Magnetic Affects on Vehicle Electronics

locsaucyElectronics - Devices

Oct 18, 2013 (3 years and 10 months ago)

92 views


1

Magnetic Affects on Vehicle Electronics

Ryan Bussis



Abstract
-

Magnetics affect the way electronics work in a
vehicle. There are certain functions on a vehicle that
introduce new magnetics. One of these items is the
defroster on a vehicle, due to the
current flow that is used
to run it. Also, in addition to the defroster a wire will
carry the voltage and current needed to run the rear
defroster. Also the air conditioning unit will also generate
magnetic fields that are strong enough to affect
electro
nics. This paper will focus on what these magnetic
fields are, actual measurements and simulation of these
magnetic fields. The paper will also focus on the theories
used to figure out the magnet forces. Another part of this
paper will focus on the magn
etization of vehicles. An
example of this is when a vehicle travels over subway
rails; due to the large amount to magnetic forces in these
rails, traveling over it will magnetize a vehicle. This can
be a concern for electronics in a vehicle. This aspect

of
vehicle magnetization will also be focused on. It will be
focused on in two areas; the cause of this phenomena, and
the effect of this magnetization.


Index Terms



Electric Field,

Magnetic Flux, Resolution,
Calibration

I.
I
NTRODUCTION


There are many th
eories that will apply to the study
that will be performed. The theories include magnetic flux
density, surface current, uniform sheet current and the Biot
-
Savart law.

These many laws will be needed to understand
the affects that magnetics have. There a
re many magnetic
forces in this world, but more specifically there are many
forces that lie in your vehicle alone, as well as those that lie

outside the vehicle and affect it.


This paper will present an overview and study that
was completed in determining

the magnetic affects on
electronics, with the study focusing on the digital compasses
that are in vehicles.

II.
MAGNETIC LAWS


The first law is
Coulombs law
which
is the basis of
everything that magnetics is built on. If one charge is in a
fixed position, an
d move a second charge slowly around,
there exists everywhere a force on this second charge; in


___________________________

Sponsered by Johnson Controls Inc,
using

US patent
5,878,370

Ryan Bussis

with the Department of Engineering at Calvin College, Gra
nd
Rapids, MI 49546, USA (e
-
mail:
rbussi88@calvin.edu
)

other words the second charge is displaying the existence of a
force called a field.[1] The force on it is called Coulombs
law:


[1]

Wri
ting this force as a force per unit charge gives:



[1]


The right side of the equation describes a vector field and is
called the electric field intensity. The electric field intensity
is a vector of force on a unit positive test char
ge.

Electric field intensity must be measured by the unit
newtons per coulomb


the force per unit charge. So the
equation for this can be seen below:



[1]


Now
let us

substitute the equation for a point charge,
and the following equa
tion can be formed:


[1]



Understanding electrical charges is important for
the understanding the magnetic fields. Another one of the
laws that is important to understand is that


The Biot
-
Savart Law is the first theory that needs to
be known since all the other theories depend on the Biot
-
Savart Law. The Biot
-
Savart Law is the magnetic equivalent
to the Coulombs Law. The Biot
-
Savart law basically states
that at any point P, the magnitude of the magnetic field
intensity produced by t
he differential element is proportional
to the product of the current, the magnitude of the differential
length and the sine of the angle lying between the filament
and a line connecting the filament to the point P.[1] The
equation is as follows:



2

[1]


Figure 2 shows a model of what the Biot
-
Savart Law is
describing, where dL is the infinitesimal length of the
conductor carrying electric current. R is the unit vector
specifying the direction of the vector distance from the
current to t
he field point. dB is the magnetic field
contribution from the current element, and dL is the
relationship between the magnetic field and its current source.



Figure
1



Biot
-
Savart Law

[2]


The law of Biot
-
Savart is sometimes called Ampere’s law for
th
e current element. In some aspects the Biot
-
Savart law is a
lot like Coulomb’s law when it is written for a differential
element charge. Both of these laws show an inverse square
law dependence on distance, and a linear relationship
between source and fi
eld, the only difference is the direction
of the field. This law can be applied in many locations. One
of those is the infinitely long wire. The direction of the
current is found by using the right hand rule. Point your
thumb in the direction of the c
urrent flow and your fingers
indicate the direction of the circular magnetic fields around
the wire. The Biot
-
Savart law can also be expressed in terms
of distribution sources such as current density J and surface
current density, which will be discussed
later in the paper.

