Tx Coil Research Articles

Determination and analysis of compensation capacitor for a robust distance-variable wireless power transfer system

Wireless power transfer (WPT) systems have been widely adopted for full autonomy in various fields due to their convenience. However, changes in the air gap between the Tx and Rx coils significantly affect efficiency. To overcome this challenge, this paper introduces the determination of a compensation capacitor for a distance-variable WPT system that is robust in varying air gap conditions. The proposed method was verified using theoretical analysis, simulation, and experimental measurement. The electrical circuit was modeled using a T-equivalent model in a series–series (SS) topology to calculate power transfer efficiency (PTE). Specifically, compensation capacitors were analyzed at distances of 10, 30, and 50 mm, considering different self-inductance values. These results are compared against varying load resistances to demonstrate the effectiveness of the proposed approach. Additionally, the PTE drop ratio was defined to facilitate comparison. The results show that the PTE drop ratio for the compensation capacitor at the farthest distance was consistently smaller than that for the capacitor at the nearest distance under varying air gaps and load resistances. In this research, the difference in the PTE drop ratio between 10 and 50 mm was measured, demonstrating that determining the capacitor at the farthest distance reduces the PTE drop ratio across a range of operational conditions.

A Wireless Miniature Injectable Device with Memory-Assisted Backscatter for Multimodal Animal Physiological Monitoring.

This paper introduces a wirelessly powered multimodal animal physiological monitoring application-specific integrated circuit (ASIC). Fabricated in the 180 nm process, the ASIC can be integrated into an injectable device to monitor subcutaneous body temperature, electrocardiography (ECG), and photoplethysmography (PPG). To minimize the device size, the ASIC employs a miniature receiver (Rx) coil for wireless power receiving and data communication through a single inductive link operating at 13.56 MHz. We propose a folded L-shape Rx coil with improved coupling to the transmitter (Tx) coil, even in the presence of misalignment, and enhanced quality factor. The ASIC functions alternatively between recording and sleeping modes, consuming 2.55 μW on average. For PPG measurements, a reflection-type PPG sensor illuminates an LED with tunable current pulses. A current-input analog frontend (AFE) amplifies the current of a photodiode (PD) with 30.8 pARMS current input-referred noise (IRN). The ECG AFE captures ECG signals with a configurable gain of 45-80 dB. The temperature AFE achieves 0.02 ̊C inaccuracy within a sensing range between 27-47 ̊C. The AFE outputs are sequentially digitized by a 10-bit successive approximation register (SAR) analog-to-digital converter (ADC) with an effective number of bits (ENOB) of 8.79. To improve the reliability of data transmission, we propose a memory-assisted backscatter scheme that stores ADC data in an off-chip memory and transmits it when the coupling condition is stable. This scheme achieves a package loss rate (PLR) lower than 0.2% while allowing 24-hour data storage. The device's functionality has been evaluated by in vivo experiments.

Multiple-Split Transmitting Coils for Stable Output Power in Wireless Power Transfer System with Variable Airgaps

In this paper, a tunable multi-split transmitting (TX) coil for a wireless power transfer (WPT) system that accommodates a wide range of distances between the TX and receiving (RX) coils is proposed. This method enables the WPT system to maintain a stable and consistent output power supply to the load, regardless of variations in coupling coefficients caused by changes in the distance between the TX and RX coils. The tunable multi-split TX coil can operate in various modes depending on how the wire connections between each TX coil are configured. This approach adjusts the inductance value of the TX coil for different conditions, using the same amount of coil as a conventional single-loop TX coil. The results show that by adjusting the TX coil to three different modes as the airgap varies from 50 mm to 250 mm, consistent output power is achieved with smaller variations in the input current and voltage compared to those in a conventional system. A conventional system requires an input voltage increase of approximately 529.64%, while the proposed system requires only a 42.93% increase. The proposed system enhances the power transfer capacity of the WPT system, particularly when operating in an over-coupling state. This approach provides a stable output power supply with a standardized and simplified TX coil structure.

