POLÍTICAS GENERALES Y EDUCATIVAS

links on the basis of mutual inductances. The fact that the magnitude of the mutual inductance

16 1655 Design Considerations for Wireless Power Delivery Using RFID

is strongly influenced by the relative orientation of the paired coil antennas means that inductive coupling RFID links are strongly directional. More energy is transferred between the reader and transponder coil antennas when they are perfectly aligned, and within the prescribed operating range. Consequently, wireless power delivery infrastructure using inductive RFID links requires intentional transmission of electrical energy from dedicated sources, rather than ambient energy scavenging.

Conceptually, designing an inductive coupling RFID-based WPT implementation involves har-nessing the energy wirelessly transferred by an inductive RFID reader to power up a suitable load.

IoT infrastructure in which inductive coupling RFID-based sensors are deployed for data gathering in scenarios that preclude wired power delivery would benefit from such WPT implementations.

An example of this WPT application is found in a recent project where near-field communication (NFC), a technology based on inductive coupling RFID, has been integrated with a sensing platform to create the NFC–Wireless Identification and Sensing Platform (NFC-WISP) (Zhao et al., 2015).

This programmable sensing and computing platform can be powered up and interrogated by induc-tive RFID readers, as well as NFC-enabled smartphones. In the cited example, the NFC-WISP was used in a data logging application, where the sensing platform is placed on milk packaging in a sim-ulated cold-supply shipment. By integrating the sensor platform with the milk packaging, the milk container is effectively converted into a smart object. The temperature of the container, its motion, and three-dimensional (3D) orientation are recorded by the NFC-WISP. Once the shipment arrives at a destination, an RFID reader can be used to download the recorded data to a host computer for further processing. Alternatively, an NFC-enabled mobile phone could be used to download the recorded data by bringing it close to the milk container. The energy used by the sensor platform to sense temperature, 3D position, and motion is acquired during the inductive coupling interactions involving RFID readers or NFC-enabled smartphones. This energy is stored onboard in a thin-film battery, which powers the sensor electronics when the sensing platform is far from a reader. Other examples of RFID WPT to smart IoT objects, as reported in the literature, include an NFC-enabled blood glucose monitor (DeHennis et al., 2013; Tankiewicz et al., 2013) and a wireless-powered smart watch (Lin et al., 2015).

In this section, we examine general system architecture for realizing inductive coupling RFID-based WPT links, as well as some design considerations, and methods to optimize link performance.

8.3.1 G

ENERAL

S

YSTEM

A

RCHITECTURE

Figure 8.2 illustrates the principal components of an inductive coupling RFID WPT scheme.

The RFID reader serves as the power transmitter in the link. In a more general sense, the RFID reader is a transceiver, since it can also detect load-modulated signaling. This facility can be employed to provide feedback control for the wireless power delivery scheme. The RFID reader can either be battery powered or powered externally. The output of the RFID reader is essentially an oscillating magnetic field, whose frequency depends on the specific RFID protocol in use. For the widely employed ISO/IEC 14443 and ISO/IEC 15693 specifications, the frequency of oscillation is 13.56 MHz. Typically, the RFID reader would have inbuilt power amplification to drive the reader coil antenna.

Load RFID reader

Power Amplifier

Transmitter antenna

Receiver antenna Magnetic

field

Rectifier/

regulator

FIGURE 8.2

FIGURE 8.2 Inductive RFID WPT system.

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More power-demanding WPT applications may require added external power amplification at the link transmitter. The power amplifier is often designed to meet the specific drive requirements of the associated power transmitter coil. For example, it could be used to increase the read range of an ISO/IEC 14443A inductive RFID system (De Mulder et al., 2009). Most power amplifier designs for inductive wireless power delivery applications are based on either Class D or Class E topologies. These are essentially switching power amplifiers. As shown in Figure 8.3, the basic Class D power amplifier uses a pair of transistors driven such that they are alternately switched ON and OFF. This arrangement realizes a two-pole switch that presents either a rectangular current or voltage waveform at the input of a tuned load circuit. The tuned load circuit filters out harmonics from the rectangular waveform, resulting in a sinusoidal output (Albulet, 2001). In a basic Class E amplifier, as shown in Figure 8.4, a single transistor serves as the switch. The arrangement is such that current and voltage waveforms do not overlap during the switching time interval. Consequently, there is virtually no power loss associated with transistor switching, resulting in very efficient power amplification (Kazimierczuk, 2008). The Class E power amplifier topology is generally preferred at higher frequencies (Pinuela et al., 2013b).