Magnetic flux density is measured in webers per
square meter, or a newer unit adopted by the International
System of Units called the tesla. The old unit is the gauss
where it equals 10,000 G to 1 tesla, which also equals 1
Wb/m2. Ma
gnetic flux is figur
ed out by the equation B = μ*
H. Where H is the integral of the equation in figure 1, and μ
is the constant 4π x 10
-
7

H/m as defined for free space. A
figure showing what magnetic flux is can be seen below in
Figure 3.


Figure 2


Magnetic flux

[3]


The

magnetic flux density vector B is a member of
the flux density family of vector fields which compares the
laws Biot
-
Savart and Coulomb, thus the relation of B and E.
If B is measured in teslas or webers per square meter, then
magnetic flux should be meas
ured in webers. Electric flux
and Gausses law state that the total flux passing through any
closed surface is equal to the charged enclosed.

The charge Q is the source of the lines of electric
flux and these lines begin and terminate on positive and
negat
ive charges. However, no source has been discovered
for the lines of magnetic flux. For lets say an infinitely long
straight filament carrying a direct current I the H field formed
concentric circles about the filament. Since we know the
equation for ma
gnetic flux B is in the same form. For this
reason Gausses law is:


Surface Current flows in a sheet of infinite thickness
and the current density is measured in amperes per square
meter.[1] Surface current density is measured in amperes per
meter width.

Figure 4 is showing how surface current theory
will be used for the purpose of this paper.


Figure 3


Sheet current theory

[4]


If there are a large number of wires running parallel to each
other within a confined distance, like on the right side of
fi
gure 4, then it can be considered an infinitely thin sheet of
current in space. A defroster on a vehicle is an example of
this. With around 20 wires that are lined up inches from
eachother they would be considered an infinite sheet of
current, sine the p
ower and current go in one end and come
out one place on the other.



3

III.
DEFROSTER

Defrosters have large currents running through them,
thus creating a magnetic field. Defrosters have a wire
running the length of the car to deliver the

Tests were perfor
med to find out how the magnetic
field will affect the location of a compass in a vehicle. Tests
were performed on two vehicles. A Chrysler Concord and an
Isuzu Rodeo. Each had a different style of defroster in the
rear window. Also, a theoretical mode
l will be used in order
to predict future magnetic strengths of defrosters by the
amount of current going through it. The result from the
testing of vehicles has proved that there are large magnetic
fields attributed to the defroster running. These field
s only
appear close to the window, and become almost negligible at
a few feet away. This is becoming more of an issue as a
result of some European vehicles providing defrosters in the
front windshields of vehicles. This could have an affect on
the compas
s that we provide to the makers of these vehicles.


The measurements of the vehicles took place at nine
different spots on the rear windshield. Four of the
measurements were taken at the corners, and 4 were taken at
the mid points of the edges. And t
he ninth measurement was
taken in the middle of the windshield. For each of the
vehicles the current and voltage was measured going through
the defroster. The field strength was measured on all three of
the axis, with the z
-
axis being perpendicular to th
e window.
The measurements were taken with a 3
-
axis gauss meter.
Measurements were taken at different distances from the
surface of the window.

In order to find the affects of the defroster,
measurements were taken of the ambient field at certain
dista
nces away. Then the measurements were taken when the
defroster was on at the same distances. Also a theoretical
model was created so that one could predict the affects of any
defroster.


The Isuzu rodeo had the following results. The
current going throu
gh this defroster was 15.61amps. The
voltage was 12.48. The result of the testing of the Isuzu
Rodeo can be seen below for the distance of .75”, the rest of
the distances and their values can be seen in Appendix A.


Table 1


Isuzu Rodeo at .75”



These

are the results of the Isuzu Rodeo. As you
can see the strength of the magnetic field even out to over a
foot is very large. A field of almost 300mG is very strong.
That, in most places of the world, would make the field the
compass sees, at least twic
e as big. Anything within 3 ft of
this style of defroster would cause too many problems. That,
in most places of the world, would make the field the
compass sees, at least twice as big.


