Inductive Wireless Power Transfer for RFID & Embedded Devices: Coil Misalignment Analysis and Design

Radio frequency inductive coupling is extensively employed for wireless powering of embedded devices such as low power passive near-field RFID systems and impla nted sensors. The efficiency of low power inductive links is typically less than 1% and is charac te ised by very unfavourable coupling conditions, which can vary significantly due to coil po sition 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 betw een the external and implanted coils. The practical issues of coil misalignment and orientation an d their implications on transmission characteristics of RF links have been overlooked b y r searchers. The aim of this work is to present a novel analytical model for near-field induc tive power transfer incorporating misalignment of the RF coil system. In this thesis the influence of coil orientation, position and geometry on the link e fficiency is studied by approaching the problem from an electromagnetic perspective . In mplanted devices some degree of misalignment is inevitable between external and implanted coils du e to anatomical requirements. First two types of realistic misalignments are studied; a latera l displacement of the coils and an angular misalignment described as a tilt of the receiver co il. A l osely coupled system approximation is adopted since, for the coil dimensions and orien tations envisaged, the mutual inductance between the transmitter and receiver coils can be negle cted. Following this, formulae are derived for the magnetic field at the implanted coil when it is la era ly and angularly misaligned from the external coil and a new power transfer fun ction presented. The magnetic field solution is carried out for a number of practical antenna coil g e metries currently popular in RFID and biomedical domains, such as planar and printed squar e, 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 nearfield loosely coupled equivalent circuit for the inductive link. This allows us to introduc e a power transfer formula incorporating for the first time coil characteristics and misalignment fa ctors. This novel power transfer function allows a comparison between differe nt coil structures such as short solenoids, with air or ferromagnetic core, planar and printed sp irals with respect to power delivered at the receiver and its relative position to the transmitter. I n 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 sugges ted that can be applied on a larger scale compared to existing methods. The main advantage of this tech niqu is that it can be applied to a wide range of implementations without the limitations imposed by nume rical modeling and existing circuital methods. Consequently, the designer has the flexibility to identify the optimum coil geometry for maximum power transfer and misalignment tole ranc that suit the specifications of the application considered. This thesis conclu des by suggesting a new optimisation technique for maximum power transfer with respect to read ra nge, coil orientation, geometry and operating frequency. Finally, the limitations of this model are reiterated and possible future development of this research is discussed.