• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.37 no.4
• /
• pp.511-519
• /
• 2013

Switching controllers for active magnetic bearings are claimed to minimize the copper losses because they do not use bias currents. In this study, we compare the performances of the switching controller with those of the widely used proportional-derivative (PD) controller. The PD controller is combined with a synchronous notch filter to reduce the effect of the unbalance disturbance. For a fair and objective comparison, the PD controller is designed systematically. The switching controller is designed so that the dynamics of the two controllers are almost identical. A system model is developed. This model includes the flexible modes of the rotor and the dynamics of the sensors and amplifiers. The simulation results show that the switching controller indeed reduces the copper loss at lower speeds. However, it fails to operate around the speed close to the bending mode of the rotor.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.62 no.9
• /
• pp.1324-1328
• /
• 2013

The reason of train location detection is generally the train interval control between the railway stations and the train path control in the railway station yard in order to avoid train collision. It is very important to know the train location for shortening the train headway, and improving the efficiency of railway maintenance as well as the safe operation of trains. Therefore, the accurate detection of train location is the prerequisite technology in railway signalling system. The track circuit and the wheel sensor of rolling stock have been used to detect the train location widely in urban railway as well as high speed train. The track circuit is continuously monitored by electrical equipment to detect the absence of the train and the tachometer and encoder is used for the wheel sensor to measure the train speed. But speed sensor failures are frequent due to the extremely harsh operating conditions encountered in rail vehicle. The CBTC(Communication based Train Control) system has been used in urban railway system recently. But the installation of CBTC system is very high and the modification of design is difficult. This paper deals with the feasibility of new train location detection method using magnetic sensors.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2014.02a
• /
• pp.370.1-370.1
• /
• 2014

Giant magnetoresistance (GMR), tunneling magnetoresistance (TMR), and magnetic random-access memory (MRAM) are currently active areas of research. Magnetite, Fe3O4, is predicted to possess as half-metallic nature, ~100% spin polarization (P), and has a high Curie temperature (TC~850 K). On the other hand, Spinel ferrite CoFe2O4 has been widely studies for various applications such as magnetorestrictive sensors, microwave devices, biomolecular drug delivery, and electronic devices, due to its large magnetocrystalline anisotropy, chemical stability, and unique nonlinear spin-wave properties. Here we have investigated the magneto-transport properties of epitaxial CoxFe3-xO4 thin films. The epitaxial CoxFe3-xO4 (x=0; 0.4; 0.6; 1) thin films were successfully grown on MgO (100) substrate by molecular beam epitaxy (MBE). The quality of the films during growth was monitored by reflection high electron energy diffraction (RHEED). From temperature dependent resistivity measurement, we observed that the Werwey transition (1st order metal-insulator transition) temperature increased with increasing x and the resistivity of film also increased with the increasing x up to $1.6{\Omega}-cm$ for x=1. The magnetoresistance (MR) was measured with magnetic field applied perpendicular to film. A negative transverse MR was disappeared with x=0.6 and 1. Anomalous Hall data will be discussed.

• Journal of Sensor Science and Technology
• /
• v.8 no.6
• /
• pp.427-433
• /
• 1999

In this paper, we have presented an one-axis fluxgate magnetic sensor with ferrite core, excitation, and pick-up coil. This magnetometer is consist of a sensing element, driving circuits for excitation coil and signal processing for detecting second harmonic frequency component which is proportional to the DC magnetic to be measured. The sensor core is excited by a square waveform of voltage through 82 turns of the excitation coil. The second harmonic output of pick-up coil(150 turns) is measured by a FFT spectrum analyzer. This result is compared to output of PSD(phase sensitive detector) unit for detecting a second harmonic component. The measured sensitivity is about 50 V/T at driving frequency of 2 kHz. The nonlinearity of fluxgate magnetic sensor is calculated about 2.0%.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2008.06a
• /
• pp.331-332
• /
• 2008

Couplings between electric, magnetic, and structural order parameters result in the so-called multiferroic phenomena with two or more ferroic phenomena such as ferroelectricity, ferromagnetism, or ferroelasticity. The simultaneous ferroelectricity and ferromagnetism (magnetoelectricity) permits potential applications in information storage, spintronics, and magnetic or electric field sensors. The perovskite BiFeO3(BFO) is known to be antiferromagnetic below the Neel temperature of 647K and ferroelectric with a high Curie temperature of 1043K. It exhibits weak magnetism at room temperature due to the residual moment from a canted spin structure. It is likely that non-stoichiometry and second-phase formation are the factors responsible for leakage current in BFO. It has been suggested that oxygen non-stoichiometry leads to valence fluctuations of Fe ions in BFO, resulting in high conductivity. To reduce the large leakage current of BFO, one attempt is to make donor-doped BFO compounds and thin films. In this study, (Bi1-x,Ndx)(Fe1-y,Tiy)O3 thin films have been deposited on Pt(111)/TiO2/SiO2/Si substrates by pulsed laser deposition. The effect of dopants on the phase evolution and surface morphology are analyzed. Furthermore, electrical and magnetic properties are measured and their coupling characteristics are discussed.

