Electromagnetic Interference in Automotive and Aerospace

Innovation Intelligence®

Electromagnetic Interference in

Automotive and Aerospace


1. Automotive

Copyright © 2014 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.

Problem description

• > 100 cable instances, > 50 connectors

• Only one of many harnesses

• Kilometers of cable in a car

• Unintended electrical interference and radiation

Courtesy Daimler AG


Case study

• Two harnesses and one windshield antenna

• Four signal conductors per harness

• Interference?


Differential cross talk

Between separate harnesses

Take a closer look at 34 MHz


Antenna causes large coupling

Important to test entire car, not just components.

Can only test entire car late in the process modifications expensive.

Software simulation essential.


EMC standard: max E on 10-m sphere

at every frequency


Account for spectrum digital signal

Clock

2 MHz

PRBS

2 Mb/s


With shielding

How much shielding is enough?

Don’t want to add much weight;

Don’t want to lose routing flexibility.


Interference on receiving antenna

Matching circuit designed with Optenni Lab. Low S11 89-91 MHz.


Received by antenna

CISPR 25 limit


2. Lightning


Model similar to measurement setup


Lightning profile and spectrum

Defined in Standard IEC 62305

Preferred over double exponential

Current (kA)

Time (s)

Need to include frequencies

at least up to 300 kHz.

In these simulations:

10 Hz – 3 MHz.


Currents with and without shielding

Important to know how

much shielding is enough.

Don’t want to add

too much weight and

don’t want to lose flexibility.

Up to 1400 mA

Up to 22 mA


Change fuselage to CFRC material


Carbon-Fiber Reinforced Composite

material

• Directions of fibers:

0, 90, 45, 135 degrees

• Anisotropic Conductivity

1000 S/m along fibers

10 S/m perpendicular to fibers

• Thickness 0.5 mm per layer

plus 0.1 mm bonding

• FEKO translates the stack-up

into a frequency-dependent

anisotropic surface impedance.


CFRC fuselage and unshielded cables

• Cables’ return path is locally PEC and is floating.

• Peak current 2.1 A, higher than with PEC fuselage but not dramatically higher.


CFRC fuselage without windows

• Cables’ return path is locally PEC and is floating.

• Currents at cable terminals larger than with windows.

• PEC fuselage: fields enter through windows

• CFRC fuselage: fields penetrate regardless of windows


Result with better cable shielding

• Denser braided shield than before


Observations

• Most lightning power enters through the windows if the fuselage is

metal, but through the material if it is CFRC.

• FEKO quantifies the currents at the ends of electrical cables, i.e. the

currents that enter the systems that are attached to the cables.

• Determining how much cable shielding is needed is straightforward.


3. High-Intensity Radiated Fields

(HIRF)


HIRF on UAV

Length body 1.21 m

Length nose to tail 2.23 m

Wing span 4.52 m

Glass dome for camera


Cables

• Two cable bundles inside: one shielded, one unshielded

• Cable length 0.67 m; dielectric medium is air.


High-Intensity Radiated Field threat

• Modeled as incident plane wave at 300 V/m

• Many directions of incidence, one at a time


Results (unshielded cables)

• Displaying only two angles of incidence to avoid clutter. Typical results were chosen.

Plexiglass

Carbon-fiber

composite

Aluminum

Note: glass dome always transparent to HIRF

97 MHz cable almost λ/4Low Q due to distance wires and return path.

Effective cable length slightly longer due to end

effects.


Results (shielded cables)

Plexiglass

Carbon-fiber

composite

Aluminum

Note: glass dome always transparent to HIRF

111 MHz

cable = λ/4


Composite material:

Comparison shielded and unshielded cables

unshielded

shielded

Displaying more angles of incidence and wider frequency range

97 MHz

3x97

5x97

1113x111

5x111


Observations

• With aluminum or carbon-fiber composite material, voltages remain modest,

in spite of transparent glass dome. Worst case 0.25 V.

• With plexiglass, interference reaches tens of Volts. This is not acceptable.

• Shielded cable (with one end open) shows remarkably strong resonance

when cable length equals ¼, ¾, … wavelength.


Copper-coated plexiglass

• Solid Lines:

Unshielded Bundle

• Dotted Lines:

Shielded Bundle

• Coating thickness

- Red 1 nm

- Blue 10 nm

- Green 100 nm

• Glass dome not coated


Observations

• Very thin coating already has large effect.

• At 100 MHz, skin depth is 6 μm

• However, 100 nm of copper can already carry enough current to provide a

shielding factor of several tens of dB!


Conclusions

• Automobiles: full-platform simulation essential.

• Metal aircraft: fields enter through windows.

• CFRC aircraft: fields enter through material and through windows.

• Lightning transient signal designing cable shielding is

straightforward.

• HIRF Continuous Wave (CW) signal resonances in cable harder

to suppress apply metal coating on the fuselage.

• 3D full-wave frequency-domain electromagnetic simulation tool

with a cable-analysis capability makes E3 mitigation easy.

Electromagnetic Interference in Automotive and Aerospace

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