From 81ee2ad5e76620605a070ae1a2161aa4e749578c Mon Sep 17 00:00:00 2001 From: jaseg Date: Mon, 9 Sep 2024 18:05:28 +0200 Subject: [PATCH 1/2] Paper intro WIP --- paper/paper.tex | 36 ++++++++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) diff --git a/paper/paper.tex b/paper/paper.tex index 5689f14..5e7d675 100644 --- a/paper/paper.tex +++ b/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 +44,40 @@ 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. + +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 small fraction of their physical size. + \section{Related Work} \subsection{Twisted Inductors in RFIC Design} \subsection{Basket-Woven Air Coils} From 5c0eb54ace38155ba918860dd465f4fe138489bf Mon Sep 17 00:00:00 2001 From: jaseg Date: Tue, 10 Sep 2024 15:47:28 +0200 Subject: [PATCH 2/2] WIP --- paper/paper.tex | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/paper/paper.tex b/paper/paper.tex index 5e7d675..f20e686 100644 --- a/paper/paper.tex +++ b/paper/paper.tex @@ -73,10 +73,21 @@ capability. In inductive WPT, the contactless interface between the transmitter 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 small fraction of their physical size. +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}