IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
2357
Method for Evaluating Shielding Factor of Double Layered Magnetically-Shielded Rooms for Uniform Magnetic Field Using Exciting Coils Placed on One Side Shunya Odawara1 , Kazuhiro Muramatsu1 , Shogo Komori1 , Kiyotaka Kamata2 , Keita Yamazaki3 , Takao Yamaguchi4 , Mitsuru Sakakibara5 , Toshifumi Shinnoh6 , Masao Simokawa7 , Noboru Ishikawa8 , and Takashi Meguro9 Department of Electrical and Electronic Engineering, Saga Univ., Saga 840-8502, Japan Department of Electronic Control Engineering, KNCT, Kirishima, Kagoshima 899-5193, Japan Research and Development Institute, Takenaka Corp., Inzai, Chiba 270-1395, Japan Daido Plant Industries Corp., Nagoya 457-0819, Japan Ohtama Co., Ltd., Inagi, Tokyo 206-0311, Japan Technical Research Institute, Kajima Corp., Chofu-shi, Tokyo 182-0036, Japan Giken-kogyo Corp., Suginami-ku, Tokyo 166-0004, Japan Shimizu Corp., 1-2-3 Shibaura, Minato-ku, Tokyo 105-8007, Japan Hitachi Metals, Ltd., Minato-ku, Tokyo 105-8614, Japan To shield the environmental magnetic noises, magnetically shielded rooms (MSRs) are used for biomagnetic measurements, etc. The sources of environmental noises are placed far away from the MSR, in such way that almost uniform magnetic fluxes are applied to the for the uniform magnetic flux density should be MSR. Therefore, to specify the shielding performance of MSR, the shielding factor evaluated. To evaluate , in a construction site, the magnetic noise is applied by coils. However, ’s depend on the size and position of the coil. Therefore, to accomplish a standardization of the method for evaluating , accurately, by coils, the method for estimating ’s by smaller coil placed near MSR is being investigated in JEITA (Japan Electronics and Information Technology Industries Asso’s by the extrapolation using simple numerical expressions ciation). In JEITA, we have already proposed the method of estimating representing the relations between the and the size, position of coil. The proposed method was verified by the actual single-layered of the double-layered MSR, using coils placed only MSR using coils placed on double sides. In this paper, the method of estimating on one side of MSR and taking account of door effect, is investigated using 3D nonlinear magnetic field analysis and measurements. By using the analysis, it is shown that the single coil should be set in front of the door. Moreover, ’s estimated by using the measured data with the optimal coil setting can almost represent the tendencies of the true . Index Terms—Exciting coil, magnetic field analysis, magnetically-shielded room, shielding factor, standardization.
I. INTRODUCTION O shield the environmental magnetic noises such as urban magnetic fluctuations due to elevators, electric trains, and motor cars, magnetically shielded rooms (MSRs) made of ferromagnetic materials are used for biomagnetic measurements, etc [1]. The shielding performance of MSR is usually represented by the shielding factor ( , and are the flux densities at the center of the room without and with MSR, respectively). The sources of environmental noises mentioned above are placed far away from the MSR, in such way that almost uniform magnetic fluxes are applied to the MSR. Therefore, the shielding factor for the uniform magnetic flux density should be evaluated. To evaluate , in a construction site, the magnetic noise is applied to the MSR by coils in place of the environmental one. If the coil is larger than the MSR or the coil is placed far enough from MSR, the uniform environmental magnetic noises can be simulated, but it is usually not possible to set such large size or far-away position coils ’s evaluated by smaller coils placed in the construction site.
T
Manuscript received October 31, 2009; revised January 20, 2010; accepted February 21, 2010. Current version published May 19, 2010. Corresponding author: S. Odawara (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2010.2045356
near MSR are overestimated compared with those by environmental magnetic noises because the magnetic flux excited by the coil are attenuated by distance. Moreover, ’s are affected by the size and position of the coil and are not unique for the same MSR. Therefore, the standardization of the method for evaluating , accurately, by coils is being discussed in JEITA (Japan Electronics and Information Technology Industries Association). In [2], the relations between the and the size, position of coil for single and double layered MSR models neglecting the effect of the door are obtained from the magnetic field analysis. Then, the method of estimating ’s by the extrapolation using simple numerical expressions representing the relations has been proposed. The proposed method was verified by the nonlinear magnetic field analysis and experiment of an actual single-layered MSR. In this verification, MSR is placed in the testing site, which has enough wide space, and two coils can be placed symmetrically in the parallel direction to the walls can be neon both sides of MSR. The effect of the door on glected. However, in the usual construction site, the coil sometimes can be placed on only one side of MSR due to limited space. Moreover, the effect of the door on cannot be neglected in the multi-layered MSR with higher . In this paper, to enlarge the coverage of the proposed method, the method of estimating of the double-layered MSR, using coil (single coil) placed only on one side of MSR and taking account of door effect, is investigated using 3D nonlinear magnetic
0018-9464/$26.00 © 2010 IEEE
2358
IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
Fig. 1. An actual single-layered MSR.
