Magnetic Affects on Vehicle Electronics

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 Asia. 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 these sensors. The magnetic field lines of earth that mu metal needs to measure can be seen below:

Figure 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 permeability 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 copper 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 direction the vehicle is pointed in. Basically it is updating the resolution 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 of earth’s field has a higher resolution. This would simulate the resolution after several turns of a vehicle.

Figure 11 – 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, seeing 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 affect 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 air 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 way 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. Other 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.

8. 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 amp 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.

9. 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.html, 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 Conditioning” http://www.edmunds.com/ownership/techcenter/articles/46869/article.html, 2000

[6] L. Ojeda and J. Borenstein, “Experimental Results with the KVH C-100 Fluxgate Compass in Mobile Robots” Proceedings from 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

10. 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.

Directory: ~pribeiro -> courses -> engr302 -> Samples
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