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Wireless Power Transfer Parameter Optimization Based on Electromagnetic and RF Exposure Compliance in the U.S. Marketplace
by Travis Michael Thul
B.S. in Electrical Engineering Technology, May 2006, Milwaukee School of Engineering M.S. in Electrical Engineering, May 2008, University of Wisconsin - Madison
A Praxis submitted to
The Faculty of The School of Engineering and Applied Science
of the George Washington University in partial fulfillment of the requirements for the degree of Doctor of Engineering
Aug 31, 2018
Praxis direct by
Bentz Tozer Professor of Engineering Management and Systems Engineering
ii
The School of Engineering and Applied Science of the George Washington University
certifies that Travis Michael Thul has passed the Final Examination for the degree of
Doctor of Engineering as of 11 July 2018. This is the final and approved form of the
praxis.
Wireless Power Transfer Parameter Optimization Based on Electromagnetic and RF Exposure Compliance in the U.S. Marketplace
Travis Michael Thul
Praxis Research Committee:
Bentz Tozer, Adjunct Professor of Engineering Management and Systems Engineering, Praxis Director Ali Jarvandi, Adjunct Professor of Engineering and Applied Science, Committee Member ` Amirhossein Etemadi, Assistant Professor of Engineering and Applied Science, Committee Member Ebrahim Malalla, Visiting Associate Professor of Engineering and Applied Science, Committee Member Justin Reed, Chief Executive Officer of C-Motive Technologies Inc., Committee Member
iii
Copyright © 2018 by Travis Michael Thul All rights reserved
iv
Dedication The author wishes to dedicate this research to his beautiful wife and Viking sons:
Bonny R. Thul, CNP
Erik T. Thul
Kai H. Thul
v
Acknowledgements There have been many heroes that inspired my path on the never ending road of
education. They include teachers who’ve shown that school is not about memorizing
facts, but enjoying the challenge of learning. This process is the culmination of steps
down that road. Educators who’ve played an oversized role on this journey include:
Ms. Collins
Mr. Wiita
Ms. Hazen
Mr. Ritchie
Ms. Johnson
Mr. Waite
Mr. Chuchwar
Mr. Schwanke
Dr. Fleischman
Mr. Heidges
Mr. Vanderloop
Mr. Jensen
Dr. Chandler
Dr. Strangeway
Dr. Kaltchev
Mr. Petted
Dr. Lorenz
vi
Abstract of Praxis
Wireless Power Transfer Parameter Optimization Based on Electromagnetic and RF Exposure Compliance in the U.S. Marketplace
The growth of battery powered devices, starting with laptops and cell phones in the 1990s,
to tablet PCs and electrical vehicles in the 21st century, has given rise to the ever present
need of charging infrastructure. This infrastructure, relying on the same conducted power
transfer technology which has been in use for decades and a near infinite variety of plugs
or connectors, is primed to undergo a change not see since the migration from DC to AC
generators. The potential of wireless power transfer to fundamentally change how users
interact with their electronic devices cannot be understated, but will not be achieved
without parameter standardization and compliance with legislation written when such a
paradigm couldn’t have been foreseen. This study will present a framework capable of
generating optimized electromagnetic parameters to meet legal constraints using
fundamental physics, Nelder-Meade optimization, known regulatory limit, and dynamic
formulations based upon rigorous magnetic field measurements. Successful
implementation of this framework is intended to assist in the push to standardize a
technology with the potential to remake the electronics marketplace.
vii
Table of Contents Dedication...........................................................................................................................iv Acknowledgements..............................................................................................................v Abstract of Praxis................................................................................................................vi List of Figures.....................................................................................................................xi List of Tables....................................................................................................................xiv List of Symbols.................................................................................................................xvi Glossary of Acronyms....................................................................................................xvii Chapter 1 : Introduction ...................................................................................................... 1
1.1 Document Organization...................................................................................... 1
1.2 Problem Statement .............................................................................................. 1
1.3 Relevance to Engineering Management ............................................................. 2
1.3.1 Marketing & Sales Management ................................................................ 2
1.3.2 Legal Issues................................................................................................. 3
1.3.3 Technology Research & Development (R&D)........................................... 3
1.3.4 Adjacent Domains....................................................................................... 3
1.4 Background......................................................................................................... 4
1.5 Research Objectives............................................................................................ 5
1.6 Research Questions............................................................................................. 6
1.7 Research Hypotheses .......................................................................................... 7
1.8 Significance......................................................................................................... 8
Chapter 2 : Background & Literature Review .................................................................... 9
2.1 State of WPT Applications, Research, & Development ..................................... 9
viii
2.1.1 Inductive Systems ..................................................................................... 10
2.1.2 Small Power Applications (<1000W)....................................................... 12
2.1.3 Medium Power Applications (>1000W, <10kW)..................................... 14
2.1.4 Large Power Applications (>10kW)......................................................... 16
2.2 Electromagnetic Interference ............................................................................ 17
2.2.1 Causes ....................................................................................................... 17
2.2.2 Mitigation.................................................................................................. 18
2.2.3 Implications for Major Infrastructure ....................................................... 19
2.3 Radio Frequency Exposure (RFX)....................................................................21
2.3.1 Causes ....................................................................................................... 21
2.3.2 Mitigation.................................................................................................. 22
2.3.3 Implications on Human Safety.................................................................. 23
2.4 Legal Issues, Regulations, & Standards Bodies................................................ 23
2.4.1 Legal & Regulatory................................................................................... 24
2.4.2 Standards Bodies....................................................................................... 27
2.5 Standardization for Regulatory & Validation Constraints................................ 31
2.6 Market & Sales Precedents for Parallel Technologies...................................... 32
2.6.1 Broadband-Over-Powerlines (BPL).......................................................... 33
2.6.2 Personal Computers .................................................................................. 34
2.7 Summary........................................................................................................... 37
Chapter 3 : Research & Methodology .............................................................................. 38
3.1 Research Methodology ..................................................................................... 38
3.2 Research Questions........................................................................................... 40
ix
3.3 Research Hypothesis......................................................................................... 40
3.4 Research Framework ........................................................................................ 41
3.4.1 Assumptions.............................................................................................. 42
3.4.2 Key Independent Variables.......................................................................52
3.4.3 Constraints ................................................................................................ 53
3.4.4 Key Dependent Variables .........................................................................57
3.5 Implementation ................................................................................................. 58
3.6 Known & Presumed Data ................................................................................. 59
3.7 Summary........................................................................................................... 59
Chapter 4 : Simulations & Analysis.................................................................................. 61
4.1 Fourier Analysis of Proposed WPT Waveforms .............................................. 61
4.1.1 Simulations of Harmonics & Amplitudes................................................. 63
4.1.2 Simulation of Near-Field Decay ............................................................... 64
4.1.3 Model Validation ...................................................................................... 65
4.2 Tabulation of Known & Potential Limitations ................................................. 68
4.2.1 EMC Limits .............................................................................................. 69
4.2.2 RFX Limits ............................................................................................... 72
4.3 Compliance Tool Generation Process............................................................... 72
4.3.1 Theory....................................................................................................... 72
4.3.2 Implementation ......................................................................................... 77
4.3.3 Results....................................................................................................... 77
4.4 Engineering Management Recommendations .................................................. 84
4.4.1 Technical Recommendations.................................................................... 84
x
4.4.2 Regulatory & Standards Recommendations ............................................. 85
4.5 Summary........................................................................................................... 86
Chapter 5 : Conclusions and Future Work........................................................................ 87
5.1 Contributions..................................................................................................... 87
5.2 Conclusions....................................................................................................... 87
5.3 Future Work ...................................................................................................... 88
References......................................................................................................................... 90
xi
List of Figures Figure 1 Critical Vertices of Optimization Solution Space ................................................ 6
Figure 2 Simple WPT block diagram ............................................................................... 10
Figure 3 Examples Emerging WPT Center Frequencies & Bandwidths.......................... 11
Figure 4 World Wide Cell Phone Sales 2007 - 2017 ....................................................... 12
Figure 5 World Wide Tablet Sales 2011 - 2016 & Installation Base .............................. 13
Figure 6 Haier WPT Television and Fulton WPT Power Tools .................................... 13
Figure 7 SAE J1772 , CHAdeMO DC Fast Charge , & Tesla Supercharger plugs ......... 14
Figure 8 Evatran's Plugless WPT System ........................................................................ 15
Figure 9 Intel funded Volocoptor VTOL Taxi ................................................................ 16
Figure 10 Proposal for Wireless Charging Bus Infrastructure ......................................... 17
Figure 11 Maxwell's Equations ........................................................................................ 18
Figure 12 GPS System Growth ........................................................................................ 20
Figure 13 Occupational Health & Safety Administration (OSHA) RF Safety Sign......... 23
Figure 14 Broadband growth for home users .................................................................. 33
Figure 15 Decline of interest in BPL since 2004 ............................................................. 34
Figure 16 Growth of Computers 1975 - 2011 .................................................................. 37
Figure 17 WPT Framework Methodology........................................................................ 41
Figure 18 Examples of circular coil geometries for Qi , PMA , A4WP , and
Hevo WPT systems.......................................................................................................... 43
Figure 19 Non-circular wireless power transmitter geometry proposed by
SAE J2954 ....................................................................................................................... 43
Figure 20 Derivation of Fourier spectrum of ideal square wave ..................................... 44
xii
Figure 21 First order formula for single inductor and associated -20 dB/decade low
pass filter response ........................................................................................................... 45
Figure 22 Derivation of on-axis Biot-Savart law for on-axis calculations ...................... 47
Figure 23 Modification of the on-axis Biot-Savart law facilitating off-axis
calculations ....................................................................................................................... 48
Figure 24 Implementation of Biot-Savart for multiple coil turns for off-axis
calculations ....................................................................................................................... 53
Figure 25 Magnetic Field Strength @ 0m for Qi Transmitter & Associated Input
Variables ........................................................................................................................... 62
Figure 26 Idealized Harmonics for WPT Signal @ 0m.................................................... 63
Figure 27 Idealized Harmonics for WPT Signal @ 0m in dBA/m.................................. 64
Figure 28 Harmonic Amplitude A/m @ 3m..................................................................... 65
Figure 29 Harmonic Amplitude dBA/m @ 3m ................................................................ 65
Figure 30 Qi Structure ..................................................................................................... 66
Figure 31 Dynamic Scaling Factors Defined.................................................................... 69
Figure 32 Normalized FCC EMC Limits.......................................................................... 69
Figure 33 FCC EMC Limits Post-Processed from 9 kHz to 30 MHz for 10m................. 71
Figure 34 RFX Limits....................................................................................................... 72
Figure 35 First harmonic H-Field Sweep.......................................................................... 73
Figure 36 EMC Safety Margin for Part 15C..................................................................... 74
Figure 37 EMC Safety Margin for Part 18 ISM. .............................................................. 74
Figure 38 EMC Safety Margin for Part 18 Non-ISM.......................................................75
Figure 39 Example of Passing Fundamental and Failing Harmonic for Part 15C............ 75
xiii
Figure 40 RFX Safety Margin for a given Tx Current ..................................................... 76
Figure 41 Maximum Currents & Frequencies for Part 15C Operation ............................ 80
Figure 42 Maximum Currents & Frequencies for Part 18 ISM Operation ....................... 81
Figure 43 Maximum Currents & Frequencies for Part 18 Non-ISM Operation............... 81
Figure 44 Minimum Safe Distance for Max Tx at Given Frequency, Part 15C............... 82
Figure 45 Minimum Safe Distance for Max Tx at Given Frequency, Part 18 ISM ......... 82
Figure 46 Minimum Safe Distance for Max Tx at Given Frequency, Part 18 Non-ISM . 83
xiv
List of Tables
Table 1 Square Wave Harmonic Amplitude Decay & Conversion Example................... 46
Table 2 Emerging WPT Protocol Parameters................................................................... 47
Table 3 Legacy RFX regulations . Note lack of limits at f < 300 kHz. ............................ 50
Table 4 Maximum Permissible Exposure (MPE) field strength limits. Framework
leverages limit established for 9 kHz to 100 kHz (see 3.4.1.7 for more information). .... 54
Table 5 Maximum Permissible Exposure (MPE) field strength limits specified by
IEEE C95-1 2005 Framework leverages limit established for 100 kHz to 30 MHz
(see 3.4.1.7 for more information). ................................................................................... 54
Table 6 Radiated field strength limits .............................................................................. 55
Table 7 Limits for Part 18 devices operating on ISM and non-ISM fundamental
frequencies. Note that WPT devices operating below 1000 MHz are not permitted the
increase in field strength otherwise permitted here for power over 500 watts. ................ 56
Table 8 Conversion of data shown in Figure 28 to dBuA/m ............................................ 66
Table 9 Comparison of calculated harmonic values for Qi parameters against
published EMC test result for FCC ID: 2AIY7--CD-1014 .............................................. 67
Table 10 Scaling of FCC Part 15 Limits to 10m .............................................................. 70
Table 11 Scaling of FCC Part 18 ISM Limits to 10m ...................................................... 70
Table 12 Part 18 Non-ISM Limits .................................................................................... 70
Table 13 Optimized Maximum Currents for Qi Device ................................................... 78
Table 14 Minimum Safe Distances for Qi Device Optimized for Maximum Current ..... 79
Table 15 Qi Device Optimized for Nearest Distance ....................................................... 79
Table 16 Maximum Currents for Qi Device Optimized for Minimum Distance to User. 80
xv
Table 17 Maximum current for Qi geometry across all frequencies ................................ 83
Table 18 Minimum distances for maximum currents for Qi geometry ............................ 83
xvi
List of Symbols
1. Hz Hertz 2. E Electric Field, expressed in
Volts/meter 3. B Magnetic Field, expressed
in Teslas 4. eeee Permittivity 5. rrrrv Volume Electric Charge
Density 6. J Electric Current Density 7. mmmm Permeability 8. H Magnetic Field Strength,
expressed in Amperes/meter 9. VVVV Del operator 10. jjjj Derivative 11. § Section mark, used to
indicate a section of legislation
12. A Amperes 13. m Meter 14. V Volts
15. e Irrational number "e", 2.7183
16. dt Time derivative 17. ∫ Integral 18. pppp Irrational number "pie",
3.14159 19. j Imaginary number, -1 20. wwww Radial frequency 21. SSSS Summation symbol 22. ∞ Infinity 23. º Degrees 24. t Variable time 25. ∠∠∠∠ Angle 26. L Inductance in Henrys 27. di Current derivative 28. v Instantaneous voltage across
inductor 29. I Direct current (constant, non
sinusoid)
xvii
List of Symbols Continued
30. dB Decibel, calculated at 20*Log10(value)
31. Ck Square wave harmonic
amplitude at "k" 32. D Diameter of Coil 33. W Watts 34. k Denotes harmonic number 35. TX Time at "x"
36. QQQQ Radial degrees 37. R Radius in meters 38. llll Wave length in meters 39. f Frequency in Hertz 40. r Near-field/Far-field
boundary 41. WWWW Ohmic resistance
xviii
Glossary of Acronyms
EMBoK A Guide to the Engineering Management Body of Knowledge WPT Wireless Power Transfer RFX Radio Frequency Exposure FCC Federal Communications Commission ICNIRP International Commission on Non-Ionizing Radiation Protection SAE Society of Automotive Engineers PC Personal Computer TCP/IP Transmission Control Protocol/Internet Protocol BPL Broadband Over Powerlines RF Radio Frequency EMC Electromagnetic Compliance Qi Pronounced "Chee", WPT standard A4WP Alliance for Wireless Power PMA Power Matters Alliance ISM Industrial, Scientific, Medical frequency bands Q Quality Factor MIT Massachusetts Institute of Technology LCD Liquid Crystal Display AC Alternating Current NEMA National Electrical Manufacturers Association CHAdeMO Charge de Move DC Direct Current OEM Original Equipment Manufacturer VTOL Vertical-Take-Off-And-Landing EMI Electromagnetic Interference AM Amplitude Modulation TTL Transistor-Transistor Logic CMOS Complementary Metal–Oxide–Semiconductor GPS Global Positioning System ARRL American Radio Relay League FOB Free On-Board LIDAR Light Detection and Ranging OSHA Occupational Health & Safety Administration ITU International Telecommunication Union FDA Food and Drug Administration CFR Code of Federal Regulation US United States MPE Maximum Permissible Exposure SAR Specific Absorption Rate UN United Nations EPRC Electronic Product Radiation Control Program FD&C Federal Food, Drug, and Cosmetic Act ANSI American National Standards Institute
IEEE Institute of Electrical and Electronics Engineers Tx Transmit Rx Receive WPC Wireless Power Consortium DVD Dynamic Video Disk HD High Definition RAM Random Access Memory WEMPEC Wisconsin Electrical Machines & Power Electronics Consortium MOSFET Metal-Oxide-Semiconductor Field-Effect Transistor IGBT Insulated-Gate Bipolar Transistor Std Standard KDB Knowledge Database ID Identification RMS Root-Mean-Square
1
Chapter 1: Introduction
1.1 Document Organization
This document is presented in five chapters. The first of which correlates the
research to the Engineering Management discipline, identifies the engineering problem,
and provides background on the technology and method under investigation.
