Jo Apr 14, 2025
It is well known that industrial processes generally have nonlinearity, time-varying features and uncertainty and that active disturbance rejection control is effective in controlling these industrial processes. The control of industrial processes, especially those with time delay, is becoming more difficult.
In several different production processes of industrial practice exist time delays such as state delay, input delay, transmission delay, output measurement delay, capacity delay, etc. Because of the time delay, the manipulated value cannot immediately reflect the disturbance that the system receives, the response characteristic has a large overshoot, adjustment time gets long, and even the system becomes unstable. Thus, the time-delay system is accepted as a difficult one to control.
Although a number of control schemes have been proposed to overcome the effect of time delay, it is very difficult to build accurate mathematical models for systems that are becoming increasingly complex, which results in the limitation of the application of current control theory and methods.
Applying active disturbance rejection control to time-delay systems can achieve good effects. The design methods for the active disturbance rejection control system of time-delay systems include time-delay neglect, dimensionality increase, output prediction, input prediction and input delay. In all these methods, a delay object is approximated or transformed into a delay-free object, and then an extended state observer is designed for observation.
Kim Ho, dean of the Faculty of Automation Engineering, has proposed a strategy for applying a multivariable linear active disturbance suppression control method to the brine heating process where there is a bias interference between the time delay and the control variables.
To compensate for the time delay, he used the input delay method in the extended state observer design. In addition, he constructed and introduced the decoupling matrix using the mathematical model information of the object to further enhance the decoupling effect.
The simulation results show that the performance of the proposed control system is very high.
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Jo Apr 13, 2025
Most of the existing estimation methods for linear models are based on mean regression by least squares or likelihood method. However, these methods are sensitive to outliers and may not be effective for many irregular errors. Quantile regression (QR) is more robust in exploring the relationship between covariates and dependent variables.
The utility of the Lp-quantile as an extension of quantile and expected quantile, the validity of the quantile regression and the expected quantile regression method have been demonstrated and many studies have been conducted, but the asymptotic properties of the Lp-quantile regression estimators have not been fully considered.
Jo Chol Min, a researcher at the Faculty of Applied Mathematics, has considered the Lp-quantile regression estimation of unknown parameters in a linear model and proved the asymptotic normality of the Lp-quantile regression estimator for different p.
The results show that when 1/2>p>0, the convergence rate of the estimator is slower than that of the usual case, and that if p>1/2 and other certain regularity conditions hold, the estimator is asymptotically normal at n1/2-rate. (See Figure)
The results can be effectively used to solve practical problems such as interval estimation or hypothesis testing.
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Jo Apr 12, 2025
There are many ring-shape faults in Phyongnam anthracite fields. These faults are called “window” or “klippe” and they are considered to be formed by the denuded allochthon from other places. However, there are some autochthonous “klippe” formed in the original place, not displaced from other places. These faults appear as conformity between Formations or as apparent normal fault on the surface. These faults are reverse faults where footwall blocks are pushed down beneath the fault surface and the dip angles of faults are similar to those of layers or smaller than them.
Hwang Myong Il, a researcher at the Faculty of Earth Science and Technology, has investigated the developmental characteristics of synformal klippe in Phyongnam anthracite fields and their correlation with structurally branched coal seam.
He has carried out a mechanical simulation by the inverse method on the basis of the geometry and developmental characteristics of synformal klippe and structurally branched coal seam found in the course of anthracite mining to analyze their genesis and kinematic characteristics of blocks of fault.
Because these synformal klippes and structurally branched coal seam are closely related to various geological structures in anthracite fields, geological processes in the study region might be analyzed from the geometry and developmental characteristics of structurally branched coal seams, and thus the buried coal seams that are difficult to be estimated on the surface might be found through the analysis of the developmental state of geological structures in the region.
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Jo Apr 11, 2025
Transparent conducting oxide (TCO) films have been widely used as transparent electrodes in various displays, thin film solar cells and other optoelectronic devices. Up to now several transparent conducting films of oxide series have been developed, among which Sn-doped In2O3 (ITO) films have been most frequently used due to their superior electrical and optical properties. However, the exhausting of resources has resulted in an increase in the price, leading to the limitation of use of ITO films. Hence, there is an increasing interest in developing transparent conducting films by using more cost-effective materials.
In recent years, doped ZnO thin films have attracted great attention due to their high conductivity, good transmission in the visible light range, abundant resource of raw materials, low cost of fabrication, non-toxicity, etc. and they are regarded as potential substitutes for ITO. Among them, Al-doped ZnO (AZO) films are the most studied due to the rich resources of Al and good electrical and optical properties.
In general, the electrical and optical properties of AZO films prepared by r. f. magnetron sputtering are greatly influenced by process parameters such as sputtering power, working pressure, substrate temperature and target-substrate distance, and many studies have been focused on analyzing the effect of these process parameters on the properties of the films and finding optimal process parameters.