Although they are commonly known as coil antennas, the coil structures employed at the front ends of RFID reader and tag infrastructure are not true antennas, as they do not radiate and inter-cept electromagnetic waves. This is because the frequency of operation of inductive RFID links is such that the sizes of these coils are significantly smaller than the wavelength by some orders of magnitude. However, these coils are closed-loop structures, which can be analyzed as derivatives of magnetic dipoles (Ramo et al., 1994). In other words, currents flowing through these structures excite more significant magnetic field components than electrical field components. Thus, a coil structure at the front end of the RFID reader enables the generation of an oscillating magnetic field.

Similarly, a coil structure at the front end of the power receiver enables the coupling of the magnetic energy in the reader-generated magnetic field. This coupled energy presents itself as a voltage across the terminals of the receiving coil structure. Physically, magnetic coupling can be achieved using conductor loop structures such as helices (Tak et al., 2011), spirals (Jonah et al., 2014), and solenoids

C

R V

O +Vdc

L

FIGURE 8.3

FIGURE 8.3 Basic Class D power amplifier topology.

RF choke

C₁

R₀ L0

+ V_dc

C0

FIGURE 8.4

FIGURE 8.4 Basic Class E power amplifier topology.

16 1677 Design Considerations for Wireless Power Delivery Using RFID

(Lee et al., 2014), among others. The printed spiral coil (PSC), where the spiral conductors are printed on suitable substrate materials, is perhaps the most widespread implementation of planar tag spiral coil structures. To eliminate losses and power reflections, impedance matching networks are usually inserted between the power amplifier and transmitter coil, and between the rectifier and receiver coil.

The oscillating alternating current (AC) voltage across the terminals of the receiver coil structure needs to be rectified to obtain a direct current (DC) voltage. This DC voltage could be filtered and further conditioned before being used to drive a designated electrical load. Consequently, the major role of the rectifier is to provide a DC voltage from the high-frequency voltage at the terminals of the receiver coil antenna. Discrete component rectifier implementations usually employ Schottky diodes in a full-bridge configuration. Alternatively, single-chip complementary metal-oxide semi-conductor (CMOS) rectifier implementations are widespread.

8.3.2 W

IRELESS

P

OWER

T

RANSFER

L

INK

D

ESIGN

Employing inductive RFID infrastructure in power-centered applications requires a shift from the more typical data- and range-centered design considerations adopted in traditional RFID systems (Sample et al., 2011). In a lot of respects, design considerations for inductive RFID-based WPT are similar to concerns for inductive power transfer systems. However, the power delivery functionality of these RFID links should not impede their traditional data transfer roles.

At the transmitter end, an AC flowing through the coil excites a magnetic field around the coil.

The energy in this field is inductively coupled to the receiving coil in proximity to the transmitter coil. The link is therefore analogous to a loosely coupled transformer circuit. Figure 8.5 is a simpli-fied circuit model of the coupling of a pair of coils.

8.3.2.1

8.3.2.1 Coil Coil AntennaAntenna

As shown in Figure 8.5, each coil can be modeled as a parasitic capacitance in parallel with a series combination of a resistance and inductance.

The coil inductance arises from the magnetic field generated by current flowing in the coil loops. A popular formula for determining the inductance of multiturn planar spiral coils is given as (Mohan et al., 1999)

L

=

^{0 5}

(

^.

^µ

n d c² avg¹

) _ ⁽

^ln c²

^{) +}

c³

⁺

c^{4 2}

_

^(8.2) The values of the coefficients c₁ − c₄ depend on the geometric layout of the PSC.μ refers to the per-meability of the conductor material, whileρ is the spiral fill ratio, namely,

ρ = − +

d d d d

o i

o i

(8.3)

d

0

and d

i

refer to the outer and inner diameters of the coil turns on the PSC, as illustrated in Figure 8.6.

Receiver coil L₂ L₁

C ₁

R₁ R₂ I ₂

C ₂

Transmitter coil M

+

–

FIGURE 8.5

FIGURE 8.5 Inductive coupling link.

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The average diameter of the spiral trace is given by

d_avg

=

^{0 5.}

(

d_o

+

d _i

)

^(8.4)

Analytic models for resistance in PSCs usually take skin effects into account. The resistance due to DCs flowing in a conductor is given by

R l

DC

=

A

σ

^(8.5)

where:l is total the conductor length

A is the cross-sectional area of the conductor

σ is the conductivity of the conductor material (Reinhold et al., 2007)

The skin effect occurs as ACs flow through the conductor, with the current migrating toward the conductor surface, away from its core. This modifies the conductor resistance to

R R t

e f

AC DC

skin t skin

c skin

= ⋅

( −

⁻

) ^{= ( )}

−

δ

_δ

µ

1

, 1, (8.6)

where:

δ_skin is the skin depth

t _c is the conductor thickness ( Jow and Ghovanloo, 2007; Reinhold et al., 2007)

In multiturn coil structures, the skin effect is further modified by proximity effects, in which magnetic fields excited by adjacent loop turns cause asymmetric current distributions. Although some studies have modeled proximity effects (Felic et al., 2013), full-wave electromagnetic simula-tions still remain a popular choice for characterizing these effects.

