Low-power near-field magnetic wireless energy transfer links: A review of architectures and design approaches

Renewable and Sustainable Energy Reviews 77 (2017) 486–505

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Low-power near-field magnetic wireless energy transfer links: A review of architectures and design approaches

MARK

⁎

Akaa Agbaeze Etenga, Sharul Kamal Abdul Rahima, , Chee Yen Leowa, Suhanya Jayaprakasama, Beng Wah Chewb a b

Wireless Communication Centre, Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia Intel Microelectronics, Halaman Kampung Jawa, 11900, Penang, Malaysia

A R T I C L E I N F O

A BS T RAC T

Keywords: Wireless energy transfer Near-field Link architectures Design approaches

The elimination of physical conductors as media for power transmission is an important step towards reducing the bulk of waste material generated when electronic gadgets are disposed of. In addition, the increasing deployment of low-power autonomous electronics in less accessible environments has provided an impetus for the development of wireless energy transfer alternatives to wired power delivery. This paper presents a review of near-field magnetic wireless energy transfer link architectures, and design approaches for realizing performance objectives specifically suited to low-power deployments. First, the paper provides a brief history of low-power magnetic wireless energy transfer development. This is followed by a fundamental description of the spatial regions surrounding an electromagnetic field source. Then, the paper presents a summary of basic topologies of magnetic wireless energy transfer link implementations, while emphasizing their distinctive features. Design approaches, which enable the realization of various link performance criteria, are also discussed. Finally, this paper highlights emergent wireless energy transfer link design trends inspired by communication network paradigms.

1. Introduction In recent times, wireless energy transfer has been the focus of considerable research interest. This technology is directed at eliminating the need for physical conductors for the transmission of electrical power. Apart from the physical convenience it provides, the elimination of conducting cables from gadgets leads to a drastic reduction the bulk of electronic waste at the end of life of such gadgets. Also, wireless energy transfer is the basis for wireless charging, further giving impetus for the replacement of hazardous disposable batteries with more environmentally sustainable rechargeable power sources. The origins of wireless energy transfer derive from pioneering contributions to the intentional transmission and reception of electromagnetic waves, made at the dawn of the last century [1,2]. Wireless electromagnetic transmission has since developed into being the core enabler of the current pervasive communications ecosystem. In contrast, there has been, until quite recently, a relative lull in the development of power-delivery systems using wireless energy transfer techniques. Rekindled contemporary interest into wireless energy transfer can be linked to two motivations: a pressing need for

alternative energy sources, and a significant upsurge in the deployment of autonomous electronic devices. The latter motive is linked to the fact that autonomous electronics are increasingly being used in scenarios for which traditional wired power delivery implementations are not only inconvenient but quite impractical. A typical case in point is the delivery of electrical power to hermetically sealed transcutaneous medical implants [3–5]. There are broadly two wireless energy transfer scenarios, namely farand near-field wireless energy transfer. Far-field schemes are based on the transmission and reception of radiated electromagnetic waves. Initially, terrestrial far-field wireless energy transfer research was geared towards intentional long-range transmission of electromagnetic waves at radio frequencies [6,7]. Current research for terrestrial applications, however, tilts more towards the harvesting of ambient microwave radiation for lowpower applications [8]. Near-field wireless energy transfer schemes, on the other hand, involve much shorter-range non-radiative energy interactions within electromagnetic fields. Energy may be transferred either through electrical or magnetic fields in the vicinity of excited field sources. However, due to the relatively limited magnetic interaction occurring between extraneous materials in the environment, magnetic energy transfer is

⁎

Corresponding author. E-mail addresses: [email protected] (A.A. Eteng), [email protected] (S.K.A. Rahim), [email protected] (C.Y. Leow), [email protected] (S. Jayaprakasam), [email protected] (B.W. Chew). http://dx.doi.org/10.1016/j.rser.2017.04.051 Received 26 February 2016; Received in revised form 20 December 2016; Accepted 19 April 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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currently more prevalent [9,10], and is the focus of this review. Applications of near-field wireless energy transfer range from highpower (kilowatt level) electric vehicle applications [11–13], to low-power (between microwatt and watt levels) consumer electronics [14–17] and autonomous sensors [18–27]. Most low-power energy transfer links are implemented to operate within either the low-frequency or the highfrequency spectral designations. A majority of low-frequency implementations typically operate within 100–400 kHz, while high frequency implementations are usually found within 1–10 MHz [28]. Depending on the requirements of the specific power-delivery application, a transmitting terminal may receive its excitation from a power source through a number of intervening sub-systems, such as oscillators, power amplifiers, and filters. Similarly, the received power from the receiver terminal may be passed through multiple stages, such as rectification, filtering, and power amplification, among others, before it is used to drive a connected load. In spite of the current diversity in the implementation of these energy transfer systems, there are broadly consistent patterns in the implementation of the wireless energy transfer link, namely the topological arrangement of coupled conductor loops. There are a number of available surveys, which review various aspects of near-field magnetic wireless energy transfer [28–34]. Lu et al. [28] focuses on the major wireless charging techniques, the available international standards and specifications, and their commercial implementations. Carvalho et al. [33], on the other hand, provides an exhaustive survey of current wireless power delivery research and development activities in Europe. Barman et al. [29], Hui et al. [30], and Wei et al. [31], on their part, focus on the magnetic resonance energy transfer technique for enabling mid-range energy transfers. In contrast, Mayordomo et al. [32,34] are concerned with short-range energy transfer schemes facilitated by magnetic induction. From these system-oriented literature surveys, one can sense a distinction between magnetic resonance-based wireless energy transfer, and magnetic induction-based methods. Ricketts et al. [35], however, has experimentally demonstrated that while there are differences in the link architectures of the these two near-field wireless transfer schemes, the fundamental physics behind both schemes are, in fact, similar. This article, therefore, undertakes a comprehensive survey of various architectural implementations of near-field magnetic wireless energy transfer links suited to low-power applications. The paper adopts a holistic view of near-field magnetic wireless energy transfer to specify the topological arrangements of coupled terminals, and the design approaches to achieve performance goals. The presented discourse shows that, topological differences aside, low-power nearfield magnetic wireless energy transfer links are generally designed with broadly similar performance considerations. Following this introductory section, this paper presents a brief history of low-power magnetic wireless energy transfer research and development. Next, a general outline of the spatial field regions surrounding a source of electromagnetic fields and waves is provided in Section 3. Section 4 describes the various near-field wireless energy transfer link architectures provided in the literature, with an emphasis on the topological arrangement of transmitter and receiver conductor loops. The design approaches employed to realize performance requirements of the various link architectures are discussed in Section 5. Section 6 then deals with trends in link design, driven by emergent network applications of near-field wireless energy transfer. Finally, the survey is concluded in Section 7. Succinct analytical descriptions of link architectures and design approaches have been provided in this paper, where necessary. Variables used in the analytical descriptions are summarized in Table 1.

Table 1 Variables used in analytical expressions. Variable

Meaning

a ω μ ε η θ ϕ Γ κ ηlink

Loop radius Angular frequency Permeability Permittivity Characteristic impedance of free space Elevation angle Azimuthal angle Intrinsic decay rate Coupling rate Link transfer efficiency

ηs − l

Source-to-load power transfer efficiency

BW C Dr d E Ediss Epeak f G H I k k12 l L M12 Q R r PL Vs

Bandwidth Capacitance Diameter of receive coil Axial separation distance between a pair of coils. Electric field strength Dissipated energy Peak energy Frequency Linear voltage gain Magnetic field strength Current Wave number Coupling coefficient between a pair of coils, numbered 1 and 2. Length of conductor section Self-inductance Mutual inductance between a pair of coils, numbered 1 and 2 Q-factor Resistance Distance from a plane Power delivered to load Source voltage

electricity and magnetism. Further work by Maxwell, published in 1873, gave further insight into how magnetic and electrical fields were influenced by each other, thereby providing a unified perspective on electromagnetism [36]. These early theoretical endeavours paved the way for practical demonstrations of wireless energy transfer. Notably, Heinrich Hertz’s 1898 demonstration of the transmission of electricity over an air-gap provided tangible proof of the existence of electromagnetic radiation. Furthermore, the invention of the induction machine by Nikola Tesla [34], showed that it was possible to couple significant amounts of energy between non-contacting parts, through magnetic fields. In later years, however, Tesla’s focus shifted to longdistance transmission of electrical power [2]. Further development in low-power wireless energy transfer was made possible by advances in electronics. By the 1960s, inductive coupling was being successfully harnessed for short-range wireless energy transfer in low-power medical applications [28]. At about the same period, there was burgeoning research into radio frequency identification (RFID) systems for electronic article surveillance (EAS). This led to significant progress in the development of passive RFID systems, based on low-power inductive coupling wireless energy transfer, and backscatter in the 1970s [37]. The proliferation of portable electronic devices in the 1990s provided the drive for research into wireless charging applications of wireless energy transfer. Mid-range magnetic wireless energy transfer was demonstrated by Kurs et al. [9], leading to a proposed Witricity wireless charging technology. The standardization of low-power wireless charging technologies has since been the focus of three major industry consortia. The Wireless Power Consortium (WPC), established in 2008, developed the Qi charging specification, which was quickly adopted in many mobile phone models. The Power Matters Alliance (PMA), founded in 2012, has developed a competing specification aimed at more generic non-mobile phone applications. More recently, the Alliance for Wireless Power (A4WP) was founded, which promotes

