The
electronic compass such as wayFinder family of digital compass
uses a patented magnetic sensor technology that was developed
by PNI for the U.S. military. This technology is called "
magneto-inductive" and is the largest advancement
in compass technology since the fulxgate was invented
60 years ago. The earth generates a magnetic field, and through
a mathematical calculation, compass heading is determined.
The
magneto-inductive technology is able to electronically
sense the difference in the earth's magnetic field from your vehicle's
magnetic field. The WayFinder automobile compasses has a embedded
microcontroller
that subtracts out your auto mobile magnetic field(distortion)
from the stronger earth magnetic fields, displaying highly accurate
compass readings.
Compass Installation
The performance of a compass will greatly depend on its installation
location. A compass relies on the earth’s magnetic field
to provide heading. Any distortions of this magnetic field by
other sources should be compensated for in order to determine
an accurate heading. Sources of magnetic fields in your automobile
include permanent magnets, motors, electric currents—either
dc or ac, and magnetic metals
such as steel or iron. The influence of these sources on compass
accuracy can be greatly reduced by placing the compass far from
them.
Some of the
field effects can be compensated by calibration. However, it is
not possible to compensate for time varying magnetic fields; for
example, disturbances generated by the motion of magnetic metals,
or unpredictable electrical current in a nearby power lines. Magnetic
shielding can be used for large field disturbances from motors
or speakers. The best way to reduce disturbances is distance.
Also, never enclose the compass in a magnetically shielded metallic
housing.
Compass
Tilt Errors
Heading errors due to the tilt sensor depend somewhat on geographic
location. At the equator, tilt errors are less critical since
the earth's field is strictly in the horizontal plane. This provides
larger Xh and Yh readings and little Z component correction [ref.
Equation (2)]. Near the magnetic poles, tilt errors are extremely
important—since there is less Xh,Yh field and more Z component.
Tilt errors are also dependent on the heading
Magnetic
Field Distortions
Nearby Ferrous Materials is Another consideration for heading
inaccuracy. Since heading is based on the direction of the earth's
horizontal field the magnetic sensor must be able to measure this
field without influence from other nearby magnetic sources or
disturbances.
The amount of disturbance depends on the material content of the
platform and connectors as well as ferrous objects moving near
the compass. When a ferrous object is placed in a uniform magnetic
field it will create disturbances as shown in Figure 8. This object
could be a steel bolt or bracket near the compass or an iron door
latch close to the compass. The net result is a characteristic
distortion, or anomaly, to the earth’s magnetic field that
is unique to the shape of the object.
Magnetic distortions can be categorized as two types—hard
iron and soft iron effects. Hard iron distortions
arise from permanent magnets and magnetized iron or steel on the
compass platform. These distortions will remain constant and in
a fixed location relative to the compass for all heading orientations.
Hard iron effects add a constant magnitude field component
along each axes of the sensor output. This appears as a shift
in the origin of the circle equal to the hard iron disturbance
in the Xh and Yh axis
To compensate for hard iron distortion is usually done
by rotating the compass and platform (your car) in a circle and
measure enough points on the circle to determine this offset.
Once found, the (X,Y) offset can be stored in memory and subtracted
from every reading. The net result will be to eliminate the hard
iron disturbance from the heading calculation.
The
soft iron distortion arises from the interaction of the earth’s
magnetic field and any magnetically soft material surrounding
the compass. Like the hard iron materials, the soft metals
also distort the earth’s magnetic field lines. The difference
is the amount of distortion from the soft iron depends
on the compass orientation.
What
Is True North?
It
is well known that the earth's magnetic poles and its axis of
rotation are not at the same geographical location. They are about
11.5° rotation from each other. This creates a difference
between the true north, or grid north, and the magnetic north,
or direction a magnetic compass will point. Simply it is the angular
difference between the magnetic and true north expressed as an
Easterly or Westerly variation. This
difference is defined as the variation angle and is dependent
on the compass short duration, making a magnetic compass a useful
navigation tool.
 Compass
Calibration
Each calibration method is associated with a specified physical
movement of the compass platform in order to sample the magnetic
space surrounding the compass. The Hard and soft iron distortions
will vary from location to location within the same platform.
The compass has to be mounted permanently to its platform to get
a valid calibration.
A
particular calibration is only valid for that location of the
compass. If the compass is reoriented in the same location, then
a new
calibration is required. It is possible to use a compass
without any calibration if the need is only for repeatability
and not accuracy.
To
see our digital compasses, click on the image |
