WIP

paper/paper.tex

@@ -30,6 +30,8 @@
 \newcolumntype{P}[1]{>{\centering\arraybackslash}p{#1}}
 \newcommand{\partnum}[1]{\texttt{#1}}
 
+\newcommand{todo}{1}{\textbf{TODO}\footnote{#1}}
+
 \begin{document}
 
 \date{}
@@ -42,6 +44,51 @@ Achieving Rotation-Invariant Coupling using Multi-Layer PCB Inductors}
 
 \section{Introduction}
 
+Inertial Hardware Security Modules (IHSMs) are a novel security technology that aims at creating tamper-proof enclosures
+for servers and other information technology using readily available off-the-shelf components. At their core, IHSMs
+provide tamper detection by creating a simple tamper sensing \emph{security mesh} cage from commodity Printed Circuit
+Board (PCB) material, then spinning this cage at a high speed such that tampering with it becomes impossible.
+
+A core challenge in IHSM engineering is exchanging both power and data between the stationary protected payload of the
+IHSM and the rotating security mesh. Slip rings, which are widely used in rotating machines, are not feasible in IHSMs
+because of their limited speed capabilities and wear life. Various contactless methods have been explored in the past,
+and a previous IHSM prototype used an optical data link for its ease of implementation and high speed along with a
+photovoltaic link to supply power. In this paper, we explore an inductive wireless power transfer link as an alternative
+providing better efficiency, higher power output capability, and a more compact implementation. While WPT has been
+employed before to provide power across a continuously rotating joint, due to their high speed of rotation, IHSMs have
+unique balancing constraints that prohibit the use of large or heavy components such as large filter capacitors or
+inductors on the secondary side, making secondary voltage ripple reduction difficult. In this paper, we solve this
+problem through an optimized PCB WPT coil geometry that provides a more uniform magnetic field and lower parasitic
+capacitance, and that results in an improvement of rotation invariance of the coupling factor of a pair of coils.
+\todo{Give concrete numbers on achievements such as better parasitics and rotational invariance.} 
+We provide a theoretical analysis, simulation resuls and practical measurements for our coil geometry, as well as a set
+of Open Source, parametric scripts for the generation of arbitrary coil configurations.
+
+Wirless Power Transfer (WPT) is used in a variety of applications, and implementations exist in several orders of
+magnitude in power capability. Excluding specialty systems used for galvanic isolation, usually WPT is employed to
+transfer power from a stationary transmitter to a moveable receiver. Applications deployed in the field include phone
+charging, RFID and powering of medical implants. \todo{provide citations.} WPT systems based on capacitive coupling
+exist, but the vast majority of systems employ inductive coupling for its more compact size and higher power handling
+capability. In inductive WPT, the contactless interface between the transmitter and receiver sides of the system is a
+pair of coupled inductors. Usually, these inductors are used in resonant circuits that are tuned to have similar
+resonant frequencies.
+
+\subsection{WPT inductor coupling}
+
+In inductive WPT systems the coupling between the coils is highly dependent on the specific geometry of the coils, their
+relative positioning, as well as any nearby magnetically permeable materials. In particular the distance between
+transmitter and receiver coil is critical, and the coupling factor of a pair of coils falls off sharply as their
+distance exceeds some fraction of their physical size. Offset and cross-axis rotation both influence coupling to a
+lesser degree.
+
+\todo{Analytical expressions, and explanation how the factors mentioned above derive from those.}
+
+In most WPT systems, distance and alignment are the factors of primary concern. In IHSM applications, where power is
+transferred through a continuously rotating joint, both take only a subordinate role, as both can easily be controlled.
+Instead, we observed a surprising third factor: If the magnetostatic field generated by the coils is not axially
+symmetric, their continous rotation periodically modulates their coupling, introducing low-frequency ripple into the
+secondary-side power output. 
+
 \section{Related Work}
 \subsection{Twisted Inductors in RFIC Design}
 \subsection{Basket-Woven Air Coils}