Kunihisa Tashiro

Simple-Box-9 Coil System: A Novel Approach to Design of a Square Coil System for Producing Uniform Magnetic Fields

This paper presents a novel approach for the design of square coil system, through considerations of a conventional square coil system. This simple approach allows us to design a coil system which contains a greater number of coils. From this design method, we introduce a new structure system of same-sized square coils, Simple-Box-9. The constant coil spacing is a fourth-part of the side length of the coil, and the number of windings is 2/1/1/1/1/1/1/1/2. From numerical calculations and experiments, it is shown that Simple-Box-9 can achieve the best uniformity compared with conventional square coil systems.

Spontaneous magnetoencephalography alpha rhythm measurement in a cylindrical magnetic shield employing magnetic shaking

This article shows a demonstration of magnetoencephalography measurement in a cylindrical magnetic shield made of cobalt-based amorphous tape with magnetic shaking. The noise levels of the first-order superconducting quantum interference device gradiometers that operated in the shield were reduced to as low as 40 fT/√Hz at 10 Hz by surrounding it with a simple rf shield made of conductive cloth. We observed spontaneous alpha rhythms from a human brain in this shielding system. Alpha rhythms and their suppression caused by opening the eyes were clearly found, which was also confirmed by electroencephalography measurement from the same volunteer under similar conditions.

Adaptive compensation of magnetic fields inside an open cylindrical magnetic shield

For the development of a magnetic field compensation system suited for the open end of the cylindrical magnetic shell, magnetic field distributions were characterized on the axis of the magnetic shell for the externally impressed low-frequency AC magnetic field and for the compensating magnetic field generated by a set of ring coils. Based on this observation, a system for actively compensating the magnetic field leakage through the opening is proposed. A cylindrical magnetic shield of desk-top size was constructed by combining an axial shell made of 2 in wide Metglas 2705M amorphous tapes encircling the outer surface of a cylindrical body in its axial direction, with a magnetic shaking coil, and then a proposed compensation system was built on top of the structure. The axial shielding factor was improved with this active compensation system by 55 times for a 2 Hz impressed magnetic field, 20 times for a 10 Hz impressed field, ten times for a 30 Hz impressed field, and five times for a 50 Hz impressed field. An interesting phenomenon observed during the characterization of the magnetic fields is that the magnetic field in the space inside the axial magnetic shell leads in phase with respect to that of the impressed magnetic field, unlike the case with a helical shell where the magnetic field is lagging in phase. A model for explaining this phenomenon is also given.

Investigations on a Novel Inductive Concept Frequency Technique for the Grading of Oil Palm Fresh Fruit Bunches

From the Malaysian harvesters perspective, the determination of the ripeness of the oil palm (FFB) is a critical factor to maximize palm oil production. A preliminary study of a novel oil palm fruit sensor to detect the maturity of oil palm fruit bunches is presented. To optimize the functionality of the sensor, the frequency characteristics of air coils of various diameters are investigated to determine their inductance and resonant characteristics. Sixteen samples from two categories, namely ripe oil palm fruitlets and unripe oil palm fruitlets, are tested from 100 Hz up to 100 MHz frequency. The results showed the inductance and resonant characteristics of the air coil sensors display significant changes among the samples of each category. The investigations on the frequency characteristics of the sensor air coils are studied to observe the effect of variations in the coil diameter. The effect of coil diameter yields a significant 0.02643 MHz difference between unripe samples to air and 0.01084 MHz for ripe samples to air. The designed sensor exhibits significant potential in determining the maturity of oil palm fruits.

Residual magnetization measurements of a motor to be used in satellites

A simple magnetic shielding system for residual magnetization measurements of a motor to be used in satellites is studied. The system consists of same-sized square coils, namely three sets of Simple Cubic-3 coil system. Because the system can generate a uniform magnetic field, we investigate the uniformity of the site. With a first-order gradient coil, the geomagnetic field can be reduced to less than 100 nT in a relatively large area. Inside of the system, the residual magnetization measurements of a step motor are demonstrated. An approach to reduce the magnetic field leakage from the step motor is also presented.

