Multi-coil Wireless Power Transfer Research Articles

A General Method for Estimating Coupling Coefficients in Multicoil Wireless Power Transfer Based on Harmonic Informatization

As the core parameter of wireless power transfer (WPT), coupling coefficient ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> ) estimation can significantly improve the system characteristics. However, the relationship between <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> and electrical signals is different under different WPT topologies, so a traditional method can only be applied to one type of topology, especially due to the large number of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> in multi-coils WPT, the traditional fundamental signal is not sufficient to solve all multi-coils <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> . In order to solve the shortcomings of traditional methods in generality, a general method of estimating <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> based on harmonic-informatization is proposed in this paper. This method uses high-frequency harmonics and band-pass/stop filters to simplify the circuit, so that <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> can be simply estimated only by harmonic signal. Compared with traditional methods, this method can be applied to all topologies (including multi-coils). Also, as the circuit is simplified under high frequency-harmonics, estimating <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> requires only a few parameters for simple operations. Thus, harmonic estimation <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> offers better accuracy, speed, and cost than traditional complex methods under fundamental. This paper also experimented with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> estimation to verify the effectiveness and accuracy of the method.

Multi-coil wireless power transfer (WPT) technique for boosting efficiency in large air gap systems

An energy-efficient high-powermulti-coil wireless power transfer system (WPT) has been studied in this paper. The proposed system was first analyzed and examined using theoretical modeling; this was followed by finite element analysis and circuit simulation to confirm the goal. As the air gap between the transmitter and receiver coils widens, the coupling coefficient of a WPT system will drop. A large-air-gap WPT system’s coupling coefficient can be raised by adding relay coils between the transmitting and receiving coils. On the other hand, an increased coupling coefficient improves the system’s overall power transfer efficiency. The suggested multi-coil system, which has an air gap of 15 cm and a lateral misalignment of 3 cm between the transmitting and receiving coils, has been simulated and verified using the Maxwell 3-D finite element tool and Ansys Simplorer circuit simulation software.

Analysis and design for constant current/constant voltage multi‐coil wireless power transfer system with high EMF reduction

The investigation on the constant current (CC)/constant voltage (CV) charging for the electric vehicle wireless power transfer (WPT) system has been widely conducted. However, the CC/CV performance under misalignment cases has been scarcely studied especially for the multi-coil WPT system. The resonant tank compensation methodology for the exciter-quadrature-repeaters (EQR) transmitter-based multi-coil WPT system is studied to achieve high power factor CC/CV charging with low leakage flux over ±150 mm lateral misalignment range. The EQR transmitter consists of one small exciter coil powered by a single full bridge inverter and two larger decoupled quadrature repeater coils which are magnetically coupled to the exciter coil. A variable switched capacitor is applied on the exciter side to maintain a high power factor in the CV mode. A 3-kW experimental setup with a 200 mm nominal vertical distance was built and a three-coil WPT system with a single large repeater coil is chosen as a comparison. Experimental results show that the EQR transmitter-based WPT system achieves 92% coil-to-coil efficiency and reduces the leakage flux density by 70% compared with the existing three-coil system in the CV mode at 150-mm lateral misalignment.

Effect of Additional Transmitting Coils on Transfer Distance in Multiple-Transmitter Wireless Power Transfer System

Wireless power transfer (WPT) technology has been widely introduced and developed over the past few years. Nevertheless, the basic WPT configuration using one transmitting (Tx) coil and one receiving (Rx) coil still achieves very limited performance in terms of power transfer distance. To overcome this limitation, various multi-coil WPT systems using three or more coils have been studied. Among them, a multiple-input single-output (MISO) structure can provide the Rx with advanced freedom in an extensive lateral area. However, few studies have investigated how far multi-Tx coils can increase the transfer distance. In this paper, we analyze the transfer distance and lateral coverage based on the generalized critical coupling condition and system energy efficiency according to the number of additional Tx coils in the MISO WPT system. The results demonstrate that the MISO WPT system using planarly arranged Tx coils improves by barely 1% in the optimal transfer distance, which achieves the maximum output power condition in a single-Tx WPT system. To validate our analysis, the MISO WPT system with seven Tx coils and seven LCC inverters was implemented and its performance was assessed. The measured results agreed well with the theoretically calculated results.

