Magnetic Affects on Vehicle Electronics

Magnetic Affects on Vehicle Electronics

Ryan Bussis

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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 electronics. 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 magnetization 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. Introduction

There are many theories 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 are 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, and move a second charge slowly around, there exists everywhere a force on this second charge; in

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Sponsered by Johnson Controls Inc, using US patent 5,878,370

Ryan Bussiswith the Department of Engineering at Calvin College, Grand Rapids, MI 49546, USA (e-mail: )

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]

Writing 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 charge.

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 equation 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 the 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:

[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 the 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 the 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 field, 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 current 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. Magnetic flux is figured 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 measured 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 negative 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 magnetic 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 figure 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 power and current go in one end and come out one place on the other.

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 performed 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 model 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 fields 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 compass 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 the 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 the 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 distances 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 through 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 twice 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 current 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, thermostatic 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]

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 tubing 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 blockage.

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 from 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 compartment. 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 it. 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 created 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 conditioner unit was then turned on. When this happened the display on the compass unit moved from west to southwest, obviously as a result to the air conditioning unit. The test was now run in the north south 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

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 magnetic 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 vehicle 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 the defroster carrying the current in the window to cross back and forth, which would cancel out the magnetic forces.