The second chapter discusses current legal implications affecting technology
adoption, validation organizations and standards bodies with potential jurisdiction over
the technology, and the current state of the research & development marketplace. All of
these factors are explored as they impact technology marketing and deployment.
The third chapter delivers the research methodology, optimization model
development, and provides quantitative assessments of current legal and validation
processes. Assumptions are made and justified therein.
The fourth chapter yields quantitative simulation results, outputs optimized
parameters for the US regulatory and commercial marketplace, and delivers engineering
management solutions pertaining to product development & deployment.
The fifth chapter discusses further research opportunities and prescribes areas
where the provided model may be enhanced.
1.2 Problem Statement
Novel technologies which require compliance with ill-suited legal constraints,
ambiguous verification standards, and platform interoperability with unsettled
requirements face significant challenges towards marketplace success. In order to guide
Engineering Managers towards successful deployment of such a novel technology, WPT,
2
an optimization model must be developed to provide optimum quantitative parameters
within the competing disciplines and qualitative guidance to maximize success across the
constraining domains.
1.3 Relevance to Engineering Management
This praxis is intended to provide qualitative and quantitative engineering
management solutions coinciding with domains discussed in A Guide to the Engineering
Management Body of Knowledge (EMBoK) [1]. Specifically, an optimization framework
is built around technology, legal, and marketplace domain constraints. The framework
outputs dependent variables which are optimized such that the system to which they are
applied will meet multiple dimensions of regulatory compliance while operating within
the bounds of existing research & development. The associated qualitative solutions
prescribe implementation of the optimized parameters and dictate strategic planning
objectives which seek to unify the marketplace, standardize validation procedures, and
achieve optimum technology performance.
1.3.1 Marketing & Sales Management This praxis begins through an examination of the needs and wants of the
marketplace via assessment of existing market fragmentation, the current installed base of
complementary goods, and historical sales trends for products faced with similar
externalities. This holistic review of the marketplace also includes emerging strategic
partnerships competing to produce a dominant design. Parameters of these emerging
designs are considered in the framework, assisting in constraining the solution space.
3
1.3.2 Legal Issues Managing the legal issues surrounding deployment of WPT is paramount to this
research. Legislation including the Federal Food, Drug and Cosmetic Act of 1938, the
Communications Act of 1934, the Occupational Safety & Heath Administration Act of
1970, and the National Environmental Policy Act or 1970 directly impact the viability of
this technology's marketplace success. This praxis considers the pertinent laws, quantifies
their constraints, and embeds them into the optimization framework. The framework
output will underpin the technology development while assuring legal compliance.
1.3.3 Technology Research & Development (R&D)
This praxis' framework combines the aforementioned legal issues with known
hardware limitations and electrical engineering principles into a simulation modeling tool.
The tool's output is intended to drive down the expensive and complex R&D process by
producing a constrained solution space and outputting system parameters optimized for
engineering performance and regulatory approval. Engineering principals reflected in the
model include power electronics switching speeds, the near-field/far-field relationship,
and magnetic field decay.
1.3.4 Adjacent Domains
While the three domains mentioned above are reflected most prominently
throughout this research, the engineering management solutions provided have much
farther reaching potential. Finances can be conserved through a decline in engineering
costs, project management can be streamlined through the inherent reduction in risk
4
associated with regulatory factors, and strategic partnerships can be evaluated based on
shared vision.
1.4 Background
Since the end of the 19th century and the age of Nikola Tesla [2], WPT has been a
dream for industry, engineers, and consumers. While novel solutions have presented
themselves in the century since Tesla (including ideas ranging from the rectifying
antenna arrays to satellite based transmitters [3]), no convergence of need, capability, and
regulatory authorization have combined to foment an environment where a commercially
successful implementation of WPT was likely. However, with advancements in
semiconductor technologies increasing the efficiency of power signal generation [4] and
the unmitigated growth of mobile battery powered devices [5][6], the first two conditions
for a prime market look to be realized. The remaining regulatory and standardized
verification criteria may be the last hurdle to achieving Tesla's dream.
The organizations and standards bodies playing a role in guiding WPT's success
run the gambit from national regulators, professional societies, and private sector
federations. This includes the Federal Communications Commission (FCC), which is
responsible for regulating electric signals from 9 kHz through 275 GHz; the International
Commission on Non-Ionizing Radiation Protection (ICNIRP), which published non-
ionizing radiation safety limits; and the Society of Automotive Engineers (SAE), which is
working to develop an open standard for wirelessly recharging electric vehicles. These
organizations, while not close to all inclusive, are a prime example of the competing
forces both impeding WPT's growth and fighting for it.
5
Much like standards based technologies which preceded it, WPT's future hinges
upon these multi-letter organizations ability to find agreement along the multitude of
variables underpinning its technology. This includes frequency of operation, magnitude
of field strength, thermal effects on biological tissue concerns, and the potential for
power transmitters to negatively impact other electronic devices due to electromagnetic
interference. While cumbersome, this type of multivariable system has precedents to look
towards for success. This includes the standardization of the personal computer (PC) and
TCP/IP communications. However, equally salient precedents exist for failure of
technologies with great promise to impact the lives of its users - most recently the failure
of broadband over power lines (BPL). None of these examples, however, had to
overcome the sheer systems complexity that WPT looks to have.
If there is a presumption that technical specifications can be quantitatively
developed to meet the emerging requirements of the interested parties, WPT may be able
to find a path forward. Such a path would require a significant amount of mathematical
simulation and an understanding of existing regulatory and legal structures. Such
parameters could allow for a simplification of WPT system design, a decrease in
regulatory engineers costs, and (ultimately) help launch the standardization of the
technology on a trajectory mirroring the PC and internet markets.
1.5 Research Objectives
The objective of this praxis is to develop a framework yielding an optimized set
of parameters for inductively coupled WPT systems. This solution will be based upon
current and emerging legal constraints for sub-30 MHz systems, rely upon international
validation standards, employ fundamental electromagnetic calculations, and known
6
magnetic-field decay properties based upon published measurements. The final output
will include dependent variables designed to assure adherence with Electromagnetic
Compliance (EMC) and Radio Frequency (RF) exposure limits. These outputs will be
based upon independent variables and seeded frequency inputs. Final optimization will
use the Nelder-Meade process, which is ideally suited to finding local maximums within
piece-wise functions. The impact of adoption of such optimized results will also be
explored and the economic and technology proliferation consequences will be established.
A graphical representation of the objective framework is shown in Figure 1. Note
that the universe of possible solutions will be optimized around dependent and
independent variables intrinsically associated with multiple legal constraints. The
engineering prescriptions are intended to justify parameters for system standardization,
allowing for WPT interoperability and increasing the likelihood of mass market adoption.
Figure 1 Critical Vertices of Optimization Solution Space
1.6 Research Questions
To achieve the aforementioned results, a set of research questions will be posed:
7
1. Given desired independent variables for WPT geometry and current, what are the
optimized parameters needed to meet RF Exposure (RFX) requirements?
2. Given desired independent variables for a WPT geometry and current, what are the
optimized parameters needed to meet EMC requirements?
3. Given optimized dependent variables for RFX and EMC operability, which
regulatory authorization method ensures greatest operable capabilities?
WPT systems are subject to multiple legal jurisdictions focusing on EMC and
RFX. These regulations, respectively, are intended to ensure that electronic devices do
not interfere with licensed or otherwise authorized radiators, as well as to ensure that
authorized devices do not present an electromagnetic hazard to users or the general public.
Due to these separate, yet coupled constraints, determining appropriate design variables
which will yield maximum performance while still maintaining compliance requires a
systems engineering analysis of those relationships and an understanding of the
underlying physics powering these devices. The automated framework presented herein
is intended to quantify these constraints, simulate the physics, and generate an output
solution set of parameters ensuring optimum device performance.
1.7 Research Hypotheses
The fundamental hypotheses of this praxis is that generation of hardware design
parameters optimized for peak WPT performance while achieving legal compliance is
possible through the development of an optimization framework.
8
1.8 Significance
Currently, the design and deployment of WPT systems is hampered due to
hardware limitations, incompatible configuration standards, and overlapping (and often
conflicting) legal regimes. While the utility of such technology is unquestionably
applicable to devices from cellular phones to electric vehicles, the market for mass
adoption of this technology is inhibited due to those aforementioned variables. Should a
framework bridging this divide between technical complexities, competing market
interests, and legalese exist and be adopted, the potential for market growth is immense.
9
Chapter 2: Background & Literature Review
WPT, while seemingly mystifying and a dream of both engineers and science-
fiction writers, is (and has been) far closer to reality than may be intuitively thought. This
includes the transfer of electromagnetic waves in the visible light spectrum into chemical
energy within plants through the process of photosynthesis [7] and the ubiquitous
transformer which uses electromagnetic induction to transfer energy from one circuit to
the next without an electrical interconnect in-between [8]. Thus, a background of
identifying the possibility of WPT is far less salient than researching the practical
implementation of such concepts into modern mobile devices while ensuring compliance
with legacy regulations guiding electromagnetic interference and radio frequency
exposure. This narrower focus will require exploration of emerging technologies, their
interaction with legal and industrial standards bodies, and a comparison of how the reality
of multiple WPT technologies in development parallels other systems which faced
similar multi-variable constraints.
2.1 State of WPT Applications, Research, & Development
WPT as it relates to the 21st century is confined to the use of electromagnetic
radiation propagating across a medium (usually air) to a receiver. The receiver converts
the electromagnetic field into electrical current which can be used to charge a battery or
directly power the receiving device (see Figure 2). This process is true for systems
operating as low as 19 kHz [9], which leverage the properties of the magnetic near field,
to devices operating at frequencies in the light spectrum [10], using photo detectors or
solar cells for the receiver.
10
Figure 2 Simple WPT block diagram The next sections will discuss the current state-of-the-art proposals for WPT using
electromagnetic properties which are governed by US regulations.
2.1.1 Inductive Systems
WPT technologies based on inductive principals have received a the greatest
degree of research thus far, with multitudes of protocols being proposed. These protocols,
many overlapping in utility, are intended to transfer power to devices ranging from wrist
watches to electric vehicles. Examples of some of the proposed systems are shown in
Figure 3. Note that the inductive technologies occupy frequencies less than 10 MHz, with
most of the known proposals falling into bands less than 1 MHz. While these lower
frequencies may decrease power transmission distances, they increase the efficiencies of
the power electronics used for signal generation.
11
Figure 3 Examples Emerging WPT Center Frequencies & Bandwidths
The underlying physics of these devices is not dissimilar to early ferrite
transformer based systems used in consumer devices dating back to the 1960s and 1970s
[11][12]. However, the critical difference enabling modern proposals to successfully
transfer energy across dynamic gaps, instead of 0mm/on-contact configurations, is the
use of resonating transmit/receive pairs combined with operating frequencies between 10
kHz and 1 MHz. While the latter difference can be attributed to semiconductor advances
since the 1990s [13], the focus on high-Q configurations and their relevance to power
transfer can be attributed to work proposed by MIT in 2006 [14].
Development of systems intending to take advantage of this new engineering
paradigm has grown in concert with the growth of batter powered devices - led by the
smart phone. However, unlike the cellular phone, the frequencies and application of
inductive WPT did not have well established regulatory guidelines - the Federal
Communications Commission would not publish its first official guidance until 2013 [15].
Due to this current state of regulatory ambiguity and plethora of varying proposals, this
12
most advanced WPT technology stands on the precipice of mass market penetration, if
only convergence of protocols and regulatory parameters could be facilitated.
Examples of the markets which stand to benefit from emerging inductive WPT
technologies are given in the subsequent sections.
2.1.2 Small Power Applications (<1000W)
The market space most primed to take advantage of these emerging WPT
technologies is that of the small and lower power applications. Such devices, included
cell phones, tablets, laptops, and flat panel televisions, have become ever present in
resident and commercial environments. Furthermore, all of these devices (save for wall
mounted televisions) require regular charging. Currently, that means wired charges in the
home, the office, and the car with variable plugs and interfaces (tragically, it almost
seems that no two devices can share the same physical charging interface). Growth of
select devices ideal for WPT implementation are shown in Figure 4 & Figure 5.
