• Journal of the Korean Magnetic Resonance Society
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• v.21 no.1
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• pp.7-12
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• 2017

The nuclear magnetic resonance (NMR) and atomic magnetic resonance (AMR) plays a fundamental role in achieving a high accuracy of magnetic field measurements. Magnetic field unit (T) was realized based on the shielded proton gyromagnetic ratio (${\gamma}^{\prime}_P$), helium-4 gyromagnetic ratio (${\gamma}_{4He}$) and related techniques. The magnetic field standard system has been disseminated by the NMR magnetometer and electromagnet, a Helmholtz coil system, and AMR magnetometer in the nonmagnetic laboratory. A magnetic field standard below 1 mT has been developed by using Cs and Cs- $^4He$ AMR with automatic compensation of an external magnetic field noise. The standards serve for the calibration of magnetometers and support the test of sensors and materials in the range from $5{\mu}T$ to 2.0 T with (1 to 50) ${\mu}T/T$ uncertainty (k=2).

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.13 no.6
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• pp.490-495
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• 2003

This paper presents investigation of damping characteristics of squeeze mode type MR (magneto-rheological) mount experimentally. Since damping property of the MR fluid is changed by variation of the applied magnetic field strength, squeeze mode type MR mount proposed in the study has variable damping characteristics according to the applied magnetic field strength. In the present work, the performance of the mount was experimentally Investigated according to the magnetic field strength and exciting frequencies. The experimental results present that the MR mount can effectively reduce the vibration in a wide range of frequency by controlling the applied electromagnetic field strength. Viscous damping and stiffness coefficients of the MR mount tend to be changed according to the variation of the applied currents in this study and MR effect is reduced by increasing exciting frequency.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.55 no.4
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• pp.151-155
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• 2006

HVAC transmission lines must be designed to satisfy environmental regulations. Therefore it is necessary to pre-evaluate environmental problems for transmission line designer using prediction program. In this study, environmental design software, TLCALC(Transmission Line CALCulation) for transmission lines was developed as a comprehensive window program. It has 6 modules that are audible noise, radio noise, television noise, magnetic field, electric field and conductor surface gradient. TLCALC solved a few problems in use of the existing foreign tools and took several advantages as follows; (1) It is a common tool that solves calculating limitations of foreign formulas, (2) It has wide application ranges and enhances accuracy of prediction. (3) It can be applied to almost transmission line configurations in Korea. (4) Experienced designers can get the results of calculation within about 15 minutes. Because the use of TLCALC is easy and practical, this program will be usefully applied to the environmental friendly design and construction of transmission lines. In the future, it is expected that public complaints and social environmental cost will be reduced by the use of TLCALC.

• Progress in Superconductivity and Cryogenics
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• v.15 no.2
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• pp.20-23
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• 2013

For sensitive measurements of micro-Tesla nuclear magnetic resonance (${\mu}T$-NMR) signal, a low-noise superconducting quantum interference device (SQUID) system is needed. We have fabricated a liquid He dewar for an SQUID having a large diameter for the pickup coil. The initial test of the SQUID system showed much higher low-frequency magnetic noise caused by the thermal magnetic noise of the aluminum plates used for the vapor-cooled thermal shield material. The frequency dependence of the noise spectrum showed that the noise increases with the decrease of frequency. This behavior could be explained from a two-layer model; one generating the thermal noise and the other one shielding the thermal noise by eddy-current shielding. And the eddy-current shielding effect is strongly dependent on the frequency through the skin-depth. To minimize the loop size for the fluctuating thermal noise current, we changed the thermal shield material into insulated thin Cu mesh. The magnetic noise of the SQUID system became flat down to 0.1 Hz with a white noise of 0.3 $fT/{\surd}Hz$, including the other noise contributions such as SQUID electronics and magnetically shielded room, etc, which is acceptable for low-noise ${\mu}T$-NMR experiments.

• Investigative Magnetic Resonance Imaging
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• v.8 no.1
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• pp.42-49
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• 2004

Purpose : The quantization noise in magnetic resonance imaging (MRI) systems is analyzed. The signal-to-quantization noise ratio (SQNR) in the reconstructed image is derived from the level of quantization in the signal in spatial frequency domain. Based on the derived formula, the SQNRs in various main magnetic fields with different receiver systems are evaluated. From the evaluation, the quantization noise could be a major noise source determining overall system signal-to-noise ratio (SNR) in high field MRI system. A few methods to reduce the quantization noise are suggested. Materials and methods : In Fourier imaging methods, spin density distribution is encoded by phase and frequency encoding gradients in such a way that it becomes a distribution in the spatial frequency domain. Thus the quantization noise in the spatial frequency domain is expressed in terms of the SQNR in the reconstructed image. The validity of the derived formula is confirmed by experiments and computer simulation. Results : Using the derived formula, the SQNRs in various main magnetic fields with various receiver systems are evaluated. Since the quantization noise is proportional to the signal amplitude, yet it cannot be reduced by simple signal averaging, it could be a serious problem in high field imaging. In many receiver systems employing analog-to-digital converters (ADC) of 16 bits/sample, the quantization noise could be a major noise source limiting overall system SNR, especially in a high field imaging. Conclusion : The field strength of MRI system keeps going higher for functional imaging and spectroscopy. In high field MRI system, signal amplitude becomes larger with more susceptibility effect and wider spectral separation. Since the quantization noise is proportional to the signal amplitude, if the conversion bits of the ADCs in the receiver system are not large enough, the increase of signal amplitude may not be fully utilized for the SNR enhancement due to the increase of the quantization noise. Evaluation of the SQNR for various systems using the formula shows that the quantization noise could be a major noise source limiting overall system SNR, especially in three dimensional imaging in a high field imaging. Oversampling and off-center sampling would be an alternative solution to reduce the quantization noise without replacement of the receiver system.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2002.11b
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• pp.222-227
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• 2002