A small-loop double-coil decoupling TEM device for high sensitivity near surface detection

Small loop transient electromagnetic (TEM) devices, several meters in size, are increasingly applied to near-surface geophysical exploration in space-limited environment. But the transmitter (TX) coil significantly affects the early time data acquisition of the receiver (RX) coil during current turn-off, resulting in the loss of shallow signals and creating a significant blind zone problem. We report a double-coil decoupling TEM (DCDTEM) device to solve this problem. The novel DCDTEM device ensures a weak primary magnetic flux region between the two coplanar transmitter (TX) coils with the same current direction and different radii. Simulation results indicate that the transmitting field of the DCDTEM device is more focused compared to the traditional central loop device, ensuring a stronger coupling with the target. 3-D TEM forward modeling results show that the DCDTEM device has the highest detection sensitivity, together with a relatively high lateral resolution compared to other small-loop TEM devices. We further carry out several physical experiments to test the response characteristics of the DCDTEM device with reference to both the offset and the ratio of turns between the TX and RX coils. The experimental results ensure that the position of the zero magnetic flux point can be accurately calibrated and mitigate interference from TX coils. The proposed DCDTEM device is also applied in a field test on water pipe detection, demonstrating its practicality in near-surface detection.

Wirelessly Powered and Bi-Directional Data Communication System With Adaptive Conversion Chain for Multisite Biomedical Implants Over Single Inductive Link.

A wirelessly powered and data communication system is presented which is implemented as a full system, designed for multisite implanted biomedical applications. The system is capable of receiving wireless power and data communication for each implant separately, using inductive links with different resonance frequencies. To achieve this, dual-band coils are presented in the system. In addition, the system supports bi-directional half-duplex data communication, utilizing amplitude and load shift keying (ASK and LSK) modulation schemes over a single inductive link. The system employs a digitally assisted active rectifier and an automatic resonance tuning system, to improve the power transfer efficiency (PTE) through various coupling coefficients, while minimizing the reverse current and power dissipation. The power control unit enables closed-loop monitoring to prevent high or low power delivery, and it can detect inefficient or excessive wireless power transmission or prevent temperature elevation by limiting the voltage to a safe level. A new structure of self-sampling separated- Vb ASK demodulator is proposed in the paper which is utilized within the data conversion chain, serving both the external and implanted units. The whole system is fabricated using a standard 180-nm 1.8/3.3 V CMOS process with a core area of 0.82 mm[Formula: see text]. The system is tested with coupled multisite inductive links and offers the maximum overall PTE of 31.2%, from the Tx coil to the implant load.

Realization of an Optimum Load for Wireless Power Transfer System

Wireless power transfer (WPT) system has been an integral part of personal living since its regained interest, especially the magnetic resonance (MR) scheme. MR-WPT scheme suffers, however, change of the coil separation distance and various alignment errors. This paper reports a realization of optimum load for MR-WPT system, which can change the loading impedance accordingly for different coupling coefficients between the Tx and Rx coils. A simple varactor circuit is adopted to realize the optimum load curve. Usefulness of this is demonstrated through both the steady and transient analysis. The proposed realization relies on an open-circuit scheme, and hence it is suitable for scenarios with low-cost and small-size requirements.

Maze-Based Scalable Wireless Power Transmission Experimental Arena for Freely Moving Small Animals Applications.

This paper presents an innovative T/Y-maze-based wireless power transmission (WPT) system designed to monitor spatial reference memory and learning behavior in freely moving rats. The system facilitates uninterrupted optical/electrical stimulation and neural recording experiments through the integration of wireless headstages or implants in T/Y maze setups. Utilizing an array of resonators covering the entire underneath of the mazes, the wireless platform ensures scalability with various configurations. The array is designed to ensure a natural localization mechanism to localize the Tx power toward the location of the Rx coil. The system is analyzed and modeled using ANSYS HFSS software to optimize design. The primary goal was to achieve uniform wireless power delivery throughout the mazes through a comparative study of different transmitter (Tx) array configurations, such as float, series, and parallel resonators. The calculated Specific Absorption Rate (SAR) in rat tissue model equals 1.7 W/kg at the power carrier frequency of 13.56 MHz. A prototype of the proposed maze-based WPT design, featuring 8 Tx resonators, a Tx coil and power amplifier, and a headstage power harvesting unit, is successfully implemented and its performance characterized for all three resonator configurations. The implemented T maze-based WPT system has a total length of 128 cm. In the overlapping Tx resonators configuration, a homogeneity of 94% is achieved for the measured power transfer efficiency at over 30%, while continuously delivering over 60 mW for series configuration.