2. Brief history of low-power magnetic wireless energy transfer development The theoretical foundations for wireless energy transfer were established through the formalization of physical laws by Ampere, Biot-Savart and Faraday, which model basic relationships between 487

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⎛ 2 jωμa 2 2 ⎞ −jkr ⎟e , cos θ ⎜ 2 + 4 jωμr 3 ⎠ ⎝ ηr ⎛ jωε jωμa 2 1 1 ⎞ −jkr ⎟e , Hθ = sin θ ⎜ + 2 + 4 ηr jωμr 3 ⎠ ⎝ r Hϕ = 0.

the magnetic resonance-based Rezence specification [38]. In 2015, however, the PMA and A4WP merged to form the AirFuel Alliance. The various wireless charging specifications provided by these industry consortia have been adopted in numerous electronic products currently available in the market.

Hr =

The excited electric field components from this structure are:

3. Spatial regions around an electromagnetic field source

Er = 0, Eθ = 0,

Classical electromagnetic theory provides a description of the electromagnetic interactions occurring in the vicinity of a currentcarrying conductor. The description provided in [39,40] begins with an assumption of a current element at the origin of a spherical coordinate system, shown in Fig. 1. The current element is oriented in the z direction, while its length l and cross-sectional radius a both satisfy the condition a < < l < < λ , with λ being the wavelength of the electromagnetic wave. In addition, equal and opposite charges are assumed to exist at both ends of the element, effectively constituting an electrical dipole. The magnetic field components at any point P , at a distance r can be written as [39]:

Eϕ = −

⎛ jk I0l 1⎞ sin θ ⎜ + 2 ⎟e−jkr , ⎝r 4π r ⎠

(1)

where the wave number k = ω / με at the frequency ω , and μ and ε are the permeability and permittivity of free-space, respectively. Similarly, the electric field components are [39]:

⎛ 2η I0l 2 ⎞ −jkr ⎟e , cos θ ⎜ 2 + 4π jωεr 3 ⎠ ⎝r ⎛ jωμ Il η 1 ⎞ −jkr ⎟e , Eθ = 0 sin θ ⎜ + 2 + 4π r jωεr 3 ⎠ ⎝ r

Er =

Eϕ = 0.

⎛ jk jωI0a 2 1⎞ sin θ ⎜ + 2 ⎟e−jkr . ⎝r 4 r ⎠

(4)

The excited fields from the electric and magnetic dipoles, described in (1)–(4), comprise of three distinct terms, namely terms that vary with 1/ r , 1/ r 2 , and 1/ r 3. At the point where the range radius r = 1/ k , the fields consist of components with an inverse dependence on r 2. At greater distances from the source, r > > 1/ k , and the only significant terms are those with a 1/ r dependence, and represent radiation. However, with r < 1/ k , the fields are dominated by the terms with 1/ r 2 and 1/ r 3 dependencies, while the radiation is negligible. Within close proximity of the field source, however, r < < 1/ k , and the field varies by 1/ r 3. This resulting field is usually described as quasistatic [39]. Kim and Ling [41] experimentally demonstrates that the 1/ r , 1/ r 2 , and 1/ r 3 terms separately give rise to three spatial regions around an electromagnetic source, with distinct power profiles. The region with a dominant radiation term is the far-field region, and has a 1/ r 2 power profile. Dominance of the two non-radiative terms occurs within the near-field, with 1/ r 4 and quasistatic 1/ r 6 power profiles. While radiative wireless energy transfer is facilitated in the far-field region, mid- and short-range non-radiative wireless energy transfers are facilitated within the 1/ r 4 and 1/ r 6 power profile near-field regions, respectively. Unlike energy transfer links based on the electric field, electromagnetic sources and sinks used in near-field magnetic energy transfer links tend to more closely resemble magnetic dipoles. Consequently, the excited axial field components, resolved along θ and r , are essentially magnetic. The current-loop structures required to excite axial magnetic fields are realized in various configurations, such as spirals [42], helices [43], printed spiral coils [44], and solenoids [45], among others. However, in subsequent sections of this paper, these structures are generically referred to as coils.

Hr = 0, Hθ = 0, Hϕ =

(3)

(2)

The characteristic impedance of free-space is defined as η = μ / ε . The dual of the current element in Fig. 1 is the closed circular loop of radius a , which is illustrated in Fig. 2. This structure is effectively a magnetic dipole, and the excited magnetic field components can be written as [39,40]:

4. Near-field magnetic wireless energy transfer link architectures In order to establish an energy transfer link, coils are typically

Fig. 1. Current element at origin of spherical coordinate system [39].

Fig. 2. Current loop at origin of spherical coordinate system.

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comprising of a single transmitter coil, magnetically coupled to a single receiver coil. This link configuration has variously been described as being based on the ‘transformer concept’ [48–55], which lends itself to a tractable circuit theoretic modelling approach [56–59]. There are two main implementations of direct-fed two-coil links. The older implementation is based on basic electromagnetic induction, as described by Biot-Savart and Faraday’s laws [39], enabling energy transfer only at very short distances [10]. The more recent implementation employs coils tuned using lumped elements, such that the energy transfer link operates at resonance. This improves the range at which an adequate link transfer efficiency can be obtained [30]. There are various combinations of series or parallel capacitive compensation arrangements which can be applied to achieve resonance and impedance match conditions in resonant direct-fed two-coil links [60–66]. The circuit shown in Fig. 4, however, is a generic model based on a series-parallel compensation topology. A series compensating capacitance C1 is used in the transmitter circuit, and a parallel compensating capacitance C2 is employed in the receiver circuit. In the circuit model, R1, R2, L1 and L2 are the resistances and inductances of the transmitter and receiver coils, respectively. The mutual inductance between both coupled coils is represented by M12, while k12 = M12 / L1L 2 is the normalization of the mutual inductance as a coupling coefficient.

arranged in such a manner as to facilitate coupling interaction between the axial components of the magnetic field. Fig. 3 is a classification of near-field magnetic wireless energy transfer link architectures arising from various coil arrangements. The illustrated link architectures are separately described in the following sub-sections. 4.1. Dual-terminal topologies Dual-terminal magnetic near-field wireless energy transfer links basically engage two terminals, coupling a single transmitter terminal to a single receiver terminal. These dual-terminal links are available in various topologies, which can broadly be classified in terms of the intervening distance between coupled terminals. Short-range links are sometimes described as links operating within a few centimetres while mid-range links operate within a range of tens of centimetres to a few meters [42,46]. An alternative description of range can be made in terms of the relationship between the energy transfer distance, and the dimensions of the coupled coils. Consequently, mid-range links could be understood as having operating distances greater than the dimensions of the coupled coils, while short-range links operate at distances less than the coil dimensions [10,30]. A third classification of dualterminal link topologies is with regards to the method by which power is fed and extracted from the link, leading to direct-fed and indirect-fed links [47]. Direct-fed links have physical wired connections between the transmitter coil and the power source circuit, and between the receiver coil and the load circuit. Indirect-fed links, on the other hand, employ transmitter and receiver coils inductively coupled to the power source circuit and load circuit through source and load loops, respectively [47].

4.1.2. Indirect-fed links Indirect-fed links are typically used to facilitate mid-range energy transfers. Often referred to as magnetic resonance links [9,10,29– 31,67–72], their origins can be traced to a series of experiments at the Massachusetts Institute of Technology (MIT) [9,10]. These experiments established the feasibility of non-radiative power transfer using a pair of self-resonant coils over distances a few times greater than the characteristic sizes of the coupled resonators. Self-resonant coils achieve resonance based on the interaction between coil distributed

4.1.1. Direct-fed links Direct-fed links are typically used to facilitate short-range wireless energy transfers. The most basic direct-fed link is the two-coil link

Fig. 3. Near-field magnetic wireless energy transfer link architectures.