field analysis and measurements. First, the estimation method proposed in [2] is illustrated by using an example of the actual single-layered MSR. Then, the optimal setting method of only single coil taking account of the door effect is examined by the nonlinear magnetic field analysis of an actual double-layered MSR. Finally, the proposed method is verified by the experiment of the actual double-layered MSR.
Fig. 2. Effect of coil (a) position L and (b) size L on SF (B = 1 T).
II. METHOD OF ESTIMATING SHIELDING FACTOR FOR UNIFORM FIELD USING TWO EXCITING COILS [2] Fig. 1 shows an actual single-layered MSR and two square coils, which are placed in symmetrical positions with each other so that the center axes coincide with that of MSR. for In [2], two methods of estimating the shielding factor the uniform flux density by the extrapolation are proposed. between and the coil distance One is that the relation using the small coil. The other is that the relation between and coil size at short distance. In the actual construction site, the one can be chosen depending on the situation. The coil of the size is used as the small coil and the distance is used as the short distance in this paper. First, to carry out the nonlinear magnetic field analysis, the initial curve ( and are the flux density and magnetic field intensity, respectively) of the shielding plate of permalloy is estimated so that the ’s at , , and the different coil currents, obtained from analyses and measurements, coincide with each other. Then, to estimate taking account of the nonlinearity, the method of setting the suitable coil current is also investigated by the analysis. It can be concluded that the coil current should be determined so that the flux density at the center point O of the room generated by the coils without MSR coincides with the specified uniform flux density . In this method, as and become larger, the both maximum and average flux densities in the shielding plate are rapidly close to those for . Fig. 2 shows the relations and obtained from the analyses and measurements. The measurements are carried out at 1 Hz in order to neglect the eddy current effects. Both results obtained from analyses and measurements are in good agreement with each other. The curve of is naturally attenuated and seems to coincide with at infinity. On the other hand, the curve of can be approximated by the straight line, and the straight line is close to when is the same with the
Fig. 3. Estimated SF for an actual single-layered MSR.
size of MSR. As a result, the following simple functions of fitted curves for and are proposed, respectively: (1) (2) where, is the length of MSR in the parallel direction to flux and is the length of longer edge of MSR in the vertical direction to flux. And coefficients , , in (1) and (2) are determined by the nonlinear regression analysis and the method of least squares, respectively, so that the measured relations and the fitted curve coincide with each other. becomes the estimated Both functions are formed so that . Fig. 3 shows the comparison between estimated using only measured data at smaller three ’s and ’s and obtained from the analysis. This figure shows that the proposed method can estimate with good accuracy. III. MODEL OF DOUBLE LAYERED MSR AND METHODS A. Model Description of Double Layered MSR Fig. 4 shows an actual double-layered MSR. In this MSR, the effect of the door on cannot be neglected. The configuration of the door is shown in Fig. 5. Only 1/4 region of the door is shown due to symmetric. In this paper, the method of
ODAWARA et al.: METHOD FOR EVALUATING SHIELDING FACTOR OF DOUBLE LAYERED MAGNETICALLY-SHIELDED ROOMS
2359
C. Analysis Method
Fig. 4. An actual double-layered MSR.
The ordinary 3D nonlinear magnetostatic analysis is carried out by using the 1st order brick edge finite element method with the method ( : magnetic vector potential). First, the curve of the permalloy is estimated by the same method described in the Section II. However, the curve is estimated using obtained by the small coil of placed at near the wall on the back setting shown in Fig. 6(a) to remove the effect of the door. In the contact part of the door and shielding wall, the magnetic characteristics of permalloy is distorted due to the stress and the gap. In this paper, the distortion is replaced to the equivalent uniform gap as shown in Fig. 5. The length of the gap is determined so that the maximum difference between ’s obtained from analyses and measurements at 8 pattern combined with the small and large coils ( and 1.885 m), short and long distances ( and 3.385 m), and small and large currents (0.1 and 30 A) on the front setting shown in Fig. 6(a) becomes minimum. The equivalent gap is set to be 1.3 mm and the maximum difference is 8.8%. D. Estimation Method In the case the is estimated by changing size of the coil set in the vertical direction to the wall of MSR, the relation is naturally attenuated but not straight line because the center position of coil is changed by . Therefore the following function is used for this case:
Fig. 5. Configuration of door.
