Inductive Wireless Power Transfer for RFID & Embedded Devices: Coil Misalignment Analysis and Design
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Date
2008Author
Fotopoulou, Kyriaki
Metadata
Abstract
Radio frequency inductive coupling is extensively employed for wireless powering of embedded
devices such as low power passive near-field RFID systems and implanted sensors. The
efficiency of low power inductive links is typically less than 1%and is characterised by very unfavourable
coupling conditions, which can vary significantly due to coil position and geometry.
Although, a considerable volume of knowledge is available on this topic, most of the existing
research is focused on the circuital modeling of the transformer action between the external
and implanted coils. The practical issues of coil misalignment and orientation and their implications
on transmission characteristics of RF links have been overlooked by researchers. The
aim of this work is to present a novel analytical model for near-field inductive power transfer
incorporating misalignment of the RF coil system.
In this thesis the influence of coil orientation, position and geometry on the link efficiency is
studied by approaching the problem from an electromagnetic perspective. In implanted devices
some degree of misalignment is inevitable between external and implanted coils due to anatomical
requirements. First two types of realistic misalignments are studied; a lateral displacement
of the coils and an angular misalignment described as a tilt of the receiver coil. A loosely coupled
system approximation is adopted since, for the coil dimensions and orientations envisaged,
the mutual inductance between the transmitter and receiver coils can be neglected. Following
this, formulae are derived for the magnetic field at the implanted coil when it is laterally and
angularly misaligned from the external coil and a new power transfer function presented. The
magnetic field solution is carried out for a number of practical antenna coil geometries currently
popular in RFID and biomedical domains, such as planar and printed square, and circular spirals
as well as conventional air-cored and ferromagnetic solenoids. In the second phase of
this thesis, the results from the electromagnetic modeling are embodied in a near-field loosely
coupled equivalent circuit for the inductive link. This allows us to introduce a power transfer
formula incorporating for the first time coil characteristics and misalignment factors.
This novel power transfer function allows a comparison between different coil structures such
as short solenoids, with air or ferromagnetic core, planar and printed spirals with respect to
power delivered at the receiver and its relative position to the transmitter. In the final stage of
this work, the experimental verification of the model shows close agreement with the theoretical
predictions. Using this analysis a formal design procedure is suggested that can be applied
on a larger scale compared to existing methods. The main advantage of this technique is that it
can be applied to a wide range of implementations without the limitations imposed by numerical
modeling and existing circuital methods. Consequently, the designer has the flexibility to
identify the optimum coil geometry for maximum power transfer and misalignment tolerance
that suit the specifications of the application considered. This thesis concludes by suggesting a
new optimisation technique for maximum power transfer with respect to read range, coil orientation,
geometry and operating frequency. Finally, the limitations of this model are reiterated
and possible future development of this research is discussed.