IN DEGREE PROJECT ELECTRICAL ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2018

Simulation Method Development for Vehicle Radiated Immunity Test

YAO WANG

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE

Acknowledgements

This thesis is accomplished at China Euro Vehicle Technology AB (CEVT),

Goteborg, Sweden, as the degree project for Master of Science in Electrical

Engineering at KTH Royal Institute of Technology. The thesis is under the

supervision of Dr. Robert Moestam (CEVT) and Associate Prof. Daniel

Mansson (KTH); Dr. Jan Carlsson (Provinn AB, Adjunct Professor at

CTH) also acts as a co-supervisor. The examiner of this thesis is Prof.

Rajeev Thottappillil (KTH).

Words are insufficient to express my thankfulness to my supervisor Robert

Moestam, Daniel Mansson and Jan Carlsson for their professional guid-

ance, suggestion, feedback and countless support. Robert has been not

only instructing me academically, but also mentoring as a senior engineer.

Also, I would like to thank Sofia Ore, as the manager of CAE Energy

Department, as well as all other colleagues at CEVT, for their support

during my work in the company.

Special gratitude goes to my family. Thank all the family members for

staying around with me during my study in Sweden.

Yao Wang

Goteborg, December 2018

Abstract

In the past few decades, on-board electronic devices have been developing

tremendously in automotive industry, and it is believed that the trend

of electrification and autonomous driving will sustain in the near future.

Thus, passenger cars are going to suffer from more severe electromagnetic

environment, especially from an EMC’s perspective.

This thesis is an investigation and overview on a possible simulation

method applied to vehicle radiated immunity test in accordance with

ISO 11451-2 standard. The preliminary work of geometry clean-up on

the model obtained from structural dynamics department and the overall

method development have been discussed. The main contribution of this

thesis is to build up a feasible workflow that is suitable for vehicle EMC

simulations based on FEM electromagnetic simulation software ANSYS

HFSS. In addition, some potential future work within this area is also

suggested by the author.

Key words

Vehicle, radiated immunity, EMC, FEM, mesh, antenna

Abstract

Under de senaste decenniernas gang har elektroniken inom bilindustrin

sett en kraftig utveckling, och att trenden pekar pa batteridrivna och

sjalvkorande bilar som en trolig verklighet inom en snar framtid. Den

utvecklingen kan ocksa leda till att personbilar kan drabbas i en alltmer

elektro-magnetisk omgivning, speciellt fran EMC’s perspektiv.

Denna uppsats har som syfte att undersoka och skapa en overblick genom

en simulering som utfardar ett stralningsimmunitets-test pa ett fordon

i enlighet med standarden ISO 11451-2. Det preliminara arbetet med

geometri rening pa modellen som erhallits fran avdelningen for struktur-

dynamik och den overgripande metodutvecklingen har diskuterats. Det

som med denna uppsats avser att bidra med i huvud-del ar att bygga upp

ett mojlig arbetsflode som lampar sig for EMC-simuleringar for fordon med

FEM elektro-magnetiska simulations-mjukvaran ANSYS HFSS. Vidare

stalls fragor for ytterligare arbete och forskning av uppsats-forfattaren.

Contents

1 Introduction 1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Rationale and Objective . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4 Finite Element Method (FEM) . . . . . . . . . . . . . . . . . . . . . 6

1.5 Outline of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 Preliminary Study: Basis of Geometry Simplification 9

2.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3 Significance to this Thesis . . . . . . . . . . . . . . . . . . . . . . . . 12

3 Radiated Immunity Simulation 13

3.1 Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2 Device under Test (DUT) . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2.1 Mechanical Structure . . . . . . . . . . . . . . . . . . . . . . . 16

3.2.2 Geometry Clean-up . . . . . . . . . . . . . . . . . . . . . . . . 17

3.2.3 Vehicle Reference Point . . . . . . . . . . . . . . . . . . . . . . 17

3.3 Material and Boundary Condition . . . . . . . . . . . . . . . . . . . . 18

3.4 Integrated Final Layout . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.5 Mesh Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.5.1 Mesher: TAU vs. Classic . . . . . . . . . . . . . . . . . . . . . 21

3.5.2 Meshing Element: Rectilinear vs. Curvilinear . . . . . . . . . 22

4 Results 24

4.1 Mesh Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.1.1 Model Import — STEP vs. STL Format . . . . . . . . . . . . 24

4.1.2 Results of Initial Meshing . . . . . . . . . . . . . . . . . . . . 25

4.2 Antenna Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 28

i

4.3 Field Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.4 E-Field Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5 Conclusions 37

5.1 Contributions of the Thesis . . . . . . . . . . . . . . . . . . . . . . . 37

5.2 Bottlenecks and Limitations of the Thesis . . . . . . . . . . . . . . . 40

6 Future Work 41

A Computing Environment in the Project 44

Bibliography 45

ii

List of Figures

1.1 Electronic systems installed on a typical automobile1 . . . . . . . . . 2

1.2 An illustration of on-board radars, LiDARs and sensors2 . . . . . . . 2

1.3 Categories of EMC problems . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Why EMC study is essential . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 The four cases investigated in literature [14] . . . . . . . . . . . . . . 9

2.2 Surface-mesh representation of spot welding in CAE pre-processing

software ANSA. The small squares (marked with red circles) are the

ends of welds and the yellow surfaces represent metal sheets . . . . . 10

2.3 Case (c) reproduced in HFSS . . . . . . . . . . . . . . . . . . . . . . 10

2.4 Maximum radiated E-field . . . . . . . . . . . . . . . . . . . . . . . . 11

3.1 Configuration of vehicle radiated immunity test . . . . . . . . . . . . 13

3.2 Log-periodic antenna used in this thesis . . . . . . . . . . . . . . . . . 15

3.3 Field regions of an antenna . . . . . . . . . . . . . . . . . . . . . . . . 15

3.4 A (incomplete) geometry model of vehicle . . . . . . . . . . . . . . . 16

3.5 Original geometry obtained from the structural dynamics department 17

3.6 Location of vehicle reference point with respect to the car (dimensions

in meters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.7 Simulation setup in HFSS interface (length of the scale = 4 m) . . . . 20

3.8 Initial mesh generation and adaptive refinement in ANSYS HFSS . . 21

3.9 Rectilinear element (left) and curvilinear element (right) . . . . . . . 22

3.10 A sub-part with typical curved geometries extracted from BIW . . . . 23

4.1 Dense triangle elements on the BIW (length of the scale = 700 mm) . 25

4.2 A zoom-in view of Figure 4.1 (length of the scale = 100 mm) . . . . . 26

4.3 A zoom-in view at B-pillar meshed with curvilinear elements . . . . . 28

4.4 Curvilinear-meshed circle and cylinder . . . . . . . . . . . . . . . . . 28

4.5 Simulated VSWR of the antenna . . . . . . . . . . . . . . . . . . . . 29

4.6 Measured VSWR of the antenna1 . . . . . . . . . . . . . . . . . . . . 29

iii

4.7 Gain at axial direction of the antenna . . . . . . . . . . . . . . . . . . 30

4.8 3D radiation pattern of the antenna . . . . . . . . . . . . . . . . . . . 30

4.9 2D radiation patterns of the antenna . . . . . . . . . . . . . . . . . . 30

4.10 A vacuum box placed in the test region instead of a vehicle . . . . . . 31

4.11 E-field without the vehicle present (f = 500 MHz, ground plane at z

= 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.12 E-field without the vehicle present (f = 500 MHz, no ground plane at

z = 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.13 Simulation setup with both antenna and BIW included . . . . . . . . 33

