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 Development of a Broad Bandwidth Helmholtz Coil for Biomagnetic Application
Tác giả hoặc Nhóm tác giả: Van-Dong Doan, Jen-Tzong Jeng*, Tsang-Hao Tsao, Thi-Trang Pham, Guan-Wei Huang, Chinh-Hieu Dinh, Tai-Hsing Lee
Nơi đăng: IEEE - The International Magnetics Conference (INTERMAG 2020); Số: HF-06;Từ->đến trang: 1390;Năm: 2020
Lĩnh vực: Kỹ thuật; Loại: Báo cáo; Thể loại: Quốc tế
TÓM TẮT
ABSTRACT
Following the advancement of magnetic sensing systems and magnetic materials, the biomagnetic technology has flourished in recent decades. To achieve a higher signal-to-noise ratio and extract more features from the object under test, the alternating-current (AC) magnetic field as an excitation source has been widely employed in biological researches [1]. To assure the accuracy of excitation amplitude, a uniform magnetic field covering the object under test is necessary. The solution for generating a uniform magnetic field is to use a solenoid or Helmholtz coil [2], [3], but the bandwidth is limited due to the inductive load at higher frequency. It is necessary to develop a coil system that can generate a uniform magnetic field in an extended frequency range. To achieve a sufficient magnetic field intensity at higher frequencies, it is straightforward to overcome the high impedance by increasing the driving voltage to keep the current amplitude unchanged. But a significant expansion of bandwidth is not possible since the ultrahigh-voltage power amplifiers are not available. In this work, we proposed a broad-bandwidth Helmholtz coil system based on a tunable tank circuit for application in the artificial deoxyribonucleic acid hybridization detection using tunnel magnetoresistance biosensor. The characteristics of the coil, including frequency response, uniformity, linearity, maximum magnetic field, and frequency spectrum of waveform distortion, were investigated experimentally. Figure 1 shows the components in the broad-bandwidth Helmholtz coil system. For biomagnetic application, the input is generated by an analog front-end circuit based on the direct-digital-synthesis technique. To generate a high frequency alternating field, a programmable capacitor matrix was cascaded with the coil to form a resistor-inductor-capacitor (RLC) resonant circuit [1]. The switchable matrix of 17 parallel capacitors for tuning the resonant frequency is digitally controlled by a microcontroller with a computer interface. The system can also be operated in a resistor-inductor (RL) mode. To assure high current capacity and stability, the metal film capacitors were used for capacitances above 68 nF and the mica capacitors were used for lower values. The 17 capacitors range from 5 nF to 200 μF and the total number of combination is 131071 for the capacitance matrix, allowing virtually continuous adjustment of frequency from 96 Hz to over 10 kHz. Figure 2 shows the frequency responses of the Helmholtz coil system with and without the capacitor matrix investigated experimentally at constant peak-to-peak voltage of 2 V. Figure 2a and 2b show the current and field intensities respectively at various driving frequencies for the system operated at the RL and RLC modes. It was found that the -3dB bandwidths of field-frequency (H-f) relation are 100 Hz in the RL mode and about 1 kHz for in the RLC mode. It was also observed that, in the RL mode, the -3dB bandwidth of current-frequency (I-f) relation is consistent with that of the H-f relation, but the bandwidth of I-f relation in the RLC mode is 6 kHz, higher than that of H-f relation. The result indicates that the expansion of bandwidth of H-f relation in the RLC mode is limited to a factor of about ten. The discrepancy in bandwidth between I-f and H-f relations is attributed to the decreased current-to-field conversion ratio (H/I) of the Helmholtz coil at higher frequencies, as shown in Fig. 2c. It can be seen that the H/I-f relation remains constant up to 200 Hz with a bandwidth (-3 dB) limit at 1.25 kHz. The frequency-dependence of H/I relation can be modeled by using the modified equation of Helmholtz coil: B/I=(μ0H)/I={8μ0Neff(ω)}/{r√125} (1) where B is the central magnetic field of the coil (Tesla), I is the coil current (Ampere), μ0 = 4π×10−7 H/m is the vacuum (or air) permeability, r is the coil radius (meter), Neff(ω) is the effective number of turns of each coil, ω = 2πf is the angular frequency, and f is the driving frequency. The Helmholtz coil used in this work has a number of turns N = 178 for each coil, an average radius r ≈ 38 mm and the inductance of coil L = 6.62 mH. From equation (1), B/I is equal to 0.004212 T/A (or H/I = 42.12 Oe/A). According to Fig. 2(c), the reduction in Neff(ω) suggests an increasing phase difference to the complex H/I ratio at frequencies above 200 Hz. As the parasitic capacitance (Cp) is sensitive to the spacing between turns of wire, the uniformity of winding for each branch of the coil must be taken into account for the more accurate simulation of the Neff(ω) behavior. The non-uniformity of field distribution is found to be 0.46% at DC and 0.76% at 440 Hz in the 1 cm×1 cm region at the center of the coil. The observed non-uniformity and the bandwidth of the coil indicate that the proposed coil design is valuable for testing of biomagnetic chips with an active area over 1 cm2 with a relatively small coil diameter of 8 cm.
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