to magnetic north or true
north at the users location.

The Philips Application Note AN00022 available on the referenced web site
is a 38 page document that goes into great detail about the each of the
the fabrication of the sensors and the SCUs, DDU's and displays used in
various electronic compass systems. The document is Adobe Acrobat format.

Best regards, Your Mad Scientist
Adrian Popa