The electrical properties of AZO films, in general, are greatly related to the doping state of Al into ZnO crystal grains, which could be qualitatively evaluated by considering the binding state of Al and compositional elements through XPS measurements. However, previous studies that have considered the effect of film thickness on the properties of AZO films, failed to analyze the doping state of Al in connection with XPS measurements. In addition, there is no literature regarding the effect of film thickness on the properties of AZO films in relation to deposition temperature.
Ri Kang Hyon, a researcher at the Faculty of Materials Science and Technology, has observed the structural and electrical properties of AZO films prepared at two different substrate temperatures (100℃ and 350℃) by r. f. magnetron sputtering for film thickness, and analyzed them in combination with the results of X-ray photoelectron spectroscopy (XPS) measurements.
The results showed the following.
First, the resistivity of films deposited at 100℃ or 350℃ decreased significantly by about one order with increasing film thickness from about 80nm to 600nm, i.e. from 34.16×10-3Ω·cm to 3.14×10-3Ω·cm and from 6.71×10-3Ω·cm to 0.75×10-3Ω·cm, respectively. Second, the resistivity differs greatly with deposition temperature even for the same film thickness, which means the average resistivity at 100℃ is 4-5 times greater than at 350℃. Third, Al2O3 segregates and the amount of chemisorbed oxygen at the grain boundary decrease with increase to 600nm of film thickness and to 350℃ of deposition temperature, resulting in the increase in carrier concentration.
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Jo Apr 10, 2025
Bulk high temperature superconductors (HTS) have significant potential for industrial applications such as magnetic bearings, flywheel energy storage systems and trapped-field magnets. One of the prospective applications of bulk high temperature superconductors is a superconducting magnet for magnetically levitated (maglev) trains. It is very difficult to make a big superconducting bulk for various large-scale applications.
The fabrication of bulk HTS for high trapping magnetic fields has been extensively studied and the top seeded melt growth (TSMG) process is a superconductor fabrication method developed for the improvement of flux pinning property of bulk HTS. This method, widely used in the field of superconductor manufacturing, has been applied to the superconductor magnetic levitation equipment like magnetic bearings due to its ability to raise the flux pinning property of superconductors several times more than the preceding methods.
Ri Hak Mun, a researcher at the Faculty of Physical Engineering, has confirmed the position-dependent current density distribution inside the bulk high temperature superconductor after field cooling of bulk HTS fabricated by TSMG by numerical calculations, and considered the effect of shape and aspect ratio of the bulk HTS on the z-axis trapped magnetic field.
The results have shown the following conclusions.
First, the critical current density distribution depending on the position in the cylindrical bulk HTS after field cooling of the bulk Y-Ba-Cu-O superconductor prepared by TSMG is a polynomial monotone function. Second, among the cylindrical and squared bulk HTS of the same volume, the cylindrical one has bigger z-axis trapped magnetic field. Third, in the case of uniform critical current density in the range of 4-30mm in radius and 1-50mm in height, the aspect ratio affecting the trapped magnetic field property is 0.775-0.957, and in the case of ununiformity, it is 0.625-0.829.
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Jo Apr 8, 2025
Diesel engines are widely used in various fields of national economy due to their low specific fuel consumption, high torque and high power density at low speed. Valve timing is one of the main characteristics that has a great influence on the fuel economy and exhaust gas emission characteristics of internal combustion (IC) engines. At present, supercharging of internal combustion engine, common rail fuel system and variable valve timing are the key technologies for improving the performance of IC engines by reducing specific fuel consumption and harmful components in the exhaust gas.
In order to analyze the effect of valve timing on the engine performance, a one-dimensional simulation analysis model is required to analyze the working process of the engine. GT-Power, Ricardo-Wave and AVL/BOOST are widely used for analyzing the working process of IC engines.
Many researchers have developed models and optimized their properties for different internal combustion engines using these programs and various analytical methods.
Jang Song Ho, a researcher at the Faculty of Mechanical Science and Technology, has established a one-dimensional working process analysis model of the 4D4-12.5/11 engine with the bore of 110mm, the stroke of 115mm and the engine displacement of 4.75L by using GT-Power, and determined the most suitable valve timing using the experimental design, the artificial neural network and the genetic algorithm of the GT-Power program.
Applying the optimal valve timing (variable distribution phase) for the engine speed to the 4D4-12.5/11 engine, the engine power increases by 1.37% at 1 400r/min and 2.75% at 2 200 r/min, respectively, compared to the initial valve timing. The effect of the intake opening angle (IVO) and exhaust closing angle (EVC) on the engine power is not so significant, but the intake closing angle (IVC) and exhaust opening angle (EVO) have a great influence on it. In addition, the effect of valve timing on the engine power is greater at the nominal state (2 200r/min) compared to the maximum torque state (1 400r/min).
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