4.14 Convergence of the simulation (f = 1 GHz) . . . . . . . . . . . . . . 33

4.15 Initial mesh (f = 500 MHz) . . . . . . . . . . . . . . . . . . . . . . . 34

4.16 Converged mesh (f = 500 MHz) . . . . . . . . . . . . . . . . . . . . . 34

4.17 E-field plotted on the xz-plane (f = 500 MHz) . . . . . . . . . . . . . 34

4.18 Engine, engine hood and doors are added to previous model . . . . . 35

4.19 E-field after adding engine, engine hood etc. (f = 500 MHz) . . . . . 35

5.1 Model of the car used in literature [9] . . . . . . . . . . . . . . . . . . 37

5.2 A workflow for vehicle EMC simulation . . . . . . . . . . . . . . . . . 38

6.1 Comparison between E-field plots obtained with FEM and FEBI1 . . 41

6.2 An example of wiring harness inside a vehicle . . . . . . . . . . . . . 42

iv

List of Abbreviations

ABC Absorbing Boundary Condition

AM Amplitude Modulation

BIW Body in White

CAD Computer-aided Design

CAE Computer-aided Engineering

CAN Controller Area Network

CEM Computational Electromagnetics

DUT Device under Test

EM Electromagnetic

EMC Electromagnetic Compatibility

EV Electric Vehicle

FDTD Finite-difference Time-domain

FEBI Finite Element Boundary Integral

FEM Finite Element Method

FM Frequency Modulation

HPC High-performance Computing

IEC International Electrotechnical Commission

IEEE Institute of Electrical and Electronics Engineers

ISO International Organization for Standardization

MoM Method of Moment

OEM Original Equipment Manufacturer

PEC Perfect Electric Conductor

PML Perfectly Matched Layer

RAM Random-access Memory

RF Radio Frequency

VSWR Voltage Standing Wave Ratio

v

Chapter 1

Introduction

1.1 Background

Since the birth of the first gasoline-powered car in 1886, progressively more electronic

devices have been installed on it: the first commercially produced in-car AM and FM

radio receiver came on board in 1930 and 1952, by Galvin Manufacturing Corporation

(later renamed Motorola) and Blaupunkt, respectively; from 1971, prototypical ABS1

system was launched onto market by several manufacturers (but with different names

though), e.g. Anti-skid Sure-brake System on 1971 Chrysler Imperial, Sure-track

Brake System on Lincoln Continental Mark III and Electro Anti-lock System on

Nissan President; in the past few decades, some state-of-the-art electronic techniques,

such as GNSS/GPS2, ACC3 and CAN4 bus, became available on passenger cars;

and in the anticipated next 10 years, the industry will keep pushing the envelope

of electrification and autonomous driving. In the meantime, however, the increasing

number of in-vehicle electronic devices (antenna, radar, LiDAR5, sensor, etc., see

Figure 1.1 and Figure 1.2) and off-vehicle radiation sources (base station, mobile

phone, radio broadcasting, navigation system, etc.), has led to higher complexity in

electromagnetic environment.

Spontaneously and inevitably, how to ensure that on-board electronic equipment

do not interfere with others and, how to ensure that they are ‘immune’ to external

1Anti-lock Braking System2Global Navigation Satellite System, Global Positioning System3Adaptive Cruise Control4Controller Area Network5Light Detection and Ranging

1

Figure 1.1 Electronic systems installed on a typical automobile1

Figure 1.2 An illustration of on-board radars, LiDARs and sensors2

electromagnetic interference (perform without degradation in their functionality and

performance) have become of interest from an automotive industry’s perspective —

this introduces the two most essential sub-problems under electromagnetic compatibil-

ity (EMC) — electromagnetic emission and electromagnetic immunity (the antonym

of ‘susceptibility’3). We can even further subdivide them into four categories by

coupling path, as illustrated in Figure 1.3. It is also worth mentioning that EMC-

related research is not limited to conducted emission (CE), radiated emission (RE),

conducted immunity (CI) and radiated immunity (RI) within automotive industry,

engineers are also paying attention to electrostatic discharge (ESD) issues.

1Clemson University Vehicular Electronics Laboratory https://cecas.clemson.edu/cvel/auto/systems/auto-systems.html

2NovAtel Inc. https://www.novatel.com/industries/autonomous-vehicles/technology3Electromagnetic susceptibility refers to the lack of electromagnetic immunity, i.e. it reflects the

equipment’s sensitivity to electromagnetic disturbance [1]

2

Figure 1.3 Categories of EMC problems

If we regard Figure 1.3 as a latitudinal approach of sorting general EMC concerns,

there is another longitudinal way of analyzing with respect to test object, that is,

complete vehicle and component. EMC analysis and testing on complete vehicle are

usually conducted by automotive OEMs on account of confidentiality concerns for

their unreleased prototypes, etc. As for component-level study, suppliers sometimes

play a more active role, but the negotiation between suppliers and OEMs is also

necessary such that the components can meet the requirements from both sides.

For example, the supplier can often use metal cover rather than plastic cover for

components in an attempt to get better EMC performance. The OEM may not

always agree upon this, though — it is true that a metal cover can bring better

shielding capability, but the higher cost and weight (a heavier car body worsens car’s

fuel economy) will also let the manufacturer hesitate. From this, the trade-off among

EMC performance, cost and sacrificing other properties will keep arousing researcher’s

and engineer’s interest in future engineering practices.

1.2 Literature Review

Automotive EMC study didn’t start as early as the born of the first passenger car.

Initially, there was neither pre-study nor highly standardized test procedure tailored

for vehicle EMC issues. It was the occurrence of failure that drove OEMs to have

internal EMC requirements or test methodologies which eventually became universal

standards. The International Special Committee on Radio interference (CISPR1)

and the International Organization for Standardization (ISO) are two international

organizations that have been dedicating to automotive EMC standardization. P.

Andersen and C. Fanning provide a comprehensive overview of the EMC standards

published by these organizations, and Andersen also underlines a present situation

1CISPR is a committee under the International Electrotechnical Commission (IEC) withspecialization in EMC standardization

3

that EMC requirements still remain to be well-defined for Tier II suppliers (suppliers

to OEM’s direct supplier) [2, 3]. Some of the commonly used automotive EMC

standards are summarized in Table 1.1.

Electrification plays an important role in motivating automotive EMC studies. In

2017, 1.1 million electric vehicles (EVs) were sold worldwide, and it is estimated

that the number will keep increasing to 11 million in 2025; moreover, by 2040,

55% of all new car sales and 33% of the global fleet will be electric, according to

Bloomberg’s long-term forecast of global electric vehicle [4]. For EV, two crucial

technologies are charging and electric drive, which both demand high-voltage power

source. S. Guttowski et al. addresses the EMC issues related to the integration of

an electric drive system into a conventional passenger car in [5]. Apart from electric

drive system, conducted emission under circumstances of EV charging also receives

scholar’s interest [6]. To mitigate such EMC problems, filtering and resonant reactive

shielding techniques have been proposed by M. C. Di Piazza et al. and S. Kim et al.

as two possible approaches [7, 8].

