Jo Dec 15, 2022
Production systems operated in practice consist of repairable components and such systems may mostly be modeled as a direct-parallel system. Thus, analysis of the reliability characteristics of a direct-cold standby system is important in the reliability analysis of the system.
Most of the components for a repairable system are not the same after repair as new ones and component’s lifetime decreases with the increase in the time of use. What is more, the time for repair increases more and more once they stop working, which finally leads to complete failure.
On the assumption that the lifetime and repair time of components follow a geometric process, Kim Man Su, a lecturer at the Faculty of Applied Mathematics, has studied the reliability of several dual series direct-parallel systems in consideration of such decrepitude.
The figure above shows a state transition graph of a system, where a circle stands for in operation and a square means break down.
It is assumed that the system consists of components 1, 2 and 3, and components 2 and 3 make up a dual cold standby system, which is then connected with component 1 directly. Here, component 1 gets priority in repair. There is a repairman. Components 2 and 3 are repaired in order of failure.
The system is a unidirectional closed dual series cold standby system. It means if component 1 fails, the system breaks down.
At the beginning, three components are all new, and components 1 and 2 are in operation and component 3 is under cold standby. When the three components in the system are in good condition, two are in operation and one is under cold standby. The repairman sets to work once one of them fails. At the same time, the standby one begins to work. When the failed one has been repaired, it is put on cold standby until the next failure. If one fails while the other is still under repair, it must wait for repair and the system breaks down. It is assumed that each component after repair is not ‘as good as a new one’.
A deteriorative repairable system will not support constant repair of its components. For a deteriorative repairable system, it seems more reasonable to assume that the successive working time of the system after repair will become shorter and shorter while the consecutive repair time of the system after failure will get longer and longer. Ultimately, it cannot work any longer, nor can it be repaired.
In a dual series cold standby system, after N cycle of component 2, when it finishes working, a plan to replace it with a new one (the same as the first dual series cold standby system) is called N replacement policy.
If the lifetime and repair time of components follow a geometric process in a unidirectional closed dual series cold standby system, the solution of state probability (density) is difficult to present analytically, so replacement cycle N needs to be defined and the characteristics of the system be analyzed.
In order to determine an effective replacement cycle, he studied the stationary conditions of the unidirectional closed system using the transitive property of the Markov chain assuming every state in all possible cases in the system.
The results showed that the proposed mathematical method could be effectively used in different pieces of research of other kinds of queuing models.
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Jo Dec 8, 2022
Humidity is an important physical quantity in all fields of human life. In recent years, fiber-optic humidity sensors with compact size, high sensitivity and good resistance to chemical corrosion and insusceptibility to electromagnetic interference have been widely studied.
Yu Hyong Su, a researcher at the Semiconductor Institute, has proposed a novel optical humidity sensor which is simple in structure, easy to make and cost-effective with high sensitivity, a wide range of operation, and good safety and reproductivity. It consists of light source, a gap and an acrylic resin film coated on PSD.
Experimental results verified very satisfactory performance of the humidity sensor in terms of its sensitivity, hysteresis, linear response to the humidity and long-term stability.
You can find more information about it in his paper “Widely sensitive optical humidity module by using acrylic resin and a position sensitivity detector” published in the SCI Journal “Optical and Quantum Electronics”.
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Jo Dec 3, 2022
Ti2AlNb‒based alloys, so-called ordered orthorhombic (O (Cmcm)) phase alloys, have a chemical composition of Ti ‒ (18 ‒ 30) Al ‒ (12.5 ‒ 30) Nb (mole fraction, %). Since the discovery of orthorhombic (O) phase, Ti2AlNb‒based alloys have received special attention as a promising candidate for advanced aerospace and automotive application due to their high specific strength, excellent creep and oxidation resistance at elevated temperatures as well as good workability.
Up to now, many studies on the fabrication of Ti2AlNb-based alloys by casting, rolling and forging have been conducted, in which the major focus was set on the elimination of disadvantages such as microstructure segregation and inhomogeneity. It is because the thermodynamic properties of Ti, Al and Nb elements including a melting point, a density, a diffusion coefficient, etc. are very different.
