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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Jo Aug 4, 2024
The silicon steel plate is widely used as core materials of various devices such as a transformer and an electromotor. However, in order to solve the energy problem, it is very important to develop soft magnetic materials with less iron loss.
Fe-based amorphous and nano-crystalline alloys are widely used because of its low cost and iron loss about one fifth to one third of silicon steel plates. But, they are restricted to some degree in application because the value of maximum flux density of these materials is about 80% of silicon steel plates. Therefore, researches to develop soft magnetic materials with higher maximum flux density and lower core loss have been performed. As a result, new Fe-based nano-crystal soft magnetic materials have been developed. Among them, Fe-Cu-B system nano-crystalline soft magnetic materials are attracting a great deal of attention.
In most studies, the experimental results of the influence of copper, silicon and boron on the magnetic properties were analyzed, but the thermodynamic changing process was not described in detail.
Jo Chol Min, a researcher at the Faculty of Materials Science and Technology, has conducted a study to thermodynamically solve the influence of Cu addition on the crystallization of the Fe-based nano-crystalline soft magnetic alloy fabricated by the melt spinning method. In addition, he has analyzed the influence of silicon and boron on the magnetic properties of Fe-based nano-crystalline soft magnetic alloys to determine the reasonable composite.
He found that the crystallization activation energy of Fe-based alloys containing copper was lower than copper-free alloys, which indicates that copper is effective to accelerate the crystallization of alloys and increase the magnetic property. He also demonstrated that in crystallization heat treatment, the maximum flux density and coercive force of the Fe-based nano-crystalline soft magnetic alloy containing copper were the best with 2% of Si.
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Jo Aug 2, 2024
The altar remains of the feudal Joson age have been newly discovered at the vertex of a talus in Hyangdo Peak Basin by Lake Chon on the top of Mt. Paektu.
The base of the altar remains is almost square with each side of 36 meters while the surface is 15 meters long and 12 meters wide. The remains are approximately nine meters thick.
Two inscriptions were found there. One was on a flat oval-shaped natural rock of about 140 centimeters in length and 80 centimeters in width while the other was on a plain rock 26 centimeters long and 20 centimeters wide.
The former consists of 25 characters and the latter has 26 letters. The summary of these two inscriptions implies that various kinds of memorial services were held there during the feudal Joson dynasty.
The altar remains are of great significance in studying the history of Mt Paektu more deeply.
The remains, a material evidence that Korean ancestors set up an altar for memorial services on Mt. Paektu as early as in ancient times, prove that Korean nation has historically regarded Mt. Paektu as the ancestral mountain and the holy mountain of the nation.
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