From c643d0747a2fcdca7de4481df6a3afea0100cd53 Mon Sep 17 00:00:00 2001 From: jaseg Date: Tue, 28 May 2024 18:33:26 +0200 Subject: [PATCH] Paper structure WIP --- paper/.gitignore | 7 +++ paper/paper.tex | 116 +++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 123 insertions(+) create mode 100644 paper/.gitignore diff --git a/paper/.gitignore b/paper/.gitignore new file mode 100644 index 0000000..c2548b9 --- /dev/null +++ b/paper/.gitignore @@ -0,0 +1,7 @@ +*.bbl +*.blg +*.aux +*.log +*.bcf +*.run.xml +*.out diff --git a/paper/paper.tex b/paper/paper.tex index 5689f14..acc2ee1 100644 --- a/paper/paper.tex +++ b/paper/paper.tex @@ -38,16 +38,132 @@ Achieving Rotation-Invariant Coupling using Multi-Layer PCB Inductors} \maketitle \begin{abstract} + % FIXME \end{abstract} \section{Introduction} +Inductive wireless power transfer (WPT) is a widely used technology supported by a large corpus of research literature. +% FIXME cite +While working on a novel application of Inductive wireless power transfer in a Inertial Hardware Security Module (IHSM) +as proposed by Götte and Scheuermann, % FIXME cite +we found ourselves presented with an unusual set of constraints around inductive wireless power transfer through a +rotating joint using a PCB inductor that does not yet seem to be addressed adequately in the existing literature on +inductive wireless power transfer. + +Our application poses the challenge of transferring power between a stationary and a rotating part. To reduce +manufacturing cost of both parts, and to reduce weight, and thereby inertia as well as susceptibility to vibration in +the rotating part, we decided to use inductors that are directly patterned onto the IHSM's printed circuit boards. +The primary constraint that results from this choice is a highly constrained turn count that is limited by the PCB +manufacturing processes' pattern resolution and by ohmic heating. + +We found that the limited turn count of PCB inductors results in a \emph{slightly} asymmetric field, which means that +the coupling coefficient of two such inductors oscillates at one oscillation per revolution when the inductors are +rotated on-axis, even if both inductors are perfectly coaxially aligned. + +In other inductive wireless power transfer systems, this oscillation is mitigated by one of several factors: First, for +this effect to matter in the first place, the two coils have to be rotating with respect to one another. In ferrite or +iron-cored inductors, the core shapes the magnetic field and evens out any such imperfection. In wire-wound inductors, +the (much) higher turn count and circular aspect ratio of the wires reduces this effect to almost nothing. Finally, the +output ripple caused by this oscillation can be filtered through a voltage regulator or by using a large decoupling +capacitor on the secondary side. + +While there exist a number of prior works focusing on efficient power transfer between two coils whose position relative +to one another cannot be precisely controlled as is the case in wireless phone charging systems, it is generally assumed +that the two coils remain (almost) stationary with respect to one another throughout the charging process. % FIXME cite + +There exists a small body of work on inductive power transfer through rotating joints, % FIXME cite +but here the focus lies on higher power budgets than our application requires, which often requires ferrite or iron-core +inductors. + +Our application is unique in that it requires power transfer through a joint that is constantly rotating at high speed, +while we simultaneously want to avoid heavy components on the (rotating) receiver side. (Liquid) electrolytic capacitors +cannot be used due to the large centrifugal acceleration that the rotating part experiences, and other heavy components +such as large ceramic or polymer electrolytic capacitors or ferrite-core power inductors are inadvisable since they will +exert large stresses onto the assembly due to the same centrifugal acceleration, and any imbalance caused by tolerances +in the placement of heavy components will quickly cause a strong vibration. + +\subsection{Twisted inductors} + +Applying a principle inspired by rectangular or octagonal RFIC inductor design as well as by the polygonal basket-woven +air coils used in early radio set, we propose a novel way of laying out circular PCB inductors that twists the +inductor's windings around one another using a ring of vias each on the inside and outside of the inductor's windings. +Applying some math, we show that we can layout a twisted inductor for any number of twists that is co-prime to the +inductor's turn count. + +We observe that in high-frequency applications, a moderate number of twists increases the spacing between the beginning +and end of the inductor's conductor, where the majority of the inductor's AC current flows. This decreases the parasitic +capacitance of the inductor and raises its self-resonant frequency, raising its maximum possible operating frequency and +improving its efficiency at lower operating frequencies. This is the same effect that is exploited in basket-woven +air core inductors that were commonly used in old radio sets. +% FIXME citation on this, citation on basket weaving -> It's hard to find reliable references on that. + \section{Related Work} + +\subsection{Inductive Wireless Power Transfer in Practice} + +Inductive WPT has been proposed in a large number of scenarios, each of which comese with a set of +unique constraints. When WPT is used to charge an electric toothbrush, the implementation cost of the system is +critical, while efficiency and total power output are of little concern. Mechanically, in an electric toothbrush's +charging system, the position and spacing of the transmitter and receiver coils can easily be controlled down to +millimeter precision. + +In contrast to this, wireless smartphone charging is a much more demanding application. Here, the total cost of the +system is only secondary, but the receiver's form factor is critical, and total power output as well as efficiency +become major objectives. At the same time, in wireless smartphone charging, position tolerances are very coarse, and the +two coils in the charging base and in the phone may be positioned more than a centimeter off-axis, with a gap of several +millimeters and potentially not even in parallel planes. + +Power transfer across large distances is even more of a concern in implantable medical devices. Where a wireless phone +charger must be able to bridge distances of a few millimeters, an implantable medical device might be situated +underneath several centimeter of tissue and bones. At the same time, cost is of (almost) no concern in this medical +application, which enables the use of complex manufacturing techniques, customized electronic components and exotic +materials. + +While all of the aforementioned applications transfer somewhere between a few hundred milliwatts and several watts of +power, at the other end of the spectrum there is a large body of research suggesting the use of inductive wireless power +transfer for the charging of electric vehicles (EVs). In this application, the wireless power transfer system replaces +the conventional wired charging connector, which improves the systems' user experience given the strong force required +to seat or unseat these rather large connectors, as well as the heft of the required water-cooled cables. In this +application, size is of (almost) no concern, but at several kilowatt up to dozens or even a hundred kilowatt, the +transferred power is enormous and consequentially efficiency becomes of utmost importance. When charging an EV at a +rate of 30 kW, an efficiency improvement of just $0.1\%$ corresponds to a reduction in power dissipation of 30 W. +Besides the monetary cost of the power lost this way, each small improvement enables a reduction in size of heat sinks +and other cooling components, which directly translates to a decrease in cost. + \subsection{Twisted Inductors in RFIC Design} \subsection{Basket-Woven Air Coils} +\subsection{Air-Core Inductors for Inductive Power Transfer} +\subsection{Ferrite or Iron-Core Inductors for Inductive Power Transfer} \section{Twisted Inductor Design} +\subsection{From Spiral to Twisted Inductor} + +\subsubsection{Ohmic Resistance} + +\subsubsection{Inductance} + +\subsection{CAD Integration} + +\section{FEM Simulation} + +\subsection{Ohmic Resistance} + +\subsection{Inductance} + +\subsection{Parasitic Capacitance and Self-Resonant Frequency} + +\subsection{Coupling} + +\section{Experimental Validation} + +\subsection{Inductance and Parasitic Capacitance} + +\subsection{Self-Resonant Frequency} + +\subsection{Coupling} + \section{Conclusion} \section*{Availability}