(3) In the other cases, the method of estimating with that described in Section II.
is the same
IV. OPTIMAL SETTING METHOD OF COIL
Fig. 6. Various setting methods of single coil for estimation of SF ’s in (a) y and (b) x directions.
estimating only by single exciting coil is examined. Fig. 6 shows the various setting methods of single coil for estimating ’s in the and directions when the uniform fluxes are applied in the vertical and parallel directions to the wall with door, respectively. B. Measurement Method In this MSR, the conductive layer exists between magnetic layers. Therefore, measurements are carried out at 0.01 Hz to remove the eddy current effects. To obtain , the absolute value of the flux densities at the center point O in MSR is measured using 3-axes of fluxgate magnetometer. The low-pass filter with cut-off frequency of 50 Hz is used to remove the noise.
accurately by single coil In this section, to estimate taking account of the effect of door, the optimal setting method of the coil is investigated by using the analysis. Fig. 7 shows the ’s in the direction estimated by the various coil settings shown in Fig. 6(a). This figure is corresponding to Fig. 3 in Section II. This figure shows that the taking into account the door effect can be estimated accurately if the coil is set in front of the door, in both cases of changing and . This is because the fluxes generated by the coil pass the door even if the small coil is placed near the door of MSR. Fig. 8 shows the ’s in the direction estimated by coil settings shown in Fig. 6(b). In this case, if the coil is set in front of the door, the proper results can be also obtained. Therefore, when the taking account of the door effect is estimated using only single coil, the coil should be set in front of the door. V. EXPERIMENTAL VERIFICATION To investigate the effectiveness of the proposed method for an actual double-layered MSR, the ’s in and directions are estimated using measured data by the optimal coil setting. The true cannot be obtained in the measurement because the uniform magnetic field cannot be applied by coil due to the limited space. Therefore the analyzed results are used for the true value. Fig. 9 shows the estimated in direction obtained by the coil set in the parallel direction to the wall in front of the door of MSR. The tendencies of both the estimated
2360
IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
Fig. 9. Estimated SF in y direction for an actual double-layered MSR in the measurement.
Fig. 7. Effect of coil setting on (a) L and (b) L .
SF
in
y
direction estimated by changing Fig. 10. Estimated SF in x direction for an actual double-layered MSR in the measurement.
more investigation will be carried out in future. Fig. 10 shows the estimated in direction obtained by the coil set in the vertical direction to the wall in front of door of MSR. In this coil setting, more space is required compared with the parallel coil setting. The space of this experimental site and the small coils with different sizes are not enough to estimate by changing . Therefore, only estimation of ’s by changing is per’s for the small ’s are estimated formed. Moreover, only because enough magnetic field cannot be applied to MSR due to limited capacity of power supply in the measurement. The estimated is in good agreement with true value. VI. CONCLUSION
Fig. 8. Effect of coil setting on (a) L and (b) L .
SF
in
x
direction estimated by changing
and the true are similar with each other, whereas the esti’s are not close to the true obtained from analmated ysis. This result is different from that in Fig. 7 obtained from only numerical analysis. It seems to be because of the following possibilities; In the analysis, the error may be due to the inaccurate modelings of magnetic characteristics of shielding plate and door with equivalent gap. On the other hand, in the experiment, the error may be due to the effect of the reinforcing bars in the building. Therefore, to specify the cause of discrepancy,
of the double-layered MSR, The method of estimating using coils placed only on one side of MSR and taking account of door effect, was investigated. By using the analysis, it was shown that the coil should be set in front of door to estimate accurately taking account of door effect. Moreover, the estimated by using the measured data with the optimal coil setting obtained from can almost represent the tendencies of true the analysis. To obtain more accurate estimated results, the limitations of the coil sizes and distances should be investigated. Moreover, the effect of the shape of MSR on the accuracy and eddy current effect will be investigated in future. REFERENCES [1] A. Mager, “The Berlin magnetically shielded room,” in Proc. 3rd Int. Workshop Biomagnetism, Berlin, Germany, 1980, pp. 51–78. [2] S. Odawara et al., “Method for evaluating shielding factor of magnetically-Shielded room for uniform magnetic field using exciting coil,” J. Magn. Society Jpn., submitted for publication.