Design Method of Three-Coil WPT System Based on Critical Coupling Conditions

It is well known that the power transfer efficiency (PTE) of a two-coil wireless power transfer (WPT) system is maximized at critical coupling under fixed system parameters. However, the two-coil WPT system is critical-coupled with a relatively short distance between the coils. To overcome the problem, a multicoil WPT system has been studied. In this paper, we review the impedance characteristics of the critical-coupled two-coil WPT system and reveal that the conventional three-coil WPT system cannot have the same impedance characteristics as critical coupling. Based on the impedance matching conditions with critical coupling of the two-coil WPT system, we propose a design method of a three-coil WPT system in which the free resonant frequencies of all resonators are changed. The proposed design method is successfully implemented at the operating frequency of 6.78 MHz. The experimental results are in good agreement with the simulated PTEs and show that by changing the free resonant frequencies, a higher PTE can be effectively achieved in comparison to the conventional two-coil and thee-coil WPT systems.

Design of Multi-Coil Wireless Power Transfer System for Gastrointestinal Capsule Robot

The existing wireless power transfer (WPT) systems for gastrointestinal capsule robot have the problems of small coupling coefficient and low power transmission efficiency (PTE). The reasons are due to the long distance between the transmitting coil and the receiving coil and the large difference in size. A new type of WPT system is designed, which uses three sets of small coil pairs to form a power supply unit (PSU), and utilizes multiple PSUs to form a multi-coil WPT system. Compared with single-coil system, the multi-coil system can achieve higher power utilization by switching between PSUs, instead of opening all PSUs. ANSYS Maxwell is used to perform finite element modeling on the PSU, analyzing the characteristics of the transmitting magnetic field. The results of the experiment show that when the distance between the small coil pairs in the PSU is 180 mm, the magnetic field has relatively good uniformity, and the magnetic strength change relative to the center point is less than 5%. The average received power of the system is greater than 800mW, and the PTE is up to 5.1%.

Plane-Omnidirectional Wireless Power Transfer System Based on Vector-Controlled Flux Linkage

The major challenge for wireless power transfer (WPT) is to extend transfer distance and to broaden transfer direction, while a stable power can be transferred with high and constant efficiency. The multi-coil WPT system can transfer power on the plane or space omnidirectionally, but its output power and efficiency are usually difficult to keep stability. For a nonlinear wireless power transfer system based on parity-time (PT) symmetry, when the coupling condition of the system can be satisfied, it can attain a stable power with high and constant efficiency, but it still has a dead-spot problem in the transfer direction. Therefore, this article proposes a design of a plane-omnidirectional WPT system, of which the coupling condition can be maintained across the plane. The transmitter coils are a pair of orthogonal coils which are named transmitter coils 1 and 2. Meanwhile, in the transmitting circuit, the positive direction of transmitter coil 1, which determines the positive direction of current and flux, is controlled by a reversed switch group (RSG) with a logic-check loop (LCL). Moreover, the theoretical analysis shows that the proposed system can reverse the flux direction of transmitter coil 1 while the angle of transfer direction is overly large, then the coupling condition of the system after reversal can still be satisfied. Finally, the experimental results show that when the receiver coil moves around the transmitter coils on the whole plane, the stable output power of 14 W is maintained with an approximately constant transfer efficiency of 87%.

Efficiency Enhancement in a Multi-coil Wireless Power Transfer System

Wireless power transfer technology via magnetic resonance coupling now has significant interest in industry and research with many applications. This paper proposes a linear multiple transmitter coil array (5 coils) for wireless power transfer for added gain and hence higher transfer efficiency in comparison to a single transmitter coil. The frequency splitting effect as a result of the coupling between the resonant transmitter coils due to their close proximity is shown to reduce the transfer efficiency to a receiver. The effect of the array spacing on splitting effect suppression is verified. It is shown that the splitting effect is sup-pressed as the distance between the coils is increased leading to a higher received signal and hence higher efficiency. Proposed horizontal displacement of the middle transmitter coils (2nd and 4th coils) in the coil array is shown to suppress frequency splitting. To further suppress the splitting effect due to the magnetic coupling between the transmitter coils, a multiple transmitter array is proposed with different coil turns. Thus it is shown that designing the multiple coil array with mixed number of coil turns (the 2nd and 4th coils are designed to have different number of turns as compared to the other three coils) causes uniform coupling among the coils reducing and eventually eliminating the splitting effect. Also to increase the efficiency at the receiver coil, displaced stacked coils are introduced on top of the coil array. The pro-posed stacked coil array is demonstrated to improve the transfer efficiency. Using the techniques, the proposed linear array structure achieves a transfer efficiency of 36.9% for a receiver coil at the boresight of the array at a transfer distance of 40 cm.

