Starting from Tesla's principles of wireless power transfer a century ago, this critical review outlines recent magneto-inductive research activities on wireless power transfer with the transmission distance greater than the transmitter coil dimension. It summarizes the operating principles of a range of wireless power research into 1) the maximum power transfer and 2) the maximum energy efficiency principles. The differences and the implications of these two approaches are explained in terms of their energy efficiency and transmission distance capabilities. The differences between the system energy efficiency and the transmission efficiency are also highlighted. The review covers the two-coil systems, the four-coil systems, the systems with relay resonators and the wireless domino-resonator systems. Related issues including human exposure issues and reduction of winding resistance are also addressed. The review suggests that the use of the maximum energy efficiency principle in the two-coil systems is suitable for short-range rather than mid-range applications, the use of the maximum power transfer principle in the four-coil systems is good for maximizing the transmission distance, but is under a restricted system energy efficiency (<;50%); the use of the maximum energy efficiency principle in relay or domino systems may offer a good compromise for good system energy efficiency and transmission distance on the condition that relay resonators can be placed between the power source and the load.

A Critical Review of Recent Progress in Mid-Range Wireless Power Transfer

Wireless charging is a technology of transmitting power through an air gap to electrical devices for the purpose of energy replenishment. The recent progress in wireless charging techniques and development of commercial products have provided a promising alternative way to address the energy bottleneck of conventionally portable battery-powered devices. However, the incorporation of wireless charging into the existing wireless communication systems also brings along a series of challenging issues with regard to implementation, scheduling, and power management. In this paper, we present a comprehensive overview of wireless charging techniques, the developments in technical standards, and their recent advances in network applications. In particular, with regard to network applications, we review the static charger scheduling strategies, mobile charger dispatch strategies and wireless charger deployment strategies. Additionally, we discuss open issues and challenges in implementing wireless charging technologies. Finally, we envision some practical future network applications of wireless charging.

Wireless Charging Technologies: Fundamentals, Standards, and Network Applications

Traditional technologies for virtual reality (VR) and augmented reality (AR) create human experiences through visual and auditory stimuli that replicate sensations associated with the physical world. The most widespread VR and AR systems use head-mounted displays, accelerometers and loudspeakers as the basis for three-dimensional, computer-generated environments that can exist in isolation or as overlays on actual scenery. In comparison to the eyes and the ears, the skin is a relatively underexplored sensory interface for VR and AR technology that could, nevertheless, greatly enhance experiences at a qualitative level, with direct relevance in areas such as communications, entertainment and medicine1,2. Here we present a wireless, battery-free platform of electronic systems and haptic (that is, touch-based) interfaces capable of softly laminating onto the curved surfaces of the skin to communicate information via spatio-temporally programmable patterns of localized mechanical vibrations. We describe the materials, device structures, power delivery strategies and communication schemes that serve as the foundations for such platforms. The resulting technology creates many opportunities for use where the skin provides an electronically programmable communication and sensory input channel to the body, as demonstrated through applications in social media and personal engagement, prosthetic control and feedback, and gaming and entertainment. Interfaces for epidermal virtual reality technology are demonstrated that can communicate by programmable patterns of localized mechanical vibrations.

Skin-integrated wireless haptic interfaces for virtual and augmented reality.

It is shown that the distance of wireless power transfer is significantly increased by placing intermediate resonators (repeaters) between transmitter and receiver. The guidelines are provided to use the repeaters effectively. We first briefly review how the repeaters enhance the transfer distance. Next, the aspect of resonant frequency splitting is studied separately both for even and odd numbers of repeaters. The transferred power, efficiency, and output impedance phase are evaluated for each repeater configuration. These provide the guidelines to select the optimum repeater positions and numbers. We also propose and study a new kind of repeater which can be placed (biased) even in the vicinity of transmitter or receiver. This increases the flexibility in choosing the repeater position. The net effect of such biased repeater is approximately doubling the effective coupling coefficient between transmitter and receiver. Experimental results are provided to prove the concepts.

