Wireless Power Transfer Capability Research Articles

3D Woven Liquid Metals for Radio‐Frequency Stretchable Circuits

AbstractMechanically flexible and stretchable inductive coils are critical for enabling communication, sensing, and wireless power transfer capabilities in wearable electronics that conform to the body for healthcare and Internet of Things (IoT) applications. Leading stretchable conductors such as liquid metals (LMs) offer conformability but sacrifice electromagnetic performance compared with Cu wires, leading to lossy radio‐frequency (RF) characteristics. Here, a strategy leveraging multistranded 3D woven ‘litz’ transmission lines is presented to amplify the resonant RF performance of LM inductors. Through comprehensive simulations and experiments, it is discovered that interwoven LM litz wires boost the Quality Factor (Q) by 80% compared to standard liquid metal wires. A fabrication methodology is also demonstrated for stretchable coils that retain high Q (>30), outperforming the previously reported LM coils and maintaining 98% of their wireless transmission efficiency under up to 30% biaxial strain. Moreover, the versatility of this approach is showcased by 3D printing four‐terminal ‘choke’ inductors optimized for RF filtering and inductance tunability, overcoming the fabrication limitations of traditional planar printed electronics. These results offer valuable insights into the design and implementation of 3D‐printed inductors for a diverse suite of electromagnetic device applications.

Remote terahertz wireless power transfer with self-resonating spoof plasmonic structures.

In this paper, we use a pair of self-resonating subwavelength spoof plasmonic structures to achieve remote non-radiative terahertz wireless power transfer, while nearly without affecting the electromagnetic environment of free space around the structure. The resonating frequency and quality factor of the magnetic dipole mode supported by the spoof plasmonic structures can be freely tuned by tailoring the geometric structure. By putting the weak source and detector into the self-resonating structures, we can find that the effective non-radiative terahertz power transferring distance can reach several hundred times the radius of the structures. Finally, we also demonstrate the efficient wireless power transfer capability for the multi-target receiving system. These results may provide a novel approach to the design of non-radiative terahertz wireless power transfer and communications.

Maximum Wireless Power Transmission Using Real-Time Single Iteration Adaptive Impedance Matching

Wireless power transfer (WPT) systems’ efficiency is significantly impacted by non-monotonic variations in the coupling coefficient. For very short distances or strong-coupling cases, the WPT efficiency is minimal at the natural resonant frequency, with two peaks around this frequency, known as the frequency splitting phenomenon. On the other hand, WPT capability decreases for long distances or weak coupling cases. Therefore, adaptive matching is required for WPT systems with varying distances, like wireless charging systems for electric vehicles (EVs). This paper first presents a detailed analysis of the frequency splitting phenomenon by studying the root locations of the WPT system’s transfer function. Then, a real-time fixed-frequency adaptive impedance matching (IM) method is proposed, in which the amplitude and phase of the input impedance is estimated using the average active power, the average reactive power, and the amplitude of input voltage. Unlike traditional search-and-find techniques, the proposed method calculates the optimal IM network parameters only in a single iteration, which improves the convergent speed. A scaled-down 20-Watt prototype controlled by the TMSF2812 is fabricated and used to validate the effectiveness of the proposed method over a wide range of coil-to-coil distances.

Deep learning for online computation offloading and resource allocation in NOMA

The limited battery capacity and low computing capability of wireless Internet of Things (IoT) devices can hardly support computation-intensive and delay-sensitive applications. While recent development of wireless power transfer (WPT) and mobile edge computing (MEC) technologies help IoT devices harvest energy and offload computation tasks to edge servers. While it is still challenging to design an efficient offloading policy to improve the performance of the IoT network. In this article, we consider a MEC network that has WPT capability and adopts the non-orthogonal multiple access (NOMA) technology to offload tasks partially. Our goal is to propose an online algorithm to optimize resource allocation under a wireless dynamic channel scenario. In order to obtain the optimal offloading decision and resource allocation efficiently, we propose a Deep Reinforcement learning-based Online Sample-improving (DROS) framework which implements a deep neural network to input the discretized channel gains to obtain the optimal WPT duration. Based on the WPT duration derived by DNN, we design an optimization algorithm to derive the optimal energy proportion for offloading data. Numerical results verify that compared with traditional optimization algorithms, our proposed DROS has significantly sped up convergence for better solutions.