Modified Inductive Multicoil Wireless Power Transfer Approach Based on Z-Source Network

This article presents a nonconventional approach to a multicoil wireless power transfer system based on a Z-source network. The novelty of the approach lies in the use of a Z-source as a voltage source for energy transmission through the wireless power transfer coils. The main advantage is in a reduced number of semiconductors. This article provides the design approach, simulation, and experimental study. Feasibility and possible application fields are presented in the conclusion.

Graph Sets Method (GSM) for Multicoil Wireless Power Transfer Systems—Part II: Simulated and Experimental Results

This article is the continuation of graph sets method (GSM) proposed in the companion paper Part I for deriving equations and analyzing multicoil wireless power transfer multicoil (MC) wireless power transfer (WPT) systems. After explaining GSM, categorizing WPT coils into three different types, and proposing some approximation techniques in Part I, this article, as the second part of this work, presents numerical simulations and experimental results for a three-coil MCWPT system similar to what is explained in Part I. The simulations are conducted in MATLAB/Simulink and ANSYS environments, and for the experimental model, three ferriteless cylindrical coils are built and compensated to behave as voltage driven, current driven, and passive coils, respectively, at the specific frequency of 200 kHz. After analyzing and comparing the simulation and experimental results, it is shown that GSM can be an effective and illustrative technique that facilitates comprehensive analysis of interactions amongst the coils and their characteristics, especially when WPT systems contain multiple coils.

Graph Sets Method for Multicoil Wireless Power Transfer Systems—Part I: Principles

A new approach to derive the equations of multicoil wireless power transfer (MCWPT) systems and to simplify and analyze them is proposed in this article. By parametrically solving the equations governing MCWPT systems and mapping the resultant transfer functions into graph sets (GSs), a set of rules is developed to form the transfer function among the voltages across and currents through the coils. Using these rules, some important aspects, such as effective paths for the power to flow, the effect of active coils on each other and on passive coils, dynamic behavior of the system, and reflected impedances, can be comprehensively analyzed. This can be done by following GS rules and without complex mathematical calculations. The GS method (GSM) also provides an effective tool to design compensators and power electronic converters driving MCWPT systems and to estimate the receiver (pickup) parameters. Moreover, simplifying the behavior of the coils into three basic types, current-driven, voltage-driven, and passive coils, helps to reduce the complexity of the model and to have a better understanding of the system. This simplification can be further expanded by removing ineffective couplings between the coils. This article is presented in two parts. In Part I, the GSM is explained, and its different analytical steps are established, and Part II is dedicated to show the effectiveness and validity of this approach by numerically modeling and experimentally evaluating a three-coil MCWPT system.

Single-Inductor Multiple-Output Inverter With Precise and Independent Output Voltage Regulation

In this article, a single-inductor multiple-output (SIMO) inverter with precise and independent output voltage regulation is presented. This SIMO inverter with controllable output voltage enables each individual transmitting coil in a multicoil wireless power transfer system to be driven by a tightly regulated constant-voltage source. This is especially useful for the wireless charging of rechargeable batteries when they act as a constant-current (CC) load in the CC charging phase. By precisely regulating the source voltage at each transmitting coil, a constant load current at the corresponding receiver can be achieved at a particular resonant frequency and mutual inductance between the transmitting and receiving coil. The closed-form expressions of the sinusoidal-like output voltage and resonant inductor current as a function of the duty ratio of the main switch are also analytically derived. An experimental prototype of a single-inductor three-output inverter with a single time-multiplexed analog-based controller employing the type II compensation is implemented. It is experimentally verified that the individual ac outputs can be accurately and independently controlled. The maximum error between the target and measured peak voltage values is shown to be within 1.14%.