Multifunctional coil technique for alignment-agnostic and Rx coil size-insensitive efficiency enhancement for wireless power transfer applications

This paper presents a multifunctional coil technique to enhance the transfer efficiency of an inductively-coupled wireless power transfer (WPT) system, regardless of the alignment condition and size ratio between the transmitter (Tx) and receiver (Rx) coils. The technique incorporates an auxiliary coil on the Tx side, where current is induced through coupling from the primary coil. Since the Tx coil consists of two coils, transmission to the Rx occurs through the coil with the higher coupling coefficient, determined by the misalignment state. Additionally, by controlling this current using a varactor placed on the auxiliary coil, an optimal magnetic flux is generated based on the alignment condition and/or the size of the Rx coil. In perfect alignment, the auxiliary coil focuses the flux from the Tx to the Rx coil, maximizing transfer efficiency. In misalignment scenarios, the current on the auxiliary coil is adjusted to shift the effective center of the Tx coil, achieving the strongest alignment of the magnetic flux traversing the Rx coil. This adjustment, which can be controlled adaptively based not only on the degree of misalignment but also on the size of the Rx coil, enables virtually null-free operation across varying misalignment conditions and for different Rx sizes. Furthermore, as this multifunctionality of the proposed system is achieved with a minimal number of additional components-just a single auxiliary coil and a single varactor-the impact on the overall quality factor (Q) of the system is minimized, contributing to the higher efficiency. In a size-symmetric system, where the Tx and Rx coils have the same size, the efficiency reaches 98.1% in perfect alignment and remains above 60% with up to 135% misalignment relative to the largest coil dimension. In a size-asymmetric system, with the Rx coil reduced to a quarter of the Tx coil, the efficiency is 96.1% in perfect alignment and remains above 60% up to 95% misalignment. Despite its enhanced practicality through a simple structure featuring only one auxiliary coil and an asymmetric configuration integrated solely on the Tx side, the proposed technique surpasses previous methods by delivering significantly superior performance. Moreover, it demonstrates unprecedented tolerance to both misalignment and smaller Rx coil sizes, which is frequently encountered in practical applications.

Electromagnetic Detection System with Magnetic Dipole Source for Near-Surface Detection.

This paper proposes a nondestructive, separate transmitter-receiver (TX-RX) electromagnetic measurement system for near-surface detection. Different from the traditional dual-coil integrated design, the proposed transient electromagnetic (TEM) system performs shallow subsurface detection using independent TX coil and movable RX coils. This configuration requires a large primary field so that the far-away secondary field is able to generate reliably induced voltages. To achieve this goal, a bipolar current-pulsed power supply (BCPPS) with a late resonant charging strategy is designed to produce a sufficiently large magnetic moment for the exciting coil with low source interference. The magnetic dipole source (MDS) with a large proportion of weight is separated from the field observation device and does not need to be dragged or transported during the detection process. This setup lowers the weight of the scanning device to 3 kg and greatly improves the measurement efficiency. The results of the laboratory test verify the effectiveness of the separate MDS and RX module system. Field experimental detection further demonstrates that the proposed system can realize highly efficient and shallow surface detection within a 200 m range of the MDS device.