0
200
400
600
800
1000
1200
1400
Year 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016
Year
Sal
es in
Mill
ion
s o
f U
nit
s
Figure 4 World Wide Cell Phone Sales 2007 - 2017 [16]
13
Figure 5 World Wide Tablet Sales 2011 - 2016 & Installation Base [17]
While the numbers above reflect an installed base of more than one billion units
world wide for cell phones and tablets, ideal candidates for wireless charging solutions,
they are not the only applications under consideration. Devices in the 10s to 100s of watts
are also being investigated. Most intriguingly is that of the wireless LCD televisions and
power tools, shown in Figure 6.
Figure 6 Haier WPT Television [18] and Fulton WPT Power Tools [19]
While the examples above highlight only a few applications for WPT in the
<1000W range, they give an example of the breadth of utility. However, much like the
stand 120V AC electric plug found throughout North America (officially the NEMA 5-15
Type B), if system standardization and regulatory consistency is not developed,
matriculation of these devices from proposals to the mass market may be impossible.
14
2.1.3 Medium Power Applications (>1000W, <10kW)
While lower powered devices may be the most intuitive and mission ready
application for WPT devices, higher powered opportunities are not far behind. Most
pressing of these is the electric vehicle. Current processes require that electric vehicles be
charged in the same fashion as other battery powered devices. This can mean using a
variety of interface plugs depending on the model of car being charged. For example, this
could mean use of the SAE J1772 plug for many Nissan and Toyota vehicles, the
CHAdeMO for fast direct current (DC) charging, or the Tesla Supercharger Plug for most
Tesla models. Visuals of these plugs are shown in Figure 7.
Figure 7 SAE J1772 [20], CHAdeMO DC Fast Charge [21], & Tesla Supercharger plugs [22]
In order to standardize this array of interfaces, the Society of Automotive
Engineers has proposed SAE J2954 "WPT for Light-Duty Plug-In/Electric Vehicles and
Alignment Methodology". This methodology is intended to allow OEMs and 3rd party
manufacturers to develop an interoperable WPT systems which would be cross-
compatible with any compliant receiver or transmitter. Although not yet recognized by
legal regulatory authorities, it could be poised to take advantage of an electrical vehicle
market expected to achieve 20 million potential vehicles by 2020 [23].
15
While SAE may have recommendations published, the uncertain standards and
regulatory environment has only encouraged other manufacturer's and entrepreneurs to
develop competing or proprietary systems. One of these, beating the SAE proposal to
market by three years, is Evatran's Plugless technology. This system is intended to serve
the same market as the SAE proposal, but operates on approximately 20 kHz instead of
SAE's higher 85 kHz [24]. A diagram of Evatran's technology is shown in Figure 8.
Figure 8 Evatran's Plugless WPT System [24]
While electric cars stand to gain from regulatory and standards convergence, other
medium power technologies are in the conception phase which would not only take
advantage of these systems, but be built around them. The most salient case is that of
electric vertical-take-off-and-landing (VTOL) aircraft designed to taxi customers around
large, heavily congested, metropolitan areas. These devices have been proposed from San
Francisco to Dubai and including backing from corporations ranging from Uber to
Tencent [25][26][27]. Unlike traditional taxis, which are parked by a driver every
evening and can be refueled as needed, the proposed VTOL taxis are intended to be
wholly autonomous. Under such scenarios, the vehicle would need to be able to
automatically interface with charging infrastructure. It is intuitive that landing on a
charging pad would improve operationals over developing a robotic mechanical charging
linkage. An example of such an electric VTOL taxi device is shown in Figure 9.
16
Figure 9 Intel funded Volocoptor VTOL Taxi [28]
Whether powering the electric cars of today or the electrical air-taxis of tomorrow,
the market for medium powered WPT applications is only poised to grow. Ensuring that
optimized power transfer protocols are specified to meet regulatory needs will be one key
component to ensure that such growth is realized.
2.1.4 Large Power Applications (>10kW)
Emerging WPT applications in excess of 10 kW are not nearly as prevalent as the
small technologies discussed above. However, the need to rapidly recharge electric mass
transit vehicles with minimal human interaction is emerging as an engineering necessity.
This is best exemplified by the growth of electric busses across the planet. Whether it is
the deployment of 300 such busses in Poland in March of 2018 [29] or nearly 90,000
electric busses sold in China in 2017 [30], charging infrastructure will have to follow.
In order to meet this demand, preliminary experimentation has already begun to
wirelessly charge such busses using high power wireless systems. Such experimentation
is being carried out in Korea, as well as Europe. In both cases, engineers are working to
embed wireless power transmitters into the road or bus stop infrastructure such that
compliant busses will be able to opportunity charge between bus stops - eliminating the
need to make stops for diesel fuel. An example of such a system is shown in Figure 10.
17
Figure 10 Proposal for Wireless Charging Bus Infrastructure [31]
Consistent with the lower and medium powered cases previously discussed,
generation of engineering parameters to meet regulatory demands, while also allowing
for optimum performance could fundamentally transform the large power application
market and contribute to an increased adoption of such technology.
2.2 Electromagnetic Interference
Electromagnetic Compliance (EMC) is the science and engineering behind
ensuring electronic devices do not interfere with other electronic devices and, in some
cases, ensuring that electronic devices can accept interference from external sources.
While the physics of the interference is invisible to the naked eye, failure to ensure
electronic devices adhere EMC regulations can have catastrophic impacts on everything
from emergency radio communications to pacemaker operations.
2.2.1 Causes
When discussing causality, typically the term Electromagnetic Interference (EMI)
is defined as the cause necessitating EMC. Such interference is generated by the flow of
current inside of an electrical device. When current flows inside of a circuit, a
corresponding electromagnetic field is generated. This field propagates through free
space at an amplitude and pattern dependent upon the source of generation, magnitude of
current flow, circuit geometry, and other parasitic parameters. The interfering signal can
18
be sourced from not only the primary transmitting antenna and intended signal of
transmission, but also through harmonics generated within the internal electronics and
through any current carrying element within the interfering device.
When the infringing field interacts with a receiving electronic device, secondary
affects, described through Maxwell's equations (Figure 11), can cause electronic system
inconsistencies which may result in unintended operations. Examples may be as mild as
static induced onto an Amplitude Modulation (AM) radio receiver from a vacuum cleaner
(resulting in crackling overlaying the music) or the disruption of 802.11x (Wi-Fi) from an
improperly shielded microwave oven, cutting off wireless TCP/IP communications.
It is also important to note that EMI can be both radiated and conducted, as the
two mediums (electron flow and field propagation) are dual properties of an
electromagnetic field. Further, conducted EMI may radiate and radiated EMI will
ultimately be realized through electron conduction.
E - Electric Field B - Magnetic Field eeee - Permittivity
H - Magnetic Field Intensity
rrrrv - Volume Electric Charge Density
J - Electric Current Density mmmm - Permeability
jjjj - Derivative
Figure 11 Maxwell's Equations [32]
2.2.2 Mitigation
As noted above, the causes of interference depend upon both physical and
electrical properties of the source devices. Thus, if the goal is to reduce the amount of
electromagnetic noise generated by an electrical apparatus, then engineering
considerations must be taken into account during the design of the source. While there
isn't one answer providing for absolute mitigation, there are common techniques which
19
can be used to capture "low hanging fruit" which may allow for system compliance.
These include, but are not limited to:
1. Reduction of system harmonics. This can be achieved by altering the switching
speed of internal semiconductors or considering various signal generating
techniques. An example would be to use analog signal generation (if possible)
over pulse-width wave generation.
2. Shielding about the non-antenna portions of the device. Shielding can be used to
dampen rough E and H field radiation from the body of the device. Examples
include ferrite shielding and Faraday cage shielding.
3. Reduction of conducted emissions through insertion of inductive and capacitive
filters. This is most commonly used when filtering out EMI noise emanating from
switched mode power supplies.
4. Decrease in switched signal amplitude. Examples in this case are moving from 5V
TTL logic to 3.3V CMOS logic. The lower dV/dt decreases the potential for
current to flow through parasitic capacitance and its associated EMI.
2.2.3 Implications for Major Infrastructure
With the growth of cellular phones (see Figure 4) for communications, the use of
Global Positioning Systems (GPS) for navigation of everything from cars, ships, and
airplanes (Figure 12), and perceived need of Wi-Fi communications across the planet, the
need to protect the electromagnetic spectrum has never been more paramount.
20
0
100
200
300
400
500
600
700
800
900
Year 2007 2008 2009 2010 2011
Year
Mill
ions
of U
nits
Figure 12 GPS System Growth [33]
Security of this spectrum is achieved through acknowledgement and mitigation of
sources of EMI, including emerging WPT technologies.
There is precedence for growth of radio frequency devices without drastic
interference with electromagnetic infrastructure. Most recently, cellular phones, Wi-Fi,
and Bluetooth devices have been able to enter nearly every home without any obvious
negative impacts to critical spectrum (pacemakers continue to function, airport radars
continue to work, and the Amateur Radio band remains intact). However, due to the
potential for wireless power systems to generate signals using 10s to 1000s of watts
(instead of the milliwatts associated with Wi-Fi and cell phones), the prospect for higher
amplitude interference is real. Further, with most WPT devices operating at lower
frequencies (with potentially large harmonics), concern for AM radio bands and Amateur
Radios bands is warranted. Finally, the reliance on a strong reactive near field for power
transfer may increase the prospects for interference with localized electronic devices such
as transcutaneous electrical nerve stimulation implants, key FOBs, and cell phones.
21
Should WPT devices fail to ensure EMC, or if regulators fail to adapt to this
emerging technology, the negative impacts on existing electronic devices may be
unpredictable and, potentially, catastrophic. Should any of these emerging devices be
shown to have caused spectrum harm, from a GPS blackout to inadvertent activation of a
pacemaker, the ensuing fallout could destroy the prospects for a wireless power future.
2.3 Radio Frequency Exposure (RFX)
In addition to electronic devices interfering with other electronic devices, the
prospect for electromagnetic radiation to cause physical harm to users is also real. Such
impacts can manifest in conditions ranging from thermal heating to optical impairments.
2.3.1 Causes
The causes of adverse biological RFX symptoms are not dissimilar to the causes
of EMI described in 2.2.1. Specifically, the incidence of electromagnetic fields onto users
can result in stray currents flowing through skin and other human tissues. This current
flow interacts with the tissues' own resistive properties and results in thermal heating
affects. This type of effect is consistent with eddy current heating.
In addition to induced currents in tissue, RF fields can also cause molecules
within tissues to oscillate. This effect is due to the RF wave's force acting upon positive
and negative charges in the skin (or other tissue) and causing different polarity charges to
move in opposing directions. When this happens, polarized tissue molecules will begin to
move in an oscillatory pattern, resulting in friction between particles. This friction also
induces heat. This heat generation property is key to cooking food within a microwave
oven (such ovens may use the same frequencies as Wi-Fi and Bluetooth systems) [34].
22
Another effect of radio frequency exposure on human tissue is that of nerve
stimulation. This stimulation effect may induce an optical reaction similar to viewing a
flickering light or of optical phosphors in the field of vision. These symptoms are
consistent with induced current in neurological tissues but are only known to be
generated when the current induced is of a significantly low frequency (< 100 kHz) [35].
It should be noted that the frequency dependence of this effect falls within the same
frequency band as many emerging or proposed WPT technologies.
2.3.2 Mitigation
Mitigation of RF effects on users differ from EMC mitigation efforts primarily in
the sense that EMC can be impacted due to harmonic minimization and shielding of non-
antenna enclosures. However, in most cases, the prime radiator contributed to the RF
hazard is the antenna or transmitting coil. Any attempt to shield or diminish the output
could reduce the transmitter's utility. Thus, techniques specific to exposure must be
implemented. Common techniques include the following:
1. User detection mechanisms. Such mitigation techniques usually rely on sensing
the presence of a user or unintended object to disable radio frequency
transmission. This could include thermal sensors, Light Detection and Ranging
(LIDAR), or capacitive proximity sensors to determine if a user has entered the
proximity of the transmitting aperture or is handling the radio transmitting device.
2. Warning signs. While not a technological instrument, the use of RFX signage is
not only critical to warning users that the environment is not safe but is also
required by some regulatory bodies (which will be discussed in a subsequent
section). An example of such a warning sign is shown in Figure 13.
23
Figure 13 Occupational Health & Safety Administration (OSHA) RF Safety Sign
2.3.3 Implications on Human Safety
Without appropriate consideration of RFX impact to human users and those in
close proximity, the negative impacts of WPT systems could be considerable. When
examining the applications most likely to migrate to wireless power (such as cellular
phones and electric vehicles), each poses a unique and tangible concern. Specifically,
many users charge their cell phones at their desks and keep them in close proximity to
their limbs. Vehicles, on the other hand, may not be close to users at all times while
charging, but will require very strong electromagnetic fields to function, which may pose
a hazard during the limited time a user is in proximity to the transmitting coil. If these
concerns were realized in the commercial space, the viability of WPT technology to the
mass market would be extremely compromised.
2.4 Legal Issues, Regulations, & Standards Bodies
WPT, like all electromagnetic devices, is governed by a host of agencies ensuring
that the radio frequency emanations do not interfere with other devices, do not cause
harm to by-standers, and that interoperability is ensured when necessary. These
organizations often have the force of law behind their requirements and tend to interact
with one another so that governments, manufacturers, and users have visibility into the
standardization and regulatory process. These organizations often span nations,
24
continents, and private sector competitors. However, due to the emerging status of WPT
proposals, many of these organizations have not yet caught up to the new technical
imperatives or are relying on legacy rules which were not intended for this new paradigm.
2.4.1 Legal & Regulatory
Regulatory bodies are those which have the force of law or governmental charters.
While initial concerns with radio frequency transmission pertained to interference with
one another (i.e. AM radio broadcasts interfering with radar or broadband over
powerlines interfering with amateur radio), many regulatory agencies would expand their
missions to ensure mitigation of harmful human exposure to such radiation.
In addition to agencies regulating explicitly radio transmissions, other agencies
may tangentially regulate electromagnetic interference or susceptibility based on
corresponding impacts to the primary area of regulation. These would be cases where an
agency may regulate medical implants but would accordingly have to consider their
susceptibility to transmitters or an agency which regulates workplace safety which must
take into account radiators in the workplace.
2.4.1.1 FCC/ITU/FDA/OSHA Specific agencies charged with regulating radio frequency emissions (or who may
do so while protecting other devices or persons) are discussed below. Their current
policies, as they pertain to WPT (or how they fail to pertain to WPT), are also discussed.
This list of agencies is not all-inclusive due to each nation or multi-national organization
having, to a large degree, their own governing bodies.
� United States Federal Communications Commission. The US FCC is charged
with protecting interstate and foreign commerce by radio [waves] under the
25
Communications Act of 1934, which was subsequently amended by the
Telecomm Act of 1996 [36]. The rules under which the FCC governs WPT can be
found in 47 Code of Federal Regulations (CFR) Part 1.1307, Part 15, and Part 18.
