Satoshi Shimokawa

A numerical study of power loss factors in resonant magnetic coupling

We numerically studied the affect of power loss factors in a wireless power transfer system using resonant magnetic coupling. Resonant magnetic coupling is regarded as one of the most promising methods for mid-range wireless charging systems. To make this method practical, it is important to accurately estimate power transfer efficiency and effect of each loss factor in the device-designing stage. We conducted a numerical simulation using an equivalent circuit model and electromagnetic analysis for a mobile-device model. Resonance at 7 MHz between the transmitting and receiving coils was achieved using lumped capacitors attached at the coil ends. In addition to the skin effect, we consider various loss factors such as proximity effect, loss tangent of lumped capacitors, and so on. The results show that the proximity effect significantly decreases the power transfer efficiency of the system, and the loss tangent of lamped capacitor also decreases it by a few percentage points.

Phase and intensity control of multiple coil currents in resonant magnetic coupling

We studied the effects of the phase and intensity of multiple coil currents in a wireless power transfer system using resonant magnetic coupling. Resonant magnetic coupling is regarded as one of the most promising methods for mid-range wireless charging systems. For mid-range charging, the charging device can assume various positions and postures, and some of the conditions make wireless charging difficult. To solve this problem, we propose a method to control the phase and intensity of multiple coil currents. By performing numerical simulations using an equivalent circuit model and electromagnetic analysis, we confirmed that it had the desired effect.

Transfer efficiency control using switching devices in resonant magnetic coupling

The resonant magnetic coupling (RMC) method has advantageous features for mid-range wireless power charging. However, if a receiver is moved closer to a transceiver as transmission output remains fixed, excessive power is supplied to a load, which may cause considerable damage to the receiving device. In this paper, we propose a new efficiency control technique using nonlinear devices. Two switches with diodes are connected to the resonant capacitance in parallel and are turned on/off alternately at the transmission frequency. With this technique, receiving power control can be achieved without any communication between the transceiver and the receiver.

An optical flying head system that features a dual-stage tracking mechanism for use in high-density MO recording

Much research work has recently been carried out on increasing the recording density on a first-surface optical disk using the combination of a blue-violet laser diode and high-NA lens. In order to achieve higher recording densities for removable magneto-optical (MO) disks, an optical flying head (OFH) with an integrated magnetic field modulation (MFM) coil and high-NA lens has been developed. The combination of the OFH and high-density media technology is expected to lead to an areal density of 100 Gb/in/sup 2/ and a data-transfer rate of 100 Mbps. In this paper, we give an overview of an optical disk system using an OFH that is suitable for use with removable MO media. We also present the structure of a mechanism for high-precision tracking by this system.

Power distribution control using switching devices for multiple charging system in resonant magnetic coupling

The resonant magnetic coupling (RMC) method has gained tremendous attention for its prospective features such as a large charging range or multiple device charging (multi-charging) application. In multi-charging systems, however, the receiving power ratio among the receivers is determined by coupling coefficient (k) or quality (Q) factor of each resonant coil. Therefore, it is difficult to supply appropriate power from single transmitter to plural receivers at the same time in various situations. To solve this problem, we apply a new efficiency control technique using non-linear devices. Two switches are connected to the resonant capacitance of receiving resonant coil in parallel and are turned on/off alternately at the transmission frequency. With this technique, receiving power ratio among the receivers can be controlled over a wide range at high efficiency.

Effect of load dependence of efficiency in a multi-receiver WPT system

Most batteries require constant voltage to charge; therefore, wireless charging receivers need a constant voltage circuit, commonly a DCDC converter (DDC). On the other hand, Resonant Magnetic Coupling enables multi-receiver charging. As a result, we need to design a wireless charging system for multi-receivers with a DDC. In cases of multi-receiver charging with different positions, the input voltage for a DDC in the stronger coupling receiver rises. Since there is a practical limit of DDC input voltage, we need to design the wireless charging system, not to exceed the limit. Through our work, we investigated the character of the load dependence of efficiency, and made clear that robustness to load variation determines the DDC input voltage by simulation and experiment. In conclusion, efficiency robustness to load variation is advantageous for single receivers, but not for multi-receivers in different coupling conditions.

A New Optical System for Holographic Memory Applying a Finite Objective Lens

We have developed a new objective lens for a holographic storage system to increase areal density. To balance focal length and working distance, we adopted a finite objective lens. By incorporating phase conjugate reproduction into the finite objective lens, we obtained practically applicable and adequate signal quality. We measured shift selectivity and the effect of crosstalk and evaluated book size, which was reduced by ca. 0.8. The areal density was estimated to increase by about 1.6 times when using the new objective lens.

Optical Flying Head System that Featuring Dual-Stage Tracking for Use in High-Density Magneto-Optical Recording

An optical flying head having an integrated magnetic field modulation coil and high-numerical aperture (NA) lens has been developed. This component can be used for first-surface magneto-optical (MO) recording with a NA of 0.9 and a wavelength of 405 nm. To achieve highly precise tracking with this system, we developed a track-access mechanism that has a sled positioner and a galvano mirror. The experimental use of a dual-stage control technique resulted in a tracking error of 10 nm.