Additionally, study on weak-electric modules is another topic that captures atten-

tion. In 2002, P. Ankarson and J. Carlsson present a joint EMC project by institutions

and companies of Chalmers University of Technology, SAAB Automobile, Volvo Truck

Corp., Volvo Car Corp., Scania and Swedish National Testing and Research Institute

in [9]. They start from the CAD-model of a vehicle, followed by geometry treatment

with commercial software ANSA and CADfix, and eventually summarize results of

E-field distribution as well as coupling between antennas derived by FDTD-based

software EMA3D. For some more particular topics, FEM method is also used for

investigating e.g. the significance of model’s complexity on simulations [10, 11]. Finite

Element Boundary Integral (FEBI), as a hybrid approach, is an alternative to the

traditional CEM methods mentioned above in vehicle EMC simulations for its shorter

simulation time and lower computational requirement, as J. Mologni highlights in [12].

Literature [9] by P. Ankarson and J. Carlsson is one of the inspirations for this

thesis: their research with FDTD method has stimulated the emergence of the in-

triguing idea of substituting it with FEM. It should be noted that there is no certain

best CEM method — any method has its own pros and cons. FEM is used in this

thesis not for the purpose of replacing FDTD but investigating the feasibility of itself.

4

Table 1.1 Some automotive EMC standards

Number Title Level

ISO 11451Road Vehicles — Vehicle Test Methods forElectrical Disturbances from NarrowbandRadiated Electromagnetic Energy

International

ISO 11452

Road Vehicles — Component Test Meth-ods for Electrical Disturbances fromNarrowband Radiated ElectromagneticEnergy

International

ISO 7637Road Vehicles — Electrical Disturbancesfrom Conduction and Coupling

International

ISO 10605Road vehicles — Test Methods forElectrical Disturbances from ElectrostaticDischarge

International

CISPR 12

Vehicles, Boats and Internal CombustionEngines — Radio Disturbance Char-acteristics — Limits and Methods ofMeasurement for the Protection of Off-board Receivers

International

CISPR 25

Vehicles, Boats and Internal CombustionEngines — Radio Disturbance Char-acteristics — Limits and Methods ofMeasurement for the Protection of On-board Receivers

International

ANSI C63.5

American National Standard for Elec-tromagnetic Compatibility — RadiatedEmission Measurements in Electromag-netic Interference (EMI) Control — Cali-bration and Qualification of Antennas (9kHz to 40 GHz)

National

ANSI C63.12American National Standard Recom-mended Practice for ElectromagneticCompatibility Limits and Test Levels

National

5

1.3 Rationale and Objective

EMC problems can always start to be treated and refined when it arise on a physical

product, yet this is often cost-inefficient and the engineers’ hands are tied such that

there might be only a few feasible measures available to resolve it (see Figure 1.4).

Therefore, it has become a driving force which motivate automobile manufacturers to

initiate EMC studies at an earlier stage. A pre-study of vehicle’s EMC performance

may sometimes bypass potential issues and profit margins for e.g. underestimated

situations and unavoidable variants. With this being the vision, the objectives of this

thesis is:

Investigate and develop a simulation method applied to vehicle radiated

immunity test

Phases of product

Research Design Test Manufacture & Launch

Number of feasible measures

Cost per measure

Figure 1.4 Why EMC study is essential

The thesis is not intended as a thorough simulation report focusing on the ge-

ometry of a certain car model, nor does it provide any specific or accurate design

guideline. Indeed, it expounds the process from obtaining finite element model from

structural dynamics department to yielding results by EM simulation software within

CAE (Computer-aided Engineering) department.

1.4 Finite Element Method (FEM)

In the area of Computational Electromagnetics (CEM), some commonly used full-

wave numerical methods are: Finite Element Method (FEM), Method of Moments

6

(MoM), Finite-difference Time-domain Method (FDTD), etc. They solve the electro-

magnetic field in either time domain or frequency domain with differential- or integral-

form Maxwell’s Equation. In recent years, researchers also endeavor to develop hybrid

methods (or, say, hybrid solvers), seeking for integrating advantages from different

methods to achieve faster method with higher accuracy on specific applications.

As mentioned in Section 1.2, FDTD-based software has been applied to some

vehicle electromagnetic simulations due to its strength on computation for electrically

large objects and transient analysis. However, as its name suggests, FDTD is a

method in time domain; therefore, it might not be the most ideal choice for deriving E-

field as a function of frequency in narrowband analysis. In addition, FEM, comparing

to FDTD, has a stronger capability to handle geometry with higher complexity (e.g.

vehicle). Taking all of these aspects into account, HFSS, a FEM electromagnetic

simulation software produced by ANSYS, Inc., is adopted for simulations in this

thesis.

To solve the electromagnetic field, HFSS first subdivide the entire geometry into

small tetrahedral elements, and the entire collection of tetrahedra constitutes the

finite element mesh. A more detailed introduction to mesh generation will be given

in Section 3.5. In each element, HFSS solves Equation (1.1) and (1.2) to calculate

the electric field E and the magnetic field H excited by source Jsource:

∇× (1

µr

∇×E)− k20rεrE = Jsource (1.1)

H =1

ωµ∇×E (1.2)

where k0 = ωc

is the wave number in free space; ω is the angular frequency; c is the

speed of light in vacuum; εr, µr are the relative permittivity and permeability, respec-

tively. The remaining electromagnetic quantities are derived using the constitutive

relations [13].

1.5 Outline of the Thesis

In Chapter 1, the author sheds some light on basic concepts of EMC and road vehicle

EMC studies; also, the author reviews some relevant research from previous years

and thus introduces the motivation of this thesis. Chapter 2 acts as a preliminary

study in the project, providing a theoretical basis for geometry simplification which is

7

used before EM simulations. Chapter 3 explains the simulation set-up and techniques

utilized in this thesis. To continue, Chapter 4 & 5 discuss the obtained results based

on Chapter 3, summarizes the methodology of the entire project and highlights some

key findings. As an epilogue, Chapter 6 gives the outlook for potential future studies

within this area.

8

Chapter 2

Preliminary Study: Basis ofGeometry Simplification

The scenario in which a cable carrying common mode current positioned over a

conducting plane has been investigated with MoM-based software WIPL-D, including

four cases (see Figure 2.1): (a) one solid sheet, (b) two sheets with horizontal gap, (c)

two overlapped sheets without direct electrical connection, and (d) two overlapped

sheets with spot connections [14]. It depicts the electrical model of wiring harness

together with its adjacent metal sheet (or metal sheets connected by spot welding)

in the simplest way.

(a) A solid sheet (b) Two sheets separated by a horizontal gap

(c) Two overlapped sheets without electrical contact (d) Two overlapped sheets with electrical contact

Figure 2.1 The four cases investigated in literature [14]

A normal passenger car typically contains 2000 – 6000 spot welding joints to con-

nect metal structures mechanically. However, the small welds can result in problems

when translating a vehicle’s geometry in computer software (see Figure 2.2); and it

is also time-consuming to mesh and compute electromagnetic field within such tiny

geometries. Inspired by the research mentioned above, a similar study is done for the

purpose of evaluating the impact of neglecting spot welding joints.

9

(a) Incorrect representation: squares with shadow indicatethat the welds are not recognized as connected to the sheet

(b) After manual correction: the ends of welds are mergedinto the sheet (mesh cells)

Figure 2.2 Surface-mesh representation of spot welding in CAE pre-processing software ANSA. Thesmall squares (marked with red circles) are the ends of welds and the yellow surfaces represent metalsheets

2.1 Methodology

Here, case (c) in [14] is reproduced in FEM-based software ANSYS HFSS: as shown in

Figure 2.3a, a cable (l = 750 mm) fed by current source is 5 mm above two 500 mm ×500 mm overlapped rectangular conductive sheets. The vertical separation between

the two sheets is defined as d = 0.1 mm, 0.7 mm or 1.0 mm, and the overlapped

distance in horizontal direction is 10 mm (see Figure 2.3b).

