Jo Jun 26, 2022
Ri Yong Ho, a section head at the Faculty of Heat Engineering, has developed an instrument that can measure and analyze vibration of rotating machinery such as steam turbine generators real-time.
As a steam turbine generator is composed of rotors that are supported by their pedestals, vibration state of all the pedestals should be measured and analyzed at the same time by a multi-channel instrument.
The multi-channel vibration instrument acquires vibration signals of up to 16 channels at high speed at the same time and transmits them to the computer so that one can measure and analyze vibration state of all the pedestals of turbine generators, diagnose vibration fault and decrease vibration level by taking necessary measures like balancing.
The multi-channel vibration instrument is composed of analog signal processing circuit, key-phase pulse input circuit, digital signal processing circuit, USB communication circuit and power supply circuit.
The analog signal processing circuit does overvoltage protection, filtering and amplification of vibration signals, and then sends them to the digital signal processing circuit.
Digital signal processing is performed by a microchip, in which AD conversion of 16-channel analog signals, measurement of the rotating speed and vibration phase by key-phase pulse, saving of vibration waveforms and communication control take place.
The acquired vibration waveforms of 16 channels are transmitted immediately to the microchip which has USB communication function by means of parallel communication mode and then they are real-time transmitted to computers through USB ports.
In the power supply circuit, AC 220V power is converted to stable direct current voltages 5V and ±12V and then supplied to every signal processing circuit.
The computer performs digital filtering and integral of multi-channel vibration waveforms received from the vibration instrument, transforms vibration acceleration signals into vibration velocity signals and vibration displacement signals, calculates vibration amplitude to measure vibration level, and then makes vibration analyses such as spectrum analysis by fast Fourier transformation, phase analysis and trend analysis.
Besides, it can perform one-plane balancing and two-plane balancing by method of influence coefficient, so field balancing of steam turbine generators can be realized.
We can measure pedestal vibration by linking this instrument with piezoelectric acceleration sensors and electromagnetic speed sensors, and shaft vibration is also measurable when it is connected with eddy displacement sensors.
The measurement range is 0~200㎛ in the case of pedestal vibration and 0~500㎛ in the case of shaft vibration, and the measurement accuracy is 2%.
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Jo Jun 25, 2022
The research group led by Pae Ho Rim, a section head at the Faculty of Shipbuilding and Ocean Engineering, has developed an experiment device for measuring natural frequency of a ship hull.
The device measures the natural frequency of a ship hull model by the principle that resonance occurs when natural frequency is coincident with the frequency of forced vibration caused by decentration mass.
The device consists of ship hull model, vibration generator, unit for motor velocity control, frequency sensor, acceleration sensor, amplification and integration circuit, communication interface, user interface, etc.
The model is 1.2 metres long, 0.1 metres wide and 0.1 metres high. The cross section is rectangular and the bow and stern assume a pyramidal form.
The user interface shows the graphs and values of real-time signals of revolution, displacement, acceleration and velocity.
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Jo Jun 22, 2022
Cast aluminum alloys are quite widespread and they find more and more applications in modern industry such as aerospace, navigation and railway transport thanks to their good mechanical property, castability, corrosion resistance, etc.
Quality indices are a measure to quantify quality of the alloy on the basis of its mechanical properties.
The indices are Q, QR, QC, QD(Q0), QE, etc. Each one has its own unique features and individual one cannot display the overall performance of cast aluminium alloys.
The most representative indices to evaluate their overall performance can be chosen by two methods ― method of correlation analysis between quality indexes and mechanical properties and method of correlation analysis and rank deviation analysis between overall performance scores obtained from quality indices and MCDM method.
The analysis result showed that Q0 proved to be the most representative one of all the indices Q, QR, QC and Q0, and that Q0 can be used instead of MCDMs for evaluation of overall performance of cast aluminum alloys and selection of optimal cast aluminum alloys.
Detailed information about this can be found in the paper “ Method to evaluate overall performance of cast aluminum alloys using most representative quality index instead of MCDM method” presented by Yang Won Chol, a researcher at the Faculty of Materials Science and Technology, to the SCI Jounal “International Journal of Metalcasting”.
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Jo Jun 14, 2022
Kim Ryul, a researcher at the Faculty of Electrical Engineering, has realized selection of crack insulators from a pile of 10kV pin insulators by data processing of thermal shock sample breaking test and a corona short-circuit nondestructive test device.
Compared with previous selection methods by electric penetration breakdown test, this method provides more safety and lower production cost. In addition, as it uses nondestructive electric tests to sort out crack insulators, the reliability of production pile of insulators can be further improved.
