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.
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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.
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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).
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Jo Nov 30, 2023
Rotating machinery, which plays an important role in modern industries, is in extensive range of mechanical applications such as steam and gas turbines, air craft and automobile engines, pumps and even domestic applications. In the rotating machinery shaft one of the most serious damages is crack, and catastrophic failures and dangerous accidents may occur from cracks in aircraft engines and other rotating machinery. A very rich amount of literature about cracked rotors has appeared in the last four decades.
A lot of researches were conducted on cracked general shafts, but few studies of the modeling of hollow shafts with cracks were found. Most studies used classical models in modeling the crack breathing in hollow shafts.
Yang Hyong Ju, a researcher at the Faculty of Mechanical Science and Technology, has studied the actual breathing mechanism of a hollow shaft and presented generalized breathing functions of cracks. On the basis of it, he formulated an exact time-varying stiffness matrix of cracked elements and derived a finite element model of a rotor system. Then, he employed HBM to find the response, orbits and critical and subcritical speeds of a cracked rotor system. Finally, he analyzed the nonlinear vibration of a rotor system and compared the results with some published results, considering the eccentricity of disk, dimensionless crack depth, ratio of the inner radius to the outer radius of the rotor and angle between the crack and imbalance directions.
The results showed that the proposed generalized functions are more accurate than classical models, and they are superior to the functions introduced in previous studies in terms of generality.
If further information is needed, you can refer to his paper “Generalized breathing functions for stiffness model of transversely cracked hollow shaft” in “International Journal of Structural Stability and Dynamics” (SCI).
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Jo Nov 29, 2023
The most important open problem of the studies of quantum mechanics is to elucidate why probability density is the modulus square of a wave function.
This problem has been a fundamental subject of a long-standing debate that began soon after quantum theory was basically formulated in 1920s.
Within the framework of the standard theory of quantum mechanics, the relation between wave function and probability density is assumed rather than derived.
Successful works which aim to elucidate this assumption theoretically are not yet known.
This problem is related to in what space quantum mechanics should be constructed.
For the purpose of solving this problem, Jong Chol, a researcher at the Faculty of Materials Science and Technology, has deduced a fundamental equation of quantum mechanics by starting with probability density.
To do so, it was necessary for him to formulate a new theory of quantum mechanics distinguished from the previous ones. His investigation shows that it is possible to construct quantum mechanics in phase space as an alternative autonomous formulation and such possibility enables us to study quantum mechanics by starting with probability density rather than wave function.
This direction of research is contrary to configuration-space formulation of quantum mechanics starting with wave function.
The work leads to a full understanding of the wave function as mathematically and physically sufficient representation of quantum-mechanical state which supplements information on quantum state given solely by probability density with phase information on quantum state.
The final result of the work is that quantum mechanics in phase space satisfactorily elucidates the relation between wave function and probability density by using the consistent procedure starting with probability density, thus withdrawing a main assumption of quantum mechanics.
You can find more information about this in his paper “Explanation of Relation between Wave Function and Probability Density Based on Quantum Mechanics in Phase Space” in “World Journal of Mechanics”.
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Jo Nov 28, 2023
Mining Method Selection (MMS) is the first and the most critical problem in mine design, and it depends on some parameters such as geometric and geological features and economic factors.
The ultimate goals of mining method selection are maximizing profit, enhancing mining recovery rate and providing a safe mining environment.
Selection of an appropriate mining method is a complex task that requires consideration of many technical, economic, social, and historical factors.
Pak Myong Chun, a section head at the Faculty of Mining Engineering, determined the factors affecting MMS with the help of some mining experts, and selected the most suitable mining method using the hesitant fuzzy group decision-making (HFGDM) and technique for order performance by similarity to the ideal solution (TOPSIS). These factors included type of deposit, slope of deposit, thickness of orebody, depth below the surface, grade distribution, hanging wall Rock Mass Rating (RMR), footwall RMR, ore body RMR, recovery, capital cost, mining cost, annual productivity, and environmental impact.
Firstly, he proposed a group decision-making (GDM) method to determine the weights of several attributes based on the score function with decision-makers’ weights, in which the n-dimensional hesitant fuzzy environment takes the form of hesitant fuzzy sets (HFS). Then, he calculated the weights of these factors using the HFGDM method. He compared seven mining methods for an apatite mine to select the optimal mining method using the TOPSIS method.
The results showed that the sub-level stoping method with priority of 0.811 3 was the best for the studied mine.
For more information, please refer to his paper “Suitable Mining Method Selection using HFGDM-TOPSIS Method: a Case Study of an Apatite Mine” in “Journal of Mining and Environment” (EI).
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