(a) Two overlapped sheets without electrical contact (b) A detailed view at the overlapped area

Figure 2.3 Case (c) reproduced in HFSS

Case (d), which is more close to reality, is created by adding conductive connec-

tions between the two sheets on the basis of case (c) to act as a reference case.

For the sake of simplicity, all the materials are assigned as ‘PEC’ (perfect electric

conductor) in simulations here.

10

2.2 Results and Discussion

For frequency f ranges from 30 MHz to 1 GHz, maximum radiated E-field in both

case (c) and (d) is plotted as a function of f in Figure 2.4.

Figure 2.4 Maximum radiated E-field

In Figure 2.4, peaks are observed at f ≈ 200 MHz, 400 MHz, 600 MHz, 800 MHz

and 1 GHz for all the four curves, which corresponds to wavelength λ = 1.5 m, 0.75

m, 0.5 m, 0.375 m and 0.3 m, respectively. The length of the cable, l, equals to

0.5λ, λ, 1.5λ, 2.0 λ and 2.5λ at those frequencies, and thus causes the occurrence of

resonances.

As frequency increases, especially when f is larger than 200 MHz, the maximum

radiated E-field of case (c) (blue, green and yellow curve) starts to approach the

reference case (red curve). Moreover, the result for case with smaller separation has

higher proximity to the reference case: for instance, the ‘0.1 mm separation’ curve

has a difference within a few dB at all frequencies, while for the other two curves, it’s

usually ∼ 10 dB at 30 – 200 MHz and roughly 5 – 10 dB at higher frequencies.

The phenomenon above is due to crosstalk or, more specifically, capacitive cou-

pling, i.e. the E-field can propagate from one metal sheet (as ‘source’) to the other

(as ‘victim’) through mutual capacitance, even though there is no direct electrical

contact between them. The typical transfer function for capacitive coupling between

two neighboring current loops is

V2V1≈ jωRC12

4(2.1)

11

where ω = 2πf is the angular frequency of the signal, R is the simplified resistance

in the circuit (all resistances in the circuit are set to be R) and C12 is the mutual

capacitance.

Considering a capacitor constructed of two parallel plates separated by a distance

d, the capacitance C can be approximated to

C =ε0A

d(2.2)

where ε0 is the vacuum permittivity (electric constant) and A is the area of overlap

of the two plates.

Qualitatively, it can be observed from Equation (2.1) and (2.2) that capacitive

coupling is proportional to f and, inversely proportional to d. Similarly, it is not

a coincidence to see the impact of neglecting spot welding joints is mitigated as f

increases or, as d decreases.

2.3 Significance to this Thesis

In practices within automotive industry, problems like Figure 2.2 have been noticed,

and it takes excessive time and considerable manual operation to heal it for thousands

of times such that the geometry is correctly represented. As discussed in previous

sections, the removal of spot welding joints will not significantly block the transmis-

sion of high-frequency E-field between two metal sheets due to capacitive coupling,

especially when the distance between them is smaller than 1 mm; hence, neglecting

spot welding connections has been taken into consideration as a way of simplifying

geometry in later study in this thesis.

It’s noteworthy that this simplification shall not be applied to low-frequency

(several tens of MHz or even lower) electromagnetic simulations because capacitive

coupling attenuates by 20 dB/decade as frequency decreases.

12

Chapter 3

Radiated Immunity Simulation

The simulations in this thesis aim to replicate the vehicle radiated immunity (refer

to Figure 1.3) test set-up in accordance with ISO Standard 11451-2 [15]. The test

is usually performed in RF anechoic chambers (refer to [16] for an overview of

automotive EMC anechoic chambers), with antenna being the radiation source and

vehicle as the victim (DUT) that subject to the emission from the source. Its purpose

is to determine the immunity of passenger cars to electrical disturbances from off-

vehicle radiation sources. Examples illustrating the test configuration are shown in

Figure 3.1.

(a) Schematic (dimensions in meters) [15] (b) Real test scene at RISE1

Figure 3.1 Configuration of vehicle radiated immunity test

Numbers in Figure 3.1a refer to following apparatuses:

1 — Absorber-lined shielded enclosure (the jagged structure surrounded by Ap-

paratus 1 is RF absorbing material)

2 — RF signal generator

1RISE (Research Institutes of Sweden AB) was formerly known as SP (Swedish National Testingand Research Institute, Sv. Sveriges Provnings- och Forskningsinstitut) before 2017

13

3 — Power amplifier

4 — Dual directional coupler

5 — Power meter

6 — Coaxial feedthrough

7 — Field generating device (antenna)

8 — Vehicle reference point

Sometimes a turntable is also provided in the test region for adjusting the angle

of incidence (not included in Figure 3.1a).

The main considerations for simulating open-field environment with indoor facili-

ties are: (a) minimizing external interference or noise from other sources on simulation

results and (b) avoiding hazardous voltage and field emission towards public area. To

realize this, Apparatus 1 and RF absorber are needed: the shield enclosure prevent the

test region from external electromagnetic field and the absorbing material attenuates

reflected energy by at least 10 dB.

3.1 Source

According to some previous vehicle EMC testing practices, a log-periodic antenna

(see Figure 3.2) is selected to act as the disturbance source due to its wide-band

frequency range coverage at 75 – 1500 MHz [17]. Table 3.1 lists some antenna types

recommended by international standard CISPR 25 for different frequency ranges [18].

Table 3.1 Antenna types recommended by CISPR 25

Frequency Antenna Type

0.15 – 30 MHz Vertical monopole antenna

30 – 300 MHz Biconical antenna

200 – 1000 MHz Log-periodic antenna

1000 – 2500 MHz Horn antenna & log-periodic antenna

14

(a) 2D side-view (b) Dimetric projection

Figure 3.2 Log-periodic antenna used in this thesis

As shown in Figure 3.3, the surrounding space of an antenna is usually subdivided

into three regions by distance R: (a) reactive near-field region, (b) radiating near-field

(Fresnel) region, and (c) far-field (Fraunhofer) region [19].

Reactive near-field region

Radiating near-field region(Fresnel region)

Far-field region (Fraunhofer region)

𝑹𝟏

𝑹𝟐

Figure 3.3 Field regions of an antenna

R1 and R2 in Figure 3.3 are defined by following equations:

R1 = 0.62√D3/λ (3.1)

R2 = 2D2/λ (3.2)

where D is the largest dimension of the antenna, λ = cf

is the wavelength of the

electromagnetic wave at frequency f , and c is the speed of light in vacuum. For the

above-mentioned Schwarzbeck VULP 9118 E log-periodic antenna, D = 1.94 m [17].

Substituting D = 1.94 m into Equation (3.1) and (3.2), it is obtained that R1 =

0.95 m, R2 = 2.46 m for antenna operating at 100 MHz and R1 = 2.16 m, R2 = 12.55

m for 500 MHz, respectively.

15

3.2 Device under Test (DUT)

3.2.1 Mechanical Structure

In automotive industry, body-in-white (hereinafter called “BIW”) refers to the stage

in which the metal structural components of a vehicle have been assembled together

by e.g. spot welding joints and rivet joints, but covering components such as engine

hood, glass, doors (may vary among OEMs, though), trunk lid, interior, etc., are not

yet installed. It consists of the most essential structural components and forms the

skeleton of a vehicle. Figure 3.4a offers a visual understanding of the concept BIW.

