digital compass for cars

I bought one of those things. It's probably got some kind of hall effectg
sensor talking to a microporcessor -- all that for >$20! -- and is calibrated
by turning the car in a circle. I can appreciate how measuring the horizontal
component of the magnetic field can 'teach' the compass about stray fields in
the car, but I'm at a loss to understand how it knows where north is. I mean,
plot a graph with a quasi sinusoid waveform, vertical axis magnetic field,
horizontal axis time, with some fixed bias then figure out, from that, where
the cardinal points are. You couldn't take the maximum magnetic field to be
north, since the field distortions in the car could screw that up (couldn't
they?).

The damned thing reports heading in 5 degree increments, not just cardinal
points. I'm tempted to take my car to the compass rose at a nearby airport to
see how good the calibration really is. (FWIW, lots of airports have a painted
compass rose on the tarmac so that we can 'swing' our compasses. Yes, I'm CP,
SEL, IFR, and the deviation card on my bird shows up to 4 degree errors.)

"AJW" <ajw27703@wmconnect.comremove> wrote in message
news:[Only registered users see links. ]...
calibrated
horizontal
in
mean,
where
be
(couldn't
to
painted
CP,

Sounds like the Wayfinder VT220:
[Only registered users see links. ]

For how it works:
[Only registered users see links. ]

The voltage measured on the Hall effect sensor will be the opposite when it
is pointed south rather than north.

To check the calibration, just find a parking lot on a clear night and point
your car towards Polaris (about 3/4° away from true north, at the most).
That will give you a "true" north (at least as close as you can get lining
up your car visually).


Tom Davidson
Richmond, VA

Tom my VFR charts show compass deviation well enough. As to opposite output for
N vs south, if you think about eartrh field and supperimpose on it a stray
field at a suitable angle, V(north) will not be - V(south) out of the Hall
effect, will it?

I'll check the URL you provided, thanks

"AJW" <ajw27703@wmconnect.comremove> wrote in message
news:[Only registered users see links. ]...
output for

The compass declination (i.e. deviation from "true north") varies somewhat
with location:
[Only registered users see links. ]

If your vehicle has a "stray" field - an inherent magnetic field of its
own - then, no matter which way you turn the vehicle, the part of the stray
field that the digital compass will see will be constant. The relative
direction of the earth's magnetic field changes with the orientation of the
vehicle.

As a result, the contribution of the stray field will merely add a constant
bias, which is easy to correct for. When you calibrate your digital
compass, it detect the orientations that create maximum and minimum
responses in your Hall effect sensor. These are used to identify "north"
and "south". North and south are still opposites, it is just that when a
constant bias is present, the magnitudes of the responses to north and south
are not necessarily going to be equal:

Response with no bias:

N
... .
. .
. .
--------------
. .
. .
...
S

Response with negative bias:

N
... .
. .
--------------
. .
. .
. .
...
S

The peaks and valleys are still the peaks and valleys.

If your digital compass is sophisticated enough, you can also pre-set the
approximate compass declination for the regions in which you expect to be
traveling. This feature is widely available on hand-held *analog* compasses.
It does require you to know what your compass declination is.


Tom Davidson
Richmond, VA