Jo Aug 16, 2024
In recent years, a number of modal identification methods based on continuous wavelet transform (CWT) have been developed. The time variation of the instantaneous amplitude and phase of each mode component within measured signals can be considered by time-frequency representation. In the time-frequency plane, wavelet analysis decomposes the measured signals into a series of curves called ridges which directly express the amplitude and phase of each mode component within the measured signals. Thus, by the extraction of ridges and the value of CWT along the ridges, modal parameters are identified from the measured signals. The measured signals are free-decay responses of mechanical structures which are processed by time-frequency transform. In this process, the challenge is to determine ridges exactly. When the frequency window of CWT includes closely spaced natural frequencies, the ridge is distorted and a pseudo mode is generated.
Existing methods for modal identification by CWT give correct results when the frequency window includes one natural frequency. Thus, it is necessary to build a theoretical basis for correctly determining closely spaced natural frequencies by considering the distortion of ridges and determining the reasonable bandwidth when the frequency window includes two closely spaced natural frequencies.
Ri Yong Ho, a researcher at the Science Engineering Institute, has proposed a formula for mathematically considering the distortion of characteristics of a ridge when the frequency window includes two close mode components, and conducted a numerical simulation on MATLAB to verify it.
The simulation verified the accuracy of the proposed formula.
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Jo Aug 15, 2024
Most rare earth minerals contain uranium, thorium and their decay products, so the technology for removing radioactivity plays a key role in the production of rare earths, especially in reduction of production cost, environmental protection and application of rare earth products. Furthermore, complete removal of trace radioactive substances existing in rare earth products is highly significant for wide application of rare earths and in medicine for health protection and cure.
Up to now, many methods for separating and removing radioactive substances have been reported: ion exchange method, solvent extraction method, co-precipitation method, etc. among which co-precipitation method is very effective for infinitesimal amount of radioactive materials such as radium and actinium. In an extremely dilute system, trace radioactive substances are usually separated by co-precipitation with carriers. Porous materials such as diatomaceous earth, activated carbon, fire brick, silica gel and alumina, barium sulfate, strontium sulfate , iron hydroxide(Ⅲ) and aluminum hydroxide are used as carriers.
In an attempt to remove trace radioactive substances existing in industrially produced rare earth carbonate, Kim Chol Ju, a researcher at the Faculty of Chemical Engineering, has conducted an experimental study of the radioactivity removal characteristics of [Ba-Fe] co-precipitation method, the combination of BaSO4 and Fe(OH)3 co-precipitation methods.
In the [Ba-Fe] co-precipitation process, the total removing rate is over 92% and loss rate of rare earths is less than 0.1% in the case of industrially produced rare earth carbonate.
As this method enables prompt and effective removal of radioactive elements such as uranium, thorium, radium, radiolead, etc., remaining in extremely small quantities in rare earth compounds, it could be applied in the field of hydrometallurgy in which rare earth compounds are produced from several kinds of rare earth minerals and in other fields of production and application of rare earth materials from rare earth compounds.
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Jo Aug 12, 2024
It is very important to correctly model and identify damping in all vibration systems of physical devices that involve damping.
A variety of methods and techniques of damping identification have been developed, most of which can be classified into frequency domain (circle-fitting method, half-power bandwidth method, wavelet transform method, etc.) and time domain (logarithmic decrement method, least squares complex exponential (LSCE) method, Smith least squares method, Hilbert transform method, etc.). Some of the damping identification methods in frequency domain have high accuracy, but the algorithm is very complicated, which restricts its wide application in practice.
Matrix method and modal method are also used for damping identification. In the modal method, modal identification is a must as the damping characteristics are decided by means of modal damping ratio. Modal identification is complicated and it is difficult to ensure the correctness of modal parameters. In the matrix method, which directly identifies the damping matrix by experimentally measured FRF, the imaginary part of experimentally measured FRF is decided for damping identification. This method is highly accurate and it is possible to decide the damping ratio of mechanical structures of lower DOF. However, it is very difficult for all numbers of DOF as mechanical structures are of high DOF in practice.
Kim Won Ju, a researcher at the Faculty of Mechanical Science and Technology, has proposed a method for correctly and simply deciding the damping characteristics of the whole system using the experimentally measured FRF and the FE model. Using the experimentally measured frequency response function matrix, he built an experimental model and combined it with the FE model to enlarge the experimental identity data to the whole system.