(a) Body-in-white (BIW) (b) Engine, engine hood, fender and doors

Figure 3.4 A (incomplete) geometry model of vehicle

The final stage of a passenger car is known as complete vehicle from a OEM’s

standpoint which requires no further work prior to vehicle registration at authorities.

Neverthelss, it is neither realistic nor efficient to simulate a complete vehicle in an

electromagnetic simulation software, since information can be insufficient for materi-

als, cable routing, precise interior shapes, etc. and the simulation time will increase

tremendously after introducing a large amount of dielectric objects. Simulating a

complete vehicle is just a vision but not a practical goal to achieve. As a result, only

the geometry in Figure 3.4 is taken into account as the DUT in this thesis.

The coordinate system used for the vehicle as well as other objects in the thesis

are defined as follows:

x-direction: from front to rear of the vehicle

y-direction: from driver’s left-hand side to right-hand side

z-direction: from floor side to roof side

xz-plane: the plane of symmetry of the vehicle (not strictly symmetrical)

16

If here the tires of the vehicle exist, they will be exactly tangent to the z = 0

plane (xy-plane).

3.2.2 Geometry Clean-up

Figure 3.5 shows the original structure before geometry clean-up (captured in ANSYS

SpaceClaim) — it is the raw geometry of a car body obtained from the structural

dynamics department, which is not the same as Figure 3.4; it contains extra informa-

tion of windshield and sunroof as well as configurations for both left- and right-hand

drive, etc., which is not in high necessity in terms of electromagnetic simulation. All

of these non-metal and/or minor structures are excluded with pre-processing software

ANSA. Besides, some tiny holes, apertures and gaps are either filled or stitched in

ANSYS SpaceClaim.

Figure 3.5 Original geometry obtained from the structural dynamics department

Furthermore, representations of spot welding joints included in the original file

are also excluded with ANSA before proceeding to EM simulations. On the basis of

Chapter 2, they are believed to have minor impact on simulation results and hence

the simplification.

3.2.3 Vehicle Reference Point

The reference point is a virtual point that is introduced for the purpose of locating

the vehicle with respect to the source. According to ISO 11451-2 standard [15], one

possible set of criteria for defining the vehicle reference point is as follows (see Figure

17

3.6):

(a) On the xz-plane

(b) 1±0.05 m above the floor for vehicles with a rood height 63 m

(c) 0.2±0.2 m behind the front axle

Figure 3.6 Location of vehicle reference point with respect to the car (dimensions in meters)

3.3 Material and Boundary Condition

Although it is possible to model the wall and ceiling of anechoic chamber with arrays

of pyramidal radiation absorbing material (usually made of rubberized foam material

impregnated with controlled mixtures of carbon and iron) [12], the RF absorber

in Figure 3.1a is substituted with certain boundary condition in HFSS to reduce

geometry and computation complexity. Table 3.2 demonstrates the features of the

three radiating boundaries that available in HFSS [20]. PML acts as a good absorbing

boundary condition by surrounding the solving domain externally with several layers

of lossy material that attenuates outgoing waves, but the drawback is the solving

domain is enlarged so that both memory usage and simulation time will increase.

Eventually, the ABC boundary is selected, for its lower requirement on computation

resource.

18

Table 3.2 Radiating boundary conditions in ANSYS HFSS

BoundaryComputation

Resources

Minimum Distance

from RadiatorShape

ABC1 Lowest λ/4 Concave only

PML2 Medium λ/8 Planar and concave only

FEBI3 Highest No limit Arbitrary

A detail that easy to be ignored is the floor. Since the floor is typically not covered

with absorber in real practices4, we shall not assign the same boundary to the floor

as the wall and ceiling, otherwise all the energy towards the floor will be absorbed.

In this case, it is resolved by utilizing the feature ‘apply infinite ground plane’ at the

z = 0 plane in HFSS: this feature is believed to be able to represent the existence of

floor or ground and thus model the backward reflection from it.

The major vehicle structural components are usually made of metallic material,

e.g. steel and aluminum alloy, which are simplified with ‘Perfect E’ boundary in the

software without significantly sacrificing the accuracy.

3.4 Integrated Final Layout

Having all the geometry treatment done, the final simulation layout is presented in

Figure 3.7.

The configuration above fulfills following instructions of Figure 3.1a:

(a) The lowest part of the antenna is > 0.25 m above the floor (the x = 0 plane)

(b) The vehicle is located such that its reference point is > 2 m away from the

antenna in x-direction

Note that the airbox (radiation boundary) is artificially set to be invisible in Figure

3.7 for a clearer view.

1Absorbing Boundary Condition (ABC)2Finite Element Boundary Integral (FEBI)3Perfectly Matched Layer (PML)4Such covering is allowed but can lead to different results, according to ISO 11451-2 [15]

19

Figure 3.7 Simulation setup in HFSS interface (length of the scale = 4 m)

As calculated in Section 3.1, R1 is 2.16 m and R2 is 12.55 m in the case of f = 500

MHz. The DUT will be > 3λ away from the antenna, and most likely in the radiating

near-field (Fresnel) region (because the separation between the DUT and the antenna

shall be at least 2 m) wherein the radiation field dominates and the angular field

distribution is dependent upon the distance from the antenna [19]. Likewise, for f

= 100 MHz, the DUT is in (or almost in) the far-field region where the angular field

distribution is independent on the distance from the antenna.

3.5 Mesh Generation

One of the most essential steps in FEM electromagnetic simulations is to discretize

the geometry domain into small elements (e.g. tetrahedra, in HFSS) and thus forms

the initial finite element mesh in which Maxwell’s equations are solved. The flowchart

in Figure 3.8 (redrawn from HFSS Online Help [20]) explains the meshing process in

ANSYS HFSS.

The process depicted by the flowchart can be summarized as follows [20]:

Step 1: HFSS generates initial mesh

Step 2: Seed refinement (if assigned) and Lambda refinement occur on the initial

mesh

Step 3: Based on the obtained mesh, HFSS computes the electromagnetic fields

within the structure

Step 4: From the current solution, HFSS produces error estimates and refines the

mesh in regions of high error and uses the refined mesh to generate another solution

20

FEMSolver

Converged?

Adaptive MeshRefinement

Finish

Initial MeshGeneration

LambdaRefinement

Start

Yes

No

Adaptive MeshRefinement

Figure 3.8 Initial mesh generation and adaptive refinement in ANSYS HFSS

Step 5: HFSS performs the iterative process of solve, error analyses and adaptive

refinement until the convergence criteria are satisfied or the maximum number of

adaptive passes is completed

3.5.1 Mesher: TAU vs. Classic

There are two optional methods provided by ANSYS HFSS to generate initial mesh

— TAU and Classic. The origin of the TAU method dates back to the late 1990s: it

was initially implemented for computing 3D flow field of aircraft-scale configurations

under sub- and transonic regimes and named as DLR-τ -Code in its infancy [21, 22].

Later it was adopted by ANSYS since HFSS version 12.0 and became an extra option

to the default Classic mesher [23].

21

It is believed that in most cases the Classic mesher converges with fewer resources

whereas TAU may converge faster, and TAU is also well-optimized for paralleled

computing environment [20]. The desktop and the cluster used during the project

has quite sufficient resource (refer to Appendix A for configuration details), thus the

TAU mesher should be a better choice. Still, both meshers are investigated to validate

the prediction.

