Jo Dec 12, 2023
Rotating machinery is widely used in industries and, in practice, it faces inevitable faults because of the errors in manufacturing, assembling and operating conditions.
Mass unbalance is one of the most common faults encountered in rotating machinery.
Rotors are classified into two types: rigid and flexible rotors. Flexible rotors are served at speeds exceeding their critical speeds and rigid ones are not. In general, flexible rotors are balanced at service speed because their dynamical characteristics change with speeds. Balancing of flexible rotors is relatively complicated, expensive and dangerous because they have to run at high speeds around critical speeds.
Therefore, methods for low-speed balancing of high-speed rotors have been developed and presented.
Ri Yong Ho, a researcher at the Science Engineering Institute, has proposed a new low-speed balancing method for high-speed rotors by using Component Mode Synthesis (CMS).
The proposed method was tested numerically by simulating harmonic response of a rotor bearing system which is constructed of a steel shaft, three discs and two rolling elements bearings. The shaft was supported at two ends by bearings and worked at the maximum speed of 9 000r/min. The system was modeled by FEM. The shaft was divided into two dimensional finite beam elements with two translational and one rotational DOF at one node and the discs were modeled as concentrated masses and inertia moments at the corresponding nodes. And the support bearings were modeled as spring elements and proportional damping was supposed.
He confirmed by his method that a high speed flexible rotor can be exactly and effectively balanced without test runs at speeds higher than critical speed.
...
Jo Dec 8, 2023
Our country has a huge number of cultural relics and remains worth boasting to the world.
The 80 000 blocks of the Complete Collection of Buddhist Scriptures, which is on show in Pohyon Temple on Mt. Myohyang, is one of the precious national treasures demonstrating the advanced level of typography in our country.
The 80 000 blocks of the Complete Collection of Buddhist Scriptures are a systematic Buddhist library of Buddhist scriptures and the books concerned with Buddhism.
As it was compiled during the Koryo dynasty, it is also called the Koryo Tripitaka.
The very first complete collection of Buddhist scriptures had already been born in our country in the first half of the eleventh century but it was burnt away by foreign aggressors.
The present collection was completed in over 1 530 kinds and 6 793 volumes from 1236 to 1251.
It is permeated with the creative wisdom of people including lots of typographers. The excellent typography employed for it makes it one of the proud cultural legacies of our people and the world’s cultural assets.
In those days, the people and handicraftsmen managed to make more than 80 000 blocks of pakdal trees and birch trees.
Each block has 23 lines with 14 letters each. Wooden ties are attached at both ends of blocks to avoid twist and the surfaces are lacquered to keep worms away.
The wood blocks are elaborate enough to resemble a fine sculpture.
The collection, a precious cultural legacy of our nation, shows the level of development and promotion of woodblock printing. It serves as a material evidence demonstrating that firm technical foundations for invention and use of metal types for the first time in the world were laid in Koryo at that time.
The collection is now recognized as the most perfect standard collection of all the old collections of Buddhist scriptures existing in the world.
The complete collection is shining brilliantly as our priceless national treasure and as a world cultural legacy, proudly boasting of the wisdom and resourcefulness of our nation.
...
Jo Dec 7, 2023
Lumen degradation is a common failure mode in white LED light sources. Lumen maintenance life, defined as the time when the maintained percentages of the initial light output fall below a failure threshold, is a key characteristic for assessing the reliability of white LED light sources.
Most widely studied among various reliability tests for LEDs has been thermal degradation because heat is known to be one of the main factors affecting the LEDs performance and durability. However, the effect of humidity on LED lifetime is also important as many high-power LEDs including street lamps and floodlights are used in outdoor conditions with high humidity.
Ri Pyong Sol, a section head at the Faculty of Electronics, has conducted research into the reliability of high-power white LEDs. He aimed to predict the rated lifetime and to estimate the reliability of 50W white LEDs using their degradation data so that he could find an available method for testing the accelerated life of 50W high power white LEDs.
According to his research results, the relative error in the predicted value of lifetime by temperature accelerated test is 17.4%, which is 4.1% greater than that by temperature-humidity accelerated test.
By his method, the rated lifetime of LEDs can be predicted quite accurately within one day at the scene of high-power LED production.
...
Jo Dec 6, 2023
Janggu dance, one of the typical folk dances of our country, is rich in national colour. Dancers do janggu dance with a janggu, a national instrument, on their shoulders.