Part 1.1307 prescribes RFX concerns as they pertain to Actions that may have a
significant environmental effect, while Parts 15 and 18 regulate EMC for
intentional radiators and for Industrial, Scientific, and Medical (ISM) devices.
Lack of convergence with WPT technologies within 47 CFR includes:
o No Maximum Permissible Exposure (MPE) limits less than 300 kHz
(§1.1310).
o No Specific Absorption Rate (SAR) limits at less than 100 kHz (§1.1310).
o No conducted EMC limits at less than 150 kHz (§15.107, §18.307).
o Radiated limits for core WPT frequencies at distances outside of practical
measurable range (§15.109, §18.305).
o Limits specified for electric field measurements, whereas WPT fields are
magnetic dominant (§15.109, §18.305).
o Extrapolation factors which do not reflect actual field decay if RF
radiation associated with emerging WPT devices (§15.31,§18.305).
� International Telecommunication Union. This is a United Nations body
representing public and private entities towards the cohabitation of RF emissions.
While now part of the UN, its charter was passed prior to the establishment of the
super-national organization - having been conceived in 1865 [37]. Some of the
ITU's non-convergence with WPT include:
26
o Inability to force compliance with published limits, instead relying on
partner nations to implement ITU recommendations.
o Recommends limits for mobile handsets or other radiating devices used
against the head, which are not pertinent to WPT [38].
o Lacks jurisdiction in the United States (with the FCC maintaining separate
regulations).
� The US Food & Drug Administration (FDA). This agency regulates implanted
medical devices which may be susceptible to electromagnetic interference. Due to
the strong field strengths emanating from some proposed WPT devices, input
from the FDA would be intuitive. Areas the FDA is unable address include:
o While authorized to issue regulations under the Electronic Product
Radiation Control program (EPRC) provisions of the Federal Food, Drug,
and Cosmetic (FD&C) Act have not taken action pertinent to WPT [39].
� The Occupational Safety & Health Administration (OSHA). This is a United
States federal institution established under the Occupation Health & Safety Act of
1970 [40]. While not explicitly a regulatory of electronic devices or radio
frequency spectrum, OSHA does publish regulations ensuring that devices which
are found in the modern workplace do not present a hazard to workers. Areas
where OSHA regulations diverge from WPT considerations include:
o The specification of power density and energy density limits at
frequencies between 10 MHz to 100 GHz (29 CFR 1910.97). These limits
are set at frequencies higher than emerging wireless power technologies
27
and do not include explicit field strength limits/reference levels (i.e.
magnetic field limits).
2.4.2 Standards Bodies
Unlike regulatory bodies, standards bodies do not have the power of law to
enforce their recommended limits or protocols. However, as standards bodies are often
inclusive of private sector, non-profits, and governmental agencies, their specifications
may be adopted for incorporation by reference by regulators, thus codifying the
standards' text. Further, as these bodies consist of a variety of stake-holders, their final
drafts encourage utilization due to the wide support across sectors. Examples of such
organizations with a potential to impact WPT are discussed below.
2.4.2.1 ANSI/IEEE/ICNIRP/Consortiums
� The American National Standards Institute (ANSI) is a 501(c)3 non-profit
organization founded in 1918 and dedicated to the standardization of practices
and procedures spanning safety glasses to EMC measurement procedures [41].
The ANSI substandard most concerned with RF emissions is that of ANSI C63.
o C63.4 is titled Methods of Measurement of Radio-Noise Emissions from
Low-Voltage Electrical and Electronic Equipment in the Range of 9 kHz
to 40 GHz. While the frequency band does indeed cover frequencies found
in emerging WPT devices, the standard's procedure in this band lacks
discussion of the nuances associated with WPT devices (i.e. delta between
Tx/Rx, size constraints, extrapolation, vehicle considerations, etc.).
Further, the FCC does not recognize this standard for intentional radiators.
28
o C63.10 is titled American National Standard of Procedures for
Compliance Testing of Unlicensed Wireless Devices. This standard is
accepted by the FCC for intentional radiators and is explicitly called out in
47 CFR Part 15. While far more detail is given with respect to
extrapolation and A/m to V/m conversions, this standard also fails to
address nuances associated with WPT (similar to those regarding C63.4).
o C63.30 is a working standard titled American National Standard for
compliance testing of WPT Products and is intended to bridge gaps found
in .4 and .10, but isn't published. This standard does not include
specifications for RFX, instead focusing on EMC test techniques.
� The Institute of Electrical and Electronics Engineers (IEEE) is a professional
association which, among other missions, is a leading developer of industry
standards [42] including wireless networking (802.11) to legacy parallel data
communications (1284). Much like ANSI, IEEE has standards which may impact
WPT or have been adopted by regulators having jurisdiction over the technology.
o The IEEE publication C95.1 is titled "IEEE Standard for Safety Levels
with Respect to Human Exposure to Radio Frequency Electromagnetic
Fields, 3 kHz to 300 GHz.” This standard is intended to prescribed RFX
limits and covers the frequency bands most likely to harbor WPT. Further,
this standard has been adopted by the FCC, giving the limits legal standing.
However, while this standard has been most recently revised for the year
2005, the edition incorporated by reference is from the year 1992. While
not all standards change dramatically in a 13 year span, this standard's
29
prescriptions underwent substantial modifications. For example, the
reference level limits for magnetic field exposure increased from 1.63 A/m
@ 300 kHz to 54.3 A/m @ 300 kHz (an increase of 3,333%). Such major
changes call into question the validity of the existing regulations and do
not reflect modern consensus regarding magnetic fields similar to those
associated with wireless power.
� The International Commission on Non-Ionizing Radiation Protection (ICNIRP) is
a European commission dedicated to providing safety limits for human exposure
to radio frequency emissions [43]. Much like IEEE C95.1, ICNIRP has prescribed
exposure limits within the frequency bands pertinent to WPT. However, these
limits exceed those currently adopted by the FCC (C95.1-1992) and do no
converge with the more recent C95.1-2005. To increase the ambiguity of
prescribed limit's validity, they have been adopted as recommendations by the
ITU but remain without legal standing in the USA.
� In addition to standards bodies dedicated do RFX limits and the prescription of
magnetic field measurement techniques, others are working to develop
interoperability protocols to ensure cross compatibility between vendors.
o Wireless Power Consortium (WPC) Qi standard. This body is dedicated to
power devices up to 15W, with primary target devices including cell
phones, laptops, and tablet PCs [44]. This organization includes members
such as Nokia, Panasonic, and Dell.
o The Air Fuel Alliance's Power Matters Alliance (PMA) standard was a
standalone standard until 2015 providing a similar, yet incompatible,
30
architecture to WPC's Qi and was intended to serve similar devices.
Founding members included General Motors, Sony Pictures, and Duracell
[45]. In 2015, PMA merged with the Alliance for Wireless Power (A4WP)
to provide a lower frequency option to A4WP's higher frequency, and also
incompatible, architecture.
o The Air Fuel Alliance's Alliance for Wireless Power (A4WP) Rezence
standard was initiated in 2012 and intended to power devices from 5W up
to 50W [46]. Unlike Qi and PMA, which used sub-500 kHz frequencies,
Rezence operated on an ISM band of 6.8 MHz. This increase in frequency
theoretically allows for great spatial freedom between the Rx and the Tx,
while also allowing for unlimited power within the ISM band. This
Rezence standard is the higher frequency component to the Air Fuel
Alliance's dual protocol standard (inclusive of PMA). A4WP founding
members include Qualcomm, Samsung, and Broadcom.
o The Society of Automotive Engineers (SAE) is much akin to the
automotive industry as the IEEE is to electronics. Having been founded in
1905, this professional organization focuses primarily on standards
development within the transportation industries [47]. Therein,
development of a WPT standard for electric vehicle wireless power
charging is considered a core competency.
� SAE J2954 is SAE's standard for interoperable wireless charging
for electric vehicles. Much like Qi and PMA, J2954 is intended to
operate at less than 500 kHz, but at power levels up to 10 kW.
31
J2954 is also non-compliant with the other standards discussed
above, nor does it interoperate with competing proprietary
automotive wireless power charging systems (such as Evatran's
Plugless technology).
The above list of standards bodies contains organizations working in collaboration
towards some facets of WPT standardization (such as Intel and Apple on the ANSI
C63.30 committee), while working against one another in other areas (PMA and Apple
on charging protocols). Further, some companies may have committed to multiple
protocol paths, such as Apple's proprietary charging for it's iWatch and adoption of the Qi
standard for its phones [48]. Simultaneously, regulators are working with competing
limits and standards for test procedures and exposure limits, many of which were not
intended for such modern systems. This dissonance of competing interests, standards,
protocols, and guidance must be addressed before the WPT technology can evolve from a
novelty to an integral part of our technological lives.
2.5 Standardization for Regulatory & Validation Constraints
While many types of complex systems seek standardization clarity to ensure
maximum product adoption, a decrease in re-engineering costs, as well as interoperability,
the reality can be much more complicated. In even less complex cases than WPT,
competing organizations may find convergence elusive. This is demonstrated in the
electrical vehicle charging plugs discussed in 2.1.3, as well as in the video disc format
competitions of the 2000s (DVD+R, DVD-R, DVD-RAM, DIVX, and then to HD-DVD,
Blu-ray, etc.). These competing standards did not require complex legal and regulatory
hurdles, nor were consumers concerned about radiation hazards from these devices. Yet,
32
the disc format process lasted from the conception of the DVD in 1997 until the last HD-
DVD release in 2010 [49][50], with electric vehicles plug standardization continuing to
remain an enigma.
While no known WPT parameter optimization framework based upon EMC, RF
Compliance, and other engineering criteria is known to exist, it does not mean that
tangential research pertaining to these items is completely unknown. Examples include a
comprehensive survey performed by the Wisconsin Electrical Machines and Power
Electronics Consortium (WEMPEC) cataloging known WPT parameters across devices
[51], as well as the ITU's proposal for limits and methods of measurement for WPT [52],
which spans frequencies of interest to the technology. While this research is absolutely
beneficial to the greater development of the technology, this praxis is understood to be
the first systems engineering framework intended to provide optimized parameters
consistent with EMC and RFX regulations or standards.
2.6 Market & Sales Precedents for Parallel Technologies
WPT is a technology which has the potential to revolutionize how electronic
devices are charged and powered. This potential could increase the use of electric
vehicles, decrease the amount of power cables and cords found throughout households,
and help to continue the move to an electrified world. However, these changes cannot
take place until the technology reaches a critical mass, which is currently subject to a
20th century regulatory environment and competing interpretability standards. While the
future of this technology may be unknown, historical parallels can be drawn from other
complex proposals which lived or died due to a rapid convergence of standards and
clarification of regulatory hurdles.
33
2.6.1 Broadband-Over-Powerlines (BPL)
In the early '00s, the divergence between urban population centers with high
speed internet and rural communities with dial-up modems was stark (see Figure 14). The
low population density of these rural communities was a disincentive for broadband
operators to invest in DSL, ISDN, and cable internet capabilities. In order to address this
disparity, using existing power line infrastructure to drive down installation costs and
connect homes already plugged into the power grid was proposed [53].
0
5
10
15
20
25
30
35
40
2000 2001 2002 2003
Year
% o
f u
sers
w/ h
igh
sp
eed
co
nn
ecti
on
s
% of rural users % of suburban users% of urban users % of users nationally
Figure 14 Broadband growth for home users [54]
While leveraging existing powerlines to provide broadband access to underserved
regions of the nation, all while driving down costs of deployment would seem intuitive,
the reality was far different. Due to uncertainty in the regulatory regime, it was not clear
if this technology would be considered an unintentional radiator (under 47 CFR Part 15B),
an intentional radiator (under Part 15C), or would require a license for operation. The
reason for this uncertainty was that the unshielded powerlines would act as de-facto
radiators of the high frequency signals BPL would need to run across them.
34
To address these items, the FCC would issue formal rules in 2004 [55] marketed
at "empowering" BPL, while realistically adding significant restrictions to it [56]. The
impact of the revised FCC regulations regarding BPL was combined with litigation
stemming from the American Radio Relay League (ARRL) - a non-profit technical
association dedicated to radio communications. This litigation sought to ensure protection
of communications on frequencies potentially impacted by BPL, but ultimately resulted
in uncertainty in the marketplace and decreased interest in the technology (see ).
Figure 15 Decline of interest in BPL since 2004 [57]
BPL, much like WPT, had the capacity to revolutionize the home and usurp an
entrenched technology (in this case, dial-up modems). However, due to regulatory
ambiguities or hindrance, combined with lack of coordination with a technical non-profit
organization, a product which could have effected millions, faded into history.
2.6.2 Personal Computers
Although not intuitive, Personal Computers (PCs) are another example of a
complex technology which was only able to see major market penetration after an
increase in regulatory and standards certainty. Unlike BPL, PCs benefited from a more
concise regulatory environment combined with the rapid adoption of protocols in the
1980s and early 90s.
35
In 1980s and early 1990s, the world of the personal computer consisted of
competing technologies, a plethora of protocols, and an unknown future. Brands vying
for adoption included IBM, Commodore, Tandy, Apple, Atari, and Timex. Unlike current
PC interoperability, these hardware systems lacked little, if any, compatibility. Further,
the software interface overlaying these devices varied from Mac OS, DOS, Windows,
UNIX, O/S2, and other proprietary operating systems. Confounding matters, the FCC
was unsure if computer systems should be tested as stand-alone devices, as complex
systems, or if new rules were required.
Throughout this decade of incompatible and expensive options, convergence
began to take root. IBM released its "Personal Computer" in 1981 [58], with the first
clone entering the market in 1982 [59]. This competition between the quasi-open
architecture of the IBM PC compatible clones, proprietary systems such as the Apple
Macintosh, and a multitude of systems falling somewhere in-between would continue up
through the early 90s. At this point, a confluence of events transpired which would
remove interoperability ambiguity, streamline regulatory hurdles, and encourage use of
international standards. Examples of some of these events include:
1. Microsoft releases of Windows 95 in August 1995 only for IBM PC compatible
systems. This operating system merged two of Microsoft's competing operating
systems, DOS and Windows, while significantly increasing usability of graphical
user interface and including built-in network capability [60]. These features (as
well as specific hardware support) increased mass adoption of this operating
system across the clone platforms, while diminishing further adoption of non-.
36
Further, the advanced features (or perception thereof) reduced the adoption of
other PC compatible operating systems (most notably IBM's O/S 2).
