Calibrating Auto-Compass (my experiments)
gardner oral e
Here are a couple of things I have tried (unsuccessfully) to calibrate
a compass near the center or lower winshield area: On my 99 Buick
Century Custom, the compass was completely silly due to the inherent
magnetism in the car. (The same model compass could be calibrated quite
nicely in the center of the winshield of my 86 Buick LeSabre).
So I got a bright idea and bought a "powerful" small flat magnet and
was able experimentally place it on the dash and "neutralize" out the
car's magnetism.....and it worked very well initially...BUT..after
driving it on a 200 mile trip, it went completely silly again.
Apparently the magnet (which had been taped down securely) gradually
changed the magnetism of the car to something different.
So then I thought I would try to "deguass" the car (i.e. de-magnetize)
by using about 15 or 20 turns (about 16" dia.) of #16 copper wire with
probably 25 amperers (60 Hz) max....gradually lowered back to zero using
a variac on a 6.3 volt filament transformer (rated at 10A continuous).
I tested it on bench with a piece of soft sheet metal which I first
magnetized (so that it affected the compass), and then deguassed it so
that it had almost no effect on the compass. Then I tried the same
on the upper dash area of the car, but could notice very little improve-
ment. I was a little concerned about making the experiment too power-
full in case it could destroy some electronic device under the dash.
I have the components to make a much more powerful field ( have a
75 ampere filament transformer), and I may try again later using a
smaller diameter coil with a more powerful field (presumeably would not
radiate so deeply below the dash to endanger electronics)...moving it
around to deguass small areas at a time.
Does anyone have a workable solution for this very common problem?
I remember an earlier car that intially calibrated nicely, but gradu-
ally became magnetized and silly after 2 or 3 years.
For the moment however, I have settled for a compromise: I found that
on this 99 Buick Century, the compass WOULD calibrate fairly well when
placed in the roof area just behind the top-center of the winshield.
The soft covering material provides no strength for support, so I
improvised an aluminum bracket that is supported by the structure that
covers the wiring to the rear-view mirror (used a small hose-clamp).
Do automobile dealers have any solution for this compass problem??
Jerry Greenberg
Hi have an Altima that I mounted in a good quality marine compass on the
windsceen. I mounted it 2/3 rd the way up from the dash ( 1/3 rd down from
the top ). This seemed to be the best place. It is not as accurate in
some directions. I set it for west and north as the references. In the
north-east and south-west there is an offset of about 12 degrees at times.
I've put compasses in other cars as well. I found that in some models it is
the electrical wiring that is effecting the compass more than anything
else. There are many amperes going through these wires and their fields are
strong. There is a lot of DC and some pulsating electromagnetic radiation
comming from an automobile or truck. It is infact much stronger than what
you would have from your appliances in your home.
Good marine or aircraft compasses will have swing magnets inside of them for
the adjustments, as I believe you are aware of. In some cases they may add
some external bar or button magnets at the correct distance and orientation
to correct the orientation a bit better. This is done to attempt to correct
for errors that the internal swing magets can not take out. This procedure
takes a lot of practice, and developed skill. It is very tricky to do.
As for electrical interference, it is extremely difficult to control. The
amount of power that is going through the wiring is dependent on the
activity required. The amount of torque on the windscreen wipers,
defroster, fan motors, fuel pump and injector actvity, electronic control
activity, and a lot more that I cannot even think of will effect the summed
magnetic field around the car.
Depending on how the wiring is run, and the overall shielding effect of the
body parts, will determine how much the compass will be effected in some
models. I've gone as far as doing some tests wtih a DC magnetimeter. I
found things were very unpredictable.
For example, when I turn on the fan (heater/aircondiioning) in my car, the
compass will swing about 6 degrees. in some directions. Since I leave the
fan motor at about 1/3 to 1/2 way up on the average, I set the compass with
the fan motor at that setting range. I also made sure that the driving
lights were on at the low setting, since I use this setting on the average.
The power to the lights had an effect on it in some directions as well. The
windscreen wipers realy drive the compass to be unusable. It follows the
wipers like crazy!
The best answer would be to get an electronic compass and put the sensor in
the best spot. This would also be a good puzzle to solve.
The very best answer or solution would be to purchase a low cost GPS. You
just sit it on the dash or the seat where it can get a good view of the
satelites. I have a GPS that I use for hiking, and camping. It works even
sitting on the seat next to me. As long as the car is moving more than about
3 mph or more, it will read with extreme accuracy of the direction the car
is moving in. The bonus you get with it, is that you can program in where
you want to go, and it will keep a record of the time and milage for you as
well. The GPS will be more accurate than any compass you can purchase for
the same price. You can get a low cost GPS for under $100 US these days.
It is good enough for the type of application I am under the impression you
would want for it.
--
Jerry Greenberg GLG Technologies GLG
==============================================
WebPage <http://www.zoom-one.com>
Electronics <http://www.zoom-one.com/electron.htm>
Instruments <http://www.zoom-one.com/glgtech.htm>
==============================================