• Journal of the Korean Magnetics Society
• /
• v.8 no.4
• /
• pp.231-240
• /
• 1998

We investigated that the fabrication and reproducing signal characteristics of tri-layered magnetoresistance (MR) element for the high density magnetic thin film heads and sensors. Magnetoresistance curve of tri-layered MR element predicted by computer modeling was saturated above external field of -15 Oe~+22 Oe, and it was shifted to linearized region as large as 4 Oe. In the case of fabricated real device, magnetoresistance curve was saturated above external field of $\pm$15 Oe, and it was shifted to linearized region as large as 4 Oe. As shown in real device, MR response curve was in good agreement with the simulation results. As a result of experimental data of reproducing output signal in real device, it retained normal sinusoidal waveforms in 1~4 Oe external magnetic field. In this magnetic field region, the fabricated heads with tri-layered MR element can be operated with good reproduced characteristics. This will be beneficial to the use of efficient processes of manufacturing elements and the vacuum deposition techniques which control thin film properties.

• Journal of Biomedical Engineering Research
• /
• v.27 no.4
• /
• pp.154-163
• /
• 2006

A 64-channel magnetocardiogram (MCG) system using low-noise superconducting quantum interference device (SQUID) planar gradiometers was developed for the measurements of cardiac magnetic fields generated by the heart electric activity. Owing to high flux-to-voltage transfers of double relaxation oscillation SQUID (DROS) sensors, the flux-locked loop electronics for SQUID operation could be made simpler than that of conventional DC SQUIDs, and the SQUID control was done automatically through a fiber-optic cable. The pickup coils are first-order planar gradiometers with a baseline of 4 em. The insert has 64 planar gradiometers as the sensing channels and were arranged to measure MCG field components tangential to the chest surface. When the 64-channel insert was in operation everyday, the average boil-off rate of the dewar was 3.6 Lid. The noise spectrum of the SQUID planar gradiometer system was about 5 fT$_{rms}$/$\checkmark$Hz at 100 Hz, operated inside a moderately shielded room. The MCG measurements were done at a sampling rate of 500 Hz or 1 kHz, and realtime display of MCG traces and heart rate were displayed. After the acquisition, magnetic field mapping and current mapping could be done. From the magnetic and current information, parameters for the diagnosis of myocardial ischemia were evaluated to be compared with other diagnostic methods.

• Journal of Radiation Industry
• /
• v.9 no.3
• /
• pp.143-151
• /
• 2015

A PET-MR system is particularly useful in diagnosing brain diseases. We have developed a prototype positron emission tomography (PET) system which can be inserted into the bore of a whole-body magnetic resonance imaging (MRI) system that enables us to obtain PET and MRI images simultaneously with a reduced cost. Silicon photomultipliers (SiPM) are appropriated as a PET detector at PET/MR system because detectors have a high gain and are insensitive to magnetic fields. Despite of its improved performance compared to that of PMT-based detectors, there is a problem of the photo-peak channel shift which is due to the increase of the temperature inside the ring detector. This problem will occur decreasing sensitivity of the PET and image distortion. In this paper, I quantitative analyze parameters of the KAIST SiPM depending on temperature by experiments. And I designed cooling methods in consideration of the degradation of sensors for correction of the temperature in the PET gantry. According to this research, we expect that distortive images and degradation of the sensitivity will not be occurred with using the above idea to reduce heat even if the PET system operates for a long time.

• Journal of Magnetics
• /
• v.21 no.4
• /
• pp.509-515
• /
• 2016

We report on development of a ground-based bi-axial Search-Coil Magnetometer (SCM) designed to measure time-varying magnetic fields associated with magnetosphere-ionosphere coupling processes. The instrument provides two-axis magnetic field wave vector data in the Ultra Low Frequency or ULF (1 mHz to 5 Hz) range. ULF waves are well known to play an important role in energy transport and loss in geospace. The SCM will primarily be used to observe generation and propagation of the subclass of ULF waves. The analog signals produced by the search-coil magnetic sensors are amplified and filtered over a specified frequency range via electronics. Data acquisition system digitizes data at 10 samples/s rate with 16-bit resolution. Test results show that the resolution of the magnetometer reaches $0.1pT/{\sqrt{Hz}}$ at 1 Hz, and demonstrate its satisfactory performance, detecting geomagnetic pulsations. This instrument is scheduled to be installed at the Korean Antarctic station, Jang Bogo, in the austral summer 2016-2017.

• Journal of Sensor Science and Technology
• /
• v.1 no.1
• /
• pp.23-34
• /
• 1992

In order to fabricate magnetoresistive sensor, Fe-Ni and Co-Ni alleys were evaporated on the slide glass and the silicon wafers. Saturation magnetic induction($B_{s}$), coercive field strength($H_{c}$) and magnetoresistance were measured for fabricated samples. The evaporated Fe-Ni thin films show that the saturation magnetic induction was 0.65 T, and coercive field strength was 0.379 A/cm, and this value was changed to 0.370 A/cm(//), 0.390 A/cm(${\bot}$), respectively after magnetic annealing. For the measurement of coercive field strength, magnetizing frequency of 1 kHz was used. For the fabricated sensor element, the change of magnetoresistance (${\Delta}R/R$) was excessively unstable due to oxidation in the process of fabrication. The evaporated Co-Ni alloy thin films show that saturation magnetic induction was 0.66 T, and coercive field strengthes were 5.895 A/cm(//), 5.898 A/cm(${\bot}$), respectively, after magnetic annelaing. The change of magnetoresistance(${\Delta}R/R$) was $3.6{\sim}3.7%$ of which value was excessively stable to room temperature. Fe-Ni thin film could have many problems due to large affinity in the process of fabrication of magnetoresistance sensor, but Co-Ni thin film could be a suitable material for fabrication of magnetoresistance sensor, because of its small affinity and definite magnetoresistance effects.