Adaptively Smart Wireless Power Transfer Using 2-Bit Programmable Metasurface

An adaptively smart wireless power transfer (WPT) system is proposed by using a 2-bit programmable metasurface (PMS), in which an indoor dynamic charging application scenario is described and near-field focused (NFF) technique is adopted to transmit wireless powers. Theoretical analyses of NFF PMS are presented to illustrate its WPT capabilities. To implement dynamic WPTs, digital phase-shift quantization of PMS element is investigated, indicating that 2-bit quantization scheme is a preferred solution. As experimental demonstrations, a 2-bit 12 × 12 PMS operating at 5.8 GHz is designed, fabricated, and tested. High-power focal spots that can be regulated to focus at different near-field positions are measured in an anechoic chamber. We conduct multiple WPT experiments by using the 2-bit PMS as the transmitter and a 4 × 4 microstrip array antenna as the receiver. Experimental results show that the proposed 2-bit PMS can continuously transmit the wireless powers to the receiver even though the target is moving, and the WPT efficiency is maintained over 9.1%, which has about 16.3 dB improvement at the end of the trajectory compared to the fixed NFF beam. This work is one of the first attempts to thoroughly examine the possibility of the dynamic WPTs using PMS, offering a brand-new manner for the adaptively smart WPT systems.

Differential Chaos Shift Keying-Based Wireless Power Transfer With Nonlinearities

In this paper, we investigate conventional communication-based chaotic waveforms in the context of wireless power transfer (WPT). Particularly, we present a differential chaos shift keying (DCSK)-based WPT architecture, that employs an analog correlator at the receiver, in order to boost the energy harvesting (EH) performance. We take into account the nonlinearities of the EH process and derive closed-form analytical expressions for the harvested direct current (DC) under a generalized Nakagami- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula> block fading model. We show that, in this framework, both the peak-to-average-power-ratio of the received signal and the harvested DC, depend on the parameters of the transmitted waveform. Furthermore, we investigate the case of deterministic unmodulated chaotic waveforms and demonstrate that, in the absence of a correlator, modulation does not affect the achieved harvested DC. On the other hand, it is shown that for scenarios with a correlator-aided receiver, DCSK significantly outperforms the unmodulated case. Based on this observation, we propose a novel DCSK-based signal design, which further enhances the WPT capability of the proposed architecture; corresponding analytical expressions for the harvested DC are also derived. Our results demonstrate that the proposed architecture and the associated signal design, can achieve significant EH gains in DCSK-based WPT systems. Furthermore, we also show that, even by taking into account the nonlinearities at the transmitter amplifier, the proposed chaotic waveform performs significantly better in terms of EH, when compared with the existing multisine signals.

Differential Chaos Shift Keying-based Wireless Power Transfer with Nonlinearities

In this paper, we investigate conventional communication-based chaotic waveforms in the context of wireless power transfer (WPT). Particularly, we present a differential chaos shift keying (DCSK)-based WPT architecture, that employs an analog correlator at the receiver, in order to boost the energy harvesting (EH) performance. We take into account the nonlinearities of the EH process and derive closed-form analytical expressions for the harvested direct current (DC) under a generalized Nakagami-m block fading model. We show that, in this framework, both the peak-to-average-power-ratio of the received signal and the harvested DC, depend on the parameters of the transmitted waveform. Furthermore, we investigate the case of deterministic unmodulated chaotic waveforms and demonstrate that, in the absence of a correlator, modulation does not affect the achieved harvested DC. On the other hand, it is shown that for scenarios with a correlator-aided receiver, DCSK significantly outperforms the unmodulated case. Based on this observation, we propose a novel DCSK-based signal design, which further enhances the WPT capability of the proposed architecture; corresponding analytical expressions for the harvested DC are also derived. Our results demonstrate that the proposed architecture and the associated signal design, can achieve significant EH gains in DCSK-based WPT systems. Furthermore, we also show that, even by taking into account the nonlinearities at the transmitter amplifier, the proposed chaotic waveform performs significantly better in terms of EH, when compared with the existing multisine signals.