Convex optimization of coil spacing in cascaded multi-coil wireless power transfer

AbstractIn this paper, we use convex optimization to maximize power efficiency through cascaded multi-coil wireless power transfer systems and investigate the resulting characteristic spacing. We show that although the efficiency is generally a non-convex function of the coil spacing, it can be approximated by a convex function when the effects of higher-order couplings are small. We present a method to optimize the spacing of cascaded coils for maximum efficiency by perturbing the solution of the convex approximation to account for higher-order interactions. The method relies on two consecutive applications of a local optimization algorithm in order to enable fast convergence to the global optimum. We present the optimal configurations of coil systems containing up to 20 identical coils that transfer power over distances up to 4.0 m. We show that when spacing alone is optimized, there exist an optimal number of coils that maximize transfer efficiency across a given distance. We also demonstrate the use of this method in optimizing the placement of a select number of high-Q coils within a system of low-Q relay coils, with the highest efficiencies occurring when the high-Q coils are placed on either side of the largest gaps within the relay coil chain.

A Systematic Methodology for Optimal Design of Wireless Power Transfer System Using Genetic Algorithm

This paper presents a systematic methodology for the optimal design of wireless power transfer (WPT) systems. To design a WPT for a specific application, the values of coil geometric parameters and the number of resonators should be chosen such that an objective function is maximized while satisfying all the design constraints. The conventional methodologies, which are based on cyclic coordinate optimization, are not comprehensive and efficient methods. This paper presents a design methodology based on the genetic algorithm (GA). The optimization method has been applied to designing different WPTs with series and parallel connections of load and different design constraints. Moreover, the number of resonators is considered as the design parameters. In addition, WPTs with parallel and series connections of load are compared from different aspects. The results of calculation, simulation and measurements demonstrate that the 2-coil WPT can be optimized to achieve maximum efficiency compared to the previously reported 2-coil and multi-coil WPTs. In a fabricated 2-coil configuration, a power transfer efficiency (PTE) of 82.7% and a power delivered to the load (PDL) of 173.89 mW are achieved.

An Approach for Selecting Compensation Capacitances in Resonance-Based EV Wireless Power Transfer Systems With Switched Capacitors

Wireless chargers for electric vehicles (EVs) can achieve high power-transfer efficiency by utilizing magnetic resonance. However, the efficiency depends on the position of the receiver coil on board the EV relative to the charging pad, which may present a challenge in some coil topologies. In multi-coil topologies, the additional auxiliary coils increase the magnetic coupling between the primary transmitter and receiver coil, helping to improve the misalignment tolerance of a wireless power transfer (WPT) system. In this paper, an approach is presented for selecting compensation capacitances in resonance based multi-coil WPT systems that utilize switched capacitor compensation. The approach for selecting the compensation capacitors is based on maintaining operation within the split resonant frequency region while misaligned. In this paper, the compensation capacitor design approach is applied to a four-coil WPT system with overlapping auxiliary coils with switched capacitors on each auxiliary coil. Experimental results are presented which show that with the selected compensation capacitances the power factor is maintained above 0.9 from 0- to 20-cm misalignment compared to a power factor that decreases to 0.34 at 20 cm when the compensation capacitors are not retuned based on misalignment.

Comparative Analysis of Two- and Three-Coil WPT Systems Based on Transmission Efficiency

It is obvious that the conventional two-coil wireless power transfer (WPT) system has a short power transmission distance, which is determined by system parameters such as the loaded Q of the resonators and the coupling coefficient. There have been many attempts to improve the power transfer efficiency (PTE) or transmission distance. A typical approach is to use multiple coils like a three- or four-coil WPT system. In this paper, we propose a new method to obtain the PTE of a multi-coil WPT system based on the scattering parameter and impedance parameter. Also, we compare the two- and three-coil WPT systems in terms of their transmission efficiency rather than their system energy efficiency. For high transmission efficiency, we determined that the three-coil WPT system should have a symmetric structure and the coupling coefficient between the transmitting and receiving coils has to be as zero as possible. Additionally, we found that 1/√3 of the conventional critical coupling is the boundary coupling coefficient at which the two- and three-coil WPT systems have the same transmission efficiency. We successfully verified these theoretical analyses by implementing two- and three-coil WPT systems at the operating frequency of 6.78 MHz and measuring their transmission efficiency and spectra.