Machine Learning Based Foreign Object Detection in Wireless Power Transfer Systems

As power electronics continue to drive advancement in electronic devices by managing the system's energy flow, it is advantageous to examine all areas of energy transfer. Wireless power transfer is one section seeing increased adoption in consumer, industrial, and healthcare applications. This consequently increases the concern for safety, reliability, and efficiency. Power transmission to any unintended load is a primary safety issue in high-power wireless power transfer applications, such as unmanned aircraft systems. Foreign object detection is utilized to selectively transmit power to only the intended target to mitigate this issue. This paper seeks to demonstrate a novel alternative method of using machine learning and artificial intelligence techniques for performing foreign object detection solely on the transmitter portion of the system by only monitoring the TX coil current. This method is first evaluated in SPICE simulation software. Then by transitioning to a closed-loop hardware-based testbed designed to be deployed in unmanned aircraft systems. During the test process for the novel FOD method, the aggregate detection accuracy achieved was 83%, including worst-case misalignment conditions with the RX coil. When the outlying worst-case conditions are excluded, the detection accuracy improves to 97%. To measure the FOD effectiveness in preventing temperature rise in a metallic object, the metallic object was placed at the peak power draw location on the transmitter coil. When the metallic object is left at this location for extended periods, the novel FOD method lowers the maximum temperature reached by 86.8°C when compared to no FOD being active. The primary advantage to this novel method is the simplistic hardware additions, which only require minimal alterations to a WPTS without any FOD implemented. Along with this, implementing this foreign object detection method in the transmitter would mitigate the need for a direct communication link between the receiver and transmitter. As a result, the entire system complexity would decrease, thus increasing the UAS power density while maintaining a safe, effective, and reliable wireless power transmission method.

Efficiency Optimization and Power Allocation of Omnidirectional Wireless Power Transfer for Multiple Receivers

Omnidirectional wireless power transfer to multiple devices has become a research hotspot due to its high spatial freedom and flexible design. However, this power supply method often faces the problems of high leakage flux and different receiver power requirements. This article studied the effects of currents of transmitting coils on transmission efficiency mathematically. A method of maximum efficiency tracking and power allocation for omnidirectional wireless power transfer to multiple receivers was proposed, and a mutual inductance identification method based on primary feedback is presented. Through the parameters identified by the primary side feedback, the current amplitudes and phases of the Tx coils were adjusted to achieve maximum efficiency, and the flexible power requirements of multiple devices can be realized. Finally, the above methods are experimentally verified. The two receivers can be charged with 58.8% efficiency and the power ratio can be precisely controlled from 0 to 100 percent, which is difficult for conventional method.

Ferrite-Loaded Inverted Microstrip Line-Based Artificial Magnetic Conductor for the Magnetic Shielding Applications of a Wireless Power Transfer System

In this paper, we propose a ferrite-loaded inverted microstrip line (IML)-based artificial magnetic conductor (AMC) with a novel design that can provide complete magnetic shielding at the backside of the transmitting (Tx) coil while slightly improving the power transfer efficiency (PTE) of a wireless power transfer system (WPTS). The target frequency of the WPTS application is approximately 6.78 MHz. In the proposed design, the AMC is placed behind the Tx coil, and its magnetic shielding capability and PTE performance were verified through simulations and measurements. The size of the proposed AMC is 528 × 528 × 6.6 mm3. The measurement results verified that, compared with the Tx coil without an AMC surface, the proposed ferrite-loaded IML-based AMC can provide complete magnetic shielding while improving the PTE of the WPTS by approximately 8.05%.

Experiment and analysis of a high-efficient stacked multi-Tx WPT system with identical Tx currents

Conventional multiple-transmitter (multi-Tx) wireless power transfer (WPT) systems require cumbersome operation in that the Tx currents need to be adjusted to be proportional to the mutual inductance between transmitting and receiving coils for optimal efficiency. In addition, planar-arranged multi-Tx systems can hardly improve the transmission distance even with fine Tx-current control. In this paper, we disclose that when the ratio of the minimum value to the maximum value of mutual inductances between Tx coils and Rx coil is 0.5 or more, multi-Tx WPT systems with identical Tx currents can achieve near-optimal efficiency. That is, under this condition, the WPT system does not need to fine-tune Tx currents. Additionally, we suggest a stacked multi-Tx system with high mutual inductance ratios that satisfies the condition. For validation of the proposed scheme, COTS coils were used to implement the single series coil, and planar multi-Tx systems as well as the stacked multi-Tx system. The experiment confirmed that the stacked multi-Tx WPT system can dramatically improve the critical transmission distance without any tuning of currents or voltages.