Compact circularly polarized beam-switching wireless power transfer system for ambient energy harvesting applications

In this work, we propose a circularly polarized (CP) beam-switching wireless power transfer system for ambient energy harvesting applications operating at 2.4 GHz. Beam-switching is achieved using a low profile, electrically small CP antenna array with four elements and a novel miniaturized 4×4 butler matrix. The CP antenna is designed with an e-shaped slot and four antennas. The CP antenna measures 0.32 λ0×0.32 λ0×0.006 λ0 at 2.4 GHz. The antenna has a gain of 3 dBic and an axial ratio less than 3-dB at 2.4 GHz. A linear antenna array consisting of four elements is designed with the CP antenna element with an inter-element distance of 0.29 λ0. A 4×4 butler matrix with miniaturized couplers and crossovers are used to feed the four antenna array elements. Based on the input port of excitation, the main beam of the antenna array is demonstrated to be switched to four directions: −5°, 65°, −55°, and 20°. A CP rectenna is used to demonstrate the wireless power transfer capability of the combination of the butler matrix and the CP-antenna array. The rectenna consists of a Teo-shaped CP antenna and a rectifier. The open circuit voltage at the output of the rectenna is found to peak value of 30 mV at −3°, 61°, −53°, and 17°. Thus a complete system for CP wireless power transfer including the power transmission system as well as the RF energy harvesting sensor is designed and experimentally verified.

A 3D wireless charging cube with externally enhanced magnetic field for extended range of wireless power transfer

More mobile devices such as mobile phones and robots are wirelessly charged for convenience, simplicity, and safety, and it would be desirable to achieve three-dimensional (3D) wireless charging with high spatial freedom and long range. This paper proposes a 3D wireless charging cube with three orthogonal coils and supporting magnetic cores to enhance the magnetic flux outside the cube. The proposed system is simulated by Ansoft Maxwell and implemented by a downsized prototype. Both simulation and experimental results show that the magnetic cores can strengthen the magnitude of B-field outside the cube. The final prototype demonstrates that the power transfer distance outside the cube for getting the same induced electromotive force in the receiver coil is extended approximately by 50 mm using magnetic cores with a permeability of 2800. It is found that the magnitude of B-field outside the cube can be increased by increasing the width and the permeability of the magnetic cores. The measured results show that when the permeability of the magnetic cores is fixed, the induced electromotive force in the receiver coil at a point 300 mm away from the center of the cube is increased by about 2V when the width of the magnetic cores is increased from 50 to 100 mm. The increase in the induced electromotive force at an extended point implies a greater potential of wireless power transfer capability to the power pickup.

A Novel Electric Insulation String Structure With High-Voltage Insulation and Wireless Power Transfer Capabilities

High-voltage insulation (HVI) strings are commonly used to hold high-voltage electric cables and electrically isolate them from the grounded transmission tower. In this paper, a novel concept of an electric insulation string with HVI and wireless power transfer (WPT) capabilities is presented. Based on the concept of the domino-resonator WPT system, this new structure consists of coil resonators embedded inside totally sealed insulation discs, which are then connected in series to form the new insulation string structure with the simultaneous HVI and WPT functions. This structure allows energy harvested from the ac magnetic field around the high-voltage cable to be transmitted wirelessly to power an online monitoring system in high-voltage transmission tower continuously, thereby reducing the storage requirements of the battery. The design and analysis of this new WPT structure based on the dimensions of commercially available HVI rod are included. Practical measurements obtained from a hardware prototype of about 25 W have been obtained to confirm the WPT capability of the proposal. An energy efficiency of more than 60% has been achieved for a transmission distance of 1.1 m over a wide range of load.