The results of the Chrysler Concord will now be
looked at. The
current going through the defroster is
19.37amps. The voltage is 12.73. The Concord had
measurements taken at the following distances: .75”, 4”, and
8.75”. The results of the measurements can be seen in Tables
6


8, which are in Appendix B.


Table 2


Chrysler Concord at .75”



The results from the Chrysler Concord are not as bad
as the Isuzu Rodeo. Yet, the defroster will still have an affect
on the compass if it is close enough. The Concord has a
different style of defroster on it than the Rodeo
does. The
defroster on the Concord has two wires that go vertically in
the window, thus making the current going through the
windshield a lot less. Essentially this is creating parallel
current paths for the current to flow through, thus dividing the
cur
rent by three, since the resistance is the same throughout
the defroster. This style of defroster reduces the current flow
through the windshield to about 8 amps, instead of the 19.37
that is sent to the defroster.


One theoretical defroster model was
created. This
model only simulates an infinite sheet that is at a certain
width. The width used is .55 meters. The flux strength was
modeled using the infinite sheet current conductor equation.

IV.
AIR CONDITIONER UNIT

Air conditioners are another part of a

vehicle that
can create magnetic fields that disrupt the functioning of
electronics. But first it is important to understand how an air
conditioner in a vehicle works.

There are six basic components: the compressor,
condenser, receiver
-
drier, thermostati
c expansion valve, the
evaporator and the life
-
blood of the A/C system, the
refrigerant. An air conditioning unit can be seen in the
following figure.


Figure 4


Diagram of the functionality of an air conditioner
[5]


4


First there is the compressor part
of the air
conditioner unit.
The compressor is the power unit of the
A/C system, it is powered by a drive belt connected to the
engine's crankshaft. When the A/C system is turned on, the
compressor pumps out refrigerant vapor under high pressure
and high
heat to the condenser.

Second there is the condenser unit. The condenser is
a device used to change the high
-
pressure refrigerant vapor to
a liquid. It is mounted ahead of the engine's radiator, and it
looks very similar to a radiator with its parallel tu
bing and
tiny cooling fins. If you look through the grille of a car and
see what you think is a radiator, it is most likely the
condenser. As the car moves, air flowing through the
condenser removes heat from the refrigerant, changing it to a
liquid state.

Third refrigerant moves to the receiver
-
drier. This is
the storage tank for the liquid refrigerant. It also removes
moisture from the refrigerant. Moisture in the system can
freeze and then act similarly to cholesterol in the human
blood stream, causing b
lockage.

As the compressor continues to pressurize the
system, liquid refrigerant under high pressure is circulated
from the receiver
-
drier to the thermostatic expansion valve,
which is the fourth part of the air conditioner. The valve
removes pressure fro
m the liquid refrigerant so that it can
expand and become refrigerant vapor in the evaporator.

Fifth there is the evaporator. The evaporator is very
similar to the condenser. It consists of tubes and fins and is
usually mounted inside the passenger compar
tment. As the
cold low
-
pressure refrigerant is released into the evaporator,
it vaporizes and absorbs heat from the air in the passenger
compartment. As the heat is absorbed, cool air will be
available for the occupants of the vehicle. A blower fan inside
the passenger compartment helps to distribute the cooler air.

Sixth there is the heat
-
laden, low
-
pressure
refrigerant vapor is then drawn into the compressor to start
another refrigeration cycle. This is basically how an A/C unit
in a vehicle works.


Air
conditioners create magnetic fields when they
are turned on due to the electric current that is needed to
power the compressor. This current going creates an electric
field, and since the magnetic fields lie at 90
-
degree angles to
the magnetic fields it
creates a substantial field enough to
affect the compass of a vehicle, if not accounted for. Such is
the case with the Mitsubishi Endeavor. Tests were run on the
Yazaki compass in this vehicle. The results will now be
discussed. First tests were run to

make sure that the compass
calibrated in a different fashion as the Johnson Controls
compass to check for patent infringements. Once it was
determined that Yazaki was performing the calibration of a
compass differently more tests were run to benchmark i
t.
Then tests were run to see how fast and accurately the
compass updated the display since we could not read into the
registers of the compass due to lack of knowledge. Finally,
tests were run to see if Yazaki correctly accounts for
magnetic fields crea
ted by functions on a vehicle. So the
following test was performed. First the vehicle was
calibrated and turned for many circles so that it was fully
calibrated and knew the magnetic fields of the earth. Then
the vehicle was pointed directly west. The
car was then
driven in a straight line. The air cond
itioner unit was then
turned on.