Fig. 4. Direct-fed two-coil energy transfer link, (a) schematic model; (b) simplified series-parallel compensated circuit model.

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are examples of Single-Input Single-Output (SISO) links. Emergent wireless energy transfer applications have necessitated the development of alternative multi-terminal topologies, where multiple transmitters and multiple receivers are simultaneously involved in a wireless energy transfer interaction [80]. These multiple transmitter and receiver terminals could be based on either direct-fed or indirect-fed arrangements. Single-Input Multiple-Output (SIMO) links typically consist of a single transmitter terminal interacting with multiple receiver terminals simultaneously, as illustrated in Fig. 7. The SIMO concept is currently implemented in a number of wireless charger designs, which are able to simultaneously charge multiple devices [16,17,67,81–83]. Multi-Input Single-Output (MISO) link topologies, on the other hand, consist of an array of multiple transmitter coils, which are usually in interaction with a single receiver coil [84], as implied by Fig. 8. MISO link topologies are

inductance and capacitance. There are broadly two implementations of indirect-fed links. Fourcoil indirect-fed links are established by magnetically coupling the transmitter and receiver coils to their associated power source circuits and load circuits through source and load coils, respectively. Three-coil indirect-fed links, on the other hand, either have the receiver coil physically connected to the load circuit [73–75], or have the transmitter coil physically connected to the source circuit [76–79]. In a sense, three-coil links can be considered as degenerate forms of four-coil links. Figs. 5 and 6 illustrate the four-coil, and three-coil indirect fed links, respectively. 4.2. Multi-terminal links The dual-terminal topologies discussed in the previous subsection

Fig. 5. Indirect-fed four-coil energy transfer link; (a) schematic model; (b) simplified circuit model.

Fig. 6. Indirect-fed three-coil energy transfer link; (a) schematic model with indirect-fed transmitter coil; (b) schematic model with indirect-fed receiver coil (c) simplified circuit model.

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Fig. 7. Simplified circuit model of three-coil SIMO link topology. Fig. 9. Simplified circuit model of a four-coil MIMO link topology.

multiple transmitter coils are focused on a single receiver coil. In this case, up to 40 cm increase in operating distance was reported, while still allowing for a flexible orientation of the receiver coil. The MagMIMO technique has subsequently been extended to a full MIMO near-field wireless energy transfer system implementation, known as MultiSpot [88]. This technology emulates the operation of a miniature WiFi hotspot, and has been demonstrated to enable the support of simultaneous wireless charging of up to 6 heterogeneous devices at distances up to 50 cm.

4.3. Relay links Relay links are essentially extensions of the previously described dual- and multi-terminal link topologies in which the transmitter and receiver terminals are magnetically coupled to each other through intermediate resonators as illustrated in Fig. 10. One of the earliest investigations of relay links is made in [89], where a three-coil arrangement is investigated. The topology was one in which an intermediate resonant coil was placed in the energy transfer link, equidistant from the resonant transmitter and receiver coils. Inspired by a quantum interference phenomenon—Electromagnetically Induced Transparency (EIT), the study attributes the energy transfer mechanism to an adiabatic evolution of an instantaneous eigenstate of the three-coil link. This link configuration differs from the earlier discussed three-coil indirect-fed link in the sense that the intermediate coil is designed to equivalently couple strongly to both the transmitter and receiver coils. More recently, metamaterial resonators have been investigated as implementations of intermediate resonators. In [90–92] numerical and experimental studies are reported, which demonstrate a significant improvement in the link transfer efficiency of an indirect-fed link through the inclusion of a metamaterial slab between a pair of coupled transmitter and receiver coils. This improvement is made possible by the amplification of the evanescent near-field surrounding each coupled resonator. Typically, electromagnetic fields are confined within resonators, and the field strength drops off as one moves further away from the resonators. Energy is transferred over a greater distance if the tails of evanescent fields from a pair of resonators are coupled. Negative permittivity and permeability metamaterial slabs can be used

Fig. 8. Simplified model of a three-coil MISO transcutaneous energy transfer link.

often employed to continuously energize dynamic receivers [85]. An example of a MISO link design is reported in [86], which has been employed in an experimental system for focused wireless power delivery to multiple randomly moving objects [23]. The MISO concept has also been applied in an indirect-fed link design to develop a matbased system for wireless power delivery to biomedical implants [20]. Multiple-Input Multiple-Output (MIMO) topologies extend the dual-terminal energy transfer link concept to multi-node wireless energy transfer networks [80], as illustrated in Fig. 9. A reported recent design of MIMO energy transfer is the MagMIMO system [87], which employs a field shaping approach, much akin to beamforming in MIMO antenna systems, for focused power delivery. However, it should be noted that the deployment of the MagMIMO discussed in [87] is essentially a MISO topology, where the magnetic fields of 491

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Fig. 10. Simplified circuit model of an n-coil relay link, with an assumption of nearest neighbour coupling.

to focus and amplify the evanescent fields from the pair of resonators, thereby increasing the distance at which significant coupling can be maintained. In reality, metamaterials with both negative permittivity and permeability require complicated fabrication methods. However, near-field magnetic wireless energy transfer links require negative permeability alone to facilitate the amplification of evanescent magnetic fields. In [92], the intermediate metamaterial slab is composed of two-sided square spirals. The magnetoinductive waveguide concept [93,94] has also inspired the development of relay links consisting of multiple intermediate resonators [30,92,95–98]. Typically, these links consist of a series of resonators placed in sequence to guide the magnetic field over a distance in a particular direction, and at a particular frequency [99]. Planar implementations of relay links place a series of coils lying in the same plane, and in proximity with one another [92,100,101]. In [92,101], planar relay implementations have been used to support energy transfer to non-stationary receiver terminals. Axial implementations, on the other hand, place the relay coils either in axial alignment or at some degree of axial misalignment with one another. Different topologies of axial implementations of relay links have been demonstrated, namely straight, curved, branched [95,97], and circular chains [97]. Branched chain relays can either be used to split energy from a single transmitter terminal to energize two receiver terminals, or to combine energy from two transmitter terminals to energize a single receiver terminal [95]. The resulting relays from these branched chains can be regarded as extensions of SIMO and MISO link topologies, respectively. A combination of a MIMO topology with a relay arrangement, using a single intermediate resonator between the transmitters and receivers, has also been described in [102]. In this example, the authors realize a MIMO link consisting of two transmitter and receiver terminals each. The inclusion of the intermediate resonator between the multiple coupled terminals is reported to increase the overall system transfer efficiency from 8.42% to 45.94%. Table 2 contains a summary of the salient features of the surveyed wireless energy transfer link architectures.

designed link is critical. In applications in which coupled coils are susceptible to changes in their spatial positions, it is also necessary for the designed links to maintain uniform transfer efficiency levels irrespective of coil positioning. On the other hand, there are some instances where the transfer efficiency, per se, is not the most critical consideration. The critical design criterion may be the actual power level delivered to a connected load, or even the sizes of the coupled coils. Design approaches to achieve these various design criteria are discussed in the following sub-sections.

5.1. Optimal transfer efficiency The link transfer efficiency is one of the more widely investigated performance characterizations of near-field wireless energy transfer links. It is typically defined as the ratio of the output power sensed at the port of the receiver coil to the power present at the input of the transmitter coil [60]. A commonly used analysis to determine the link transfer efficiency is based on reflected impedance [57,76], and is briefly summarized below. Fig. 11 modifies the circuit earlier presented in Fig. 4, to enable the application of the reflected impedance concept in the link analysis. The link transfer efficiency is enhanced if both coupled coils are tuned to a single resonance frequency, defined by ω = 1/ L1C1 = 1/ L 2C2 . In this condition, the coupled coils can be characterized in terms of their quality factors, namely

Q1 =

ωL 2 ωL1 ; Q2 = . R2 R1

(5)

The reflected resistance at resonance is

Rref = k12 2ωL1Q2L ,

(6)

where, from [103]

Q2L =

Rp ωL 2

=

Q2QL , Q2 + QL

(7)

and

5. Design approaches

Rp =

Near-field magnetic wireless energy transfer link are designed to fulfil certain performance criteria, which depend on the specific application. Quite often, the transfer efficiency delivered by the

Q2 2R2RL Q2 2R2 + RL

.