This method is constructed with several stages: determination of size of 10kV pin insulator piles and samples, thermal shock sample destructive test, state decision of pass or pass failure of production piles by processing test data, determination of acceptance probability by quality control software and of the number of faulty insulators, and selection of crack pin insulators by the corona short-circuit nondestructive selection device.
This thermal shock test costs less and provides 1.4 times higher accuracy than the previous electric penetration breakdown test. To estimate cracks in 10kV pin insulator piles, previous binomial distribution was replaced with noncentral t-distribution, which reduced the number of samples to half and improved the accuracy of acceptance probability of piles and the calculation of the number of crack insulators by 1.2 times. Instead of electric penetration breakdown test devices, corona nondestructive test devices were used, which made it possible to select 100% of crack insulators from passed piles.
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Jo Jun 13, 2022
Mausoleum of King Kogukwon is a tomb with mural paintings for King Kogukwon, 21st king of Koguryo, situated in Oguk-ri, Anak County, South Hwanghae Province. It dates back to mid-4th century and it is also called Anak Tomb No. 3.
The mausoleum, precious heritage of our national culture, is by far the number one of all Koguryo tombs ever known in terms of scale and richness of content of mural paintings. As a world-famous relic representative of the eastern culture in 4th century that gives an impression of an underground palace, it fully demonstrates the high level of might and culture of Koguryo.
It has a side chamber with figure and genre paintings. It is colossal in size ― about 33 metres from north to south, about 30 metres from east to west and 6 metres high. Located on the mountain and merged with it into one, it looks really magnificent like a huge mountain. It consists of a doorway, a front chamber, two side chambers in the east and west of the front chamber, inside chamber, a corridor, etc. that are built of stone semi-underground on the mountaintop cut downward flat.
The entrance facing the south of the doorway is blocked by a standing plain rock. The doorway is 2.17 metres long from east to west, 2.12 metres wide from north to south and about 3.48 metres high. Two stone doors are attached between the doorway and the front chamber. The front chamber is 4.88 metres long from east to west, 2.73 metres wide from north to south and 3.47 metres high. Between the front chamber and the inside chamber stand 3 octagonal pillars and between the two side chambers and the entrance to the corridor stand 2 square pillars, on the tops of which crenelles (layers of "cow's-tongue" ornamentation) or caps are placed.
The inside chamber is 3.8 metres long from east to west, 3.32 metres wide from north to south and 2.8 metres high. The eastern and the northern sides of the inner chamber are connected by an L-shaped corridor. The corridor is 10.13 metres in length, 0.69~0.87 metres in width and 2.5 metres in height. The ceiling of the corridor is supported by double parallel stays.
The murals are painted on bare stone walls. On the walls of the doorway are painted warriors guarding the king buried in this tomb and in the front chamber are found paintings of guards of honour, subakhui (Korean style of martial art using only the hands), singing and dancing. In the west side chamber are seen the scenes where the king, the main character, wearing “Paekragwan” (crown worn by Koguryo kings) and colourful silk robes is conducting state affairs together with civil and military officers, and the queen is being waited by the court ladies. In the east side chamber there are murals of kitchen, meat storage, well, mill, stable, cowshed, coach house, etc. The corridor is decorated with the mural of the main character wearing “Paekragwan” on an oxcart followed by the grand procession advancing majestically under the guard of warriors, civil and military officers, instrumental march music band, ceremonial flags such as “Songsangbon”, “Ttukki”, “Jongjol”, etc. owned only by the Koguryo kings. Each mural is different in painting technique.
The best-painted ones of all the murals are those with main characters in them. Although they have the limitation of the period when the portraits of the main characters were painted differently in size according to their social standings, they are invaluable ones that ascertain the very beginning of portrait painting in our country. The procession of several columns overlapping each other is portrayed skillfully by the technique of a bird’s-eye view. Demonstrating that its technique of picture painting was contemporarily of the highest level, this mural, in terms of the number of characters of over 250, the size and the complicated and colourful scenes, ranks first of all the murals in our country and it is a leading masterpiece that holds an important position in the history of the world fine arts.
At first there was a huge amount of valuable objects made of gold, silver, copper, iron, jade, etc. in the mausoleum, but at the time when it was discovered, only earthenware like plates, small bowls, pots, etc. was found due to grave robbery a long time ago.
The mausoleum is a precious cultural heritage that vividly shows the history, custom, architecture, art, music, dance, weapons, armament, etc. through the structure of chambers and the murals in them.
The architecture of bolsters and the murals in the mausoleum are the precious national cultural heritage that clearly shows the wisdom and resourcefulness of the Korean nation as well as the high level of its cultural development and world-famous relics representative of the contemporary eastern culture.
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