3.5.2 Meshing Element: Rectilinear vs. Curvilinear

In ANSYS HFSS it is also possible to apply curvilinear meshing to curved surfaces

(disabled by default). A curvilinear element, as can be understand literally, consists

of curved edges and/or faces in comparison with a rectilinear element (see Figure

3.9 [20]); thus, even though it might not identically conform to the original curve,

more accurate representation of the curvature in a car’s geometry (wheel arches,

connections between the roof and the A- or C-pillar, etc.) and less elements usage

can still be expected when utilizing curvilinear meshing, and usually generates more

accurate and faster FEM solutions [20].

Figure 3.9 Rectilinear element (left) and curvilinear element (right)

In order to investigate the effect of curvilinear meshing, a sub-part containing

typical curved geometries (marked with red arcs) is extracted from the BIW (see

Figure 3.10).

The reason for using the geometry in Figure 3.10 rather than the complete BIW

model is to demonstrate the problem with a minimal working example and thus

shorten the time for validating the function ‘curvilinear meshing’.

22

Figure 3.10 A sub-part with typical curved geometries extracted from BIW

In this chapter, both the ISO 11451-2 test set-up and some simulation configu-

rations are discussed. Since the meshing process and result of a geometry with

high complexity is often of interest from a company’s perspective, a benchmark

regarding the mesh settings (i.e. TAU/Classic mesher, with/without curvilinear

meshing applied) is performed at 100 MHz and 500 MHz on a DELL Precision T7610

Desktop Workstation.

23

Chapter 4

Results

4.1 Mesh Generation

4.1.1 Model Import — STEP vs. STL Format

Automotive companies use software like CATIA and Creo Parametric (previously

known as ProE, Pro/ENGINEER) to create CAD models and, tools like ANSA to pre-

process geometry before sending to e.g. EM, CFD1 and collision simulation software.

Some commonly used CAD formats are: STEP, STL and IGES, etc.

STL (an abbreviation of ‘stereolithography’) is a file format that has been widely

applied in 3D printing and computer-aided manufacturing. The typical structure of

a STL file (in ASCII format) is shown below in plain text: for each triangle, the

normal vector (ni, nj, nk) and the xyz coordinates of its three vertices (vmx, vmy, vmz),

(m = 1, 2, 3) are defined with strings in line 2 – 8.

1 solid <name >2 facet normal ni nj nk3 outer loop4 vertex v1x v1y v1z5 vertex v2x v2y v2z6 vertex v3x v3y v3z7 endloop8 endfacet9 ...

10 endsolid <name >

Due to its simplicity, STL is chosen as an alternative to STEP format, expecting

1Computational Fluid Dynamics

24

for shorter loading and meshing time in EM simulation software. However, a STL-

imported BIW in ANSYS HFSS may have more than 1.4 million tiny triangle faces

which results in longer rendering time and tardy response while moving, rotating or

zooming in/out the geometry in software GUI. And the length of triangles’ sides is

typically smaller than 15 mm, which is the corresponding wavelength of 20 GHz —

far beyond the frequency range of this thesis (yet STL format could still be of interest

for research in higher frequency band, e.g. 5G), and thus causes over-resolution (see

Figure 4.1 and 4.2). In addition to response and resolution, it can occupy more than

13 GB RAM for the GUI1 to display such STL model while STEP format requires less

than 10 GB for even larger geometry (an idle HFSS GUI normally occupies slightly

less than 7 GB RAM).

One possible approach to resolve the issue above is to coarsen the STL mesh

in ANSA before exporting to HFSS. However, coarse STL mesh may result in poor

representation of model since it does not support surfaces with curvature. Further

study is yet to be done.

Figure 4.1 Dense triangle elements on the BIW (length of the scale = 700 mm)

4.1.2 Results of Initial Meshing

A benchmark is carried out on a desktop workstation (configuration of the workstation

is listed in Appendix A) to evaluate the initial meshing process with various setting

combinations (see Table 4.1). In Group 1 – 4, a basic vehicle EMC test set-up (as

1Graphical User Interface

25

Figure 4.2 A zoom-in view of Figure 4.1 (length of the scale = 100 mm)

shown in Figure 3.7) is meshed; for efficiency reason, only the BIW or a sub-part1 of

the BIW (rather than the complete set-up with an antenna and a car) is taken into

account in Group 8 & 9 and Group 5 – 7, respectively.

Table 4.1 Specification of different combinations

Group Frequency GeometryInput

FormatMesher

Mesh

Element

Faces

Merging

1

100 MHzBIW

+

AntennaSTEP

TAURectilinear

N/A

2 Curvilinear

3 Classic

Rectilinear4500 MHz

TAU

5

Sub-part6100 MHz

Curvilinear

7

Rectilinear8500 MHz BIW STL

None

9 Planar

Table 4.2 shows the corresponding information regarding the meshing process of

each specific group.

Combining Table 4.1 and Table 4.2, some noticeable facts are as follows.

STEP vs. STL If we compare Group 4 to Group 8 or 9, it turns out that much

longer time is needed for meshing even smaller STL-imported geometry. Moreover,

1Refers to the geometry in Figure 3.7

26

Table 4.2 Profile of the initial meshing process

GroupMeshing Time (hh:mm:ss) Number of Elements

(Tetrahedra)

RAM Usage

(GB)Real Time CPU Time

1 19:47:50 31:08:02 4,521,570 12.42

2 38:28:24 100:16:31 4,478,181 48.66

3 36:15:26 48:02:30 4,476,487 21.68

4 20:03:54 31:55:45 4,669,005 12.42

5 00:19:08 00:21:08 246,496 0.76

6 03:56:04 04:31:51 210,046 1.53

7 00:18:32 00:20:47 209,988 0.75

8 111:01:18 1869:03:08 5,703,927 37.41

9 105:45:38 1849:10:07 5,707,936 35.23

selecting ‘None’ or ‘Planar’ when importing STL file into ANSYS HFSS doesn’t make

big difference in time and RAM usage for meshing the given BIW geometry.

TAU vs. Classic As can be observed from Group 1 and Group 3, the TAU

mesher generates similar amount of mesh elements with approximately 45% less time

and memory usage than the Classic mesher. It is claimed that both mesher can

provide converged mesh with good accuracy but the TAU mesher will be automatically

selected by the software in most cases if no preference is specified; nevertheless, the

user can always explicitly override the default option with TAU or Classic whenever

necessary [20].

Rectilinear vs. Curvilinear Meshing with curvilinear elements is more time-

and RAM-consuming in comparison with rectilinear elements (two times more CPU

time usage and three times more RAM usage), as indicated by Group 1 & 2 and 6 &

7. In addition, it doesn’t manage to creat mesh elements as expected on given curved

geometry: as can be seen from Figure 4.3, the mesh around the curved geometry

is still quite dense, indicating that utilizing curvilinear element cannot significantly

reduce the number of mesh elements (reduced by less than 1%, indeed) on the certain

BIW geometry. To double check, a circle and a cylinder, as minimal 2D and 3D

examples, are also meshed with curvilinear meshing being applied, and yet, the mesh

elements do not conform to the curved arcs.

Consequently, settings in Group 1 turn out to be more satisfactory, and therefore

be adopted for later simulations. More precisely, the geometry will be imported in

27

Figure 4.3 A zoom-in view at B-pillar meshed with curvilinear elements

Figure 4.4 Curvilinear-meshed circle and cylinder

STEP format and meshed with TAU mesher with rectilinear elements applied in

further EM simulations.