Originated from the appearance and wide playing of janggu in our country, janggu dance has a long history.
The murals in Jiban Five Tombs of Koguryo show some dancers dancing and playing the janggu.
It indicates that janggu was already used in dancing as a prop and janggu dance was widely spread in the Koguryo age.
Later, janggu dance with such a long history became more fashionable with development of a peasant dance in the latter half of the feudal Joson dynasty, when playing skills and dancing skills were combined.
Janggu dance was found in several folk games and dances as well as in a peasant dance. In modern ages, it was made artistic by professional dancers and put on the stage, too.
Janggu dance that was loved by our people for a long period of time was reproduced in the form of a solo dance to be congenial to the aesthetic tastes of the era under the wise guidance of our Party.
As janggu serves as a main prop of janggu dance, skills in playing the janggu are given great importance in the dance composition.
With a variety of beating techniques and rich and sophisticated tones, janggu has a high possibility of expression. Therefore, janggu dance produces unique descriptive charm with the beating techniques of janggu and artistic rhythms of a dance combined harmoniously with each other.
Under the scrupulous guidance of our Party, janggu dance has been improved into a great diversity from a solo to an ensemble, a group dance, a large group dance, etc. and it has now become a dance of strong national coloring that preserves the flavor of folk dances.
...
Jo Dec 4, 2023
With downscaling of MOSFET to nanometers, gate induced drain leakage (GIDL), short channel effect (SCE), etc. greatly affect its characteristics.
Adaptation of a double-gate (DG) in MOSFETs can improve the SCE and GIDL can be alleviated by a structure of dual materials. DMDG-MOSFETs have been proposed and studied in some aspects.
Ri Hui Chol, a section head at the Semiconductor Institute, has proposed a model for potential distribution and subthreshold swing through the entire channel. In the DMDG-MOSFET structure, he adopted the structure of two gate materials that have different work functions. This structure enables potential to increase through two steps so that the potential change rate may be decreased. Thus, the intensity of the electric field on the position close to the drain can get lower. This feature is useful for improving electron mobility and for hot electron generation.
When VGS was relatively small, the simulation on the model and the previous results agreed well. But as the applied bias increased, so did the deviation.
It is because he employed depletion approximation that only accounts for ion concentration. When VGS is relatively small, carrier concentration is low, so the influence of neglecting this term is not so significant. But VGS reaches a certain extent, the influence of the gate voltage on channel potential is eliminated by inversed charge so that it may result in deviation.
Especially, under the double-material gate configuration the band voltage of part M2 is relatively small and the effective gate voltage is big, so the inversed charge can be generated more easily. Thus, though both models (double-material gate model and single-material gate model) are obtained in the same method, the accuracy is different, that is, the accuracy of the DMDG-MOSFET model may be lower than SMDG-MOSFET’s.
The simulation results indicate that the subthreshold swing increases with the decrease in the channel length like general DG MOSFETs, and that the subthreshold feature is deteriorated with the increase in the thickness of the gate oxide and the channel.
If further information is needed, please refer to his paper “Simulation for characteristics of the Dual-Material Double-Gate MOSFET” in “Journal of Analog and Digital Devices” (EI).
...
Jo Dec 1, 2023
The winter solstice is the time of the year with the longest night and the shortest day.
It usually falls on the 21st or 22nd of December every year.
On the winter solstice the sun rises the latest and sets the earliest.
The true noon in Pyongyang is 12:35 and the sun’s altitude is 27.6°.
After the winter solstice, the night gets shorter gradually in all regions (the northern hemisphere) while the day becomes longer until the summer solstice.
From ancient times, our ancestors celebrated the winter solstice, regarding it as the small New Year’s Day (called Ase).
On the day people used to cook tongjijuk (special adzuki-bean gruel taken on the winter solstice) over which they looked back upon the year thinking of themselves growing a year older. They usually shared it with neighbours to promote harmonious relations.
Specially prepared for the gruel were glutinous rice balls which mean good luck in the coming year.
This reflects both the love of mothers who want to please their children and the sincerity of housewives wishing every family member good luck.
People have called it Aedongji if it falls in early November by the lunar calendar and Rodongji if it comes later.
It has been said that Aedongji brings a hard winter while Rodongji makes a mild one.
...