Wireless power transfer (WPT) is a promising new solution to provide convenient and perpetual energy supplies to wireless networks. In practice, WPT is implementable by various technologies such as inductive coupling, magnetic resonate coupling, and electromagnetic (EM) radiation, for short-/mid-/long-range applications, respectively. In this paper, we consider the EM or radio signal enabled WPT in particular. Since radio signals can carry energy as well as information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) is pursued. Specifically, this paper studies a multiple-input multiple-output (MIMO) wireless broadcast system consisting of three nodes, where one receiver harvests energy and another receiver decodes information separately from the signals sent by a common transmitter, and all the transmitter and receivers may be equipped with multiple antennas. Two scenarios are examined, in which the information receiver and energy receiver are separated and see different MIMO channels from the transmitter, or co-located and see the identical MIMO channel from the transmitter. For the case of separated receivers, we derive the optimal transmission strategy to achieve different tradeoffs for maximal information rate versus energy transfer, which are characterized by the boundary of a so-called rate-energy (R-E) region. For the case of co-located receivers, we show an outer bound for the achievable R-E region due to the potential limitation that practical energy harvesting receivers are not yet able to decode information directly. Under this constraint, we investigate two practical designs for the co-located receiver case, namely time switching and power splitting, and characterize their achievable R-E regions in comparison to the outer bound.

MIMO Broadcasting for Simultaneous Wireless Information and Power Transfer

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

Diet is a major lifestyle-related risk factor of various chronic diseases. Dietary intake can be assessed by subjective report and objective observation. Subjective assessment is possible using open-ended surveys such as dietary recalls or records, or using closed-ended surveys including food frequency questionnaires. Each method has inherent strengths and limitations. Continued efforts to improve the accuracy of dietary intake assessment and enhance its feasibility in epidemiological studies have been made. This article reviews common dietary assessment methods and their feasibility in epidemiological studies.

https://www.e-epih.org/upload/pdf/epih-36-e2014009.pdf

Dietary assessment methods in epidemiologic studies.

Resonance-based wireless power delivery is an efficient technique to transfer power over a relatively long distance. This technique typically uses four coils as opposed to two coils used in conventional inductive links. In the four-coil system, the adverse effects of a low coupling coefficient between primary and secondary coils are compensated by using high-quality (Q) factor coils, and the efficiency of the system is improved. Unlike its two-coil counterpart, the efficiency profile of the power transfer is not a monotonically decreasing function of the operating distance and is less sensitive to changes in the distance between the primary and secondary coils. A four-coil energy transfer system can be optimized to provide maximum efficiency at a given operating distance. We have analyzed the four-coil energy transfer systems and outlined the effect of design parameters on power-transfer efficiency. Design steps to obtain the efficient power-transfer system are presented and a design example is provided. A proof-of-concept prototype system is implemented and confirms the validity of the proposed analysis and design techniques. In the prototype system, for a power-link frequency of 700 kHz and a coil distance range of 10 to 20 mm, using a 22-mm diameter implantable coil resonance-based system shows a power-transfer efficiency of more than 80% with an enhanced operating range compared to ~40% efficiency achieved by a conventional two-coil system.

Design and Optimization of Resonance-Based Efficient Wireless Power Delivery Systems for Biomedical Implants

The intent of this chapter is to enhance understanding of various dietary assessment methods so that the most appropriate method for a particular need is chosen. This review focuses only on individual level food intake assessment. It is intended to serve as a resource for those who wish to assess diet in a research study using individual measurements for group level analysis. The chapter reviews major dietary assessment methods, their advantages and disadvantages, and validity; describes which dietary assessment methods are appropriate for different types of studies and populations; and discusses specific issues that relate to all methods.

Dietary Assessment Methodology

Wireless power transfer using strongly coupled electromagnetic resonators is a recently explored technology. Although this technology is able to transmit electrical energy over a much longer distance than traditional near field methods, in some applications, its effective distance is still insufficient. In this paper, we investigate a relay effect to extend the energy transfer distance. Theoretical analysis is performed based on a set of coupled-mode equations. Experiments are conducted to confirm the theoretical results and demonstrate the effectiveness of the relay approach. Our results show that the efficiency of power transfer can be improved significantly using one or more relay resonators. This approach significantly improves the performance of the present two-resonator system and allows a curved path in space to be defined for wireless power transfer using smaller resonators.

Relay Effect of Wireless Power Transfer Using Strongly Coupled Magnetic Resonances

Dietary reporting by individuals is subject to error (1–3). Therefore, a research program has been initiated to develop a small electronic device to record food intake automatically. This device, which contains a miniature camera, a microphone, and several other sensors, can be worn on a lanyard around the neck. It collects visual data immediately in front of the participant and stores them on a memory card in the device. The data are transferred regularly to the dietitian’s computer for further processing and analysis. The device is designed to be almost completely passive to the participant, and thus hopefully will not intrude on or alter the participant’s eating activities. In addition to this function, in the future the device will have other functions, such as the measurement of physical activity, human behavior, and environmental exposure (e.g., pollutants).

A Wearable Electronic System for Objective Dietary Assessment

This paper investigates wireless electricity (witricity) and its application to medical sensors and implantable devices. Several coupling scenarios of resonators are analyzed theoretically. In vitro experiments are conducted in open air and through an agar phantom of the human head. An in vivo animal experiment is also carried out. Our studies indicate that witricity is a suitable tool for providing wireless power to a variety of medical sensors and implanted devices.

In vitro and in vivo studies on wireless powering of medical sensors and implantable devices

A wireless energy transfer system includes a first energy transfer unit having at least one resonant frequency, a second energy transfer unit having the at least one resonant frequency, and a load. The first wireless energy transfer unit includes a first coil magnetically coupled to a first wireless energy transfer cell, and the second wireless energy transfer unit includes a second coil magnetically coupled to a second wireless energy transfer cell. The first coil receives first energy and through the magnetic coupling between the first coil and the first wireless energy transfer cell, the first wireless energy transfer cell is caused to generate second energy, wherein the second wireless energy transfer cell receives the second energy and through the magnetic coupling between the second wireless energy transfer cell and the second coil, the second coil is caused to provide third electromagnetic wave energy to the load.

Wireless energy transfer system

Witricity is a newly developed technique for wireless energy transfer. This paper presents a frequency adjustable witricity system to power medical sensors and implantable devices. New witricity resonators are designed for both energy transmission and reception. A prototype platform is described, including an RF power source, two resonators with new structures, and inductively coupled input and output stages. In vitro experiments, both in open air and using a human head phantom consisting of simulated tissues, are employed to verify the feasibility of this platform. An animal model is utilized to evaluate in vivo energy transfer within the body of a laboratory pig. Our experiments indicate that witricity is an effective new tool for providing a variety of medical sensors and devices with power.

Steven A. Hackworth

Papers

Relay Effect of Wireless Power Transfer Using Strongly Coupled Magnetic Resonances

A Wearable Electronic System for Objective Dietary Assessment

In vitro and in vivo studies on wireless powering of medical sensors and implantable devices

Wireless energy transfer system

Wireless energy transfer platform for medical sensors and implantable devices