(8)

Similarly, the reflected capacitance is given by

Table 2 Comparison of features of near-field wireless energy transfer link architectures. Link topology

Features

Typical applications

Direct-fed

Two-terminal links, with physical connections between coils and their corresponding circuits. Two-terminal links without physical connection between coils and corresponding circuits. A hybrid arrangement could have one direct-fed terminal coupled to an indirect-fed terminal. Links consisting of multiple transmitter or/and multiple receiver terminals

Short-range (few millimetres to a few centimetres) wireless charging of electronic devices. Mid-range (few centimetres to few meters) power delivery; midrange wireless charging of electronic devices.

Indirect-fed

Multi-Terminal Relay

Two-terminal links with intermediate resonators placed between the terminals. Feeding arrangement can be considered as direct or indirect.

492

Simultaneous charging of multiple devices; emerging energy transfer networks. Enhanced mid-range (few meters) power delivery; non-line-ofsight power delivery; merging and splitting of energy paths.

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transfer efficiency in indirect-fed link configurations are based on an implicit assumption that the coils in the link are arranged in a manner such that only the coupling between each coil and the next coil in sequence is significant [9,76]. For the four-coil indirect link, the transfer efficiency is provided by ηlink =

k122k232k342Q1Q22Q32Q4L 2 , 2 (1 + k34 Q3Q4L + k23 Q2Q3)[(1 + k342Q3Q4L )(1 + k122Q1Q2) + k232Q2Q3]QL 2

(14)

where

Q4L =

Q1 =

Q4QL , Q4 + QL

ωL1 , R1

(15)

Q2 =

ωL 2 , R2

Q3 =

ωL 3 , R3

ωL4 , R4

Q4 =

(16)

and

M12 , L1L 2

k12 =

k 23 =

M23 L 2L 3

,

k 34 =

M34 L 3L4

. (17)

With the elimination of either the driver or load coil (see Fig. 5), the four-coil link configuration reduces to a three-coil configuration, for which the transfer efficiency is given by [76]

ηlink =

k12 2k 232Q1Q2 2Q3L 2 2

(1 + k12 Q1Q2 + k 232Q2Q3L )(1 + k 232Q2Q3L )QL

.

(18)

Transfer efficiency expressions for n resonator relay links can be deduced in a similar manner based on Kirchhoff’s voltage law description of such links, namely ⎛ ⎛ ⎞ ⎜ R1 + j ⎜ωL1 − 1 ⎟ ωC1 ⎠ ⎜ ⎝ ⎜ ⎜ jωM21 R1 + ⎜ ⎜ ⋮ ⎜ ⎜ ⎜ jωM2(n −1) ⎜ ⎜ ⎜ jωM2n ⎜ ⎝

Fig. 11. Application of the reflected impedance concept to a direct-fed two-coil link.

Cref

1 = 2 2 . ω k12 L1

(9)

As shown in [76], the reactance due to Cref is counteracted at resonance by the reactance of k12 2L1, while L1 and L 2 resonate with C1 and C2 , respectively, Consequently, R1 and Rref form a voltage divider. The transferred power is delivered through Rref , part of which is dissipated across R2 , and the other part delivered to the load RL . The link transfer efficiency can therefore be defined as

Q2 2R2Rref

ηlink =

(Rref

⋅

(10)

where

QL =

RL . ωL 2

(11)

An optimum load resistance needed to maximize the link transfer efficiency can be obtained by equating the derivative of (10), with respect to QL , to zero. Using the definition of QL in (11), this leads to

RL opt =

ωL 2Q2 1 + k12 2Q1Q2

. (12)

The assumption of an optimal loading condition, which is equivalent to a receiver coil impedance match condition, enables a consideration of the link performance in terms of the link elements, independent of the impact of the load resistance. Consequently, the link transfer efficiency defined in (10) reduces to [60,104]

k 2Q1Q2

ηlink = (1 +

1 + k 2Q1Q2 )

2

jωM13 …

⎛ 1 ⎞ j ⎜ωL2 − ⎟ jωM23 … ωC2 ⎠ ⎝ ⋮ ⋮ ⋱ …

…

…

…

…

…

⎞ ⎟ ⎟ ⎟ ⎟ jωM2n … ⎟ ⎟ ⋮ ⋮ ⎟ ⎟ ⎛ ⎞ 1 ⎟ Rn −1 + j ⎜ωL n −1 − jωM(n −1)n ⎟ ωCn −1 ⎠ ⎟ ⎝ ⎟ ⎛ ⎞ 1 ⎟ jωM(n −1)n RL + Rn + j ⎜ωL n − ⎟⎟ ωCn ⎠ ⎠ ⎝

⎛ I1 ⎞ ⎛V ⎞ ⎜ ⎟ ⎜ 1⎟ ⎜ I2 ⎟ ⎜ 0 ⎟ ⎜ ⋮ ⎟ = ⎜0⎟ ⎜I ⎟ ⎜ ⎟ ⎜ n −1⎟ ⎜ 0 ⎟ ⎝ In ⎠ ⎝ 0 ⎠

…

jωM1n

(19)

Assuming optimal loading conditions, and nearest-neighbour coupling interactions, the transfer efficiency of a typical dual-terminal relay link with n − 2 intermediate resonators is a function of n − 1 coupling coefficients and n coil Q-factors. Analytic expressions for the transfer efficiency of SIMO, MISO, and MIMO link configurations are typically more complex, as they need to account for the coupling interactions among transmitter coils, receiver coils, and between the various transmitter and receiver coils. For these multi-terminal link architectures, the simplifying assumption of a nearest-neighbour coupling interaction no longer holds. Generally, for a multi-terminal link with m transmitter terminals and n receiver terminals, the transfer efficiency is a function of m + n coil Q-factors, and mn + (m − 1)(n − 1)coupling coefficients. An alternative description of the energy interaction between a pair resonant coils, is provided by coupled-mode theory (CMT) [9,10,76]. According to the CMT, the field amplitudes a1(t ) and a 2(t ) from a pair of resonant coils, 1 and 2, satisfy the linear system [10,76]:

k12 2Q1Q2L

Q = ⋅ 2L , + R1)(Q2 2R2 + RL ) 1 + k12 2Q1Q2L QL

jωM12

a1̇ (t ) = − (jω1 + Γ1)a1(t ) + jκa 2(t ) + F1(t ) , a 2̇ (t ) = − (jω2 + Γ2 + ΓL )a 2(t ) + jκa1(t )

(20)

where the rate of intrinsic decay due to resistive and radiative losses in both coils are Γ1 and Γ2 . ΓL is the intrinsic decay rate due to the resistive load. The eigenfrequencies of both coils are ω1 and ω2 , and κ refers to the coupling rate. If the pair of coils operate at the same resonance condition, so that ω1 = ω2 , the link transfer efficiency can be written as [9]

. (13)

The analysis provided in [76] shows that the basic reflected impedance analysis can be expanded to determine the transfer efficiency of indirect-fed links. The analytical expressions for link 493

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ηlink =

ΓL κ 2 Γ2 ΓΓ 12

⎡ κ2 ⎛ ⎢ ΓΓ ⎜1 + ⎣ 1 2⎝

⎡⎛ + ⎢⎜1 + ⎦ ⎢⎣⎝ ⎤

ΓL ⎞ ⎟⎥ Γ2 ⎠

ΓL ⎞ ⎟ Γ2 ⎠

2⎤

⎥ ⎥⎦

matching topologies can be employed, with the essential tradeoff being between complexity and performance. Indirect-fed links employing self-resonant coils depend on the interplay between the distributed capacitance and coil self-inductances to achieve resonance and impedance matching [9]. It is also increasingly common to have parallel capacitances connected to the transmitter and receiver coils [10], a technique that is later shown to reduce leakage field emissions from the energy transfer link.

. (21)

The system described by (20) can be expanded in a linear manner to cater for dual-terminal link interactions employing more than more than 2 coils. The resulting expression for link transfer efficiency for these interactions, will similarly be functions of inter-coil coupling rates, and coil intrinsic decay rates. Although the CMT provides an analytical framework to describe the wireless energy transfer interaction, it is generally not well suited for use as a practical synthesis tool to dimension physical link parameters, such as coil geometries or separation distances, for optimal energy transfer [80]. Consequently, the bulk of link synthesis methodologies are based on circuit theory descriptions of wireless energy transfer link performance. The earlier described circuit theoretic expressions of link transfer efficiency for direct- and indirect-fed energy transfer links show that the efficiency of link energy transfer depends on coil Q-factors, and the coupling coefficients between the coupled coils. Under impedance matched conditions, the product of the coupling coefficient and the geometric mean of coil Q-factors is a figure-of-merit for energy transfer [60]. Inevitably, therefore, strategies to achieve high transfer efficiency levels revolve around the design of high Q-factor coils, and methods to achieve high coupling coefficients [15].

