WIP

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}