Practical Wireless Power Transfer System Research Articles

Data-Driven Modeling and Control Considering Time Delays for WPT System

A controller for output voltage and power regulation is important for the wireless charging system. For the controller design, it is crucial to establish the accurate system model with considering the time delay, as high time delay exists due to wireless communication, data sampling, and processor calculations for a practical wireless power transfer system. Presently, the common modeling methods, such as the coupled-mode method are based on circuit theories and the component parameters. However, the exact parameters are hard to get due to the diversity of the equipment and application scenarios. Especially, the circuit theories modeling cannot deal with the time delay from hardware limitation and wireless communication. Alternatively, a data-driven modeling method based on the simplified refined instrumental variable method is proposed in this article to get the control-oriented model with time delay. Moreover, based on the estimated model, a controller based on the internal model control is designed to regulate the output voltage. It is shown that the models obtained based on different sampling times are able to accurately describe the system dynamics and the controllers designed based on them are able to achieve the desired control performance.

On wireless power transfer between coils in the presence of radiation

Wireless power transfer (WPT) between magnetically coupled coils is necessarily accompanied by radiation that may hamper efficient power transfer and may cause harmful electromagnetic pollution. Hence careful control of the ratio between WPT and radiation is required. We have a formulation that allows the simultaneous determination of both the transferred and the radiated power by relying on both field and circuit theory and introducing the concept of a complex mutual inductance. Emphasis is placed on the visualisation of a radiative WPT system by using the streamlines of the Poynting vector. We perform several optimisations. We can find the parameters, coil radius, coil separation and load resistance, that maximise the ratio of load power to radiated power and at the same time minimise the radiated power. We identify the location of the singularities of the Poynting vector streamlines, the so-called P-points, and of the boundaries of a ‘power bubble’ surrounding the wireless two-coil system and forming an impenetrable barrier between the wireless power flow to the load and the power radiated out. A relationship between the size of the power bubble and the ratio between the useful load power and the harmful radiated power is established in a series of parametric studies. Field theory of Poynting vector streamlines is shown to quantitatively agree with the predictions of the circuit model. The new physical picture emerging for the power moving in space is expected to aid the design of complex practical WPT systems when radiation needs to be closely watched and for which a simple circuit model would fail to describe power distribution around the structure.

Investigating human exposure to a practical wireless power transfer system using and the effect about key parameters of dosimetry.

Accurate dosimetry for a real wireless power transfer system (WPT) using electromagnetic resonance and electromagnetic induction requires an accurate description of the field formed by the system. In particular, the electromagnetic field depends on factors such as the construction of the transmitting and receiving coils, the circuit configuration, the input source of the front end of the transmitting coil, and the input impedance of the rear end of receiving coil. However, both circuit and electromagnetic simulations need to be performed to analyze the entire system, which is a difficult task. In order to overcome this difficulty, a method using an equivalent circuit model is proposed and verified through experiments. Moreover, the worst exposure condition to a magnetic field was examined by considering three variables: the charging mode, the state of charge, and the alignment and misalignment between the transmitting and receiving coils. Accordingly, the strongest magnetic field was created in the constant current mode in the fully charged state with misalignment. For example, the magnetic field strength in the case of 80% state of charge and misalignment was 1.397 times greater than in the case of 20% state of charge and alignment at a point 10 mm from the transmission pad. Finally, the induced electric fields and induced current densities were calculated by using a Japanese adult male whole-body voxel human model, and the results were compared with the values recommended by international guidelines to ascertain their compliance.

Modeling and Experimental Validation of a Butterfly-Shaped Wireless Power Transfer in Biomedical Implants

Wireless power transfer (WPT) presents a safe and robust method for powering biomedical implants. The range and misalignment tolerance restrictions of the WPT system are the main factors, conducive to the transfer efficiency degradation. Especially, the implanted device is invisible and difficult to arrange in alignment with the transmitting coil. This paper proposes a butterfly-shaped transmitting (BS-Tx) coil of a magnetic resonance WPT system for biomedical implants. The BS-Tx coil prototypes with the changing driven current directions, parallel and anti-parallel current directions, are verified and analyzed. Besides, an equivalent circuit model of a coupling two-coil system with an optimal pair of capacitors method for the matching network is mathematically derived to obtain the maximized power transfer efficiency. The BS-Tx coil enhances the efficiency of the system in the distance, angle, and axial misalignment tolerances. A practical WPT system is fabricated and measured to verify the transfer efficiency through the real biological tissue and exhibits a significant efficiency improvement. Finally, a hypothetical context of retinal prosthesis application is demonstrated as a potential application to retinal implants. The proposed system shows a low specific absorption rate, that is satisfied the human safety regulations.

Design, Measurement and Analysis of Near-Field Focusing Reflective Metasurface for Dual-Polarization and Multi-Focus Wireless Power Transfer

A new design of dual-polarization and multi-focus near-field focusing (NFF) reflective metasurface for wireless power transfer (WPT) system is proposed in this paper. In terms of multi-beam phase synthesis of reflective metasurface, the dual-polarization metasurface with independent regulation characteristics is introduced to realize multi-focus and high-efficiency WPT. The single-feed single-focus and the single-feed dual-focus metasurface working at X-band (10 GHz) with $26\times 26$ elements in two polarizations are designed, fabricated and measured by planar near-field scanning experiments. Furthermore, a compact antenna is designed and fabricated as the receiver to form a practical WPT system. The measurement results show that a NFF transfer system is 15 dB higher than a non-NFF transfer system. Through full-wave simulation and experiments of three cases, single focus, dual-focus, and single focus with dual-polarization can respectively realize the maximum focusing efficiency of 71.6%, 68.3% and 65.9%. The relative bandwidth with 50% power focusing efficiency of these three cases are all about 12%, which demonstrates the stability and feasibility of the NFF reflective metasurface for practical WPT applications.