Design of dual-band coil for wireless power transfer system via magnetic resonant coupling

For wireless power transfer (WPT) systems, communication between the two terminals of power transfer system is also an important problem to be solved. This paper proposes a design method of multi-turn dual-band coil for implementing synchronous transfer of power and data in WPT system with one set of WPT coils. An extra compensation circuit is inserted into the multi-turn spiral coil, so the new coil has two resonant frequencies that lie on both sides of the original resonant frequency of the multi-turn coil. Just one set of dual-band coils is used to provide the data and power transfer via two different resonant frequencies, so the undesirable interference in multi-coil WPT system is avoided. The equivalent circuit model is built to analyze the characteristic of the dual-band WPT system. The data pickup circuit is analyzed and designed for the receiver of WPT system. Finally, a dual-band WPT prototype with data pickup circuit is built to demonstrate the effectiveness of the proposed dual-band WPT system.

A Smart Wirelessly Powered Homecage for Long-Term High-Throughput Behavioral Experiments.

A wirelessly powered homecage system, called the EnerCage-HC, that is equipped with multicoil wireless power transfer, closed-loop power control, optical behavioral tracking, and a graphic user interface is presented for longitudinal electrophysiology and behavioral neuroscience experiments. The EnerCage-HC system can wirelessly power a mobile unit attached to a small animal subject and also track its behavior in real-time as it is housed inside a standard homecage. The EnerCage-HC system is equipped with one central and four overlapping slanted wire-wound coils with optimal geometries to form three- and four-coil power transmission links while operating at 13.56 MHz. Utilizing multicoil links increases the power transfer efficiency (PTE) compared with conventional two-coil links and also reduces the number of power amplifiers to only one, which significantly reduces the system complexity, cost, and heat dissipation. A Microsoft Kinect installed 90 cm above the homecage localizes the animal position and orientation with 1.6-cm accuracy. Moreover, a power management ASIC, including a high efficiency active rectifier and automatic coil resonance tuning, was fabricated in a 0.35-μm 4M2P standard CMOS process for the mobile unit. The EnerCage-HC achieves a max/min PTE of 36.3%/16.1% at the nominal height of 7 cm. In vivo experiments were conducted on freely behaving rats by continuously delivering 24 mW to the mobile unit for >7 h inside a standard homecage.

An Investigation of Electromagnetic Radiated Emission and Interference From Multi-Coil Wireless Power Transfer Systems Using Resonant Magnetic Field Coupling

Wireless power transfer (WPT) technology has recently emerged as an innovative and promising technology, and its electromagnetic compatibility (EMC) has become a significant issue. In this study, we investigated the electromagnetic (EM) radiated emission and interference generated by WPT systems using resonant magnetic field coupling, especially in applications with multi-coil configurations. The change in coil resonance associated with multi-coil configurations was analyzed via the impedance profile. We measured the EM radiated emission and analyzed the results with respect to the coil resonance. An analog-to-digital converter chip was designed and fabricated to analyze the effect of electromagnetic interference (EMI). Based on measurement and simulation results, we verified that the EM radiated emission and interference increase at the series or parallel resonance peaks, depending on the source type. In addition, we verified that EMI can be reduced by using ferrite sheet shielding.

A smart homecage system with 3D tracking for long-term behavioral experiments.

A wirelessly-powered homecage system, called the EnerCage-HC, that is equipped with multi-coil wireless power transfer, closed-loop power control, optical behavioral tracking, and a graphic user interface (GUI) is presented for long-term electrophysiology experiments. The EnerCage-HC system can wirelessly power a mobile unit attached to a small animal subject and also track its behavior in real-time as it is housed inside a standard homecage. The EnerCage-HC system is equipped with one central and four overlapping slanted wire-wound coils (WWCs) with optimal geometries to form 3-and 4-coil power transmission links while operating at 13.56 MHz. Utilizing multi-coil links increases the power transfer efficiency (PTE) compared to conventional 2-coil links and also reduces the number of power amplifiers (PAs) to only one, which significantly reduces the system complexity, cost, and dissipated heat. A Microsoft Kinect installed 90 cm above the homecage localizes the animal position and orientation with 1.6 cm accuracy. An in vivo experiment was conducted on a freely behaving rat by continuously delivering 24 mW to the mobile unit for > 7 hours inside a standard homecage.