W
hen this happened the display on the compass
unit

moved from west to southwest, o
bviously as a result to
the air conditioning unit. The test was now run in the north
s
outh direction. Starting with the vehicle facing north it was
driven south and the air conditioning unit was turned on with
the same result. Now, these tests were performed again to see
the affect on the compass. Only this time the air conditioning
unit

would be turned on and off while traveling in a straight
line. This was done to see if the compass would correct itself
again. The compass would not correct itself until a couple
more circles were turned.

V.
MAGNETIC MODELS

The following model was created
using MathCAD.
The equation that was used to calculate the following graph
can be seen below:

[1]



The results can be seen in the following table. The
model has to be recreated but that
will not

be hard.



Figure 5


Magnetic flux
in relation of distance from
defroster


5

As can be seen for the data above, the affects of this magnetic
field can be seen from quite a distance. The first graph is the
magnetic flux with respect to the distance in inches away, and
the second graph is magne
tic flux with respect to the the
distance in meters.


Overall it appears that placing a compass close,
within 2 ft of a defroster, the defroster will create a field that
will greatly affect the performance of the compass in a
vehicle. Also affecting the

compass would be the wire that is
controlling the defroster since it needs to run the length of the
vehicle. Also, this wire has the same current running through
it as the defroster, which will create as large magnetic field
running the length of a vehic
le on a side. The best solution to
solving this issue for the compass would be to have it placed
in the middle of a vehicle. Or on the side of the vehicle
where the defroster wire is not put. The other solution to this
issue would be for the lines of th
e defroster carrying the
current in the window to cross back and forth, which would
cancel out the magnetic forces.

VI.
EXTERANL MAGNETIC FO
RCES


There are also many magnetic affects that a vehicle
incurs while traveling. An example of this is subway rails.
These rails have large currents that run through them in order
to drive the subway vehicles. Due to these large currents
going through, they will basically magnetize your vehicle by
a few gauss. Well this can provide difficulties in getting
electronics i
n a vehicle to work. So tests were run to
simulate this phenomena, and see how long the effects where
and how to reduce the magnetic flux.


There were many different tests that were run, to
simulate different theories. The first theory is that vibrati
ons
such as rain hitting a vehicle would reduce the magnetics.
Also, being tested are vibrations from everyday driving, heat,
and time. The results of these tests will now be discussed.

To get an accurate analysis of the data, the amount
of magnetizatio
n during the time spent in the coil had to be
determined. This would give a better understanding of how
the demagnetizing tests worked. When looking at the
magnetization of a vehicle the three axes reacted differently
to the 8 Gauss field. The magnetiza
tion of the x
-
axis on the
Ford Winstar decreased slowly as measurements approached
the rear of the vehicle. The difference was about 100 to
150mG change. The y
-
axis increased dramatically in
measurements from the front of the vehicle towards the rear
of
the vehicle. The increase was about 1 Gauss. The z
-
axis
decreased dramatically from the front of the vehicle
measurements as the measurements approached the rear. The
total change in the z
-
axis was approximately 2 Gauss. The
average magnitude of change

over all three axes for the
Winstar was 2.026 Gauss. The changes were slightly
different for the Audi. The x
-
axis decreased slightly in
measurements from the front towards the rear. This was
approximately a 150mG change. The y
-
axis measurements
incre
ased by about 1 Gauss from the front towards the rear of
the vehicle. The z
-
axis increased by approximately 1.5
Gauss. The average magnetization for all three axes for the
Audi was approximately 1.580 Gauss. There are two tests
that decreased the magnet
ization of a vehicle more than the
rest. Those tests are the bumpy road test and the
precipitation/heat test. The bumpy road test for both the Ford
Winstar and the Audi reduced the magnetization by about
317mG. The precipitation test for the Ford Winsta
r reduced
the magnetization by approximately 203mG. The heat test
was run on the Audi; the average demagnetization for that
test was 130mG. A bumpy road test was run on a Jeep as
well. The average demagnetization was 266mG.