2. The FCC simplifies its EMC procedures for personal computers. Specifically, the
FCC adopted a new "Declaration of Conformity" (DoC) procedure that permitted
PCs to be authorized based on a manufacturer's or supplier's declaration that the
computer product conforms with all FCC requirements [61]. These new
regulations drove down costs to build clone style PCs (due to the elimination of
the need for manufacturers to obtain FCC approval before marketing new PC
products), thus allowing a lower barrier of entry for competition. The results were
a decrease in regulatory costs and increase adoption of commodity systems.
3. The novel exploitation of information technology standards' communications
protocols (primarily TCP/IP) by Netscape (and subsequent web browsers) in
1995 and 1996 [62]. In this case, software was developed to create a visual web
browsing tool allowing computer users to connect over the fledgling internet in a
fashion not requiring complex usage of a command prompt. Netscape led this
revolution, in part leveraging Microsoft's adoption of TCP/IP in Windows 95.
Decreasing the technical barrier of entry to the internet helped make it an must-
have resource for every home in America and took PCs from being a tool in
schools and offices to being as ubiquitous as the toaster.
These events, consisting of standardization, regulatory simplification, and mass
adoption of standardization bodies' open protocols has resulted in a world where nearly
37
every home as a computer and the internet is an indispensible way of life. This meteoric
rise in the adoption of the PC can be seen in Figure 16.
Figure 16 Growth of Computers 1975 - 2011 [63]
2.7 Summary
WPT stands at the same point in history as technologies which have either
floundered our flourished based upon the multi-variable environment consisting of
regulatory pressures, standards adoptions, and a user-space containing a critical mass of
symbiotic systems. If a framework can be developed to help lead standards organizations
and regulators down a path of convergence, this emerging technology may become as
common as the internal combustion engine. If not, the world may remain connected to
power plugs for the foreseeable future.
38
Chapter 3: Research & Methodology
3.1 Research Methodology
With the growth of battery powered devices spanning cellular phones, laptop
computers, and electric vehicles, the need to conveniently and rapidly recharge these such
devices has grown. This need has manifested in the development of WPT technologies,
with those operating using resonant magnetic coupling at frequencies less then 30 MHz
leading the way. In order for such paradigm shifting technology to successfully transition
from idea to operable infrastructure, a solution to radio frequency exposure and
electromagnetic interference must be developed which does not rely upon traditionally
expensive and time consuming simulations such as Finite Time Difference Time Domain
Method or Finite Element Analysis. Accordingly, the research presented herein is
intended to prescribe technical parameters which would conform to regulatory constraints,
allow the WPT device to meet its power transfer requirements, present dependent
variables ideal for adoption by industry standards organizations, and to do so far quicker
than traditional electromagnetic simulations methods.
This research provides a framework developed to accept power transfer device
geometries and transit coil current requirements, and subsequently prescribe optimized
frequencies of operation, minimum safe distance between user and transmitter, optimum
regulatory authorization method, and shielding factors. Specifically, this framework
estimates magnetic field strength generated from independent variables dictated by the
physical structure of the transmit coil. This geometry, combined with the desired input
current, is processed using off-axis magnetic field strength calculations reliant upon a
39
modified version of the Biot-Savart law, inclusive of surface integrals to compensate for
the 90 degree offset from the vector perpendicular to the circular transmitting coil. Field
strength calculations are found for a distance of 3m, ensuring values are found within the
reactive near-field boundary. In order to ensure compliance with not only the
fundamental operating frequency, but also constituent frequencies out to the 10th
harmonic (as required by the Federal Communications Commission), a simplified Fourier
analysis conducted against the field amplitude to approximate the strength of each
harmonic is conducted.
With transmitting device magnetic field strength properties accounted for,
assessment against regulatory constraints is then conducted. Here, both RFX and EMC
limits are taken into account. EMC limits are interpolated from their prescribed distances
using dynamic factors experimentally validated for the American National Standards
Institute by the Federal Communications Commission. RFX limits, defined by maximum
field strength at the user, not maximum field strength at a prescribed distance, are
compared against the source field at variable distances until the closest distances meeting
regulatory bounds is identified for prescribed independent variables.
Once data has been generated for the defined transmitting device, the framework
cross references RFX compliance criteria, EMC compliance criteria, and uses Nelder-
Mead simplex method to identify the optimized dependent variables needed to meet
regulatory constraints and prescribe the regulatory authorization method most favorable
to the design. Should the prescription conflict with the desires of the original equipment
manufacturer, the framework can also output a necessary shielding factor to compensate
for non-compliant field strength.
40
3.2 Research Questions
A multi-step optimization framework using the Nelder-Mead method was
developed to answer the following questions:
1. Given desired independent variables for WPT geometry and current, what are
the optimized parameters needed to meet RFX requirements?
2. Given desired independent variables for a WPT geometry and current, what
are the optimized parameters needed to meet EMC requirements?
3. Given optimized dependent variables for RFX and EMC operability, which
regulatory authorization method ensures greatest operable capabilities?
3.3 Research Hypothesis
The aforementioned research questions may be found through testing against research
hypotheses. These hypotheses are presented below:
H1o - The framework will successfully resolve dependent variables meeting RFX and
EMC regulatory constraints for the regulatory method ensuring maximum performance
based on the defined WPT architecture.
H1a - The framework will be unable to successfully resolve dependent variables meeting
RFX and EMC regulatory constraints for the defined WPT architecture.
41
Test of the Hypothesis
Variable wireless power transmitter geometries, frequencies, and currents will be input
into the framework, with the expectation that optimized solutions will be generated.
These solutions can then be compared against existing technologies, hard regulatory
limits, and emerging industry standards for validation.
Figure 17 WPT Framework Methodology
3.4 Research Framework
The research framework is predicated upon independent variables, ostensibly
supplied by the OEM, processed through two competing regulatory structures, with
42
dependent output variables chosen for optimal device performance. Although such a
framework could be constructed to take many degrees of input variables and optimizing
for the unknowns, the framework presented herein focuses on the input of geometry
parameters, with subsequent production of electrical and utilization parameters.
Specifically, the tool is intended to generate optimized solution sets based on
specification of transmitter diameter and number of turns. The methodology applied to
this framework will then iterate through a combined 45,000,000 possible solutions
examining frequencies between 9,000 Hz and 30 MHz, transmitter current between 0.1A
and 499.1A, and a 0.001m to 1m RFX distances between the transmitter and the user.
Based upon the user's selected seed frequency (or frequency range), the tool will output
dependent variables optimized for the regulatory regime (47 CFR Part 15C or 18) which
would yield maximal performance.
3.4.1 Assumptions
Due to the complexity and variability inherent in this emerging market, as well
nuances associated with reactive near field measurements, approximations were
incorporated into the optimization framework. These assumptions are intended to provide
conservative solution sets to circular topologies operating under 30 MHz under
traditional testing methods (i.e. receiving test antenna parallel to device under test and
testing at 10m). A discussion of these assumptions follows.
3.4.1.1 Circular Coil Emerging WPT technologies have established circular (or quasi-circular)
topologies as the topology of choice. This include Qi, Air Fuel Alliance PMA, Air Fuel
43
Alliance A4WP, and Hevo vehicle charging systems. Although not the only type of coil
possible for wireless power (SAE J2954 has proposed a Double-D geometry), the circular
coil assumption is predicted to apply to a greater variety of WPT proposals than not.
Figure 18 Examples of circular coil geometries for Qi [64], PMA [65], A4WP [66], and Hevo [67] WPT systems.
Figure 19 Non-circular wireless power transmitter geometry proposed by SAE J2954 [68]
3.4.1.2 Perfect Square Wave Generation Due to the need of WPT technologies to transfer power (not information),
switched mode devices (MOSFETs, IGBTs) are typically used to generated the power
transfer signal (for information transfer, such as AM radio, linear devices generating a
continuous wave are typically used). The goal of switched devices is to operated in the
switched-mode, thus reducing losses stemming from current flow during the transition
between on and off. Ideally, a switched device would never operate in this transition
region, thus forming a perfectly rectangular output pulse. In reality, all switched devices
have turn-off and turn-on times which yield a slightly trapezoidal signal.
44
In order to decrease the complexity of the framework, while still ensuring that
harmonics are taken into account, a perfectly square approximation of the power signal is
chosen. By making this assumption, even harmonics can be eliminated from the
simulation due to the inherent properties of the Fourier series of the square wave.
Figure 20 Derivation of Fourier spectrum of ideal square wave [69]
The above derivation makes clear that, assuming a perfect square wave as the
power transfer signal, only odd harmonics need to be calculated. Further, as EMC
regulations dictate that compliance devices must meet specific field strength limits, the
phase angle of each harmonic may be disregarded for the purposes herein. This
assumption decreases the amount of harmonics needing to be calculated by 50%.
3.4.1.3 Emulating 1st Order Low Pass Filter Response on
Harmonics
In addition to assuming even harmonics would be negligible, odd harmonics are
assumed to decrease with a -20 dB/decade gradient. This assumption is based upon the
inductive properties of the transmitting coil. If it is understood that the coil functions as
45
an ideal inductor, the impact on the electrical signal would conform to Ohm's law for an
inductor, replicating the effects of a low pass filter.
Figure 21 First order formula for single inductor [70] and associated -20 dB/decade low pass filter
response [71] In addition to treating the transmitting circular coil as a first order low pass filter,
the system response of the coil is to resonate at the fundamental frequency. Due to WPT
systems relying on resonance to achieve high levels of efficiency, assuming -
20dB/decade for harmonics is reasonable.
Since the framework is only concerned with harmonics required for assessment
by regulators, frequencies greater than the 10th harmonic are ignored. In doing so, the -20
dB/decade response must be tailored to frequencies which fall within the decade between
the fundamental and 10th harmonic. The conversion from the logarithmic notation of -20
dB/dec can be converted to a (1/n) calculation as shown below (with n representing the
harmonic number).
46
Table 1 Square Wave Harmonic Amplitude Decay & Conversion Example
Harmonic F (Hz) Hn (A/m) Hn (dBA/m)
1st Order Response dBA/m
1st Order Response A/m
1 20,000.00 2,731.73 68.73 68.73 2,731.73
2 40,000.00 0.00 0.00 0.00 0.00
3 60,000.00 910.58 59.19 49.64 303.53
4 80,000.00 0.00 0.00 0.00 0.00
5 100,000.00 546.35 54.75 40.77 109.27
6 120,000.00 0.00 0.00 0.00 0.00
7 140,000.00 390.25 51.83 34.92 55.75
8 160,000.00 0.00 0.00 0.00 0.00
9 180,000.00 303.53 49.64 30.56 33.73
10 200,000.00 0.00 0.00 0.00 0.00
1st order harmonic attenuation of square wave with amplitude of (H0) of 2145.5 (A/m) - Notice assumption that even harmonics are of amplitude zero. It is interesting to note that the harmonic amplitude calculated using the proof
demonstrated in 69 results in a -20 dB/decade decay for a square wave harmonics. This,
combined with the -20 dB/decade low pass filter response of the transmitting coils, yields
a total attenuation of -40 dB/decade for harmonics when compared to the fundamental
(i.e. the magnetic field amplitude of the 10th harmonic in free space would be 40 dB less
than the fundamental magnetic field in free space). A validation of this observation will
be shown in the results section of this praxis.
3.4.1.4 Biot-Savart Frequency Dependence
In order to calculate magnetic field strength for both RFX and EMC assessments,
the Biot-Savart law is used. This law assumes a steady state current by ignoring the time
dependency of Ampere-Maxwell’s law (see derivation below). Because of this
simplification, Biot-Savart would be inappropriate for field strength calculations for
electrically large devices, but may be used for electrical small devices consisting of
apertures much smaller than the wavelengths of their radiation - so long as the calculation
is performed in the reactive near field [72]. In this case, the calculations are found for a
47
distance of three meters from the transmitter. This distance is chosen due to three meters
generally falling within the reactive near field of WPT devices operating at less than 30
MHz, while also applying to traditional EMC test measurement distances.
Figure 22 Derivation of on-axis Biot-Savart law for on-axis calculations [73]
In order to justify three meters as a simulation distance, parameters of emerging
WPT technologies can be compared against their wavelengths to determine near-field
boundaries. As shown, with aperture sizes much smaller than the radiating wavelengths,
using the Biot-Savart law within the near-field/far-field boundary may present a
sufficient approximation. This can be equivocated as follows [74][75][76]:
� If D << λ, then the device is considered to be electrically small
� If the device is electrically small, the near-field/far field boundary can be
calculated as: r = λ/(2∗π)
Table 2 Emerging WPT Protocol Parameters SAE Qi PMA A4WP
Center frequency (f) 85.7 kHz 157.5 kHz 317 kHz 6.78 MHz
Wave length (λλλλ) 3,500 m 1,905 m 946 m 44 m
Approximate Diameter of Coil (D) 0.62 m 0.044 m 0.036 m 0.22 m
Near-field/Far-field boundary (r) 557 m 303 m 151 m 7 m
Using the most mature emerging WPT protocols as examples, it can be shown
that approximating field strength to three meters, while also staying within the reactive
near-field, is possible. However, as the frequency increases or as the transmitting device
48
size increases, the near-field/far-field boundary will migrate towards the radiator,
eventually diverging from this approximation.
In addition to the traditional Biot-Savart derivation shown above and assumption
of applicability in the near-field, modifications must be made to find field strength 90
degrees off-axis in order to emulate the traditional EMC measurement procedure. In
order to do so, Biot-Savart is integrated over a circular current loop to find the magnetic
field at any point in space [77]; this requires usage of complete elliptic integral functions
of both the first and second kinds. Implementation of this process is explained in [77] and
summarized as follows:
K(k) is the complete elliptic integral function, of the first kind. E(k) is the complete elliptic integral function, of the second kind.
Figure 23 Modification of the on-axis Biot-Savart law facilitating off-axis calculations [77]
3.4.1.5 Characteristic Impedance of Free Space
Many of the assumptions above rely on the properties of near-field measurements
as justification. Conversely, the properties of the near-field make predicting phase shift
between the magnetic and electric components of the field quite difficult. In order to
simplify the approach taken in the framework, the constant characteristic impedance of
free space, 377W, and the static relationship between electric field, magnetic field, and
free space impedance found in the far-field is used. This is primarily used to convert
49
EMC limits defined in volts/meter into magnetic field limits necessitating an amps/meter
threshold. This method is consistent traditional EMC practice [78].
3.4.1.6 Dominance of H-Field over E-Field
EMC regulations specify compliance limits in both magnetic and electric field
values (as well as power density). When performing measurements on traditional
radiators which are electrically small, electric field measurements typically are made in
the far-field and measured in volts/meter. For near-field WPT devices, measurements are
made using a loop antenna and measured in amps/meter. Due to the magnetic field
dominating the electric field within the near-field region, it is understood that electric
field limits and power density limits may be ignored [79].
