Applications of the Hybrid Shielding in Biomagnetometry M.C. Rau1, O. Baltag2, and I. Rau2 1
2
“Gh. Asachi” Technical University, Faculty of Electrical Engineering, Iasi, Romania “Gr.T. Popa” University of Medicine and Pharmacy, Faculty of Biomedical Engineering, Iasi, Romania
Abstract— The work presents the experimental results concerning the electromagnetic characteristics of a shielded room destined to bio-electromagnetism researches and to the utilization of a high resolution SQUID magnetometer. The experimental results regarding the dependence of the shielding factor on the applied field frequency are presented. The phase of the residual magnetic field and its dependence on the frequency of the external field has also been determined. The experimental results confirm the theoretical studies on the utilization of the non-ferromagnetic material shields. Keywords— shielding.
shielded room, passive shielding, active
I. INTRODUCTION The bioelectromagnetism measurements require special conditions, given the very small signals and detected fields. These conditions concern the necessity to reduce the magnetic fields external to the space where the measurements are carried out, i.e. both the geomagnetic field and the interfering magnetic fields existing outside this space. This kind of space where the magnetic activity is almost zero can be obtained by combining various compensation means. These are represented by the magnetic shields which can be used independently or with other means meant to increase the shielding factors. The magnetic shields [1], [2], [3] built for bio- electromagnetism measurements are shaped as cubes or parallelepipeds and are made of various materials, using various assembling and construction methods. The materials they are made of can be ferromagnetic [4], [5] or non-ferromagnetic [6]. For the shielded rooms made of ferromagnetic materials, the shielding is due to the fact that the magnetic flux prefers the path with the highest value of the magnetic permeability. The utilization of multiple layers increases the shielding factor [7] – [9], [10], [11]. To increase the permeability of a ferromagnetic material, an alternating magnetic field produced by coils wound on the shield is applied, i.e. the shaking method is used. In order to increase the shielding factor of the passive shields, other systems can be added. The mostly used method is to add systems of uniaxial or tri-axial coils disposed in Helmholtz configuration, as well as negative feedback assembly, see Table 1.
Table 1 Classification of the hybrid shielding methods Materials
Combination Cube or Ferromagnetic parallelepiped with high shape with 1, 2, magnetic 3, 6 or 8 layers permeability - with uniaxial or mumetal, triaxial coils permalloy, Fe- system and negative feedSi alloy back assembly Walls with 1, 2, Non3 layers and ferromagnetic uniaxial or (Al, Cu) triaxial coils materials with system and high electric negative feedconductivity back assembly
Ferromagnetic and nonferromagnetic
sandwich type with air, wood, plastic, glass in combination, permalloy with Al or Cu and with uniaxial / triaxial system coils and negative feedback assembly
Compensation
Applications
Based on high magnetic permeFor passive shielding and ability of peractive shielding malloy, for ELF shielding
for passive and dynamic shielding
Based on Lenz law, for EHF shielding
Biomagnetism, satellites, generally equipment using electron guns or ions sources and mass spectrometry, masers and atomic passive shield- clocks, electron ing and dymicroscopy and namic shielding transmission electron microscopy, SEM (scanning electron microscopy), SQ UID, MRI, electron beam instruments
The feedback loop closes through a field sensor, a magnetometer, power amplifiers and compensation coils. The sensor is located in the area which must have a minimum magnetic field, inside the compensation coils. These systems are especially used to obtain minimum magnetic fields within different volumes in which the equipment are due to operate. Several terms are used in literature, such as: active compensation [12], [13], [14], active shielding [15], [16], [17], dynamic shielding [18], magnetic field cancellation [19], magnetic field stabilization [20] or hybrid technique [21]. All these terms define compensation systems which operate in negative feedback regime.
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II. THEORETICAL CONSIDERATIONS In order to measure the biomagnetic fields, besides using these systems for compensating and shielding the disturbing fields, the utilization of high resolution magnetometers is also necessary. The high resolution biomagnetometers have very good sensitivity, but also some limitations, such as: - reduced immunity to electromagnetic and magnetic fields; - sensitivity to shocks, vibrations and microseisms; - necessity of periodic refilling with liquid He or nitrogen. The disturbing magnetic fields are both continuous, like the geomagnetic field, and natural slowly variable components produced by geomagnetic field micro pulsations or geomagnetic storms. Among the low frequency fields, those produced by the supply networks and by the 50 Hz supply plants and power transformers are the most present in the disturbing signal spectrum, both by their basic frequency, and the second and third order harmonics especially. These frequencies are hard to eliminate from the complex signal of the biomagnetometer, because they can be found in the spectrum of both bioelectric and biomagnetic signals. Usually, the high frequency fields in the long wave and microwaves range do not penetrate in the channel of biomagnetic signal of the SQUID, but they easily penetrate in the end processing modules. That is why it is recommended to ensure the compatibility of the SQUID installation with the electromagnetic medium within the entire spectral range. The compatibility can be reached by classical shielding methods, introducing the SQUID installation together with the subject in a room shielded against both continuous and alternating magnetic fields. There are several shielding techniques and methods, depending on the SQUID performances, destination and location, and the designer and builder imagination. In the case of shielded rooms made of nonferromagnetic (Cu, Al) materials, the shielding is based on the Lenz’s law. The time-variable electromagnetic field induces in the conducting material eddy currents which, in turn, generate a field opposite to the external field. Simultaneously, the energy absorption through eddy currents occurs. The shielding effect is determined by the skin depth δ, which represents the distance at which the electromagnetic wave is attenuated by a factor 1/e. For the calculation of the skin depth, an electromagnetic wave with intensity H0 is considered, with normal incidence to the external surface of a plane conducting wall with the thickness z, the intensity Hx of the wave emerging from the conducting wall being diminished. Let’s consider a plane wall made of a non-ferromagnetic high conductivity material - Cu or Al, having the separation surface from the medium in the Oyz plane, Figure 1.