These results show some inter
esting things. These
results show that the best way to decrease magnetization of a
vehicle is to introduce vibrations to it. Although through
testing this way of demagnetizing only reduces the overall
magnetization of a vehicle a small amount. The other

tests
that did not include any sort of vibration did not have much
of an affect on the overall demagnetization of a vehicle. The
precipitation on a vehicle seems to be another decent way of
reducing the magnetization, although probably due to the fact
th
at vibrations were introduced through precipitation. The test
was run with snow, and not rain. The next test to run is a
controlled rain test.

Hard rain does quite well reducing the magnetization
of a vehicle. When rain was dumped on a vehicle at a rate
of
5.85 GPM for one hour, it reduced the magnetics of a vehicle
to within an average of 60
-

70mG of the original
magnetization. This should be close enough to the original
strength of the vehicle not to worry about taking it down any
further. Normal dri
ving should reduce it back to the original
values after a while.

It appears as though the best way to get way to get a
the vehicle magnetics back to the original state would be to
let the vehicle sit in a hard rain for an hour. This is the best
action tha
t could be taken so far based on the testing that has
been performed.

The main purpose of the testing with the mega
-
helmholtz would be to see how products in a vehicle would
work at a large offset, as well as to figure out what he
vehicles offset would b
e at when placed in an field of that
magnitude. This is a stepping stone to testing electronic
products in a fixture so the effects of the magnetics can be
seen on the product as well as the corrections that might be
made in the product to account for thi
s.

VII.
EFFECTS

What affects does this have and how does it affect a
product such as a compass. How a compass calibrates.

But
first the sensors of an electronic compass will be discussed.

Desciption of earths magnetic field can be seen in the figure
below:


6


Figure 6


Components of earths magnetic field

[6]


Figure

7


Earths magnetic poles

[6]

Sensors of a compass work by sensing the difference
in the natural magnetics of the earth. The magnetics of the
earth vary from about 60mG to 400mG in central A
sia.
These sensors need to be very sensitive since they need to
sense changes of 1mG. In order to do this a special metal
called mu metal is used
for the
se sensors. The magnetic field
lines of earth that mu metal needs to measure can be seen
below:


Fi
gure
8



Magnetic
field lines of earth
[6]

Mu
-
metal is a nickel
-
iron alloy (77% Ni, 15% Fe,
plus Cu and Mo) which has extremely high magnetic
perme
ability at low field strengths.
Mu
-
metal can be used as
a very effective magnetic screen as it exhibits high

attenuation at low levels of the interference field (i.e earths
magnetic field).
[7] This metal is formed in strips that are
placed in a trough of plastic. The metal is then held in by an
epoxy. This is then surrounded by approximately 8000 winds
of cop
per wire. This set
-
up forms an inductor. A picture of
one can be seen below.


Figure 9

-

Compass sensor break down
[7]

The black section is the mu metal while rest is the plastic
trough that the sensor sits in. Then not shown is the wire
wrapping that
goes around it.


Now these sensors are basically inductors. As the
earth

s magnetic field changes (i.e. the sensor changes
direction) the mu metal will change the current flowing
through the wires. Thus the current difference going into the
circuit is

how the direction is measured.


Now to discuss how a compass uses these sensors as
well as how the software works. First, a compass needs to
calibrate, which means it has to learn the field that it is
in. In
order to do this the

compass needs to spin
circles. In a
vehicle it should calibrate within 1.25 circles turned.
Calibration is done be reading in 3 points in a plane. Once
these points are figured out, a circle is generated. This circle
will represent the 2 dimensional field of the earth for

the
software to reference when it reads the current values. As a
vehicle continues to turn circles the software continues to
update the circle to get a more exact data as to the direct
ion
the vehicle is pointed in. Basically it is updating

the
resoluti
on of the circle. Below this process can be seen.


