Development of a measuring system for low-frequency, extremely low-level magnetic field-Introduction of high-performance magnetic shield and a adaptive vibratory noise cancelling mechanism

开发低频极低磁场测量系统-引入高性能磁屏蔽和自适应振动噪声消除机制

基本信息

批准号：
06555122
负责人：
SASADA Ichiro
金额：
$ 8.77万
依托单位：
KYUSHU UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A)
财政年份：
1994
资助国家：
日本
起止时间：
1994 至 1996
项目状态：
已结题

项目摘要

In this study, the following two themes were investigated to develop ; (1) a human-size, light-weight, cylindrical magnetic shield with open ends by applying magnetic shaking technique, and (2) a system canceling vibratory magnetic noise adaptively.We first constructed half-sized model (diameter=52cm, length=120cm) using Metglas 2705M amorphous ribbons amounting to 15kg in a five-shell structure with narrow spacing (less than 1cm) in-between. The shielding factor for transversely applied alternating magnetic field was 90900 at 1 Hz and 21000 at 10 Hz. It shows high value for relative permeability obtained with magnetic shaking helps multi-shell to work well despite narrow spacings. We then developed a human-size, four-shell cylindrical shield (diameter=90cm, length=250cm) with the same amorphous ribbon of 70kg and Permalloy tapes of 40kg which was wound helically to form the innermost shell used to prevent unwanted leak of the shaking field into the shielded space. Background magnetic noise in the shield was measured using a SQUID magnetometer as 54 fT/ (Hz)^<-1/2> at 1 Hz and 23 fT/ (Hz)^<-1/2> at 10 Hz. The residual dc magnetic field was only 1-2nT.An algorithm to cancel nonstationary magnetic noise due to vibration was evaluated using a model system in which vibration of two order of freedom was assumed and linear gradient ac magnetic fields were used as a reference frame. Noise rejection ratio was 20dB.This system was then implemented to the human-size cylindrical magnetic shield. The rejection ratio was 10dB.Finally, the shielding system was used for a heart magnetic field measurement, showing magnetic field from a human heart was successfully measured by both a SQUID magnetometer and a first-order gradiometer.

在这项研究中，研究了以下两个主题：（1）采用磁振动技术，制成一个人体大小、重量轻、两端开口的圆柱形磁屏蔽，以及（2）自适应消除振动磁噪声的系统。我们首先构建了半尺寸模型（直径=52cm，长度= 120cm），使用重15kg的Metglas 2705M非晶带材，采用五壳结构，中间间距窄（小于1cm）。横向施加交变磁场的屏蔽系数在1Hz时为90900，在10Hz时为21000。它显示出通过磁振动获得的相对磁导率的高值，有助于多壳在狭窄的间距下良好地工作。然后，我们开发了一个人体大小的四壳圆柱形屏蔽（直径= 90厘米，长度= 250厘米），采用相同的70公斤非晶带和40公斤坡莫合金带，螺旋缠绕形成最内壳，用于防止不必要的泄漏。震动场进入屏蔽空间。使用SQUID磁力计测量屏蔽中的背景磁噪声，1Hz时为54fT/(Hz)^<-1/2>，10Hz时为23fT/(Hz)^<-1/2>。残余直流磁场仅为1-2nT。使用模型系统评估了消除振动引起的非平稳磁噪声的算法，其中假设二阶自由度振动，并使用线性梯度交流磁场作为参考系。噪声抑制比为20dB。然后将该系统应用于人体大小的圆柱形磁屏蔽。抑制比为10dB。最后，将该屏蔽系统用于心脏磁场测量，结果表明SQUID磁力计和一阶梯度计均成功测量了人体心脏磁场。