5.1.2. Realizing high Q-factor coils Q-factors are a figure-of-merit describing the ratio of peak energy storage in a coil to the energy dissipation per cycle [107]:

Q = 2π

Epeak Ediss

.

(22)

This closely translates to the ratio of the reactive to the resistive part of the coil self-impedance, as shown in (5). High Q-factor coils are therefore realized using coil designs with large reactance-to-resistance ratios. The reactance of a typical coil is a function of the coil selfinductance and the parasitic capacitance that develops between the coil turns. Usually, high coil reactance can be achieved by enhancing the coil self-inductance, whilst limiting parasitic capacitance [108]. The fabrication of coils from highly conducting materials leads to lower coil resistance values. Further, if the operating frequency for the energy transfer link is not determined by regulatory concerns, an optimal operating frequency needs to be carefully determined. This is because, although coil reactance increases with frequency, the same can also be said of coil resistance, which increases with frequency due to the skin effect [43,44,47,77,95]. In summary, coil designs techniques to achieve high Q-factors involve geometric, material, and operational considerations to realize the appropriate combination of coil reactance, resistance, and link operating frequency. Jow and Ghovanloo [53] have demonstrated the impact of different operating frequencies on the link transfer efficiency. The studies performed at 1 MHz and 5 MHz showed that the efficiency for the particular design scenario was maximized at the latter frequency due to the high coil Q-factors at this frequency. However, numerous investigations of wireless energy transfer implementations seek to leverage the available industrial scientific and medical (ISM) frequency band designations in the megahertz and sub-megahertz ranges. The more popular frequencies in the megahertz range are 13.56 MHz [3,77,109– 113], and 6.78 MHz [38,103,113–117], while sub-megahertz implementations are typically at a few hundred kHz [118–124]. An approach to achieve high coil reactance is to design coils with appropriately dimensioned number of turns, conductor diameters and inter-turn spacings. These physical coil parameters are often determined by employing algorithmic methods [53,125–127], supported by electromagnetic simulation tools. Generally, closely spaced coil turns lead to higher coil self-inductance, suggesting the use of the closest spacing supported by the implementation technology [53,128]. Coil self-inductance is also enhanced by small conductor diameters [129]. A double-layer turn layout can also be employed, as proposed in [130], to increase the number of turns that can be realized on a planar coil. Different materials have been employed in order to realize lowresistance coils. Litz wire is a popular implementation [18,131–136], which counteracts the increase in resistance with frequency as a result of the skin effect. Other material enhancements to achieve low coil resistance include the use of magnetoplated wire [137], copper conductor strips on a Titanium Oxide (TiO2) nano-powder-(C4H6O2)x latex composite dielectric substrate [72,138]. Methods for realizing high coil Q-factor are summarized in Table 3.

5.1.1. Impedance matching and coil tuning Hui and Lee [30] point out a significant difference between the energy transfer link interfacing employed in low-power energy transfer links, and that employed in high-power links. High-power links are designed with the overall energy efficiency of the system in mind. This consideration necessitates the design of systems with the minimum possible source impedance [68]. Low-power links, however, are interfaced with the other system components based on the maximum power transfer criterion, which requires impedance matching. Inagaki [60] shows that impedance matching requires the fulfilment of the conjugate matching criterion. The resistance of connected external circuits should be equal to the resistance presented by the energy transfer link at the interface port, while the circuit reactance should be the conjugate of that presented by the energy transfer link. Satisfying this criterion prevents power reflections at the interface ports of the energy transfer link. The same circuit arrangements employed to achieve impedance match conditions are usually employed to provide the dual utility of coil tuning for link resonance. Single-component topologies are frequently adopted to achieve resonant direct-fed links at the least cost of complexity and power loss. Although the tuning topology illustrated in Figs. 4 and 11 is a series-parallel capacitive arrangement, other variations of single-component compensation topologies are possible, namely, series-series; parallel-series; and parallel-parallel, all illustrated in Fig. 12(a)–(d). However, the transfer efficiency of the link is influenced by the compensation topology of the receiver circuit, irrespective of the arrangement present in the transmitter circuit [105]. Jegadeesan and Guo [106], in comparing series and parallel receiver compensations, note that the performance offerings from both topologies are largely frequency dependent. Typically, parallel resonance offers better transfer efficiency in the sub-MHz to low-MHz range, while series resonance is more efficient in the sub-GHz to low-GHz range. In addition, the parallel resonance technique provides a larger voltage swing, which eases power rectification. In contrast, series resonance is reported to be more suitable for small loads, while offering greater robustness to coupling fluctuations [106]. Linhui et al. [66], however, argues that the gains provided by both series and parallel resonance topologies can be harnessed by adopting two-component capacitive L-match arrangements, as shown in Fig. 12(e). Generally though, other impedance

5.1.3. Improving coupling and operating range Coupling, as characterized in circuit theoretic analysis, is a function of self-inductances of the coupled coils, and the mutual inductance 494

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Fig. 12. Simple coil tuning circuits (a) series-series, (b) series-parallel, (c) parallel-series, and (d) parallel-parallel compensation layouts as degenerate forms of (e) L-match layout [66].

relatively low eddy current loss potentials. Ferrite materials can therefore by used to redirect excited magnetic fields towards the intended coupling directions [47]. Generally, coupling strength is directly related to the sizes of the interacting coils [147]. This implies that coupling between a pair of coils can be enhanced by using larger coils. However, most practical scenarios place restrictions on the coil sizes. Coil design studies, consequently, place more emphasis on optimal geometries within the available footprint [3,53,125]. Usually, the adopted methods maximize conductor lengths for a given coil geometry, through an appropriate number of turns, spacing between turns, and width of turns. The realized coil structures with high coupling potentials are often characterized by high Q-factors as well. Alternatively, Zierhofer and Hochmair [49] demonstrate a method of distributing coil turns away from the coil edge, resulting in an increase in the coupling coefficient between paired coils. However, since the coil self-inductance and

between them. The mutual inductance is proportional to the magnetic flux enclosed by the receiving coil as a consequence of the magnetic field excited by current flow in the transmitter coil. Also, the mutual inductance is influenced by the geometry of the interacting coils, since a larger receiving coil encloses more flux than a smaller coil. It also follows that mutual inductance is a function of the strength of the magnetic field excited by the transmitter coil. Consequently, research undertakings to enhance coupling between paired coils focus on two areas, namely field enhancement, and coil geometry. Magnetic field enhancement is a widely employed coupling enhancement strategy for short-range direct-fed links. Typical techniques involve the use of ferrite materials to alter the distribution of the excited magnetic fields from the transmitter coil. Popular methods include the use of ferrite cored transmitter coils [139–143] ferrite plates [141,144], and ferrite sheet support for receiver coils [145,146]. Ferrites are typically characterized by high magnetic permeability, and Table 3 Summary of methods for realizing high coil Q-factors. Method

Features

Typical Transfer Efficiency

Geometrical design of coils.

Iterative optimization of number of coil turns, turn widths and turn spacing [50,123–125]; doublelayer turn layout [128]. Use of litz wire [18,129–134]; use of magnetoplated wire [135]; copper conductor strips on a Titanium Oxide (TiO2) nano-powder-(C4H6O2)x latex composite dielectric substrate [69,136].

> 80% [123].

Coil implementation using enhanced materials.

495

> 85% [69].