Figure
10



Calibration of a compass

Figure 15 is showing the first step of calibration. Figure 16
will show how the resolution has updated and how the circle

7

of earth

s field has a higher resolution.

This would simulate
the resolution after several turns of a
vehicle.


Figure
1
1



Resolution updated


Now that it is know how a compass works, how does
external magnetic affects effect the compass. Well first due
to the sensitivity of the compass, seein
g a field of 90mG,
would shift the center of the circle 90mG in the direction of
the field generated. What this will do is shift the circle in that
direction. Let

s say that the circle is shifted at 45 degrees in
the positive x
-
y plane. This would affec
t the compass greatly
since the only readings it would have would be in the north to
west directions. Basically when you would turn a circle in a
vehicle
all you would read would be north, northwest and
west on an eight point compass. The circle that is

being red
will be the shifted circle, but the software will think it needs
to read values along the original circle. Figure 17 will show
what the software and hardware are seeing.


Figure
12



Magnetic shift due to external field


This is what happens

to a compass when a small
effect happens like the ai
r conditioner being turned on. But
what happens when a large offset occurs, like one that is
over
1 gauss. Well that will rail the compass, which means that it
will lock up and be useless. The only wa
y to undo a rail of a
compass is to re
-
calibrate it.


The affects that a compass sees while in a vehicle
will be worst case scenario for electronics in a vehicle. Due
to the fact that the specific purpose of a compass is to
measure magnetic fields. Oth
er electronics in vehicles will
not be affected to this degree. However tolerances and values
on components could change like the inductor on the
compass.



Capacitors are a component which could be affected
by a large field, since they themselves are a

form of a magnet.
But it would be rare to see this affect a circuit. Since the
larger the capacitance, the longer it will hold the voltage of a
circuit. So, in most cases the circuit could perform just as
well. However, if there are inductors in the
circuit, those can
be affected by magnetics, since it can change the current flow
through the circuit.

VIII.
CONCLUSION

The affects of magnetics to a vehicle’s electronics
are mostly seen by the compass in a vehicle. And since
almost if not all of vehicles
have compasses in, you might see
this affect at some point in your life. Also, magnetics can be
generated from any
wire that has a current in it, although the
wire has to have significant current flowing through it.
Significant current would be in the am
p
range, not the
milliamp range. Magnetic forces are everywhere and there
are a lot in vehicles since there is a lot of DC current flowing
throughout the vehicle. There are also many affects that
come from outside the vehicle that can have a major affect

on
the
electronics inside as well.

IX.
REFERENCES

[1]

W. Hayt Jr, and J. Buck,
Engineering Electromagnetics
, Copyright ©
2001

McGraw Hill
, pg 27


254, 2001, The McGraw
-
Hill Companies
Inc. 1221 Avenue of the Americas,
New York, NY

10021.

[2]

C.R. Nave, “Biot
-
Savart Law”
http://hyperphysics.phy
-
astr.gsu.edu/hbase/magnetic/biosav.html
, 2000

[3]

C.R. Nave, “
Magnetic Flux

http://hyperphysics.phy
-
astr.gsu.edu/hbase/magnetic/fluxmg.ht
ml
, 2000

[4]

K. Jones, “The Magnetic Field of an Infinite Current Sheet”
http://www.physics.uq.edu.au/people/ficek/ph348/sols/sol3/node5.html
,
2000

[5]

B. Romans, “Air Co
nditioning”
http://www.edmunds.com/ownership/techcenter/articles/46869/article.h
tml
, 2000

[
6
]

L. Ojeda and J. Borenstein, “Experimental Results with the KVH C
-
100 Flux
gate Compass in Mobile Robots” Proceedings f
rom

the
IASTED International Conference
,
http://www
-
personal.engin.umich.edu/~johannb/Papers/paper77.pdf
,
2000

[7]

Digikey
, “
Honeywell Sensors”

http://dkc3.digikey.com/PDF/T042/1301.pdf

X.
BIOGRAPHY

Ryan Bussis

received a BSE in May of 2004 from Calvin College in
Grand Rapids MI. Presently is considering options for future

employment. Electrical engineering interests include digital systems
and communications.





8






















9