4.2 Antenna Characteristics

The VSWR of the antenna is simulated without BIW’s presence and the result at 100

– 1000 MHz is plotted in Figure 4.5.

As Figure 4.5 shows, VSWR is below 1.7 at most frequencies, which indicates that

more than 93% energy can be radiated from the antenna as expected. Although the

curve is not in perfect agreement with the manufacturer’s test result in Figure 4.6, it

would be sufficient as long as it is radiating adequately within the frequency range of

28

the test.

Figure 4.5 Simulated VSWR of the antenna

Figure 4.6 Measured VSWR of the antenna1

In Figure 4.7, the gain at antenna’s axial direction from both measured (blue

curve) and simulated (red curve) results is drawn in the same plot.

The advantage of using directional antenna like log-periodic antenna in real tests

is that directional antenna can converge the majority of energy into one main lobe

at expected orientation, while omnidirectional antenna (e.g. biconical antenna) can

also be used but it usually requires higher input energy to realize same field strength

at a certain location.

Figure 4.8 and Figure 4.9 illustrate the 3D and 2D radiation patterns, in which

the main lobe covers ±30◦ angle at axial direction. An excessively narrow main lobe,

in other words, a highly directional antenna, is not what one expects here because it

will converge too much energy in a small angular range such that the entire vehicle

cannot be radiated upon uniformly.

1Extracted from the datasheet [17]

29

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

100 300 500 700 900

Maxi

mu

m G

ain

[d

Bi]

Frequency [MHz]

Simulated value

Measured value

Figure 4.7 Gain at axial direction of the antenna

Figure 4.8 3D radiation pattern of the antenna

(a) E-plane (b) H-plane

Figure 4.9 2D radiation patterns of the antenna

30

4.3 Field Calibration

Again, the simulation in this section is performed in absence of the vehicle; a dummy

vacuum box (the light blue cuboid in Figure 4.10) is placed at the test position

instead to maintain the solving domain. The red point in the vacuum box is the

vehicle reference point, which is modeled as a ‘non-model’ sphere, that is, a visible

object that the software treat as it does not exist.

Figure 4.10 A vacuum box placed in the test region instead of a vehicle

The purpose of this simulation is to efficiently verify all boundary conditions with

minimal geometry. Also, field calibration can be performed at this stage if multiple

frequencies are going to be simulated.

Figure 4.11 E-field without the vehicle present (f = 500 MHz, ground plane at z = 0)

The E-field in Figure 4.11 penetrates continuously through the edges of the vac-

uum box and no reflection or refraction is observed at the boundaries, indicating that

31

the existence of dummy box does not affect the propagation of electromagnetic wave;

on the right half of the plot, the wave no longer propagates in a concentric-ring shape

due to the superposition effect of reflected wave from the conductive ground plane at

z = 0.

Figure 4.12 E-field without the vehicle present (f = 500 MHz, no ground plane at z = 0)

Figure 4.12 provides a comparative case to Figure 4.11 in which no ground plane

is assigned, and thus no energy reflects back from the z = 0 plane.

The comparison between Figure 4.11 and Figure 4.12 highlights the necessity of

ground plane for its reflecting effect. Such effect should be taken into account since

it will affect the distribution of E-field near the vehicle’s chassis.

Before onsite tests, a procedure called field calibration is performed periodically,

i.e. tuning the forward power to the antenna such that a specific value of field

strength can be obtained for each test frequency. From a simulation’s perspective,

the difference in field strength can be calibrated by post-processing.

4.4 E-Field Distribution

Now the geometry of a BIW (as shown in Figure 3.4a) is added into HFSS in STEP

format and forms up a “Antenna + BIW” configuration (see Figure 4.13).

With the BIW being included, the simulation time has increased significantly to

56, 78 and 163 hours for single-frequency simulation at 100 MHz, 500 MHz and 1000

MHz, respectively.

32

Figure 4.13 Simulation setup with both antenna and BIW included

For simulation at 1 GHz, 3 adaptive passes are needed (4 passes in total) to achieve

convergence criteria ∆S < 0.1 (see Figure 4.14), whereas 2 adaptive passes would be

sufficient at 100 MHz and 500 MHz due to shorter wavelength.

Figure 4.14 Convergence of the simulation (f = 1 GHz)

As previously discussed in Section 3.5, adaptive mesh refinement is a iterative

process that increases the mesh density in regions with high field strength. The

initial mesh is purely based on the geometry and the converged mesh is achieved by

automatic refinement on the basis of initial mesh.

In the plot of initial mesh (Figure 4.15), the elements are quite dense around the

geometries (i.e. the antenna and BIW) and homogeneously distributed but sparser

elsewhere. After two adaptive passes, the mesh plot becomes Figure 4.16. Evidently

denser mesh is generated and even the field distribution towards the BIW becomes

visible.

33

Figure 4.15 Initial mesh (f = 500 MHz)

Figure 4.16 Converged mesh (f = 500 MHz)

The E-field under this “Antenna + BIW” circumstance is plotted on a cross section

(xz-plane) in Figure 4.17. Maximum E-field measured at the vehicle reference point

is 72.8 V/m when antenna’s input power is 50 W.

Figure 4.17 E-field plotted on the xz-plane (f = 500 MHz)

One fact worth to be noticed is the ‘hot spots’ (places with high field strength)

inside the cabin. This is because the metallic roof and floor act as a loosy reverbera-

tion chamber with some Q-value (or a rectangular waveguide cavity) such that some

of the electromagnetic wave is restricted in it, and thus increases the field at certain

34

locations. The exact distribution (location, magnitude, etc.) of hot spots mainly

depends on frequency as well as the shape and material of BIW.

If we further add the engine, engine hood and doors (pink) into previous model

(grey), a more complete layout (as shown in Figure 4.18) can be obtained. The

corresponding E-field is plotted in Figure 4.19.

Figure 4.18 Engine, engine hood and doors are added to previous model

Figure 4.19 E-field after adding engine, engine hood etc. (f = 500 MHz)

The engine model contains substantial amount of tiny, complex geometry which is

predicted to result in much longer meshing time, or even failure in performing initial

mesh. A technique called wrapping is used to handle this issue. The principle of

wrapping is just like putting an object into plastic bag and suck all the air out, then

use the plastic bag to represent the object. With this approach, we minimize the

engine model by excluding all the information, except its contour. However, even

though treatment as wrapping has been applied, the simulation time still increases

from roughly 78 hours to more than 160 hours after including new geometries.

35

Comparing Figure 4.19 to Figure 4.17, it is obvious that the introduction of new

geometries has changed the distribution of E-field. The region has been affected most

is believed to be the engine bay — the maximum of E-field at vehicle reference point

has decreased from 72.8 V/m to 51.9 V/m due to the shielding effect of engine hood.

Besides, the reflection and shielding of four doors are also not negligible when the

antenna radiates laterally.

36

Chapter 5

Conclusions

5.1 Contributions of the Thesis

Although vehicle EMC analysis is attracting more academic attention nowadays,

there is not much comprehensive and thorough discussion start from ‘raw model’

obtained from e.g. structural dynamics department. Literature [9] has summarizes

such work completed with FDTD method but much simpler geometry (see Figure

5.1), as mentioned in Section 1.2. However, new problems appear when increasing the

integrity of the model: for instance, inability of translating large geometry, incorrect

representation of geometry and extremely long time usage for meshing and simulating

large model.

