SMPC chapter work on WPT foo

chapter-smpc/chapter.tex

@@ -278,6 +278,41 @@ of our design.
 \subsection{Rotation-Invariant Envelope Power Supply}
 % Twisted Inductor paper
 
+A central engineering challenge in inertial HSMs is transferring power and data between the payload and the rotating
+mesh cage. Industrially, power and data transfer through rotating joints is usually done using slip ring assemblies. A
+slip ring consists of one or more contacts that wipe on a rotating circular surface. Industrially, metal spring contacts
+plated with hard gold or other common surface coatings are used for transferring small currents and data signals, and
+carbon brushes are used for higher currents. Slip rings are widely used in motors and other rotating machinery.
+
+For use in IHSMs, slip rings have several limitations. First, they are complex precision-machined components and thus
+are rather expensive. Beyond cost, they also have performance limitations. Generally, slip rings are most well-suited to
+slow rotation, as high rotation increases the wear of the contacts. The design target of \qty{1000}{rpm} we use in IHSMs
+are at the upper end of what commercial slip rings usually support. A third disadvantage is that they are sensitive, and
+any misalignment or contamination by dust can increase wear and cause intermittant contact.
+
+An IHSM's data link can easily be realized using optical communication. Although power transfer using light is also
+possible---and we have in fact demonstrated it in our first prototype IHSM---it comes at the disadvantage of a heavy
+rotating assembly since large solar cells are needed, and it has poor end-to-end efficiency. For the large-scale meshes
+needed in a high-performance IHSM tailored to SMPC applications, we engineered a better solution: A rotation-invariant
+inductive Wireless Power Transfer link.
+
+While Wireless Power Transfer (WPT) can be implemented in many different ways, the vast majority are variants of
+Inductive WPT, where the primary and secondary side are linked primarily through the magnetic component of the
+electromagnetic field, and coils are used as the transmitting and receiving antenna. Inductive WPT uses low frequency,
+which reduces circuit complexity, and it is well-suited for transferring high power across short distances. The
+electronic realization of a WPT link is usually similar to that of a DC/DC converter, except that in place of the
+inductor or flyback transformer, the pair of transceiver coils is used. Compared to a flyback transformer, the WPT
+link's transceiver coil pair has a lower coupling coefficient that varies with distance.
+
+A challenge in WPT links is the strong dependency between link inductor coupling coefficient and distance. In a naïve
+implementation that uses the link coils as a simple transformer, link efficiency would drop sharply with distance. To
+decrease the impact of this distance dependency, almost all WPT implementations combine the transceiver coils with
+capacitors to form a pair of tuned tank circuits that are driven like they would be in a resonant converter. Like in
+resonant converters, a variety of topologies such as series, parallel, or series-parallel LC are used for these tuning
+circuits.
+
+
+
 \subsection{Software Considerations}
 
 \subsection{Fast Zeroization of Non-Customizable Memories}