Analysis of coupling between magnetic dipoles enhanced by metasurfaces for wireless power transfer efficiency improvement

In this paper, we investigate the possibility of improving efficiency in non-radiative wireless power transfer (WPT) using metasurfaces embedded between two current varying coils and present a complete theoretical analysis of this system. We use a point-dipole approximation to calculate the fields of the coils. Based on this method, we obtain closed-form and analytical expressions which would provide basic insights into the possibility of efficiency improvement with metasurface. In our analysis, we use the equivalent two sided surface impedance model to analyze the metasurface and to show for which equivalent surface impedance the WPT efficiency will be maximized at the design frequency. Then, to validate our theory, we perform a full-wave simulation for analyzing a practical WPT system, including two circular loop antennas at 13.56 MHz. We then design a metasurface composed of single-sided CLSRRs to achieve a magnetic lensing based on the calculated equivalent surface impedance. The analytical results and full-wave simulations indicated non-radiative WPT efficiency improvement due to amplifying the near evanescent field which can be achieved through inserting the proposed metasurface.

An optimization model for wireless power transfer system based on circuit simulation

Recently, the market promotion of wireless power transfer (WPT) products has generated great expectations for systematic design and optimization of practical WPT system (especially the coupler). The existing optimization methods usually employ the Finite Element Analysis (FEA) to simulate the magnetic field, where the effects and requirements of overall circuit are neglected. To address this issue, an optimization approach for the coupler based on circuit simulation is proposed in this paper, and the secondary compensation capacitor is further optimized to improve the system performance. The simulation model is embedded into Simulink and the calculation part is embedded into MATLAB, where the objective is to achieve the maximum efficiency by searching three variables under adequate constraints. The numerical results imply that the proposed method is more effective and accurate than the traditional trial-and-error way, and that it suits the practical WPT system design excellently.

Misalignment Sensitivity of Strongly Coupled Wireless Power Transfer Systems

A traditional strongly coupled magnetic resonance (SCMR) system is highly sensitive to the alignment between the transmitter and receiver elements, which is an issue that has limited their applicability in practical wireless power transfer systems. This paper proposes, for the first time, a novel set of SCMR-based topologies that are less sensitive to misalignment while providing large wireless powering efficiencies. Specifically, instead of using planar coils for the transmitter and the receiver, we connect two orthogonal coils together into a 3-D model to provide misalignment insensitivity. Three SCMR systems (standard SCMR, conformal SCMR, and hybrid SCMR systems) are studied and compared to the proposed 3-D SCMR system in terms of angular azimuth, angular elevation, and lateral misalignment. Also, the ranges of these SCMR systems are compared. It is shown that the proposed misalignment system achieves above 40% efficiency for the entire range of 360° of angular misalignment. Also, our results show that the proposed misalignment system provides longer range.

New Insight of Maximum Transferred Power by Matching Capacitance of a Wireless Power Transfer System

Most research on wireless power transfer (WPT) has been focused on how to achieve a high-efficiency power transfer. Our study found that under the impedance matching for achieving maximum WPT efficiency, the power transferred to the load cannot reach the maximum when a WPT system is supplied by an AC voltage source with constant amplitude. However, the load power or the voltage across the load is essential for a low-power electric device such as the implanted medical device where the transfer efficiency is not the priority to be considered. The paper presents a method for achieving maximum power on the load by matching capacitance in a WPT system with given two-coupled-coils. Three sets of matching capacitances for extreme load power were deduced based on the circuit model considering the coil's resistance, and all these three matching make the WPT system operate at the resonant state. Two sets can make the system achieve the global maximum of load power. One set can make the system achieve the local maximum of load power and reach the power transfer efficiency next to 1. Experimental results verified the theoretical calculations. The results can contribute to the compensation design of a practical WPT system for transferring the maximum power to the load.

APPLICATION OF ULTRA-THIN ASSEMBLED PLANAR METAMATERIAL FOR WIRELESS POWER TRANSFER SYSTEM

Magnetically coupled resonant wireless power transfer (WPT) has been employed in many applications, including wireless charging of portable electronic devices, electric vehicles, etc. However, the power transfer efficiency (PTE) decreases sharply due to divergence of magnetic field. Electromagnetic (EM) metamaterial (MM) can control the direction of magnetic fields due to its nega- tive effective permeability. In this paper, MMs with negative effective permeability at radio frequencies (RF) are applied to a WPT system operating at around 16.30 MHz for improvement of PTE. This ul- tra-thin and assembled planar MM structure consists of a single-sided periodic array of the capaci-tively loaded split ring resonators (CLSRRs). Both simulation and experiment are performed to cha-racterize the WPT system with and without MMs. The results indicate that the contribution of high PTE is due to the property of negative effective permeability. By integrating MM in the WPT system, the experimental results verify that the measured PTE with one and two MM slabs have respectively 10% and 17% improvement compared to the case without MM. The measured PTEs of the system at different transmission distances are also investigated. Finally, the proposed MM slabs are applied in a more practical WPT system (with a light bulb load) to reveal its effects. The results verify the efficiency improvement by the realized power being transferred to the load.