A Study on Magnetic Field Repeater in Wireless Power Transfer

Wireless power transfer (WPT) concept offers users the freedom from annoying wires, and allowing seamless powering and charging of portable devices in an unburdened mode. Since Nikola Tesla׳s early experiment, the WPT technology has observed the remarkable technological advancement on transmission methods which previously deemed unfeasible. This review paper outlines recent research activities on wireless power technology covering the history, the basic principle of magnetic resonant coupling, and early works on resonant coupled WPT. The two fundamental concepts of power transmission, the maximum power transfer and maximum energy efficiency principles, are summarized in terms of their energy efficiency and transmission distance capabilities. This paper also reviews the comparative study between coupled-mode theory (CMT) and reflected load theory (RLT) in case of analyzing the power transmission model of conventional 2-coil resonant coupled WPT with frequency splitting modes. The study shows that circuit-based RLT provides accurate results as CMT while predicting the average power transmission efficiency in steady-state analysis and is more convenient. This paper explains the effectiveness of advance 4-coil resonant coupled system adopting maximum power transfer principle in extending the operating range of WPT with better power transmission. Various efficiency enhancement techniques for resonant coupled WPT including system with multiple receivers are demonstrated in this paper. The review implies that the adaptive impedance matching using LC circuits is more prolific in practical terms to improve the efficiency of coupled coils. Moreover, the benefits of resonant coupled WPT, its implementation in both consumer and non-consumer applications, and the commercial journey of WPT along with the safety consideration to human exposure issues are also addressed in this paper.

Wireless Powering by Magnetic Resonant Coupling: Recent Trends in Wireless Power Transfer System and its Applications

This letter presents an efficiency analysis of a magnetic resonance wireless power transfer (WPT) system with an intermediate resonant coil. A helical coil and a spiral coil with an additional capacitor are considered as resonant coils for the WPT system. The intermediate resonant coil is set up coaxially and perpendicular to both the Tx and Rx resonant coils in order to observe the efficiency change according to the directions. The power efficiency is calculated using the temporal coupled mode theory (CMT). Impedance matching conditions are also shown by using the CMT. Analysis results show that using an intermediate coil properly improves efficiency and extends the distance between the transmitter and receiver. Both calculated and measured efficiencies are in good agreement. It is also shown that the intermediate resonant system has a good efficiency and is superior to nonintermediate systems.

Efficiency Analysis of Magnetic Resonance Wireless Power Transfer With Intermediate Resonant Coil

This paper proposes an optimal design method in an asymmetric wireless power transfer (WPT) system for a 150 watt LED TV. The WPT system has three self-resonators: a Tx resonator, an Rx resonator, and an intermediate resonator. The Tx and Rx resonators are perpendicular and offset, respectively, to the intermediate resonator in the geometry. For optimal design, the WPT system is analyzed using an equivalent circuit. In particular, a calculation method for mutual inductance in the system is expressed. The calculation results of mutual inductance are used to determine the optimal position of each self-resonator for maximizing the power transfer efficiency. For verification, a WPT system for a 150 watt, 47 inch LED TV is fabricated at 250 kHz. The WPT system exhibits wireless power transfer efficiency of 80%.

Optimal design of a wireless power transfer system with multiple self-resonators for an LED TV

In Table 1 of the above titled letter (ibid., vol. 10, pp. 389-392, 2011), the value 233.60pF under the "lumped C" column is incorrect. The correct value is 148.00pF.

Correction to “Efficiency Analysis of Magnetic Resonance Wireless Power Transfer With Intermediate Resonant Coil”

A planar coil of multiple loops connected in series and in parallel is proposed for wireless power transfer (WPT). The current direction in each loop is mixed in forward and in reverse. The proposed coil gives uniform mutual inductance and enables simple impedance matching for free-positioning WPT.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/el.2013.0135

Multi-loop coil supporting uniform mutual inductances for free-positioning WPT

In this paper, a wireless power transfer (WPT) system supporting free positioning in a large area is proposed. The main coupling unit of the proposed system consists of a transmitter with 5 self-resonators and a receiver with a self-resonator. By circuit analysis, the proposed system is analyzed. The fabricated coils are compact planar rectangular spiral types. The total size of the transmitter is 96 cm × 16 cm. The receiver is 10 cm × 10 cm. With the exception of the 5th resonator in the transmitter, their resonant frequencies are 6.78 MHz. There are some regions with less than 50% power transfer efficiency (called a dead zone). By changing the 5th resonator's frequency, the area of the dead zone can be reduced. When the 5th resonator's resonant frequency f 5 is 6.78 MHz, a system has 40% dead zone of the transmitter and power transfer efficiencies of 57% ∼ 87% in the other regions. On the other hand, when f 5 is 6.38 MHz, a system has 10% dead zone and power transfer efficiencies of 70% ∼ 86% in other regions. The measured power transfer efficiencies are approximately consistent with the calculated results.

Hyeon-Chang Son

Papers

Efficiency Analysis of Magnetic Resonance Wireless Power Transfer With Intermediate Resonant Coil

Optimal design of a wireless power transfer system with multiple self-resonators for an LED TV

Correction to “Efficiency Analysis of Magnetic Resonance Wireless Power Transfer With Intermediate Resonant Coil”

Multi-loop coil supporting uniform mutual inductances for free-positioning WPT

Wireless power transfer for free positioning using compact planar multiple self-resonators