Figure 5.1 Model of the car used in literature [9]

The main contribution of the project is investigating and developing a feasible

simulation method for vehicle radiated immunity test in accordance with ISO 11451-

2 standard based on a relatively complete model of car body. What’s new in the

thesis is (a) it starts from a raw model with high complexity (rather than a simple

model as Figure 5.1) and carries out further simulation based on that, (b) it analyzes

the feasibility of omitting spot welding joints for the purpose of reducing potential

37

geometry error and simplifying the model, and (c) it investigates several features that

may affect meshing results. The goal of this project is not to give an EMC analysis on

the model, but to find a viable and ‘inexpensive’ way which makes the raw model of

car body suitable for EM simulation, and also to find what results can be anticipated

for EMC analysis after post-processing.

The method is expected to enhance the synergy between structural dynamic

department and EM simulations within CAE department. Also, companies would

like to count on such method for offering the customer with cars that have better

EMC performance.

Following workflow chart in Figure 5.2 provides an overview of the entire process

of the method from dimensions of both hardware and software.

Cluster

HFSSANSA/SpaceClaim

Workstation

RawModel

GeometryClean-up

InitialMesh

GoodMesh?

No

Post-process Results

Adaptive MeshRefinement

EMComputation

Finish

Software

Hardware

CAE

Yes

Figure 5.2 A workflow for vehicle EMC simulation

The initial geometry used in the project is obtained from the structural dynamic

department which requires some extra geometry clean-up to better adapt to FEM-

based electromagnetic simulations. Specifically, the clean-up work or, say, the sim-

plification consists of removal of spot welding joints and exclusion of unnecessary

structures. On the basis of Chapter 2, spot welding joints are neglected at certain

frequency ranges due to capacitive coupling effect. Practicable software to execute

such clean-up including but not limited to CAE pre-processor ANSA and ANSYS

SpaceClaim. Once the simplification is done, the model is export to HFSS in STEP

format.

38

In HFSS, a benchmark on initial meshing process is carried out on desktop in

consideration of its stronger graphic card. The benchmark turns out that the TAU

mesher is approximately 45% faster and 42% less RAM-consuming than the Classic

mesher. However, if such benchmark is not needed, it is suggested to run the

simulations on cluster (or other similar HPC platforms) to shorten computation time

by making full use of parallel computation capability on multiple cores.

Some findings in the thesis are summarized as follows.

Minimal working example Some computation on electrically large objects, e.g.

vehicle, has been found to be extremely time-consuming as it requires several tens of

hours to complete just a initial meshing on desktop. A minimal working example that

only contains necessary information could be used to validate hypotheses efficiently,

especially when the certainty is low.

Mesh generation Meshing is an indispensable step in HFSS simulations. A

model that can be meshed not necessarily means satisfactory results, but a model that

cannot be meshed will certainly yield no result. Therefore, a suggestion for similar

simulations is to check the quality of mesh before proceeding to further simulations.

Significance of ground plane It has been proved that the ground plane shall

not be neglected in automotive EMC simulations, since the floor does not attenuates

RF energy as the absorbing materials do on walls and ceiling. Conversely, it can

reflect energy to e.g. the chassis.

Significance of engine hood The metallic engine hood is also not negligible for

its significant shielding effect.

Hot spot Hot spots, i.e. locations with high field strength, are found near or

in the vehicle. Consequently, it is likely be risky if disturbance-sensitive electronic

components are placed there. But this discovery may also provide design guidelines

to improve automotive EMC performance in an earlier stage. Note that the field

strength at hot spots might increase when resonance occurs at certain frequency.

Expandability Although this thesis originates from vehicle radiated immunity

test, it also has a potential to be applied to e.g. radiated emission simulations.

39

5.2 Bottlenecks and Limitations of the Thesis

Bottlenecks and limitation also do exist in this thesis.

Geometry Determining what geometry to use in simulations has been a tough

decision — there is always a trade-off among feasibility, efficiency (time) and accu-

racy and, thus, the best approach that suitable for each circumstance can never be

achieved. In general, the selection of geometry to be included depends on multiple

factors, including but not limited to application, availability of different parts, maxi-

mum error that can be tolerated, longest acceptable simulation time and configuration

of computer.

Frequency The computational capability has been a bottleneck in simulations:

a single-frequency simulation in this project normally takes several tens of hours and

thus a wide-band simulation will require incredibly long time. It is reported that

the E-field as a function of frequency at 30 – 100 MHz is possible to be calculated

on geometry with much lower complexity, but similar plot cannot be anticipated to

finish in acceptable time on complex geometry at e.g. 100 – 1000 MHz [10].

40

Chapter 6

Future Work

The thesis is unable to cover all of the key aspects within automotive EMC area.

Some of the intriguing topics and potential works to be studied in the future are

mentioned in this chapter.

Hybrid method As previously discussed in Section 1.4, hybrid CEM method

is currently under fast development. A successful example that has been applied

in automotive industry is FEBI, which accelerates simulation process by combining

MoM and FEM as well as shrinking solution domain (see Figure 6.1). For the upper

case in the figure below, 78% of the mesh elements is used to model the air, which

explains why the simulation time can be reduced dramatically by using the FEBI

technique [12].

Figure 6.1 Comparison between E-field plots obtained with FEM and FEBI1

The author of [12] has performed a benchmark between FEBI and FEM on a horn

antenna, which turns out the reflection coefficient at 100 – 500 MHz and the radiation

1Extracted from [12]

41

pattern computed by both methods are in good agreement. Further benchmark is

yet to complete for validating e.g. the performance and accuracy of FEBI at higher

frequencies.

Higher frequency With the rapid development in IT and mmWave technology,

diverse equipment operating at higher frequencies, e.g. automotive radar at 77 GHz

and 5G devices at sub-6 GHz / 28 GHz / 39 GHz band, has been coming on board.

Thus, EMC capability of vehicle will subject to challenges from higher part in the

spectrum and studies on simulation methodology in correspondence to such frequency

will be needed.

Model improvement Most of the large metallic structures in a vehicle has

been considered in this project so far; however, as frequency increases, the effect

of structures made of dielectric materials (glass, rubber, etc.) will also start growing.

Accordingly, geometry of e.g. windshield and tires shall be well represented in

simulations at higher frequency ranges.

Some other geometries that can be added to current model are wiring harness (see

Figure 6.2) and antenna, etc. With such information added, the coupling between

external disturbance source and on-board system or, between on-board systems (e.g.

between two antennae) can be studied for seeking optimal design.

Figure 6.2 An example of wiring harness inside a vehicle

However, the road of model improving will never reach the end of complete

vehicle — simulating a complete vehicle is a vision but not a practical goal. Neither

the information of all materials used in manufacture is completely available, nor is

it efficient in terms of timeliness to apply electromagnetic simulation on complete

vehicles.

42

Result verification The work can also be extended by comparing the results

with measured data from real tests. Bias under a certain level is anticipated since

several simplifications have been done on the model, but the overall pattern should

be in agreement.

43

Appendix A

Computing Environment in theProject

Desktop: DELL Precision T7610 Desktop Workstation

- CPU: Intel Xeon E5-2620 v2 @ 2.10 GHz (2 processors, 24 cores)

- RAM: 64.0 GB

- Graphic card: NVIDIA Quadro K4000

Cluster

- CPU: 128 cores distributed among 6 nodes

- RAM: 125.0 GB per node

44

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