Ultrafast Backscatter Modulator With Low-Power Consumption and Wireless Power Transmission Capabilities

The rapid increase in the progress and development of wireless communications and identification has led to the increase of billions of devices applied in the Internet of Things (IoT) context. Low-power wireless sensors, passive-backscatter radios such as passive radio frequency identifications (RFIDs), RFID-enabled sensors, and passive wireless sensors are seen as key-enabling technologies to realize the future ubiquitous IoT. This letter presents a system composed of a backscatter modulator at 2.45 GHz that has a consumption of 61.5 fJ/b with wireless power transfer capabilities at 1.7 GHz. This 16-QAM modulator is shown to provide 960 Mb/s of data rate with a error vector magnitude of 8.37%. Moreover, the system is also composed of a rectifier with 56% of efficiency with an input power of 5 dBm at 1.7 GHz with unbiased gate at each transistor. The demonstrated system enables low power consumption while maintaining a high modulation bandwidth.

Fronthaul Load Balancing in Energy Harvesting Powered Cloud Radio Access Networks

Enhanced with wireless power transfer capability, cloud radio access network (C-RAN) enables energy-restrained mobile devices to function uninterruptedly. Beamforming of C-RAN has potential to improve the efficiency of wireless power transfer, in addition to transmission data rates. In this paper, we design the beamforming jointly for data transmission and energy transfer, under finite fronthaul capacity of C-RAN. A non-convex problem is formulated to balance the fronthaul requirements of different remote radio heads (RRHs). Norm approximations and relaxations are carried out to convexify the problem to second-order cone programming (SOCP). To improve the scalability of the design to large networks, we further decentralize the SOCP problem using the alternating direction multiplier method (ADMM). A series of reformulations and transformations are conducted, such that the resultant problem conforms to the state-of-the-art ADMM solver and can be efficiently solved in real time. Simulation results show that the distributed algorithm can remarkably reduce the time complexity without compromising the fronthaul load balancing of its centralized counterpart. The proposed algorithms can also reduce the fronthaul bandwidth requirements by 25% to 50%, compared with the prior art.

On Wireless Power Transfer and Max Flow in Rechargeable Wireless Sensor Networks

In rechargeable or energy harvesting wireless sensor networks (WSNs), a key concern is the max flow or data rate at one or more sinks. However, this data rate is constrained by the available energy at each node as well as link capacity. To date, in order to increase the amount of data extracted from a WSN, past works have considered routing approaches or they optimize the location of sinks. In contrast, we take a novel approach whereby we aim to “upgrade” the recharging rate of a finite number of “bottleneck” nodes using the so called auxiliary chargers (ACs) equipped with wireless power transfer capability. We formulate a mixed integer linear program (MILP) for the NP-hard problem at hand and propose three novel solutions to place ACs: 1) Path , which preferentially upgrades nodes on the shortest path among paths from sources to sinks, 2) Tabu , a meta-heuristic that first uses Path as the initial solution. It then searches for a neighboring solution that yields a higher max flow rate, and 3) LagOP , which approximates the said MILP using Lagrangian and sub-gradient optimization. Our results show that Tabu has the best performance, where it is able to achieve 99.40% of the max flow rate derived by MILP in tested scenarios.

A study on Relay Effect via Magnetic Resonant Coupling for Wireless Power Transfer

Wireless power transfer (WPT) transmits electrical energy from a power source to an electrical load wirelessly or without any conductors. The capability of WPT to transmit the energy is limited. Therefore, a relay was introduced to increase the distance of the WPT capabilities. The effect of the relay has been investigated to extend the energy transfer distance. The effect of relay was demonstrated by placing a relay coil between transmitter and receiver, relay biased to transmitter and placing two relay coils in the designed system. Experimental results are provided to prove the concept of the relay effect. The power transmission efficiency can be achieved up to 75% at 1 meter distance.