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load range in indirect-fed links eases the tuning required to achieve maximum power transfer to a load for a given source voltage. This facilitation of an easier impedance transformation has been attributed in [31] to additional degrees-of-freedom made available by inter-coil mutual inductances between driver and transmitter coils, or receiver and load coils. Furthermore, the indirect feeding arrangement is an implementation of the mini-loop impedance match technique, which provides low-loss, high-ratio impedance transformation. The required impedance transformation ratio typically grows with the distance of wireless energy transfer, making indirect-fed link configurations more appropriate for longer distances [154]. Table 4 summarizes the design approaches to improve coupling and operating range.

resistance also depend on the geometrical layout of coil turns, gains in coupling may be offset by a reduction in the coil Q-factor. Consequently, the method discussed in [49] has been modified to take into cognisance the impact of turn distribution on the Q-factor as well [148–150]. Coupling also depends on the intervening distance between interacting coils, since the strength of the excited magnetic field decays with distance. Range concerns are, therefore, intertwined with coupling issues. A typical focus is on the ability of an energy transfer link to achieve high levels of transfer efficiency at extended operational distances. The major attraction to indirect-fed links is the facilitation of mid-range energy transfers over relatively greater distances than are possible with two-coil links. CMT-based analysis of two-terminal energy transfer links asserts that efficient mid-range energy transfer is contingent on the operation of a link within a “strong-coupling” regime, in which the coupling rate is much greater than the intrinsic decay rates of the coupled resonant coils [9,10]. By implication, the desired operating condition for the energy transfer link – strongcoupling – is defined by κ / ΓΓ 1 2 ≫ 1, where κ / ΓΓ 1 2 is regarded as the link figure-of-merit [9,76]. However, the term “strong-coupling” does not seem to have found widespread acceptance in the electrical engineering community. This may be due to the wider adoption of circuit theoretic analysis, and differences between its characterizations of coupling and coil quality (i.e. “coupling coefficient” and “Q-factor”), from the CMT approach (i.e. “coupling rate” and “decay rate”). Kurs et al. [9], and Bou et al. [57], however, note the following relationships between coupling rates and coupling coefficients, on one hand, and between Q-factors and intrinsic decay rates on the other:

κxy =

ωkxy 2

;

Qx, y =

ω . 2Γx, y

5.2. Spatial freedom Spatial freedom, in this case, refers to the ability of an energy transfer link to maintain a consistent performance level, irrespective of coil orientation or placement within the required operational range. Fig. 13 classifies various coil positioning variations that could occur in near-field wireless energy transfer scenarios. When the axes of a sequence of coupled coils coincide, the coils are said to be axially aligned. Axial misalignment, on the other hand, describes the situation of non-coincidence of coil axes. A rotation of the plane containing one of a pair of coupled coils, without a change in the coil alignment, results in planar rotation [155], illustrated in Fig. 14. Range variations, on the other hand, arise from changes in the distance between a pair of coupled coils. Axial misalignments could manifest in any of two forms, namely angular or lateral misalignments, as illustrated in Fig. 15. These positional variations could occur separately, or could be jointly present at varying degrees in a coupled interaction. Generally, misalignments cause a reduction in the coupling between coils, leading to a loss in transfer efficiency. The impact of axial misalignments is more pronounced in practical design scenarios in which the receiver coil is subject to stringent size constraints. Power delivery link implementations to transcutaneous medical implants is a typical example which requires small receiver coils [108]. In this case, it is typical to employ larger transmitter coils to increase the coupling potential of the link. It is therefore imperative to assess the impact of coil misalignments during the design of such near-field wireless energy transfer links. Fotopoulou and Flynn [156] have specified a compact analytical model to describe coil misalignments in direct-fed two-coil links. The utility of the developed model lies in its prediction of the impact of coil lateral and angular misalignment without recourse to time-consuming electromagnetic simulations. The model is based on formulations of expressions for the magnetic field at the receiver coil

(23)

Bearing in mind that the circuit theoretic coupling coefficient is a distance-dependent variable, one can appreciate that mid-range links operate at lower coupling coefficients relative to short-range links. Consequently, by circuit theory analysis, the CMT “strong-coupling” regime is actually characterized by “loose-coupling” [5,38,151,152] and low coupling coefficients. However, Sample et al. [153] note that even with low coupling coefficients, high efficiencies can be obtained by employing high Qfactor coils, since coupling and Q-factors equivalently determine transfer performance. This is basically how indirect-fed links sustain high transfer efficiencies at mid-range. In addition, Ramrakhyani and Lazzi [73] explain that an indirect feed enables the decoupling of high Q-factor transmitter and receiver coils from finite source and load resistances, thereby boosting the end-to-end power transfer efficiency. In addition, the authors note that the availability of a wider reflected Table 4 Comparison of methods to improve coupling and operating range. Primary Goal

Method

Typical Transfer Efficiency

Increased strength of magnetic field Increased coupling coefficient

Use of ferrite cored transmitter coils [137–141] ferrite plates [139,142], and ferrite sheet support for receiver coils [143,144]. Optimal geometries within the available space constraint [3,50,123]; distribution of coil turns away from coil circumference [48,148].

> 70% [143].

Fig. 13. Classifications of coil positional variations in an energy transfer link.

496

> 70% [148].

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coefficient of variation of the perpendicular component of the magnetic field at a plane 1 mm above the coil. This minimizes the lateral variations in the excited magnetic field. In contrast, the approach in [158] employs a transmitter coil of multiple loops connected in series and parallel to limit the variation ratio in the lateral mutual inductance profile across the coil. The spacing and layout of the series and parallel connected turns are optimized to ensure that the resultant magnetic field is a superposition of fields due to forward and reverse currents. A fully analytical approach to designing the transmitter coil to excite a uniform lateral magnetic field profile is provided by Waffenschmidt [159,160]. The method first calculates a current distribution that matches the required lateral magnetic field profile. Then a coil turn distribution is determined to match the calculated current distribution. Typically, the realized coil layout would have turns concentrated at the coil edge, and be more sparsely distributed towards the coil centre. The author notes that insensitivity to lateral coil displacements over a larger area can be realized by implementing an array of such transmitter coils. Such MISO link configurations, therefore, are conceptually link implementations developed to address coil lateral misalignment issues. Indirect-fed links have been shown to exhibit greater robustness to the impact of coil misalignments than direct-fed two-coil links [161,162]. In [74], this feature of indirect-fed links is employed in the design of a wireless power delivery system for a biomedical implant. The obtained results show that the by using the indirect-fed arrangement, the variation in power transfer efficiency due to changes in the orientation of coupled coils is reduced by half, as compared to a directfed two-coil arrangement. The loss in transfer efficiency associated with increasing degrees of misalignment can be mitigated through adaptive impedance matching techniques. Generally, a change in the orientation of coupled coils from their default positions alters the electrical model the link design is predicated upon, leading to impedance mismatches, and increased reflection of power back to the source. In [163], the authors demonstrate an improvement in the transfer efficiency of a laterally misaligned indirect-fed link by connecting the source loop to a tuneable impedance matching network. Indirect-fed links are unique in the fact that the presence of source and load loops between the transmitter and receiver coils provide tuneable networks, which reuse the distributed parameters of the coils for impedance matching [154]. For an indirect link with variations in the distance between the transmitter and receiver coils, [14,164] establish that the impedance match conditions can be maintained by corresponding alterations to the distance between the source or load loops from the transmitter or receiver

under lateral and angular misalignment conditions. Planar rotations are relatively the easiest misalignment conditions to deal with, as they are handled by employing coils with symmetric geometries. As is evident from Fig. 14, planar rotation between a pair of coupled circular coils is of no significance. Lateral misalignments in direct-fed two-coil links, however, require more involving mitigation techniques. In [16], the authors realize a spatially uniform transfer performance by designing the transmitter coil to have a uniform lateral magnetic field profile. This goal was achieved by implementing a hybrid coil structure comprising of concentrated coil turns, connected in series to spiral coil turns. For a circular structure, the concentrated coil turns provide a concave field distribution, with the minimum field strength occurring at the centre of the enclosed coil area. The spiral windings provide a convex field distribution, with the maximum field strength occurring at the centre of the enclosed coil area. With an optimization of the geometry of these two connected coil layouts, the resulting field is a superposition of the convex and concave profiles, providing a more uniform lateral profile. Other semi-empirical methods for the design of lateral misalignment tolerant transmitter coils have been reported as well [157,158]. In [157], the spiral coil geometry is defined by minimizing the

Fig. 14. Rotation of the smaller circular coil along the xy plane.

Fig. 15. Axial misalignments; (a) angular misalignment; (b) lateral misalignment.