3.4.1.7 RFX Limits
Due to the emerging technology status of WPT devices, long standing regulatory
regimes established by the Federal Communications Commission are still struggling to
catch up. An example of such a shortcoming is exemplified in the current RFX limits.
These limits, shown below, reflect the following ambiguities which must be resolved for
the framework.
1. FCC limits, published in 47 CFR §1.1310, are incorporated through reference
with “IEEE Standard for Safety Levels with Respect to Human Exposure to Radio
Frequency Electromagnetic Fields, 3 kHz to 300 GHz,” ANSI/IEEE Std C95.1-
1992. This reference has been subsequently updated in 2005 [80]. The more
modern limits are assumed applicable in this framework.
50
2. Current FCC limits do not explicitly specify field strength thresholds below 300
kHz, which is the region of RF spectrum most likely to be utilized for WPT.
Consistent with #1, the revised IEEE limits are specified in the framework down
to 100 kHz.
3. Below 100 kHz, the FCC does not specify Specific Absorption Rates (SAR), from
which field strength thresholds are developed. In order to ensure conservative
calculations, the lower RFX reference level limits found in ICNIRP 2010 [35]
(compared to those found in ANSI/IEEE Std C95.1-2005) are used at frequencies
less than 100 kHz.
4. Due to the consumer nature of the technology explored in this framework, only
General Population limits are considered.
Table 3 Legacy RFX regulations [81]. Note lack of limits at f < 300 kHz.
3.4.1.8 Interpolation of FCC EMC Limits
FCC EMC limits for devices operating at frequencies less than 30 MHz can be
specified at distances between 30 meters to 300 meters. Because these distances often
exceed the size of the test site (or, in the case of 300 meters, prove to be unfeasible), the
FCC authorizes extrapolation factors for measurements made at more practical distances.
For 47 CFR Part 15C regulations, an extrapolation factor of -40 dB/decade is prescribed;
51
Part 18 prescribes -20 dB/decade (noted as a 1/d attenuation factor). Additional
complications to these testing scenarios are introduced through a third option which
allows for an estimation of field decay, which can be used to find a tertiary extrapolation
value. These options are defined below for Part 15C and Part 18, respectively:
� When performing measurements at a closer distance than specified, the
results shall be extrapolated to the specified distance by...making
measurements at a minimum of two distances on at least one radial to
determine the proper extrapolation factor... [82]
� Where possible, field strength measurements shall be made along each
radial at several intervals and an average curve shall be drawn for
measured field strength in uV/m versus distance in meters. Where
necessary, the average curve shall be extend to show the extrapolated field
strength at the distance at which the emission limit is specified. [83]
Due to these ambiguities, accurately and repeatedly demonstrating EMC
compliance with devices operating at frequencies less than 30 MHz has proven difficult.
However, due to the historically limited number of devices operating in this frequency
range, issues stemming from these regulations have not resulted in updated regulations.
Notwithstanding the limited amount of concern stemming from extrapolation
related issues, engineers from the Federal Communications Commission developed a
seminal guide to sub-30 MHz extrapolation containing dynamic factors based upon
frequency of operation. This document, published in 1991 for the American National
Standards Institute (ANSI) C63 committee [84], will be used in the framework to
52
overcome the shortcomings associated with the ambiguous extrapolation instructions
found in the current regulations.
3.4.2 Key Independent Variables As discussed above, this framework limits independent design variables to
physical attributes of the transmitting radiator. These parameters are further constrained
by the aforementioned assumption that the radiator will be a variant of a circular coil.
Thus, the framework only necessitates entry of total number of turns in the radiator, as
well as the turn diameter. It can also be noted that the largest turn diameter represents the
largest dimension of the transmitter, thus allowing for consideration of the near-field/far-
field boundary properties formally mentioned.
This praxis is understood to be the first systems engineering framework
specifically designed to provide parameter optimization for WPT geometries based upon
regulations, standards, and electromagnetic constraints.
3.4.2.1 Transmitting Coil Turns Above, the Biot-Savart law is established for on-axis and off-axis magnetic field
calculations. However, those calculations presume a single turn of wire as the field
generation source. Wireless power transmitting devices are likely to have multiple turns
in order to increase the amount of flux linking the transmitting and receiving devices. In
order to account for multiple turns, the framework uses an iterative summation process to
add the magnetic field strength from each subsequently smaller turn. The framework
operates with the assumption that each additional interior loop will have a radius 3%
shorter than the next larger loop. This is similar to the Qi and PMA standards, but would
53
provide a conservative approximation for the A4WP standard. An encapsulation of this
code is shown below.
Figure 24 Implementation of Biot-Savart for multiple coil turns for off-axis calculations
3.4.2.2 Transmitting Coil Diameter In addition to the number of turns impacting magnetic field strength of the
transmitter, the overall diameter of the transmitter is required. This parameter is not only
necessary due to requirements elucidated in the Biot-Savart law, but is also necessary to
determine near-field/far-field boundary parameters. The framework will also use the
outer coil diameter to assess RFX, as human exposure to the radiator is measured from
the outermost boundary of the device. Also note that, with respect to Figure 24, interior
coils are considered to be measured at a 3% farther distance from the 3 meter EMC
measurement point (i.e. each interior turn diameter is assumed to be 3% farther from the
three meter test distance than the next largest turn).
3.4.3 Constraints Before optimized dependent variables can be generated, the framework must
consider the constraints which the output values will be held against. For WPT systems,
the most pressing concerns are compliance with RFX and EMC constraints.
54
3.4.3.1 RFX RFX, as it pertains to sub-30 MHz WPT devices, is the impact of a time varying
electromagnetic field on human tissues. Under some circumstances, this field can cause a
thermal reaction and burns to the user [35]. In order to mitigate this, a combination of
regulatory agencies and standards bodies have worked to establish limits which would
prevent such negative effects. For the purpose of this framework, the assumptions
discussed above will be implemented. Accordingly, the below limits will form the
foundation of the RFX constraints within the frequency bands under the jurisdiction of
the Federal Communications Commission pertinent to sub-30 MHz WPT (9,000 Hz -
30,000,000 Hz) and for uncontrolled exposure scenarios:
Table 4 Maximum Permissible Exposure (MPE) field strength limits specified by [35]. Framework leverages limit established for 9 kHz to 100 kHz (see 3.4.1.7 for more information).
Table 5 Maximum Permissible Exposure (MPE) field strength limits specified by IEEE C95-1 2005 Framework leverages limit established for 100 kHz to 30 MHz (see 3.4.1.7 for more information).
55
Upon investigation of the above RFX limits, it is clear that compliance hinges
upon their frequency dependency, as well as the distance at which the user is exposed.
This latter concern is impacted by the magnetic field decay between the source of
generation and the location of the user, thus presenting a distance dependent variable.
3.4.3.2 Electromagnetic Compliance (EMC) Unlike RFX constraints, EMC constraints are bound entirely within 47 Code of
Federal Regulations (CFR). Depending upon which regulatory framework is most
advantageous to the OEM, two different constraint sets may be chosen from1.
The first option is compliance with 47 CFR 15C - Intentional Radiators. This
section establishes limits which are frequency dependent and require various test
distances which are dependent upon frequency of operation.
Table 6 Radiated field strength limits as established in [85]
The second option for authorization is 47 CFR 18 - Industrial, Scientific, and
Medical (ISM) equipment. Although all WPT applications may not intuitively be defined
as ISM devices, this rule section allows for consumer ISM devices, under which most
emerging WPT devices may be classified.
1 Note that the FCC allows authorization under either Part 15C or Part 18 pending modulation considerations and other nuances found within these two rule sections. Although the proposed wireless power transmitter protocols mentioned herein may currently fall into either part, future systems may be more confined. Additional guidance can be found in FCC KDB Publication 680106 v02 which is available at the following link: https://apps.fcc.gov/oetcf/kdb/forms/FTSSearchResultPage.cfm?id=41701&switch=P
56
Unlike Part 15C, Part 18 has two sets of limits. The first set is for any device,
regardless of operating frequency, so long as it meets the requirements of an ISM device.
The second is for devices operating on specific "ISM frequency bands". These bands are:
� 6.765 MHz - 6.795 MHz
� 13.553 MHz - 13.567 MHz
� 26.957 MHz - 27.283 MHz
Devices operating within these bands are authorized unlimited transmit power (i.e.
no limit) for the fundamental frequency, but are required to meet the specified limits for
all harmonics. The limits for Part 18 devices operating within ISM bands and not within
ISM bands is shown below.
Table 7 Limits for Part 18 devices operating on ISM and non-ISM fundamental frequencies. Note that WPT devices operating below 1000 MHz are not permitted the increase in field strength otherwise permitted here for power over 500 watts.
Reflecting upon the emerging standards found in Table 2, it would appear that
most devices would operate on frequencies governed by Part 15C, which may appear
counterintuitive given the unlimited limit for the fundamental associated with ISM band
operation within Part 18. Two reasons WPT protocols may be focusing on lower, non-
ISM frequencies, are parasitic eddy currents associated with higher frequencies and
efficiency losses in switched power supplies due to limitations of MOSFETs and IGBTs
[86]. Regardless, the framework considers all three sets of limitations when looking for
an optimized solution.
57
Finally, when reviewing the above limits, it must be noted that Part 15C limits are
explicitly frequency dependent and will require the limits be interpolated using the
process identified in the 3.4.1.8. Part 18 limits are frequency dependent only as far as
determining whether or not ISM or non-ISM limits should be applied; interpolation will
be required from the 300 meter measurement distance specified in both cases.
3.4.4 Key Dependent Variables With the entry of the independent variables, application of Biot-Savart in an off-
axis capacity, and (if necessary) summation of fields, the framework is able to establish a
satisfactory RFX EMC solution space. This space is intended to provide optimized
frequency, distance to user, and transmitter current strength for maximum performance.
3.4.4.1 Transmitter Current As evident from the magnetic field calculations shown in 3.4.1, the magnetic field
is generated from current flowing through the transmitting coil. As current increases, so
do the fields which must adhere to EMC and RFX constraints. As the framework
generates the solution space of compliant dependent variables, it calculates the maximum
current authorized on each frequency and the associated distance a user would need to be
away from a transmitter operating at such a current.
3.4.4.2 Distance to User The distance between the radiator and the user is only required to meet RFX
limitations, for they are defined as the field incident on a human being, not for any
particular distance. When the framework generates the optimized solution set, it
prescribes the minimum distances the user may be to the transmitter while still meeting
58
RFX constraints. This multidimensional space provides distances across frequencies
operating at their maximum EMC compliant amplitudes.
3.4.4.3 Frequency
Above all other dependent variables, frequency is arguably the most salient. As
shown above, both EMC and RFX limits tend to vary with frequency. Further, when
interpolating limits inwards, the scaling factors are also found to be frequency dependent.
Finally, as noted in 3.4.3.2, the ability for electronic devices to efficiently generate
power transmission signals is largely based upon the internal semiconductors' frequency
dependent parasitic loss characteristics. Thus, while the framework does provide an all
inclusive set of optimized frequency, distance, and current parameters, it allows the user
to choose a seed frequency that can be subsequently locally optimized using the Nelder-
Mead method.
3.5 Implementation
The methodology and framework described above is implemented using pre-
processed limits and intensive computations. Pre-processing consists of loading known
limits into .csv files. Optimization computations are performed using MATLAB.
MATLAB was chosen for computational purposes due to it ease of programming,
rapid multidimensional operations on large matrices, curve fitting capabilities, and built-
in optimization processes. The major drawback of this software is a lack of continuous
time domain analytics (outside of its Simulink package - which was not considered for
this research).
59
Because of the discrete nature of the calculations, combined with the
discontinuous and nonlinear constraints found in RFX and EMC, a compatible
optimization process was also required. MATLAB is able to meet this need through
implementation of unconstrained nonlinear optimization via the Nelder-Mead process
[87], functionalized as "fminsearch". This embedded function will find a the local
minimum based upon a seeded estimate. As the optimized solutions sought in this
framework are not minimums, but rather maximum performance characteristics, the
function must be preceded with a "-" sign, inverting the solution space.
3.6 Known & Presumed Data
As with any technology still in its infancy, published WPT data is limited, with
much of the literature focusing on non-commercial experimental results. However, there
are emerging standards proposing various frequencies, amplitudes, and geometries. There
have also been a limited amount of early adopters who've successfully navigated the
credentialing process with the FCC, thus requiring that their test results be published.
This data can be compared against the framework's results and will be inspected in the
Simulation & Analysis section of this document. Further, all framework calculations are
based upon proven mathematical theorems (such as the Biot-Savart law) or published
methods based upon experimental data (such as that found in [84]).
3.7 Summary
The methodologies described in this section are intended to provide a framework
which optimizes dependent WPT variables based upon dependent geometric variables
and known regulatory constraints. This framework combines assumptions pertinent to
60
switching semiconductor signal generation hardware, Fourier analysis of idealized power
transfer signals, electromagnetic field calculations based on modifications to Maxwell's
Equations, frequency dependent limit interpolation founded on experimental research,
and implementation of a heuristic optimization process ideally suited for such an
application. Results from this framework will be compared to published data and
parameter recommendations will be made for forthcoming WPT technologies.
61
Chapter 4: Simulations & Analysis
Implementation of the framework requires simulation of the electromagnetic
principals underpinning WPT, as well as adaptation of existing regulatory guidelines. The
electromagnetic simulation can be validated against existing data from emerging WPT
devices, whilst the manipulation of limits is conducted using known and documented
compliance techniques.
The framework combines the electromagnetic and compliance results into
multidimensional arrays, eliminating combinations which are outside of RFX and EMC
limits. A Nelder-Mead local maximum optimization search is then conducted against
compliant solutions across competing regulatory regimes (specifically 47 CFR 15C and
18), with an output of independent variables which offer greatest performance (with
performance based upon desired parameters of the dependent variables discussed in
3.4.4).
The entirety of the framework process begins with OEM prescribed independent
variables and seeded dependent frequency and current variables. The seeded frequency
and current variables are used for two dimensional validation of the electromagnetic
simulations, with the frequency variable also establishing the beginning search point for
Nelder-Mead optimization.
4.1 Fourier Analysis of Proposed WPT Waveforms
All optimization within this framework is based upon assessment of a transmitted
WPT waveform for compliance with RFX end EMC constraints. As noted in 3.4,
compliant solutions are found for a universe of waveforms based upon desired geometric
62
independent variables. This universe of solutions starts with a single seed waveform
using on-axis Biot-Savart (i.e. a zero meter distance from Tx center) and the desired
geometry.
For demonstration purposes, the parameters associated with the Qi WPT protocol
are used in the simulations below (unless otherwise noted). Qi has been chosen due to
availability of data relative to other emerging WPT technologies.