Comparing his method with preceding ones through simulations and error analysis, he verified its precision and effectiveness.
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Jo Aug 10, 2024
Kirigocha is a semi-automatic machine for distance measurement.
According to “Sejongsillok” (Chronicles of King Sejong), kirigocha was first used in March, 1441, which means that it was used for measuring distances in Korea in the mid-fifteenth century.
No records at the time of building it have been found. Instead, its structure is described in the book “Juhaesuyong” written by Hong Tae Yong (1731~1783), a realist and materialist of the eighteenth century.
According to it, two cogwheels that are placed one upon another in the box on a cart engage tightly with the vertical cogwheel attached to the wheels. The rotations of the wheels make the sounds of a bell and a drum, which indicate distances.
In other words, the rolling of the wheels over a half ri (1ri=392.73m) and one ri rings a bell once and several times, respectively, while the drumbeat is heard once after 5 ri and several times after ten ri. The distance is measured in ri according to the sounds of the bell and the drum.
Distance measurement by kirigocha was more accurate than that by a tape measure.
The distance of longitude 1° measured 108 kilometers by kirigocha, which means the measurement error limit is below 3%, compared with the current value 110.95 kilometers. This proves that kirigocha was perfect for measuring distances.
To sum up, kirigocha demonstrates Korean ancestors’ high mathematical talent and creative wisdom.
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Jo Aug 8, 2024
Unijunction transistor (UJT) is a semiconductor with two ohmic base contacts and an emitter p-n junction, so it is called double-base diode. Its static(S-shaped) I-V characteristic has a region with negative differential resistances. Therefore, UJTs are used in pulse generators, sawtooth generators and converters. UJT-based circuits are well known to be simpler and more reliable than those based on diodes and bipolar transistors.
Recently, transducers where the output signal frequency depends on physical quantities such as temperature, light, magnetic field, stress, concentration of gas and radiation level, have been designed with UJTs. Thus, improvement of the switching characteristic of UJT leads to that of the characteristics of these transducers.
To improve the switching characteristic of UJT, a number of researchers have investigated the radiation effect on it. To the best of our knowledge, there are no reports in the literature regarding how the neutron irradiation of UJT affects its peak point voltage.
Pae Kyong Il, a researcher at the Semiconductor Institute, has proposed a new analytical expression for the peak point voltage of UJT considering the effect of neutron irradiation, and proved its validity by comparing the calculated values and those measured from the neutron irradiation on an Si planar UJT.
His study has shown that the calculated values of the peak point voltage of UJT are in good agreement with the measured ones, that the peak point voltage of UJT decreases after neutron irradiation, and that the peak point voltage of UJT decreases gradually with the growing intensity of neutron fluence.
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Jo Aug 6, 2024
A historical site Puyong Pavilion in Haeju City was built at the beginning of the sixteenth century during the feudal Joson dynasty.
It is a unique architecture built on the stone columns in a lotus pond.
The Puyong Pavilion forms an L-shape with a building called “Unghyang Pavilion”.
The pavilion is supported by tens of columns erected in the middle of a pond which is as large as one thousand several hundred square meters. The columns are standing on the border but no columns in the center, so the whole inside is uninterrupted for parties or something.
The Puyong Pavilion preserves the structural features of a typical pavilion and blends the surrounding environment and architecture, thus demonstrating the excellent building techniques of the Korean ancestors.
The charming scenery of the pavilion which felt like floating gently over the lotus pond gradually became widely known, which developed a saying “Missing the Puyong Pavilion denies that you have been to Haeju”.
The sound of raindrops falling onto the lotus flowers in full bloom in the pond at night was so pleasant to the ear that huge numbers of people gathered there when it was raining. It soon started to be called one of the Eight Scenes of Haeso (Haeso means ‘province with the sea in the west’).
As the scene of the pavilion became the pride of Haeju and one of the Eight Scenes of Haeso, many poems, songs and legends came into being, which are still being passed down.
Though it was severely destroyed during the past Fatherland Liberation War, the Puyong Pavilion was restored to its original state by the earnest instructions of President
The pavilion, which was used as a pleasure resort for feudal rulers in the past, turned into an excellent cultural recreation center for working people under the care of the Workers’ Party of Korea. It now serves as a place for education in patriotism that instills the national pride and self-confidence into the people.
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