Broadband Air-Core Brooks-Coil Induction Magnetometer

Frequency compensation technique to be implemented in an optimal design method of air-core Brooks-coil induction magnetometer is presented. It is confirmed that the capacitance of a cable between coil and electronics defines the limitation of upper frequency. The limitation of lower frequency is expanded by use of I/V converter with frequency compensation. The fabricated magnetometer demonstrates the noise floor level of 300 fT/Hz1/2 at 100 Hz, and the flat frequency response from 0.2 Hz to 20 kHz. It is found that the capacitance to be use in the I/V converter is also key parameter to define the noise floor level, and the parameter could be neglected if the value is small. This result may allow us to explain why the induction magnetometer based on definition of inductance is more sensitive in low-frequency region than that based on Faradays law. A design of induction magnetometer having comparable diameter to SQUID sensor for biomagnetic measurements is also discussed

Proposal of Coil Structure for Air-Core Induction Magnetometer

Proposal of coil structure for air-core induction magnetometer is presented. The key point is the lamination factor which refers to the ratio of winding number compared with an ideal Brooks-coil. A decrease in lamination factor produces an increase in flux-to-voltage transfer ratio, and a decrease in noise floor level of the magnetometer. The size of coil defines the 20 dB/decade slope in frequency response of the magnetometer. While a magnetometer consisting of a coil with low lamination factor has high cutoff frequency, the sensitivity is better than of a coil with high lamination factor even if the frequency is lower than the cutoff frequency. Some magnetometers having few hundred ft/Hz1/2 are fabricated, and demonstrate the validity of the idea. This design method may allow us to develop an ultra-sensitive, high-spatial-resolution induction magnetometer comparable to SQUID sensor.

Estimation of Effective Permeability for Dumbbell-Shaped Magnetic Cores

The objective of this paper is to provide suitable approximations of effective permeability for magnetic concentration cores. This core has a dumbbell shape so that the magnetic flux is concentrated to the coil winding part of the core. It is well known that the demagnetizing factor strongly depends on the core shape, which defines the effective permeability. Although several researchers have already proposed an approximation for a dumbbell-shaped core, they did not take into account the flange part length. In this paper, a novel approximation for a dumbbell-shaped core is proposed. It is based on the demagnetizing factor and correction terms related to the aspect ratio of both the flange and coil winding part. From experimental results, the validity of the approximation is confirmed.

Improvement in Sensitivity of an Inductive Oil Palm Fruit Sensor

Among palm oil millers, the ripeness of oil palm Fresh Fruit Bunch (FFB) is determined through visual inspection. To increase the productivity of the millers, many researchers have proposed with a new detection method to replace the conventional one. The sensitivity of such a sensor plays a crucial role in determining the effectiveness of the method. In our preliminary study a novel oil palm fruit sensor to detect the maturity of oil palm fruit bunches is proposed. The design of the proposed air coil sensor based on an inductive sensor is further investigated to improve its sensitivity. This paper investigates the results pertaining to the effects of the air coil structure of an oil palm fruit sensor, taking consideration of the used copper wire diameter ranging from 0.10 mm to 0.18 mm with 60 turns. The flat-type shape of air coil was used on twenty samples of fruitlets from two categories, namely ripe and unripe. Samples are tested with frequencies ranging from 20 Hz to 120 MHz. The sensitivity of the sensor between air to fruitlet samples increases as the coil diameter increases. As for the sensitivity differences between ripe and unripe samples, the 5 mm air coil length with the 0.12 mm coil diameter provides the highest percentage difference between samples and it is amongst the highest deviation value between samples. The result from this study is important to improve the sensitivity of the inductive oil palm fruit sensor mainly with regards to the design of the air coil structure. The efficiency of the sensor to determine the maturity of the oil palm FFB and the ripening process of the fruitlet could further be enhanced.

Desktop Magnetic Shielding System for the Calibration of High-Sensitivity Magnetometers

The objective of the study is a development of desk-top magnetic shielding system for the calibration of high-sensitive magnetometers, and the evaluation of their ability to produce a uniform magnetic field. This system consists of hexagonal tubes and a cylinder. The hexagonal tubes have a double-layered structure, an opening and an advantage in axial shielding performance. The cylinder has openings and an advantage in transverse shielding performance. A solenoid-like coil is wound around the hexagonal tubes to produce a uniform magnetic field. From the experimental results, the magnetic shielding and the calibration field properties are demonstrated. Within the half length of the system, the achieved total magnetic field (dc component) is less than 200 nT within ± 50 nT deviation, and the measured shielding factor is larger than 250. For producing a uniform magnetic field up to 10 kHz, we can use the 1/3 length of the system.