R = 0.0275; %Loop radius in m; basic Qi is R=0.0275m Turns = 25; %Number of turns; basic Qi is Turns=25 I = 1; f=125000;
Figure 25 Magnetic Field Strength @ 0m for Qi Transmitter & Associated Input Variables
The waveform shown in Figure 25 can be used as the starting point for the
electromagnetic simulation. Here, the independent and seed variables are used to
construct a perfect square wave WPT signal at the center of the transmitting coils. This
simulated waveform can be considered the idealized signal found within a device
compatible with that shown in Figure 18, reference 64.
63
4.1.1 Simulations of Harmonics & Amplitudes
Although an ideal square wave signal within the transmitter allows for a starting
point of analysis, EMC regulations dictate that harmonics be considered for compliance
up through the 10th harmonic [88]. The ideal signal can be broken apart into these
constituent signals using the simplified Fourier analysis shown in Figure 20. The
expected amplitudes for the 10 harmonics of the idealized signal shown in Figure 25 are
shown in Figure 26.
Figure 26 Idealized Harmonics for WPT Signal @ 0m The values shown in Figure 26 are given in Amps/meter, which is the
international unit of magnetic field strength. However, when comparing device
measurements against limits defined for EMC, decibels are often used (see [89] as an
example). The formulas for converting A/m to dBA/m and dBuA/m are shown below:
1. dBA/m = 20*LOG10(A/m)
2. dBmA/m = 20*LOG10(A/m*10-6)
Using #1, the constituent harmonics in dBA/m are calculated and shown in Figure 27.
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Figure 27 Idealized Harmonics for WPT Signal @ 0m in dBA/m
Note that in both Figure 26 and Figure 27, even harmonics are give a value of
zero due to the assumption of the source signal being a perfect square wave. In the case
of the latter figure, values should be understood to be undefined, as a dB value of zero is
a mathematical impossibility.
4.1.2 Simulation of Near-Field Decay
Although on-axis Biot-Savart can be used to establish the ideal magnetic field at
the transmitter center, both RFX and EMC require that field strength be evaluated off-
axis for circular coils oriented horizontally (which is considered the use case for this
framework). The mathematics behind this transformation are discussed in 3.4.1.4.
Implementation of this process is limited to field strength calculations at a distance of no
more than 3m to ensure that the frequency independence of Biot-Savart is adhered to.
Both A/m and dBA/m for the Qi parameters shown in Figure 25 are plotted in Figure 28
and Figure 29.
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Figure 28 Harmonic Amplitude A/m @ 3m
Figure 29 Harmonic Amplitude dBA/m @ 3m
4.1.3 Model Validation
With off-axis magnetic field values now known and at a distance consistent with
EMC measurement techniques, the framework can be validated against known data. In
this instance, a certified2 Qi transmitter's test data can be compared against the simulated
2 Certified is an authorization type allowed by the US Federal Communications Commission. It is defined as, "the most rigorous approval process for RF Devices with the greatest potential to cause harmful
66
data above. Prior to doing so, the calculated A/m must be converted to dBuA/m to align
with the published data. This conversion is shown in Table 8.
Table 8 Conversion of data shown in Figure 28 to dBuA/m Calculated Values for Qi Device @ 3m
Harmonic A/m uA/m dBuA/m
1 0.00011710 117.1 41.3711379
3 0.00001301 13.01 22.28554593
5 0.00000468 4.682 13.40862819
7 0.00000239 2.389 7.564322995
9 0.00000145 1.445 3.197356942 With the magnetic field strength converted to units consistent with the reference
test report, compensation for the Qi device's shielding factor must be taken into account.
This attenuation is due to a layer of ferrite about the transmission coils and has been
measured to attenuate approximately 21 dB [90]. Figure 30 shows this ferrite shielding.
Figure 30 Qi Structure [91]
Whilst taking into consideration the attenuation of the shielding factor and
conversion assumptions, the delta between the calculated magnetic field strength and the
measured magnetic field strength can be seen in Table 9. Therein, it is shown a difference
of -0.34dB between the calculated magnetic field strength and the average measurement
of the certified device. Further, for subsequent harmonics, a difference ranging between
interference to radio services." and is prescribed under 47 CFR Section 2.907. Certified devices must publish their EMC and RFX test data with the FCC.
67
0.3dB and -5.98dB is found. In all cases, save for the 9th harmonic, the framework
projects a more conservative value than what has been measured.
Table 9 Comparison of calculated harmonic values for Qi parameters against published EMC test result for FCC ID: 2AIY7--CD-1014 3.
Test Report Data @ 3m for FCC ID: 2AIY7-CD-1014
Harmonic Frequency
(Hz) Measured dBuA/m
Calculated dBuA/m
Delta (dB)
1 125,000 20.03 20.37 -0.34
3 375,000 -4.7 1.28 -5.98
5 625,000 -11.6 -7.59 -4.01
7 875,000 -15.1 -13.44 -1.66
9 1,125,000 -17.5 -17.8 0.3
While the prima facie alignment between the measured data and the calculated
data appears strong, further consideration must be given to the uncertainly in the
measured data. The comparison test report notes an uncertainty factor of approximately 4
dB. The measurement data pertaining to the shielding factor does not specify an overall
uncertainty, but does note that a delta of approximately 1 dB between calculated and
measured ferrite attenuation exists. Quantitatively, the framework's simulated
electromagnetic field profile has been shown to align with the measurements, with values
no greater than 1.98 dB outside of the experimental uncertainty factor (on the second
harmonic).4
3 For multiple tests conducted at the same distance, the average measurement is used against the calculated value. Test data presented as dBuV/m is converted to dBuA/m using 51.5dB offset derived from an assumed characteristic free space impedance of 377 ohms, described in 3.4.1.5. 4 The published experimental data also includes measured values for even harmonics which are non-zero. These values are due to imperfect square wave injection into the transmitting coil. These values are less than their adjacent odd counterparts, but still represent emissions. It will be understood that even harmonics, although existing in practice, are shown to be less than the nearest odd harmonics, thus will pass regulatory muster so long as those nearest odds harmonics are below the regulatory threshold.
68
4.2 Tabulation of Known & Potential Limitations
The limit with which the simulated electromagnetic fields will be compared
against are discussed in Constraints3.4.3. These limits are hard coded into an csv file and
loaded into MATLAB for processing.
With the limits loaded into MATLAB falling between 30m and 300m (as
specified in Table 6 and Table 7) and the Biot-Savart law limiting the distance at which
magnetic fields can be accurately calculated due to near-field/far-field boundary concerns,
a point in-between must be chosen for EMC regulatory threshold testing. A review of
emerging international EMC regulations indicates a 10m compromise to be prudent
[92][93].
In order to accurately interpolate limits inwards to 10m and extrapolate out Biot-
Savart calculations to 10m, the scaling factors discussed in [84] will be used. As noted in
3.4.1.8, these factors make use of extensive testing across frequencies and distances,
resulting in frequency dependent formulas ideal for estimating field strength beyond the
near-field/far-field boundary. These formulas are incorporated into the framework,
resulting in the dynamic factors shown in Figure 31.
69
Figure 31 Dynamic Scaling Factors Defined in [84].
4.2.1 EMC Limits
As noted above, EMC limits are loaded into the framework to provide baseline
thresholds for simulated magnetic fields generated by the WPT transmitter. Normalized
limits for 15C, Part 18 ISM, and Part 18 Non-ISM are shown in Figure 32 at 10m. The
factors for those limits and the subsequent scaling are shown in Table 10, Table 11, and
Table 12.
Figure 32 Normalized FCC EMC Limits
70
Table 10 Scaling of FCC Part 15 Limits to 10m FCC Part 15C Limits
Frequency (Hz)
Field Strength (uV/m)
Field Strength (uA/m)
Field Strength (dBuA/m)
Distance (m)
Scaling Factor
Limit @ 10m (dBuA/m)
9,000 266.67 0.707 -3.01 300 88.56 85.55
490,000 4.90 0.013 -37.73 300 75.42 37.69
490,000 48.98 0.130 -17.73 30 28.60 10.87
1,705,000 14.08 0.037 -28.56 30 25.41 -3.15
1,705,000 30.00 0.080 -21.98 30 25.41 3.42
30,000,000 30.00 0.080 -21.98 30 9.30 -12.68 Table 11 Scaling of FCC Part 18 ISM Limits to 10m
Part 18 Limits ISM
Frequency (Hz)
Field Strengt
h (uV/m)
Field Strength (uA/m)
Field Strength (dBuA/m)
Distance (m)
Scaling Factor (dB)
Limit @ 10m (dBuA/m)
9,000 25 0.066 -23.57 300 88.56 64.99
6,765,000 25 0.066 -23.57 300 34.26 10.69
6,765,000 10 0.027 -31.53 1,600 48.76 17.23
6,795,000 10 0.027 -31.53 1,600 48.71 17.18
6,795,000 25 0.066 -23.57 300 34.21 10.64
13,553,000 25 0.066 -23.57 300 29.40 5.83
13,553,000 10 0.027 -31.53 1,600 43.90 12.37
13,567,000 10 0.027 -31.53 1,600 43.90 12.37
13,567,000 25 0.066 -23.57 300 29.40 5.83
26,957,000 25 0.066 -23.57 300 29.40 5.83
26,957,000 10 0.027 -31.53 1,600 43.90 12.37
27,283,000 10 0.027 -31.53 1,600 43.90 12.37
27,283,000 25 0.066 -23.57 300 29.40 5.83
30,000,000 25 0.066 -23.57 300 29.40 5.83
Table 12 Part 18 Non-ISM Limits Part 18 Limits Non-ISM
Frequency (Hz)
Field Strengt
h (uV/m)
Field Strength (dBuV/m)
Field Strength (uA/m)
Field Strength (dBuA/m)
Distance (m)
Interpolation Factor
(dB)
Limit @ 10m
(dBuA/m)
9,000 15 23.5218 0.0398 -28.0050 300 88.5615 60.5565
6,765,000 15 23.5218 0.0398 -28.0050 300 34.2582 6.2532
6,765,000 15 23.5218 0.0398 -28.0050 300 34.2582 6.2532
6,795,000 15 23.5218 0.0398 -28.0050 300 34.2102 6.2052
6,795,000 15 23.5218 0.0398 -28.0050 300 34.2102 6.2052
13,553,000 15 23.5218 0.0398 -28.0050 300 29.4000 1.3950
13,553,000 15 23.5218 0.0398 -28.0050 300 29.4000 1.3950
13,567,000 15 23.5218 0.0398 -28.0050 300 29.4000 1.3950
13,567,000 15 23.5218 0.0398 -28.0050 300 29.4000 1.3950
26,957,000 15 23.5218 0.0398 -28.0050 300 29.4000 1.3950
26,957,000 15 23.5218 0.0398 -28.0050 300 29.4000 1.3950
27,283,000 15 23.5218 0.0398 -28.0050 300 29.4000 1.3950
27,283,000 15 23.5218 0.0398 -28.0050 300 29.4000 1.3950
30,000,000 15 23.5218 0.0398 -28.0050 300 29.4000 1.3950
71
These pre-processed limits simply adjust the thresholds first discussed in
Electromagnetic Compliance (EMC) 3.4.3.2 to a distance of 10m. However, [84] makes
clear that the scaling factors are dynamic over all frequencies (whereas the limits are
defined at single frequency points). In order to accurately interpolate the limits across the
entire frequency band of interest (9 kHz to 30 MHz), scaling must be conducted against
all possible solutions using the MATLAB framework. While it may be intuitive to
assume values similar to those shown in Figure 32 would be calculated, the reality is
more complex due to the rapid changes in slope shown in Figure 31. These post-
processed limits are show in Figure 33.
Figure 33 FCC EMC Limits Post-Processed from 9 kHz to 30 MHz for 10m It is clear from Figure 33 that the additional post processing shifts the limits a
non-insignificant amount relative to only adjusting for the points at which the limits are
defined. These post-processed limits will be used as the threshold for optimization.
72
4.2.2 RFX Limits
Unlike EMC limits, RFX limits are not defined at a specific point for compliance.
Rather, they are defined for maximum RFX at the point where the field is incident upon
the user. This negates the need for significant post-processing of limit data once loaded
into the framework. Further, as the limits are defined for the RMS signal value, the
assumption that the source signal is a perfect square wave also negates the need for
addition computations, as the RMS value for a square wave is the peak value for a 50%
duty cycle [94]. The limits discussed 3.4.3.1 are plotted in Figure 34.
Figure 34 RFX Limits
4.3 Compliance Tool Generation Process
4.3.1 Theory
The framework has proven able to simulate magnetic fields consistent with a
known system and has had RFX and EMC thresholds processed for comparison at a
distance of 10m. At this point, the framework must sweep dependent variables for
compliance against those thresholds for the specified independent variables.
73
The sweep process will use the input geometry as the foundation for calculations
and derive fields for the fundamental frequency through the 9th harmonic. These fields
will be calculated with a starting current of 0.1 amps and an initial fundamental
frequency of 9 kHz. This process is repeated for each of the remaining odd harmonics.
The sweep will then increment current in 1 amp intervals until 500A is reached. At that
point, the sweep will begin again at 19 kHz, with frequency steps of 10 kHz. An example
of such calculations for the first harmonic is shown in Figure 35.
Figure 35 First harmonic H-Field Sweep
Once a magnetic field profile is established for possible geometry, frequency, and
current combinations, each data point will be subtracted from its associated EMC limit to
establish whether the operating frequency and its harmonics, at that particular current,
will meet EMC thresholds. This is done for Part 15C, Part 18 ISM, and Part 18 Non-ISM;
this data is plotted in Figure 36, Figure 37, and Figure 38 respectively. Note how each
dataset shows the safety margin increase as current decreases towards zero. This is an
intuitive check, as the less current in the transmitter, the less magnetic field is generated.
74
Further, in Figure 37, note the ISM bands on the right side of the curve. These bands are
allowed to generate unlimited fields, so long as harmonics don't exceed their limits.5
Figure 36 EMC Safety Margin for Part 15C
Figure 37 EMC Safety Margin for Part 18 ISM.
5 Part 18 ISM bands allow for unlimited generation of field, so long at 10uV/m is not exceeded at 1,600m (or approximately a mile). Due to the logistics in making accurate measurements at such a distance, it is traditionally understood that such consideration is not required.
75
Figure 38 EMC Safety Margin for Part 18 Non-ISM
It is extremely important to note that the EMC phase of the framework evaluates
not only the fundamental frequency array (an example of which is shown in Figure 35),
but also each harmonic. Although not intuitive, due to the varied nature of the adjusted
limits shown in Figure 33, it is possible for a fundamental frequency to pass the limit,
while a subsequent harmonic may not. An example of such a configuration is shown in
Figure 39. Here, the first two harmonics would indicate a compliant device, with the
lower amplitude 5th, 7th, and 9th harmonics are failing.