Fig. 1 Wave propagation through conducting wall The plane wave of intensity H0 travels in the space on a direction perpendicular to the incidence plane of the conducting wall. Under these conditions, only the field component along the Ox axis will be considered Ddeveloping the Helmholtz equation, the component HZ becomes: z
− z H y (z, t) = H0e δ cos(ωt − )
δ
(1)
From the above equation, the field amplitude and phase at the distance z result. The phase angle is also useful to express the standard skin depth. At the standard skin depth, the phase angle is about 570. Therefore, the electromagnetic field intensity decreases exponentially with the distance run through a conducting medium. In the electromagnetic field theory, it was demonstrated that the skin depth δ depends on the electric and magnetic properties of the medium and the electromagnetic wave frequency according to the relation:
δ=
2
ωσμ
=
1 πfσμ
(2)
where: f – field frequency; σ– conductivity of the conducting medium; µ – magnetic permeability of the medium. One can notice that the skin depth decreases as the field frequency increases.
III. EXPERIMENTAL SETUP The theoretical and experimental researches are dealing with the theoretical calculation and experimental determination of the global shielding factor within different frequency ranges, from the continuous fields to medium frequency fields. At low frequency, the phenomena occur in the magnetic induction zone and, consequently, only the magnetic component of the field is of interest. Even though, from the theoretical results, it follows as obvious that there are two components of the field inside the room, a real and an imaginary one, the studies were not directed to the determination of the field phase variation inside the system. The knowledge of the field phase within the shield is important in the utilization of the systems for magnetic field dynamic control, systems which operates in the negative feedback
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loop. The realized installation makes use of mixed shielding, consisting of a room made of non-ferromagnetic material, and a group of external coils which control, through the negative feedback, the residual magnetic field inside the room. The shielded room has a parallelepiped shape, sized 2x2x3 m, and has 0.012 m thick aluminum walls. It is located in the centre of a system of coils disposed in a Helmholtz configuration, with the side of 4 m each. From the equation (1), it follows that inside the shielded room there is a residual complex signal with the amplitude smaller than the external field. One can notice that the phase of this signal strongly depends on the field frequency. That is why it is difficult to apply a negative feedback loop, given the fact that it is necessary to correct the phase of the current through the Helmholtz coils in terms of the applied frequency. In order to determine the shielding factors and the residual signal phase, the installation presented in Figure 2 is used.
Fig. 3 Shielding factors along the three directions In fact, one can consider that the wave front of the disturbing induction meet the shielded room walls under different, well defined angles, in each of the three main planes. The field phase inside the room changes differently along the three directions, within the volume comprised between the geometrical centre and the room walls. Figures 4, 5, 6 present the variation of the residual field phase inside the room along the three orthogonal directions Oxyz.
Fig. 2 The block diagram of the installation for electromagnetic characterization of the shielded room An alternating current is injected through the Helmholtz coils, its intensity being measured by means of a standard resistor and a digital oscilloscope. The residual magnetic field within the shielded room is measured with a saturable fluxgate magnetometer. For the determination of the residual field phase, the current injected through the Helmholtz coils was taken as reference. It has been experimentally found that the shielding factors along the three directions are different Figure 3. The variation of the shielding factors along the three directions results from the coupling factors between the three pairs of Helmholtz coils and the room walls, which are different along the three directions. Still another reason that the shielding factors are different along the three directions is the presence of the walls which are parallel with the field lines, and within which residual fields are induced by the components of the external magnetic induction produced by the coils.
Fig. 4 Variation of the residual field phase inside the room along the Ox direction
Fig. 5 Variation of the residual field phase inside the room along the Oy direction
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Applications of the Hybrid Shielding in Biomagnetometry
The phase variation inside is important because the system that works in the negative feedback loop, having sensors on the inside, must maintain a phase relation corresponding to a stable regime. Consequently, in the negative feedback loop of the power amplifier which injects compensating currents through the external coils, a phase correcting circuit must be introduced depending on frequency. In this way, the negative feedback coefficient can become constant within the entire frequency range, and equal to the negative feedback coefficient for the external fields.
Fig. 6 Variation of the residual field phase inside the room along the Oy direction
IV. CONCLUSIONS The shielded room presents different shielding factors along the three main directions. The phase of the residual field inside the room depends on the frequency and on the position of the measurement point. A phase variation was found within the volume of the shielded room along all the three directions. For the active control/shielding, a phase corrector must be introduced, which maintain the system within the stable zone of the negative feedback loop.
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[email protected]
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