A composite decoupling method of the 2D WPT transmitter array based on the compensation topology and orthogonal coupling

Typical dynamic wireless power transfer systems use a long-track transmitter (Tx) coil or multiple segmented Tx coils, producing a limited efficient charging area. To improve charging flexibility, the two-dimensional (2D) transmitter array is adopted. However, the cross coupling between Tx coils will exist in multiple directions, resulting in the degradation of system performance. Therefore, this paper presents a composite decoupling method for the 2D wireless power supply array based on the compensation topology and orthogonal coupling. The compensation topology eliminates cross coupling between electrically connected Tx coils, while orthogonal coils decouple Tx coils without electrical connection. Compared with traditional methods, the proposed decoupling method can eliminate cross coupling between Tx coils in multiple directions. Additionally, it reduces the cost of inverters and decoupling inductors significantly. The transmitter array is configured with three operation modes to stabilize system power transmission. The effectiveness of the proposed method is verified through the experiment.

Wireless Power Transfer System Using High-Quality Factor Superconducting Transmitting Coil for Biomedical Capsule Endoscopy

We experimentally investigated the power transfer efficiency (PTE) of a wireless power transfer (WPT) system for capsule endoscopy that uses a high-temperature superconducting (HTS) transmitting (Tx) coil and a small Cu receiving (Rx) coil. A high-quality factor Tx coil is needed to maximize the PTE of a WPT system. A previously developed high-quality factor coil using double-sided HTS coated conductor wire was used as the Tx coil to improve the PTE. The diameter of the coil was 235 mm. The Rx coil was φ10 × 20 mm. The measured quality factors of the coils were 13,007 and 105, respectively. That of the Tx coil was about 13 times that of a Cu one. We measured the PTE of the WPT system in the facing direction, axial misalignment, and angular misalignment. The measured PTEs of two systems, one with an HTS Tx coil and one with a Cu Tx coil, in the facing direction at a transfer distance of 15 cm were 48.6% and 15.3%, respectively. The PTE of the system with a HTS Tx coil under axial misalignment, and angular misalignment conditions showed sufficiently high robustness compared with a system with a Cu Tx coil. The measured quality factors of the HTS and Cu Tx coils with a liquid phantom were 1,350 and 382, respectively. That of the Tx coil was about 3.5 times that of the Cu one.

Design and Analysis of Misalignment Insensitive Wireless Power Transfer System Based on Multitransmitter for Constant Power

This paper proposes a multiple transmitting array wireless power transfer (MTA-WPT) system with constant output power charging for consumer electronics, such as tablets and mobile phones. Compared to conventional WPT systems, this proposed system can achieve constant output power by controlling the current phase shift of the transmitting (Tx) coils when the receive (Rx) coil is located at different positions and directions. The detailed circuit topology of the proposed system is modeled and analyzed to obtain the correspondence between the current phase shift and load output power. Furthermore, the excitation current distribution strategies for a system of three and four Tx coils at optimized efficiency are theoretically analyzed. The simulation demonstrates that the proposed system can achieve constant output power with modulating the phase shift at various Rx sites. Finally, an experimental prototype with four Tx coils is designed and carried out to prove the effectiveness of the phase shift modulation method and current distribution strategy. Besides, the superiority of the proposed MTA-WPT system is verified by comparison with other methodologies. The experimental results show that the output power of Rx at different positions is kept at roughly 41W with the transmission efficiency over 80% by the Tx current phase shift modulation.