Figure 39 Example of Passing Fundamental and Failing Harmonic for Part 15C
76
During the above process, the framework generates three master arrays of
3000x10x500 - one array for Part 15C, one for 18 ISM, and another for 18 Non-ISM.
This equates to 45 millions possible solutions before RFX is considered.
Once EMC safety margins are established and the associated currents and
frequencies are known, those values can be passed into the RFX component of the
framework. Here, each known EMC safe frequency and amplitude will be evaluated to
determine the closest distances the transmitter can be to the user. This is especially
important when considering human factors for the device. Should the framework only
generate parameters optimized for EMC compliance, the device may fail to meet RFX
requirements at a usable distance (for example, if a WPT system for an electric car
required a user to stand 100ft away, installation within a home's garage may be
impossible). Thus, safety margins are found for the closest possible distance associated
with the aforementioned EMC compliant frequencies and amplitudes. An example of this
is shown in Figure 40. Note that the safety margin decreases as the distance to the device
decreases. This is an intuitive check to ensure accurate implementation.
Figure 40 RFX Safety Margin for a given Tx Current
77
Each RFX safety margin array contains 3000x3000 solutions. Calculations are
performed at the maximum transmitter current identified in the EMC section which also
meets Part 15C, Part 18 ISM, or Part 18 Non-ISM regulations (whichever allows for
greater performance).
4.3.2 Implementation
Implementation requires a new simulation for each set of independent variables.
Depending upon the complexity of the geometry (number of turns, radius), the duration
of the simulation can vary from under 10 minutes to greater than a day. As noted above,
running the optimization process against the known Qi device referenced in 4.1.3 takes
approximately 12 minutes. This is inclusive of solution simulation, identification of
compliant dependent variables, and Nelder Mead optimization about a seed frequency.
For the demonstration Qi parameters, EMC calculations yield 45 million solutions.
RFX calculations yield an additional 9 million solutions. This combines for a total
universe of data approaching 1/2 GB
4.3.3 Results
Results are broken into three segments for the defined independent variables; the
first being maximum currents for each frequency between 9 kHz and 30 MHz; the second
being minimum safe distance from the transmitter to the user for the selected frequency
of operation; and the third is the local optimum parameters discerned through the Nelder
Mead function based on a seeded frequency or band. Based on the results fed back from
Nelder Mead, the framework can determine which regulatory regime (15C or 18) offers
the greatest performance for the variables selected by the OEM.
78
The example solutions presented below are based on the Qi geometric parameters
first noted in Figure 256. Although the associated reference device discussed in 4.1.3 was
designed for a maximum transmitter current of 1A, the optimization framework allows
for sweeping across a wide range of currents and is able to determine if the device can be
optimized for greater power transfer (consider if the transmitter could be upgraded to
charging an iPad instead of an iPhone through use of the optimization framework) or a
closer safe distance to the user. Further, adjacent frequencies can be evaluated to
determine if another may increase device capabilities. Note that the solutions below also
use a zero shield factor in order to establish a regulatory baseline for the geometry.
The first set of solutions pertain to EMC and span 15C, 18 ISM, and 18 Non-ISM.
If the device under consideration is intended to meet the emerging Qi protocol, it must
operate within the compatible Qi band, specifically 110 kHz - 210 kHz (this also aligns
with the Qi device used for model validation). This will be considered the seed range.
The compliant data sets for maximum transmitter current optimization are shown
in Figure 41, Figure 42, and Figure 43 for 15C, 18 ISM, and 18 Non-ISM respectively. If
it is understood that current optimization is limited to the proposed Qi band, then the 18
ISM data can be omitted due to applicability limited to operation only on ISM bands.
Calling Nelder Mead across the remaining two regulatory limits yields the following:
Table 13 Optimized Maximum Currents for Qi Device Maximum Current (A) Frequency within Qi Band
15C 24 110 kHz 18 Non-ISM 269 110 kHz
Once optimized currents are derived, the corresponding optimized minimum
distance to user can be determined. Distances based on the values shown in Figure 46 are
6 The presented solutions are based upon the Qi parameters, but could just have easily been based upon generic OEM needs pertaining to form factor, cost, and manufacturing limitations. The framework can take any circular coil geometry and provide optimized solutions based on OEM requirements.
79
found in Table 14, while solutions spanning all frequencies evaluated are shown in Figure
44, Figure 45, and Figure 46 for 15C, 18 ISM, and 18 Non-ISM respectively.
Table 14 Minimum Safe Distances for Qi Device Optimized for Maximum Current Minimum Distance (m) Frequency within Qi Band
15C 0.0716 110 kHz 18 Non-ISM 0.161 110 kHz
Although both solutions shown in Table 14 will meet EMC criteria, the 18 Non-
ISM authorization will permit an order of magnitude more current in the transmitter,
while only increasing the minimum safe distance by approximately 9cm. That increase in
distance may be impractical for wireless power chargers imbedded into vehicle consoles,
but could be ideal for those built into desks for charging iPads or laptops. If the latter case
were fit the OEM's needs, the framework would suggest pursuit of Part 18 Non-ISM
certification and an operational frequency of 110 kHz.
In an alternative case, should distance to user be more valuable than maximum
current (for example, if charging a Pacemaker through human tissue), the framework can
also consider the inverse. By using Nelder Mead on Figure 44 and Figure 46 to identify
which frequency within the Qi band will allow the closest distance, optimized RFX
parameters can be found. These are shown in Table 15.
Table 15 Qi Device Optimized for Nearest Distance Distance (m) Frequency within the Qi band
15C 0.0596 190 kHz & 210 kHz 18 Non-ISM 0.161 110 kHz
Table 15 shows that a 15C device will provide the closest distance to the user at a
frequency of 190 kHz. In the Pacemaker example above, such insight would significantly
benefit the OEM when considering signal control, transmitter enclosure design, and
hardware development. When those optimized parameters are cross referenced against
their corresponding maximum currents, shown in Table 16, the OEM would also know
that a 15C device could conduct a maximum of 8.1 amperes.
80
Table 16 Maximum Currents for Qi Device Optimized for Minimum Distance to User Maximum Current Frequency within Qi Band
15C 8.1A 190 kHz 18 Non-ISM 269 110 kHz
Using this framework, the OEM (in this case, perhaps Medtronic) would be could
choose 15C as the regulatory option knowing that 8.1A would meet EMC thresholds
while providing a minimum safe distance of 0.0596m. The value of an automated
framework selecting operable frequencies and maximum currents simply through a
known geometry would decrease both engineering time and regulatory costs.
Figure 41 Maximum Currents & Frequencies for Part 15C Operation
81
Figure 42 Maximum Currents & Frequencies for Part 18 ISM Operation
Figure 43 Maximum Currents & Frequencies for Part 18 Non-ISM Operation
82
Figure 44 Minimum Safe Distance for Max Tx at Given Frequency, Part 15C
Figure 45 Minimum Safe Distance for Max Tx at Given Frequency, Part 18 ISM
83
Figure 46 Minimum Safe Distance for Max Tx at Given Frequency, Part 18 Non-ISM
While the discussion in this section has pertained to selecting the appropriate
regularly regime for optimized RFX and maximum current parameters within a limited
bandwidth, the framework can also provide far more insight. For example, if the Qi
geometric parameters are desired, but the frequency band is not of interest, the
framework can investigate the entire frequency range (9 kHz - 30 MHz) instead of the
limited Qi band (110 kHz to 205 kHz). If the salient dependent variable is maximum
current for a non-ISM device (with distance being a secondary consideration), the
framework would identify the following optimized currents, frequency, and minimum
distances parameters shown in Table 17 and Table 18.
Table 17 Maximum current for Qi geometry across all frequencies Current (A) Frequency for Qi Geometry
15C 5007 < 49 kHz 18 Non-ISM 338.1 9 kHz
Table 18 Minimum distances for maximum currents for Qi geometry
Distance (m) Frequency for Qi Geometry 15C 0.386 < 49 kHz
18 Non-ISM 0.3376 9 kHz
7 This framework presumes a maximum transmitter current of 500A. Calculations for higher currents are possible, but would require less granularity in current steps or an increase in simulation time. 500A is also a reasonably high threshold for emerging wireless power transfer technologies.
84
In the above case, the framework's results would yield a maximum current of
500A, a minimum safe distance of 0.386m, and prescribe authorization under 15C.
4.4 Engineering Management Recommendations
4.4.1 Technical Recommendations Recommendations for any particular set of independent variables will require
insight into OEM engineering decisions and associated tradeoffs. In most cases, this will
be limited to weighing minimum distance to user (the use-case parameter) versus
maximum current (performance parameter). It is clear that some wireless power
transmission system will be incentivized to increase current, such as charging an electric
vehicle, while others will need to ensure a minimum distance to user (such as the
Pacemaker example). Other devices may put more emphasis on the frequency of
operation, as high frequencies will tend to increase the power transfer distance [95]
between the transmitter and receiver, but also tend to operate more inefficiently due to
eddy currents and semiconductor losses.
With the multivariable tradeoffs required for WPT design, the final
recommendation to the OEM would be to determine the WPT application prior to
determining topology (is the receiver powering a car or a phone?). This will also guide
geometry consideration (a car transmitter could be far larger than a phone charger). With
those two decisions made, the framework will guide selection of optimized frequencies,
output currents, and regulatory regimes. The framework may also be used to refine the
geometry if the initial design does not yield satisfactory dependent variables. Finally, if it
is determine that the optimized solutions do not meet the OEM needs, an offsetting
85
shielding factor may be applied to decrease the magnetic field incident on the user and at
the EMC limit (for example, the results shown in Figure 39 would dictate a shielding
factor of approximately 7 dB to meet EMC limits).
It is highly recommended that OEMs with shared missions work to standardized
their products around the parameters generated herein, such that maximum
interoperability be achieved, while facilitating the greatest performance possible under
the discussed legal and standards environment.
4.4.2 Regulatory & Standards Recommendations
While this framework can be used to optimize WPT parameters, it also
demonstrates the need for regulators and standards bodies to consider migration towards
a common set of validation and implementation specifications. These include the
following solutions:
1. Revision of 47 CFR §1.1310 Radiofrequency radiation exposure limits to reflect
the most recent edition of ANSI/IEEE Std C95.1, from 1992 to 2005.
2. Regulatory adoption of ICNIRP 2010 reference levels for radio frequency
exposure at frequencies equal to or less than 100 kHz.
3. Revision of 47 CFR 15C and 18 such that measurements limits at frequencies less
than 30 MHz are prescribed at distances of 3m or 10m.
4. Revision of 47 CFR 15C and 18 such that emission limits are specified in terms of
magnetic field strength (A/m) in stead of electric field strength (V/m).
5. Collaboration between PMA, Qi, SAE J2954, and the FCC to relax emission
limits between 80 kHz and 300 kHz if possible. This would include consultation
86
with the American Radio Relay League to avoid repeating issues similar to those
encountered during the BPL evolution.
6. Incorporation by reference of ANSI C63.30 once those EMC measurement
procedures are finalized.
7. Provisioning of formal regulatory guidance specific to RFX to magnetic fields
generated by WPT technologies.
4.5 Summary
The simulation and analysis has demonstrate a powerful tool towards the
optimization of WPT parameters. This framework has been successfully validated against
known measurement data and is operable across the entire < 30 MHz frequency band
regulated by the Federal Communications Commission. Although time to perform
solution calculations may vary, the ability to generate known maximum currents for any
circular geometry and their corresponding minimum RFX distance to user without having
to resort to expensive Finite Element Analysis of Finite-difference time-domain method
software suites is invaluable. Further, for emerging WPT standards already honing in on
a topology and frequency band, this framework can determine maximum currents under
existing regulations, allowing such standards bodies (i.e. Society of Automotive
Engineers) to petition regulators to decrease limits concurrent with standard development.
In totality, the framework presented has the capability to save time, money, and help
guide the development of a technology poised to disrupt the battery charging paradigm.
In summary, these results confirm the null hypothesis stated in 3.3
87
Chapter 5: Conclusions and Future Work
5.1 Contributions
The framework presented herein generates a quantitative solution set optimizing
WPT system parameters for compliance with the US legal realm, existing hardware
research & development limitations, and provides a baseline for systems interoperability.
These quantitative parameters support qualitative engineering management directions
providing for convergence of emerging standards and validation procedures such that
maximum market penetration can be achieved.
5.2 Conclusions
The framework presented in this praxis allows for parameter optimization of WPT
technologies while maintaining adherence to regulatory and standards bodies'
requirements. The methodology presented in Chapter 3 defines a framework for
optimizing electromagnetic parameters to meet RFX and EMC standards. This
framework may be applied to any flat coil geometry and compensates for known
discrepancies and ambiguities associated with current regulatory regimes. The research
presented in Chapter 4 provides explicit recommendations for a specific WPT geometry
and identifies optimal frequency, current, and operational specifications to provide
maximum performance. This combined body of research provides an expedient method
of assessing a WPT system and generating design specifics which, if adopted for
universal standards, would portend increased power transfer capabilities and acceptance
by oversight organizations. Further, the regulatory and standards recommendations
88
presented would stand to increase adoption of WPT devices, drive down engineering &
regulatory costs, while allowing maximum interoperability between WPT devices.
Prior to regulatory and standards bodies adopting the proposals herein, the
research also concludes that existing constraints on WPT systems significantly hamper
growth of the technology. Inconsistency within United States regulations (most saliently
those published by the FCC), disagreement between major RFX publications (such as
ICNIRP and the IEEE), and ongoing competition between private sector interoperability
standards will continue to pose a threat to this technology's long term viability.
5.3 Future Work
While the work presented in this praxis presents a comprehensive analysis of
regulatory requirements, standards bodies' recommendations, and electromagnetic
simulations, the need for additional work exists. Specific initiatives which would further
the mission of this praxis are discussed below.
� The mathematical models and simulation framework discussed in Chapter 3
pertain only to WPT systems using flat coil geometry. Future work considering
alternative geometries would be of great value, even if such geometries aren't
currently in the majority of proposals.
� Assumptions pertaining to regulatory constraints are discussed in Chapter 3.
However, should regulations change or the assumptions prove to be inapplicable,
revision of the framework should be undertaken, with the newly optimized results
compared to prior iterations.
� The electromagnetic simulations provided in this praxis are compared against the
published electromagnetic signature of a known device. Further research using
89
Biot-Savart in the near field to find magnetic fields usable for RFX reference
levels is recommended. Such research could allow this framework to substitute
for expensive Finite Element Analysis or Finite-Difference Time-Domain method
simulations packages for showing compliance.
� This framework defines dependent and independent variable for optimization.
However, a truly dynamic framework would allow additional dimensions of
calculations, such that such variables could be dynamic, ensuring the greatest
degree of engineering flexibility for OEMs.
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