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link parameters. Although the link transfer efficiency has received the most attention in the literature, there are other concerns to be addressed along with the link efficiency. Figures-of-merit (FoMs) are, therefore, often used a means to characterize the performance of the energy transfer link in terms of multiple performance objectives. As discussed in earlier sections of this paper, the loading effect of the receive circuit can be modelled as a reflected impedance at the transmit circuit. The maximum source-to-load efficiency of power transfer is achieved when this reflected impedance is maximized [57,171,172]. However, the maximum power delivered to a connected load at the receive terminal does not necessarily coincide with the maximum source-to-load power transfer efficiency [171]. Hence, the analysis points out the need for designers to strike a delicate balance between the source-to-load power transfer efficiency ηs − l and the actual power delivered to the load. This compromise is expressed in the FoM defined as [171]

coils. Rather than alter the distances between drive loops and transmitter coils, or load loops and receiver coils, [117,165] develop indirect-fed links that are able to switch between different sizes of source or load coils in order to maintain impedance match conditions. An analogous direct-fed link implementation is found in [166], which reports a switchable link implementation that alternates between three impedance matching networks corresponding to three ranges of link operation. Energy transfer at coil separation distances closer than the link is designed for presents a unique problem. Such scenarios lead to the appearance of multiple resonance frequencies, equal to the number of coupled resonators, a phenomenon known as frequency-splitting [41,60]. Wang-Qiang et al. [167] provide an analytical investigation to predict the even and odd splitting frequencies arising in symmetric (similar coils at both terminals), and asymmetric (dissimilar coils at both terminals) direct-fed two coil links. The analysis also enables the determination of the splitting coupling of a link, which represents a threshold coupling beyond which frequency splitting occurs. Sample et al. [153,168], on the other hand, reveals the existence of distinct range-dependent coupling regimes in an indirect-fed link. The papers note that link transfer efficiency is maximized when a link is operated at critical coupling. A decrease in coupling from the critical coupling, which results as coupled coils are moved farther apart, moves the link into an under-coupled regime. Bringing the coupled coils into closer proximity beyond the critical coupling distance moves the link into an over-coupled regime. The over-coupled regime is characterized by frequency splitting, as illustrated in Fig. 16. Both [167,153] demonstrate that a near-uniform transfer efficiency characteristic can be obtained in the over-coupled regime by tracking the split frequency providing the higher transfer efficiency. An alternative approach to achieving near-uniform transfer efficiency within the over-coupled regime is by suppressing the increase in mutual inductance as coupled coils are brought closer together. Proposed methods to suppress the mutual inductance include the use of anti-parallel turns coils [169], or capacitive loaded anti-parallel turns coils [170] as transmitter coils in an energy transfer link. The salient features of methods to ensure spatial freedom of designed links are presented in Table 5.

FoM =

ηs − l nPL Vs

(24)

where PL is the power delivered to the load, and Vs is the source voltage. Parameter n is a weighting parameter whose value is chosen depending on the relative criticality of power transfer efficiency or power delivered, as defined by the requirements of the target application. This FoM, measured in Siemens, describes source-to-load power transfer efficiency if n → ∞; or the power delivered to the load if n → 0 . Hence, these two limiting cases allow for the determination of the maximum power transfer efficiency or power delivered that can be achieved for a particular application. Depending on the relative criticality of power transfer efficiency or power delivered in an application, the designer can decide on an appropriate value for n, and proceed with a maximization of (24) on the basis of that choice. The analysis presented in [171], however, shows that n = 2 is suitable for applications in which the power delivered is just as important as the power transfer efficiency. Apart from achieving impressive levels of link transfer efficiency, and delivering adequate power to a connected load, it is often necessary that these goals are achieved without a prohibitive increase in the size of the receiver coils. In addition, these goals should be achieved without compromising the required operating range of the link. Mirbozorgi et al. [84] tie together these concerns in a figure of merit defined as

5.3. Multi-objective criteria Designers of near-field wireless energy transfer links frequently have to reach compromises between various conflicting and competing

FoM =

η × PL × d Dr

(25)

where Dr refers to the diameter of the receiver coil, while d is the distance of separation between the transmitter and receiver coils. The non-trivial size, energy consumption and additional data transfer requirements of bio-telemetry systems have necessitated the adoption of multi-parameter FoMs that allow for a quantitative comparison between the various link implementations. A two-coil telemetry link FoM developed by [123] assesses coil diameters D1,2 , range d , power transfer efficiency η , and transmission bandwidth BW , alongside the link frequency f as

⎛ d 2BWη ⎞ ⎟, FoM = 10Log10⎜ ⎝ D1D2f ⎠

∈ [FoM < 0]. (26)

Improvements in the implementation of the energy transfer link are characterized by values of the FoM tending towards zero. The design of efficient inductive bio-telemetry links often poses a unique challenge due to a conflict in requirements for maximum power transfer efficiency and data bandwidth. High coil Q-factors generally lead to high efficiencies, but at the price of bandwidth constriction [37,38], except with topological modifications to the link [73]. The utility of this multi-parameter FoM lies in its incorporating these often competing of goals of enhanced bandwidth and power transfer efficiency, as well as

Fig. 16. Spatial coupling regimes in a simulated 13.56 MHz direct-fed two-coil link. Coil separations shorter than the critical coupling distance (d=30 mm) lead to over-coupling, characterized by resonance frequency splitting. Separations greater than 30 mm lead to under-coupling, characterized by degraded of transfer efficiencies at a single resonance frequency.

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Table 5 Summary of methods to ensure link spatial freedom. Method

Design Problem

Application Link Architecture

Hybrid transmitter coil winding [16]. Minimization of variations in perpendicular component of magnetic field [155]. Series and parallel connected turns in multi-loop transmitter coil [156]. Solution of inverse field problem [157,158]. Tuneable impedance network [161]. Alteration of distances between drive loop and transmitter coil, and between load loop and receiver coil [14,162]. Switching between different sizes of source and load loops [115,163]. Switching between different impedance matching networks [164]. Frequency tracking [151,165]. Suppression of mutual inductance increase [167,168].

Lateral misalignment Lateral misalignment Lateral misalignment Lateral misalignment Lateral and angular misalignment Range variation

Direct-fed, SIMO Direct-fed, SIMO Indirect-fed, SIMO Direct-fed, SIMO Dual-terminal, indirect-fed Indirect-fed, four-coil

Range Range Range Range

Dual-terminal, indirect-fed Direct-fed, two-coil Indirect-fed, four-coil Direct-fed, two-coil

maximization of transfer efficiency are two diametrically opposed objectives, for which a compromise must be sought. Theoretical and numerical analysis by Mclean and Sutton [175], however, demonstrate that the radiated power from a direct-fed two-coil link is insignificant, even in high power energy transfer systems. The major concern lies rather with the extent of the near-field far beyond the intended operating distance and required coverage volume. This concern is exacerbated with the inclusion of more coils in the energy transfer link, either through the use of indirect feeding arrangements, relays, or multi-terminal topologies. The health impact of human exposure to electromagnetic fields has been a critical concern in recent times [176]. Key guidelines for human exposure to time-varying electrical, magnetic and electromagnetic fields up to 300 GHz have been issued by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [177]. Similar guidelines have also been issued by the ICNIRP specifically for time-varying electrical and magnetic fields below 100 kHz [178]. The Institute of Electrical and Electronics Engineers have also released recommendations for safe human exposure to radio frequency electromagnetic fields operating from 3 kHz to 300 GHz (IEEE C95.1-2005 [179]). Christ et al. [180] provides a review of these safety guidelines and exposure standards, as well as models, and methods for assessing the risk of human exposure to electromagnetic fields. In another contribution, Christ et al. [181] provides recommendations for methods of assessing the compliance of wireless energy transfer links with the human exposure guidelines. The developed methods are applied for the estimation of maximum Specific Absorption Rate (SAR) levels for a generic 8 MHz indirect-fed link transferring energy over a few meters. The numerical results reveal that the local and whole-body Specific Absorption Rate (SAR) limits are reached when the RMS transmit current values lie between 0.5 A and 1.2 A, at 8 MHz. Also, it was observed that the exposure could vary by as much as 3 dB for different human body models, using the same coil configuration. Similar numerical assessments of indirect-fed links using human body models include the works of Hong et al. [182], Hirata et al. [183], Chakarothai et al. [184], Ding et al. [185], and Song et al. [186]. Various techniques to curtail leakage fields from wireless energy transfer links have been reported in the literature. One way to limit leakage electric fields in indirect-fed links is by achieving resonance through the addition of lumped capacitors, rather than through the distributed capacitance of coils. This approach localizes the electrical field required for resonance in the connected lumped elements. The surrounding spatial region in the immediate vicinity of such coils is therefore predominantly magnetic. The resonance condition ensures minimal magnetic interaction with off-resonant extraneous objects, except if these objects have high magnetic permeability or low magnetic losses [10]. Conventionally, leakage magnetic fields emanating from such links can be curtailed either by field cancellation, or flux shielding.

reduced coil sizes, into one goal function, which can be optimized for a required bio-telemetry application. A modification to this multi-parameter FoM has been proposed in [73], to also include the achievable voltage gain by the telemetry link. The reasoning for this inclusion is that a high voltage gain precludes the need for a high source voltage at the input of the telemetry system, leading to more cost effective implementations. This modified FoM is defined as