Recently, powder metallurgy (PM) method such as spark plasma sintering (SPS) has been applied to the fabrication of Ti2AlNb‒based alloys, which made it possible to obtain fine and homogeneous microstructure. Several endeavors have been devoted to preparing PM Ti2AlNb‒based alloys from pre‒alloyed powder and elemental powders by vacuum hot pressing, and from pre‒alloyed powder by hot isostatic pressing (HIP). However, these as‒sintered compacts showed the microstructure with coarse grain size (>40μm), due to higher sintering temperature and longer sintering time. Therefore, the strength of these Ti2AlNb‒based alloys was not so high.
In recent years, several studies on the fabrication of high strength and ductility titanium alloys with nanostructured (NS) and ultrafine grained (UFG) microstructure have been reported. One process to achieve this microstructure consists of the preparation of NS or UFG powder by high energy ball milling (HEBM) and the consolidation of the powder into dense compacts by PM methods such as HIP and SPS. For instance, using the high energy ball-milled powder as a starting material, an ultrafine grained Ti‒6Al‒4V with high mechanical properties can be obtained.
In order to produce a high strength and ductility Ti‒22Al‒25Nb alloy from pre alloyed powder, Sim Kyong Ho, a researcher at the Faculty of Materials Science and Technology, has introduced HEBM with subsequent SPS. The process is as follows. Firstly, the Ti‒22Al‒25Nb pre‒alloyed powder with a weight of 75g was put in a stainless steel vial with bearing steel balls. HEBM was carried out in a high-energy planetary ball mill (QM-3SP4) under a high purity argon atmosphere for 20h at a rotation speed of 300 rpm. The HEBMed powder was directly loaded into a high-strength graphite mould, and subsequently consolidated by a LHPD250 SPS apparatus (FCT Co. Ldt., Germany) at SPS temperatures of 950℃ for SPS time of 10 min. After sintering, the sintered compact was cooled to the room temperature (RT) in the furnace. Finally, a sintered compact, with dimensions of φ50 × 15 mm, was obtained.
The Ti‒22Al‒25Nb alloy sintered at 950℃ for 10 min under the pressure of 50 MPa from the 20 h‒HEBMed powder showed the microstructure with a large amount of UFG/NS O-phase. The yield strength, tensile strength and elongation to failure were 1 092 MPa, 1 105 MPa and 9.4%, respectively. Compared with the referenced Ti‒22Al‒25Nb alloys, the Ti‒22Al‒25Nb alloy fabricated by HEBM and subsequent SPS exhibited higher strength and good ductility at the room temperature and high temperature. This indicates that HEBM with subsequent SPS is feasible for PM Ti‒22Al‒25Nb alloy with enhanced tensile properties.
More information about this is found in his paper “Fabrication of a high strength and ductility Ti‒22Al‒25Nb alloy from high energy ball-milled powder by spark plasma sintering” published in the SCI Journal “Journal of Alloys and Compounds”.
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Jo Nov 28, 2022
The research team led by Kim Hyok Chol, an institute head at the Faculty of Electrical Engineering, and Kim Nam Chol, a researcher at the Faculty of Mechanical Science and Technology, who designed and manufactured our style two-way high-speed circuit breaker for underground electric vehicles, is working for further achievements.
A two-way high-speed circuit breaker was designed and fabricated by combining a digital cut-off controller.
The device can achieve reliable high-speed cutoff even under severe operating conditions of underground electric vehicles. Accurate cutoff setting current and low mass and volume are other advantages of this device. In particular, successful bidirectional blocking is guaranteed.
The high-speed circuit breaker consists of a maintenance and release device for high-speed interruption, a blanking room, a locking control unit, and a contact system, which can sense both the current entering the electric vehicle and the current entering the grid during regenerative braking, so that the circuit can be shut down within 18 ms once an accident occurs.