MR(Magneto-rheological) fluid is smart fluid that can change its characteristics then magnetic fields are applied. Recently, many researches have been performed on this MR fluid for the application in a vareity of areas including automobile shock absorbers. This paper describes the design procedure of a MR damper and the analysis results of its dynamic characteristics. MR fluid in the magnetic field shows initial yield shear stress and increasing resistive viscosity with final saturation thereafter. Herschel-Bulkley model is used to simulate the flow characteristics of MR fluid and magnetic analysis is used to identify the magnetic property of the MR fluid in the orifice of the damper. The dynamic characteristics of the damper was predicted and compared with the experimental results for typical sinusoidal excitations.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.21 no.8
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• pp.714-719
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• 2011

This paper presents vibration characteristics of magnetorheological(MR) elastomer, whose elastic modulus are controllable by applied magnetic field. By using this property, the material can be applied to vibration absorber, so that the stiffness of the absorber can be changed and actively controlled according to the magnetic flux density. However, the various performances of MR elastomer depends on different polarized direction of particles by applied magnetic field and dimension during the manufacturing process. In this paper, in order to obtain the optimal characteristics of MR elastomer, MR elastomers with different types and dimensions are prepared for a series tests. Using this test setup, extent of natural frequency shifted against magnetic field at various excitation frequencies can be measured. Specimens are prepared with 3 types, as cylinder samples exposed to magnetic field vertically, horizontally and unexposed during cure, respectively. Also, a set of design variables are considered to produce MR elastomers. Through the modal tests of mass structure with MR elastomer, the optimal design as well as the polarization direction of MR elastomer is obtained among the various dimensions and 3 directional types of MR elastomers.

• Journal of the Korean Magnetics Society
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• v.12 no.4
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• pp.149-153
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• 2002

Flux-gate magnetometer has been still used for low field magnetic field measurement with portability, low power consumption, and high reliability. In many applications, flux-gate magnetometer measures not absolute values but changes of the earth magnetic field. For the eia magnetic field change measurements, we have constructed a high sensitive 3-axis flux-gate magnetometer of which measuring ranges is ${\pm}$1000 nT and noise level is 5pT/√㎐ at 1 ㎐. Using this magnetometer, we can compensate the earth magnetic field of ${\pm}$50,000 nT with successive approximation methods using microcomputer. After earth magnetic field compensation, we could measure earth magnetic field changes with ${\pm}$100 nT measuring ranges.

• Journal of Radiation Industry
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• v.10 no.4
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• pp.193-198
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• 2016

Perfusion magnetic resonance image of biological mechanism are independent of magnetic field strength in hyper acute ischemic stroke. 3.0 T magnetic field, however, does affect the SNRs (signal to noise ratio) and artifacts of PMRI (perfusion magnetic resonance image), which basically will influence the quantitative of PMRI. In this study, the effects of field strength on PMRI are analyzed. The effects of the diseases also are discussed. PMRI in WM(white matter), GM (gray matter), hyper acute ischemic stroke were companied with 1.5 T and 3.0 T on SNR. PMRI also was compared to the SI difference after setting ROI(region of interest) in left and right side of the brain. In conclusion, the SNRs and SI of the 3.0 T PMRI showed higher than those at 1.5 T. In summary, PMRI studies at 3.0 T is provided significantly improved perfusion evaluation when comparing with 1.5 T.

• Journal of Magnetics
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• v.18 no.3
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• pp.339-341
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• 2013

The magnetic field standards, facilities and capabilities available at NPL for the calibration of magnetometers and gradiometers and the measurement of the magnetic properties of materials will be introduced. The details of the low magnetic field facility will be explained and the capabilities this facility enables for the characterisation and calibration of ultra-sensitive room temperature magnetic sensors will be presented. Building on core material capabilities that are compliant with the IEC 60404 series of written standards, the example of a standard permeameter that has been modified for the measurement of strips for real world conditions is discussed. This was incorporated into a stress machine to measure the DC properties of the soft magnetic materials used by the partners of a collaborative industry led R&D project at stress levels of up to 700 MPa. The results for three materials are presented and the changes in the properties with applied stress compared to establish which material exhibits favourable properties.