⎛ d 2BWη ⎞ FoM = 10Log10⎜ G⎟ , ∈ [FoM < 0], ⎝ D1D2 f ⎠

(27)

where G is the linear voltage gain between the transmit and receive terminals. Similar to (26), typical values of this FoM are negative, with improved performance achieved as the FoM value tends towards zero. The presented multi-objective criteria are summarized in Table 6. 5.4. Limiting electromagnetic emissions Even though near-field magnetic wireless energy transfer mechanisms are essentially non-radiative, some leakage energy is radiated when a potential difference is applied across the terminals of a transmitter coil. The radiation potential of a current-carrying conductor loop is typically modelled by a radiation resistance [42,67,70]. To keep the radiation resistance low, a link may be designed for operation at a frequency less than the point at which the radiation resistance becomes significant compared to the resistance due to the skin-effect. Alternatively, attempts to enhance coil Q-factors by increased number of turns or coil sizes should be tempered by considerations of the corresponding increase in radiation resistance [173]. Indeed, Mclean and Sutton [174] argue that the minimization of leakage fields and the

Table 6 Summary of multi-objective criteria for assessing energy transfer link performance. Objective Balance between the source-to-load power transfer efficiency and the actual power delivered to a connected load [169]. Balance between link transfer efficiency, and actual power delivered to the load, and size of receiver coils [81].

Figure-of-Merit

FoM =

η nPL Vs

FoM =

η × PL × d Dr

Balance between coil sizes, range, transfer efficiency, and transmission fractional bandwidth [121].

⎛ d 2BWη ⎞ FoM = 10Log10⎜ ⎟ ⎝ D1D2f ⎠

Balance between coil sizes, range, transfer efficiency, and transmission fractional bandwidth and voltage gain [70] .

⎛ d 2BWη ⎞ FoM = 10Log10⎜ G⎟ ⎝ D1D2f ⎠

variation variation variation variation

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without the need for coil alignment [194–199]. Agbinya and Mohamed [199] presents the design of a near-field wireless energy transmitter node to serve in a multidimensional wireless power distribution usage scenario. The transmitter node, illustrated in Fig. 17, comprises of one primary coil and six secondary coils. The primary coil is implemented as a multi-turn circular coil, which provides strong and uniform vertical energy transfer. The secondary coils are positioned around the primary coil, using a hexagonal pattern, to orient the excited magnetic fields in six directions. Experimental results are reported to demonstrate power delivery over a distance of 6 m using the developed node, with prospects for even greater distances with the inclusion of a relay coil placed 9 cm vertically above the primary coil. An interesting prospect for the developed node topology is the deployment of multiple units of the transmitter node in a wireless energy transfer network, as shown in Fig. 18, achieving wireless energy coverage over an area using the cellular concept currently employed in mobile communications. In another development, Nguyen and Agbinya [200] apply the concept of frequency diversity, which enables multiple simultaneous transmissions on the same link, to propose a diversity scheme based on the frequency splitting phenomenon. The proposed technique is reported to enable simultaneous energy transfer to multiple receivers at different frequencies, using a single transmitter terminal.

Field cancellation employs reactive resonant loops to excite magnetic fields to cancel the original leakage magnetic field. This method is, however, predominantly applied in high-power online electric vehicle applications [47,187], and may not be suited for low-power applications. Flux shielding, on the other hand, is based on the use of magnetic materials to modify the spatial distribution of the excited magnetic fields. The incorporation of flux shields in transmitter and receiver terminals is a widely used approach [17,32,47,159,188]. Most implementations of flux shields employ ferrite-based materials. High magnetic permeability, and relatively low eddy current loss potentials, enable ferromagnetic materials act as magnetic flux guides, which can be employed to keep the magnetic flux along a path close to the field source. This reduces the leakage of the magnetic field in the vicinity of an excited coil [47]. Ferrite materials to be incorporated in a wireless energy transfer links are typically chosen on the basis of datasheet information regarding permeability and eddy current losses. Unfortunately, values for these critical design parameters may not be available for the frequency of interest in the design of the energy transfer link. Dick et al. [189], therefore, proposes a characterizing of these materials in standardized setups, which are more closely related to wireless energy transfer applications. These setups enable the characterization of the performance of ferromagnetic materials on the basis of realized circuit theoretic figure-of-merit of energy transfer links, namely the product of the coupling coefficient and the geometric mean of coil Q-factors. An emerging technique for leakage field control in multi-terminal link configurations involves the concept of adaptive phased arrays. Strictly, though, this technique is a system-oriented approach, which maximizes power delivery to an intended receiver, while minimizing unintended power delivery and leakage fields in other directions [190].

7. Conclusion Low-power magnetic wireless energy transfer links are an important step towards bridging the gap between the power requirements of today’s mobile gadgets and the present bottlenecks in battery technology. By eliminating the need for physical conductors for the transmission of electrical power, the technology helps reduce the bulk of generated waste at the end-of-life of such gadgets. Notwithstanding the current diversity in implementations of wireless energy transfer systems, there are certain common features shared by near-field magnetic wireless energy transfer links. This article has presented a comprehensive survey of near-field magnetic wireless energy transfer link architectures, and link design approaches for achieving performance criteria. Starting with a fundamental description of the spatial regions around sources of electromagnetic fields, the paper has presented basic features of direct-fed, indirect-fed, multi-terminal, and relay link architectures. This was followed by a discussion highlighting the common performance goals shared by these link architectures. The attainment of high transfer efficiencies at required operating distances is a key design objective for energy transfer links. Generally, this goal is achieved either through realizing high Q-factor coil implementations, or through the enhancement of range-dependent coupling. High performance energy links are also required to exhibit spatial independence with regards to their transfer efficiency performance. Various techniques to enable energy transfer links maintain acceptable performance levels, to some degree, irrespective of positional variations of coupled coils are discussed in the paper. Oftentimes, though, the design of high-performance energy transfer links involves tradeoffs between multiple competing objectives. Consequently, multi-objective design approaches are presented as attempts to reach a compromise between multiple parameters. In addition, energy transfer links have to be designed in full cognisance of electromagnetic emission regulations, which are mentioned in the paper. Future trends in the development of low-power near-field magnetic wireless energy transfer architectures are likely to be increasingly influenced by emergent applications of wireless sensor networks. This is projected to encourage an increase in the adoption of communication

6. Emergent trends in wireless energy transfer link design Trends in the deployment of near-field magnetic wireless energy transfer links in low-power applications are increasingly being influenced by the rapidly evolving Internet-of-Things (IoT) ecosystem. Lowpower sensing devices are anticipated to play an increasingly more critical role in the evolution towards the envisaged fifth-generation (5G) networked society. These autonomous sensors are projected to account for 75% of the growth of active wirelessly connected devices from current levels to about 40.9 billion devices by 2020 [191]. With this current drive, the provision of power-delivery mechanisms to these pervasive ‘smart objects’ within an internet-of-things (IoT) ecosystem is a critical undertaking. This is all the more so necessary as a rapidly growing number of sensor nodes are being deployed in operations requiring autonomous information acquisition in increasingly more adverse operating and environmental conditions. A direct consequence of the upsurge of sensor applications is that network deployments of wireless energy transfer links are becoming increasingly more necessary, with an attendant increase in the application of communications network topology paradigms. The development of multi-terminal links has stimulated research into energy beamforming techniques [192,193]. Energy beamforming is an attempt to mimic the beamforming approach employed in communication array antenna systems to enable the automatic steering of antenna beams towards intended coverage areas, and away from unintended locations. The need for network energy transfer deployments has also inspired the development of alternative non-planar energy terminals. Current research has begun to investigate the development of 3-D omnidirectional transmitter terminals, which enable the broadcast of energy to receiving devices over a coverage area in the vicinity of the transmitter,

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Fig. 17. Multidimensional wireless energy transfer node (a) schematic (b) implementation. Reprinted from Agbinya and Mohamed [199], with permission from Elsevier.

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[10] [11]

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[13] Fig. 18. Illustration of proposed cellular wireless energy transfer network. Reprinted from Agbinya and Mohamed [199], with permission from Elsevier.

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network-inspired energy transfer link architectures and topologies, as low-power link implementations evolve from laboratory prototypes to real-world deployments.

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Acknowledgement

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This work was supported in part by the Collaborative Research in Engineering, Science and Technology (CREST) Fund, Malaysia under Grant no. 4B151, and the Universiti Teknologi Malaysia, Research University Grant with Vote Number 12H35.

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