The parallel capacitance in the PSC equivalent circuit representation models the aggregate par-asitic capacitance forming between coil conductor turns and strips, through free space, and the dielectric substrate. Actual analytic expressions for parasitic capacitance depend on the geometrical layout and implementation of the PSCs.

Faraday’s law predicts that the voltage induced in the receiver PSC is

V_ind

= ω

j MI 2 (8.7)

d _i d o

FIGURE 8.6

FIGURE 8.6 PSC layout.

16 1699 Design Considerations for Wireless Power Delivery Using RFID

where:

I ₂ is the current in the receiver coil

M is the mutual inductance between the transmitter and receiver coils

As earlier noted, there are several handbook methods for computing the mutual inductance between a pair of inductively coupled coils. For instance, the mutual inductance between a pair of circular multiturn coils can be computed using (Raju et al., 2014)

M a b

n_a and n_b are the number of turns of the transmit and receive loop, respectively a_i and b_i are the radii of the ith transmitter and jth receiver coil turns, respectively

z is the axial separation distance between both coils

Here,μ₀ refers to the permeability of the free-space medium separating the transmitter and receiver coils.

8.3.2.2

8.3.2.2 Link Link TrTransfer ansfer EfficiencyEfficiency

The end-to-end system efficiency of an inductive RFID WPT scheme is the product of the efficien-cies of the constituent subsystems. In most WPT cases, however, the weakest link in the chain is the coupling between the interacting coil antennas (Vandevoorde and Puers, 2001). Consequently, the link transfer efficiency is a most critical determinant of the efficiency of the WPT scheme. In a general inductive WPT scenario employing a pair of biconjugate matched coil antennas, the link transfer efficiency is given by

Conventional inductive power transfer links typically employ high Q-factor coil structures to facilitate efficient power delivery. The Q-factor of a coil antenna is the ratio of its reactive self-impedance to its resistive self-self-impedance. Consequently, high Q-factor coil antennas are realized using designs with high reactance-to-resistance ratios. Unfortunately, due to the inverse relation-ship between coil Q-factors and their bandwidths, Q-factor enhancements could be detrimental to the data transfer capability of an inductive link. Usually, inductive RFID reader coil antennas are implemented to have as low a Q-factor as possible within the design specification (Aerts et al., 2008). Consequently, Q-factor-based transfer efficiency enhancements may not be the most viable approach to efficient inductive RFID WPT schemes.

The product of coil antenna Q-factors and the coupling coefficient is often viewed as a figure of merit (FoM) of traditional inductive WPT schemes (Bosshard et al., 2013; Inagaki, 2014). By impli-cation, Q-factors and coupling levels equivalently determine the transfer efficiency of an inductive WPT link ( Waffenschmidt and Staring, 2009). Consequently, coupling enhancements could be used as an alternative means to achieve efficient power transfer in Q-factor-constrained inductive RFID WPT applications. Coupling is a function of self-inductances of the coupled coils, and the mutual

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inductance 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. Since this mutual inductance is also influenced by the geometry of the interacting coils, design undertakings to enhance coupling between paired coils focus on two areas, namely, field enhance-ment and coil geometry.

Magnetic field enhancement is widely employed in short-range inductive WPT links. Typically, ferrite materials are employed to redirect excited magnetic fields toward intended coupling direc-tions (Jiseong et al., 2013). Ferrite sheets can be incorporated into the structure of inductive RFID coil antennas to alter the distribution of the magnetic field (B. Lee et al., 2014; Bauernfeind, 2013).

Alternatively, coil turns can be distributed away from the coil edge, resulting in an increase in the coupling coefficient between paired coils (Zierhofer and Hochmair, 1990). This technique has been further harnessed to strengthen the excited magnetic field from a Q-factor-constrained reader coil antenna (Sharma et al., 2013a, 2013b), and to improve the link transfer efficiency in Q-factor-constrained symmetric inductive WPT links (Eteng et al., 2016).

8.3.2.3

8.3.2.3 Spatial Spatial FreedomFreedom

Misalignment between a pair of coupled coil antennas results in a reduction in the intensity of the magnetic coupling between them. Consequently, less power is transferred across misaligned coil antennas than if they were perfectly aligned. Axial misalignments have a more significant impact in design scenarios that require smaller transponder receiver coil antennas. Usually, in such cases, larger reader transmitter coil antennas are used to compensate for the small size of the receiver infrastructure. Such design scenarios make it necessary to investigate the limits of power transfer performance under various degrees of coil antenna misalignment. Compact analytical models to describe coil misalignments in inductive RFID telemetry links have been proposed (Fotopoulou and Flynn, 2011). Such models enable a designer predict the impact of lateral and angular coil antenna misalignments without resorting to lengthy electromagnetic simulations.

