110 lines
5.4 KiB
TeX
110 lines
5.4 KiB
TeX
\documentclass[conference,compsoc]{IEEEtran}
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\newcommand{\partnum}[1]{\texttt{#1}}
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\newcommand{todo}{1}{\textbf{TODO}\footnote{#1}}
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\begin{document}
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\date{}
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\title{Wireless Power Transfer with a Twist:
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Achieving Rotation-Invariant Coupling using Multi-Layer PCB Inductors}
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\maketitle
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\begin{abstract}
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\end{abstract}
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\section{Introduction}
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Inertial Hardware Security Modules (IHSMs) are a novel security technology that aims at creating tamper-proof enclosures
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for servers and other information technology using readily available off-the-shelf components. At their core, IHSMs
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provide tamper detection by creating a simple tamper sensing \emph{security mesh} cage from commodity Printed Circuit
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Board (PCB) material, then spinning this cage at a high speed such that tampering with it becomes impossible.
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A core challenge in IHSM engineering is exchanging both power and data between the stationary protected payload of the
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IHSM and the rotating security mesh. Slip rings, which are widely used in rotating machines, are not feasible in IHSMs
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because of their limited speed capabilities and wear life. Various contactless methods have been explored in the past,
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and a previous IHSM prototype used an optical data link for its ease of implementation and high speed along with a
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photovoltaic link to supply power. In this paper, we explore an inductive wireless power transfer link as an alternative
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providing better efficiency, higher power output capability, and a more compact implementation. While WPT has been
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employed before to provide power across a continuously rotating joint, due to their high speed of rotation, IHSMs have
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unique balancing constraints that prohibit the use of large or heavy components such as large filter capacitors or
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inductors on the secondary side, making secondary voltage ripple reduction difficult. In this paper, we solve this
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problem through an optimized PCB WPT coil geometry that provides a more uniform magnetic field and lower parasitic
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capacitance, and that results in an improvement of rotation invariance of the coupling factor of a pair of coils.
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\todo{Give concrete numbers on achievements such as better parasitics and rotational invariance.}
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We provide a theoretical analysis, simulation resuls and practical measurements for our coil geometry, as well as a set
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of Open Source, parametric scripts for the generation of arbitrary coil configurations.
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Wirless Power Transfer (WPT) is used in a variety of applications, and implementations exist in several orders of
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magnitude in power capability. Excluding specialty systems used for galvanic isolation, usually WPT is employed to
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transfer power from a stationary transmitter to a moveable receiver. Applications deployed in the field include phone
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charging, RFID and powering of medical implants. \todo{provide citations.} WPT systems based on capacitive coupling
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exist, but the vast majority of systems employ inductive coupling for its more compact size and higher power handling
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capability. In inductive WPT, the contactless interface between the transmitter and receiver sides of the system is a
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pair of coupled inductors. Usually, these inductors are used in resonant circuits that are tuned to have similar
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resonant frequencies.
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\subsection{WPT inductor coupling}
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In inductive WPT systems the coupling between the coils is highly dependent on the specific geometry of the coils, their
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relative positioning, as well as any nearby magnetically permeable materials. In particular the distance between
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transmitter and receiver coil is critical, and the coupling factor of a pair of coils falls off sharply as their
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distance exceeds some fraction of their physical size. Offset and cross-axis rotation both influence coupling to a
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lesser degree.
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\todo{Analytical expressions, and explanation how the factors mentioned above derive from those.}
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In most WPT systems, distance and alignment are the factors of primary concern. In IHSM applications, where power is
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transferred through a continuously rotating joint, both take only a subordinate role, as both can easily be controlled.
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Instead, we observed a surprising third factor: If the magnetostatic field generated by the coils is not axially
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symmetric, their continous rotation periodically modulates their coupling, introducing low-frequency ripple into the
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secondary-side power output.
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\section{Related Work}
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\subsection{Twisted Inductors in RFIC Design}
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\subsection{Basket-Woven Air Coils}
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\section{Twisted Inductor Design}
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\section{Conclusion}
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\section*{Availability}
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This is version \texttt{\input{version.tex}\unskip} of this paper, generated on \today.
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% The git repository with the
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% LaTeX source for this paper as well as our data analysis and demo code can be found at:
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% \center{\url{https://git.jaseg.de/nice-coils.git}}
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\printbibliography[heading=bibintoc]
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\end{document}
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