Yuhua Cheng

A New Analytical Calculation of the Mutual Inductance of the Coaxial Spiral Rectangular Coils

The mutual inductance between two coils is a key parameter in the inductively coupled wireless power transmission. The spiral rectangular coils are easier to implement than the circular ones, but the study of the mutual inductance of the spiral rectangular coils is not enough. In this paper, a new analytical calculation of the coaxial spiral rectangular coils is presented, which is simpler than the greenhouse method. The impact of the track thickness and width is also analyzed. The calculation results are verified by the existing methods and the experimental results. Based on these results, some guidelines for optimizing the coupling coefficient are given, when the received coils size is limited by space constraints.

Analytical Modeling and Optimization of Small Solenoid Coils for Millimeter-Sized Biomedical Implants

The new trend towards minimally invasive millimeter-sized and free-floating distributed implants promises to enable emerging applications, such as chronic neural recording with minimal damage to the surrounding tissue. However, wireless power transmission (WPT) to these medical devices is quite challenging. The magnetic field produced by external transmitter (Tx) coils at the position of small implants can be considered homogeneous to separate the optimization of Tx and receiver (Rx) coils for efficient WPT. This paper focuses on the optimization of the solenoid-type Rx coils, which are suitable for this application. We have developed an analytical model of solenoid coils that includes the impact of tissue and coating around the coils, verified through simulations and measurements. Using the proposed model, under a given size restriction and a specific load, we find the optimal operating frequency and coil geometry to maximize a figure of merit (FoM) for the Rx that includes the loaded quality factor and its internal efficiency as well as a factor related to the coupling coefficient. For a millimeter-sized coil, the optimal operating frequency for the Rx and the number of turns are found to be 500 MHz and six, respectively, if the coil is closely wound using AWG36 copper wires. If the pitch is also optimized, then 700 MHz and four turns provide the best FoM for the solenoid Rx.

Mutual inductance calculation between arbitrarily positioned rectangular filaments

Abstract. The mutual inductance between the transmitted and received coils is an important parameter in the inductively cou-pled wireless power transmission system. The relative position between the linked coils may be arbitrary according to theapplications. In this paper the analytical calculation of the mutual inductance between two arbitrarily positioned rectangu-lar filaments is proposed based on a more general expression of mutual inductance calculation of two arbitrarily positionedstraight filaments than Grover’s. The calculation results are validated by using the finite element simulation tools. The previouspublished approximation methods for square coils in the coaxial, lateral misalignment or angular misalignment situations areverified that they are accurate when the two coils’ relative distances are large.Keywords: Mutual inductance, rectangular filament, arbitrary position 1. IntroductionIn the wireless power transmission system, the power is transferred from the transmitted coil to thereceived coil through the inductively coupled method whose coupled coefficient and the transfer effi-ciency are related to the mutual inductance between the two coils [1]. In addition to the inductivelycoupled power transmission application, the mutual inductance is also a basic parameter in the applica-tions like the electromagnetic induction sensors[2], and the electrical machines [3].The mutual inductance is related to the coils’ shapes,sizes, and relative positions [4]. The commonlyused shapes of the coils are circular or rectangular/square. No matter what shapes are, the relative posi-tionwillbecoaxial,coplaner,perpendicularorevenarbitraryaccordingtotheapplications.Furthermore,in some applications the relative position will be changed with the time. A typical example is the ap-plication of the wireless power transmission for the capsule endoscope, where the received coils insidethe body are movable irregularly [5]. Although two coaxial coils are needed in some applications, thelateraland/orangularmisalignmentswillbeinevitable.Insummary,itisnecessarytoanalyzethemutualinductancein the most general situation that the two coils are arbitrarily positioned.The mutual inductance of the

Optimizing three-phase three-layer coil array for omnidirectional wireless power transfer

Angular misalignment between transmitting and receiving coils leads to “dead spots”, i.e., no power is received for some receivers in electromagnetically coupled wireless power transfer (WPT) systems. Multi-layer multi-phase coil array is an alternative that can generate omnidirectional and homogeneous magnetic fields. In this work, the magnetic field from a coil array is calculated and explored for omnidirectional magnetic field characteristics. In order to improve the magnetic field at the “dead spots” in the receiving region, the worst-case power transfer efficiency is optimized, which is a different scheme from the conventional one. As an example, a WPT system for powering many fully implanted, distributed, and tiny neural recording implants under the restriction of 20-mm power transfer distance is optimized by this new scheme.

Modeling and optimization of single-turn printed coils for powering biomedical implants

A new coil optimization procedure is proposed for inductively coupled wireless power transfer when square single-turn printed coils are used, by means of analytically modeling the self- and mutual-inductances, parasitic resistances, and parasitic capacitances of these coils. As an example, the average diameter and trace width of the transmitting coil and the trace width of the receiving coil are optimized when the average diameter of the receiving coil and the transfer distance are assumed to be fixed. The efficiency is improved by 30% (i.e., form 40% to 70%) in comparison with the existing optimization method.

Modeling and optimization of mm-sized solenoid coils for biomedical implants

The new trend towards minimally invasive mm-sized and free-floating distributed implants imposes a significant challenge in wireless power transmission to these medical devices. The magnetic field produced by external transmitter (Tx) coils at the position of the implants can be considered homogeneous to separate optimization of the Tx and receiver (Rx) coils for efficient power transfer. This paper focuses on optimization of the solenoid-type Rx coils, which are suitable for this application. We have developed an analytical model of solenoid coils that includes the impact of tissue and coating around the coils and verifies through simulations and measurements. Using the proposed model, under a given size restriction and a specific load, we find the optimal operating frequency and coil geometry by maximizing a figure of merit for Rx. For a mm-sized coil, the optimal operating frequency and the number of turns are found 500 MHz and 6, respectively, if the coil is closely wound using AWG36 copper wires.

A study of wireless power transfer for implantable devices by using wearable devices

Wireless power transfer (WPT) is an effective method for power supply from wearable devices to implantable devices (IMDs). To reduce the IMD size, the received coil can be integrated with other functional circuits, whereas the efficiency is low if the traditional inductively coupled method is used. In this paper, limiting factors of the WPT efficiency including the quality factor of coils and the coupling coefficient are analyzed. A circuit model of magnetic resonance coupled (MRC) WPT is established and simplified for this application. It shows that the MRC method is feasible to improve the efficiency for IMD.

Parallel connected transmitting coil for achieving uniform magnetic field distribution in WPT

Transmitting coil with uniform magnetic field distribution allows the received coil to be positioned freely and keeps the coupling coefficient, the received power, and the power transfer efficiency stable. Traditionally, the transmitting coil is realized by connecting the coil turns in series and each turn has the same current. In this paper, a new transmitting coil is proposed, in which all the coil turns are connected in parallel. The current in each turn is not necessarily the same, instead, determined by a series connected capacitor. It is shown that this new transmitting coil improves the uniformity of the magnetic field.