A unique problem arises when inductive coupled coils are at separation distances closer than the link is designed to operate at. Such scenarios lead to the appearance of multiple new resonance frequencies, a phenomenon known as frequency splitting (Kim and Ling, 2007; Inagaki, 2014).

Frequency splitting is associated with link overcoupling, and is usually accompanied by a loss in transfer efficiency at the srcinal frequency the link was designed to operate in. Efficiency loss at the link operating frequency can be prevented by proper link design. One proposal is to introduce a reverse coupling to counteract the overcoupling arising from coil antennas being brought closer together. This can be achieved through the design of the reader coil antenna with antiparallel loop turns (Lee et al., 2013), or with capacitance-controlled reverse-current flow (Lee et al., 2014).

Spatial freedom can also be achieved by incorporating control schemes in the inductive RFID link. The general system architecture described in Figure 8.2 is essentially an open-loop configura-tion, whose performance will be significantly impaired by changes in the physical orientation of the coupled coil structures. This can be counteracted by implementing a closed-loop RFID link (Kiani and Ghovanloo, 2010). The RFID implementation in the cited reference employs the commercially available TRF 7960 RFID reader (Texas Instruments, Dallas), and an external control unit com-posed of a digital potentiometer and a microcontroller. The implemented WPT link harnesses the back telemetry capability of the RFID reader to provide the feedback required by the control unit for corrective measures against voltage fluctuations arising from coupling and loading variations.

8.3.2.4

8.3.2.4 Multiobjective Multiobjective Link Link ConsiderationsConsiderations

Designers of inductive power transfer links frequently have to reach compromises between various con-flicting and competing link performance parameters. These competing interests are often expressed in FoMs, which characterize the power transfer performance in terms of multiobjective criteria.

It has been demonstrated that the maximum power delivered to a load at the receiver terminal of an inductive coupling WPT link does not necessarily coincide with the maximum source-to-load

17 1711 Design Considerations for Wireless Power Delivery Using RFID

power transfer efficiency (Kiani and Ghovanloo, 2013). Consequently, it is imperative for design-ers to strike a delicate balance between the source-to-load power transfer efficiency η and the actual power delivered to the load. This compromise is expressed in a FoM defined as ( Kiani and Ghovanloo, 2013)

FoM

= η

^{n L}

s

P

V (8.10)

where:

P _L is the power delivered to the load V _s is the source voltage

The value of weighting parameter n is chosen to reflect the relative criticality of either power transfer efficiency or delivered power, as required by 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. These two extremes allow for the determination of limits of power transfer efficiency or delivered power that can be achieved in a usage scenario.

It is necessary that power is delivered to connected loads without a prohibitive increase in the size of the inductively coupled coil antennas. Usually, the receiver coil is subject to more severe size constraints than the transmitting coil antenna, as it is embedded in the IoT object. Since coil antenna sizes have an impact on the achieved inductive coupling, the operating range of the induc-tive coupling link must not be compromised in the attempt to achieve small antenna form factors.

These concerns can be addressed in a FoM defined as (Mirbozorgi et al., 2014)

FoM

= × × η

_D^{P L}_r ^{d (8.11)}

where:

D_r refers to the diameter of the receiver coil

d is the distance of separation between the transmitter and receiver coils

In order to leverage the data transfer capability of inductive RFID in a power transfer scenario, it would be necessary to note the impact of data bandwidth on efficient power transfer. This impact can be characterized in a FoM that evaluates coil antenna diameters D_1,2 at both terminals, coupling range d , power transfer efficiencyη, transmission bandwidth BW , and the link frequency f (Catrysse et al., 2004), namely,

FoM

= 

FoM

  

  ∈ [ < ]

10 10 0

2

1 2

Log d BW D D f

η

, (8.12)

This FoM has been further modified to include the achievable voltage gain by the telemetry link, namely,

FoM

= 

FoM

  

  ∈ [ < ]

10 ₁₀ 0

2 1 2

Log d BW D D f

η

G

, (8.13)

where:

G is the linear voltage gain between the transmit and receive terminals (RamRakhyani and Lazzi, 2013)

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The inclusion of voltage gain in the performance assessment of the inductive coupling link allows for links to be designed for high-voltage gain, thereby enabling more cost-effective implementations using lower source voltages at the transmitter input.

In document Memoria Consejo Escolar de Castilla y León Consejo Escolar de Castilla y León (página 150-156)