Improvement in Power Transmission Efficiency of Wireless Power Transfer System Using Superconducting Intermediate Coil

We have demonstrated that using a high-quality-factor intermediate (IM) coil with a high temperature superconductor (HTS) wire improves the power transmission efficiency (PTE) of a wireless power transfer (WPT) system for long-distance power transmission. The IM coil is placed at the center of Cu transmitting (Tx) and receiving (Rx) coils and consists of a previously developed double-sided HTS coated conductor (CC) wire. A WPT system with the HTS IM coil was designed on the basis of the design theory for a third-order bandpass filter. The measured PTE of the system was compared with that of one with a Cu IM coil. The resonant frequencies of the Tx, Rx, and IM coils in both systems were each about 9 MHz. When the distance between the Tx and Rx coils was 160 cm, the PTEs of the systems were 49.7% and 17.5%, respectively, demonstrating that the PTE of a WPT system can be greatly improved by using an HTS IM coil rather than a Cu one.

Phase Synchronization and Magnitude Control With Interference Between Transmitters in Wireless Power Transfer

With coupling between transmitter (TX) coils, the weakly-activated TX experiences interference current from adjacent strong TX. The strong interference has prohibited the magnitude and phase control. We reveal two operating modes (called negative and positive) that simultaneously achieve magnitude and phase control of TX coil currents. The negative mode is for TX which is a victim of strong interference current originating from adjacent TX. This mode is chosen when the target level of TX coil current is lower than the interference level. At such mode, its inverter voltage should be significantly delayed with respect to adjacent TX to maintain phase synchronization and interference regulation. On the other hand, the positive mode is chosen when the target level of TX coil current is higher than interference current. Hence, the proposed controller first chooses one of the two modes depending on the target magnitude of TX coil currents. The phase of two TX coil currents are synchronized and their magnitude ratio is regulated to coupling ratio even with strong interference level. No additional power components or RF communication is required. High cross coupling between TXs is tolerable, increasing the flexibility in coil geometry.

Reconfigurable PCB Coil Array Loaded With PIN Diodes for Improving Transmission Efficiency in MCR-WPT Systems

In this letter, we propose a reconfigurable coil array by loading PIN diodes to solve the problem of transmission efficiency degradation due to lateral misalignment in magnetically coupled resonant wireless power transfer (MCR-WPT) systems. The reconfiguration of the coil array is achieved by changing the capacitance of the coil array through the ON/OFF of the PIN diodes, thus adjusting the magnitude of the induced current and achieving higher transmission efficiency. By specifying one of the four loaded PIN diodes in the transmitting (Tx) coil array as ON state by the relative position of the receiving (Rx) coil to the Tx coil array, a significant improvement in transmission efficiency with a wider area of strong coupling can be obtained compared to a conventional Tx coil array without loading PIN diodes. This can be used in high efficient multi-coil versus single-coil WPT systems to provide more positioning freedom for Rx coil.

A Study on the Optimal Magnetic Beam Forming of Coil Arrays for Long Distance Wireless Power Transmission.

Multiple-input multiple-output (MIMO) wireless power transfer (WPT) technology which employs multiple transmitter (TX) coils to simultaneously couple power to the receiver (RX) coil has proved to be an effective technique to enhance power transfer efficiency (PTE). Conventional MIMO-WPT systems rely on the phase-calculation method based on the phased-array beam steering concept to constructively combine the magnetic fields induced by the multiple TX coils at the RX coil. However, increasing the number and distance of the TX coils in an attempt to enhance the PTE tends to deteriorate the received signal at the RX coil. In this paper, a phase-calculation method is presented that enhances the PTE of the MIMO-WPT system. The proposed phase-calculation method considers the coupling between the coils and applies the phase and amplitude to calculate the coil control data. From the experimental results, the transfer efficiency is enhanced as a result of the transmission coefficient improvement from a minimum of 2 dB to a maximum of 10 dB for the proposed method as compared to the conventional one. By implementing the proposed phase-control MIMO-WPT, high-efficiency wireless charging is realizable wherever electronic devices are located in a specific space.