The shutoff coil of the hold and trip device has no vibration-induced malfunction because there is no current flow during normal operation, and it takes less than 1ms to output a shutoff signal immediately after the current-cut signal from the current sensor enters the shutoff control device.
The turn-off controller has a double current sensing and a circuit to turn off analog and digital double, and the current setting has been simplified on a button.
The contactor is a two-point open type with an inclination angle of 45° and there is an arc-extinguishing component and a main contractor at each contactor. There is little electrical wear at the main contactor because there is no structural repulsion during turn-on.
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Jo Nov 25, 2022
Self-mixing (SM) technique has been a promising optical measurement technique for a few decades.
This technique is developing toward increasing the measurement distance and the limit of displacement measurement, enhancing the measurement accuracy and improving the real-time quality of the measurement. In addition, it is also important to drop the development cost of a sensor.
Estimation of the optical feedback factor C is the key to SM displacement measurement with a better resolution than half a wavelength. But this estimation is very complex and tricky, and it thus leads to its time-consuming nature, which is one of the important causes that increase the cost and labor for the development of a self-mixing sensor. There have been many methods reported for the C estimation, but they are very time-consuming, very sensitive to noise, or can only work in a certain feedback regime.
Kim Chol Hyon, a researcher at the Faculty of Physical Engineering, has proposed a method of the C estimation using the derivative of an SM signal. So to speak, he has solved a problem of fast estimation of the parameter of an SM interferometric system with the derivative in the jumping point of an SM signal. This method is very simple, and, what is more, it can work in all regimes including weak, moderate and strong feedback regimes.
The estimation precision of the proposed method is not only comparable to that of the data fitting technique but also much simpler than that. Therefore, it is effective in low-cost high-precision SM displacement measurement.
You can find the details of this in his paper “Fast estimation method of feedback factor based on the derivative of the self-mixing signal” presented to the SCI Journal “Applied optics”.
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Jo Nov 24, 2022
Water spraying itself is a dynamic course, and it is affected not only by the structure of an ultrasonic gas atomizer and the compressed air pressure but also by the movement of the compressed air flow in an atomizer.
The fundamental principle of formation of fine droplets in the ultrasonic gas atomizer is that a liquid sheet is disrupted into droplets by the energy of high-velocity air. If the ultrasonic energy is supplied, finer droplets with high sphericity and uniform size distribution can be achieved.
Kim Kang Dok, a section head at the Faculty of Mining Engineering, has observed water atomization of an ultrasonic gas atomizer by numerical simulation with CFD software ANSYS Fluent, and found out about the generation of ultrasound and water fog in the ultrasonic gas atomizer with a resonance chamber together with ultrasound characters (frequency, pressure level, power level).
Recently, experimental researches to improve the features of ultrasonic atomizers by air spray have been conducted, but only a little about numerical simulation was presented except for some experimental data.
Thus, he has simulated the characters of ultrasonic air atomization using the finite volume method based on the Roe solver in the structure similar to the Hartmann resonance chamber. Then, he has looked closely at the occurrence of oscillation and the factors affecting the oscillation, and investigated the distribution character of water droplets.
The experimental conditions for the simulation were compressed air pressure 3.5~4atm, air amount 0.1㎥/min, acoustic power in the cavity resonator 242dB, acoustic power at the outlet 172dB.
The range of frequency is 20~120 kHz, where 21~28 kHz has the highest density. The acoustic power decreases with the distance when the pressure of the compressed air is 0.3MPa.
The determination of the distribution characteristics of the atomizer indicated that condensation of water droplets decreased rapidly from the distance of 3m. Thus, it is clear that the rational spray distance of the atomizer is 3m. The results show that the number density of water particles is 1.52×1010 in the center, but it is 6×108 in the diffusion region.
You can find more information about this in his paper “Numerical simulation on the generation of ultrasound and Formation of Water fog in the Ultrasonic gas atomizer” presented to the SCI Journal “ULTRASONICS”.
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