Jo Oct 13, 2024
It is difficult to ensure ideal symmetric conditions during hot plate rolling, so rolled plates may bend upwards or downwards. The factors affecting the plate curvature are temperature difference, roll diameter difference, rolls speed mismatch, frictional difference, reduction rate, ingoing thickness, inclination angle, etc.
In the single strand mill, the difference in the height of the upper parts of the lower roll and the breast roll is fixed. So, during multi-pass rolling, the plate sometimes faces inclined entry. Reprocessing of the work roll also causes the difference in the roll diameter. Therefore, it is very important to determine the admissible roll diameter ratio that causes very narrow range of plate curvature for normal rolling.
Ri Chun Ho, a researcher at the Faculty of Materials Science and Technology, investigated the influence of the inclination angle of the plate on strip curvature during the 7 passes of reversing rougher at the hot mill when the plate enters with inclination, by using the three-dimension finite element analysis software, DEFORM-3D. Based on this, he determined the range of admissible roll diameter ratio that leads to the very narrow range of plate curvature when the plate enters with inclination.
He found that the roll diameter ratio of reprocessed work rolls is not allowed to exceed 1.067.
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Jo Oct 11, 2024
Coal is a main energy resource and many households in our country use honeycomb briquettes made of coal for heating and cooking. In summer honeycomb briquettes are mainly used as fuel for cooking. But they reveal some shortcomings such as difficulty in ignition and low combustion efficiency due to the need for high ignition temperature.
The most important thing is to reduce the time and energy needed for ignition of honeycomb briquette as much as possible, and to raise the combustion rate up to the level of gas combustion for instant cooking. 150 to 200 grams of wood sticks are needed for ignition of honeycomb briquettes. If the calorific value of wood is 16MJ/kg, as much energy as 2 400 to 3 200kJ is consumed for ignition alone. What is more, a large amount of flame and harmful gases are released due to the difference in the activation energy between coal and biomass, which pollutes the environment.
Therefore, many researchers have conducted studies to lower the ignition temperature and raise the combustion rate by lowering combustion activation energy of coal. To this end, they added combustion additive to coal or mixed coal and biomass. To prevent the release of much flame during ignition with wood sticks, some researchers have developed double-layer or multi-layer ignition honeycomb briquettes to be ignited with a sheet of paper or a match stick. The problem is that if biomass like raw sawdust is added to the upper sides of briquettes as it is, it produces smoke and exerts bad influence on coking power.
Jon Chol, a researcher at the Faculty of Mining Engineering, on the basis of the analysis of biomass ignition characteristics for its size, has studied the ignition and combustion characteristics of upper side ignition honeycomb briquettes and determined appropriate crushing size and blending rate of biomass.
The results showed that the upper side ignition honeycomb briquette added with biomass has ignition time of two to three minutes on average, combustion time of 75 minutes and combustion rate of 156.12g/min, and that the most appropriate biomass crushing size is 125㎛.
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Jo Oct 8, 2024
In order to protect critical electronic equipment against damped sine transient currents induced into its cables due to transient electromagnetic fields, switching phenomena, platform resonances, etc., a proper measure should be taken.
In any major equipment, there must be several individual units (LRUs), which are interconnected by cable harnesses in varying lengths. When such equipment is used in severe electromagnetic (EM) environment, the loops consisting of the shields of cable harnesses, the grounded chasses of LRUs at the two ends and the ground path provide coupling with the interfering fields. The induced voltages, particularly due to the current in the cable shields, can cause temporary upset or permanent damage to the LRUs connected at either end of the cable harnesses.
To ensure satisfactory performance of the LRUs under such conditions, it is necessary to simulate appropriate conditions in a laboratory and conduct an evaluation.
In order to protect the critical electronic equipment against damped sine transient currents, Jong Hak Chol, a section head at the Faculty of Electrical Engineering, generated and induced the necessary damped sine currents into the cables of the electromagnetic compatibility test (EMCT) equipment in the laboratory. Then he developed a test equipment CS116 of Standard 461E/F and measured other factors using this equipment.
Many approaches using LC discharge have been investigated as a general way of producing damped sine current waves, but those need tiny amount of inductance and capacitance and they tend to cause wave distortion and power loss. Especially, demanding value of inductance for building a damped sine wave generator for 30MHz and 100MHz are around 5 to 14nH.
Thus, he designed and manufactured a damped sine wave generator with small inductance and capacitance, and conducted some experiments to verify its output.
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Jo Oct 3, 2024
The least mean square algorithm is widely used in many areas due to its simplicity and robustness. Within the LMS algorithm, the tap-length of the filter, defined as the number of tap coefficients of the adaptive filter, is an important factor that influences the performance and complexity of the LMS adaptive filter. If the tap-length is too long, it causes heavy computational burden and slows convergence rate, whereas if the tap-length is underestimated, the mean squared error (MSE) tends to increase. Therefore, segmented filter LMS (SF-LMS) algorithm, gradient descent LMS (GD-LMS) algorithm, and fractional tap-length LMS (FT-LMS) algorithm have been used to regulate the tap-length of filters. Among these variable LMS algorithms, the fractional tap-length least mean square (FT-LMS) algorithm has attracted attentions recently due to its less computational complexity and better convergence performance than other variable tap-length algorithms.
According to the steady-state performance analysis of the FT-LMS algorithm, correct choice of the step size for tap-length adaptation is very important. It provides a trade-off between the convergence rate and the steady-state bias of tap-length.
Jon Kwan Hak, a section head at the Faculty of Communications, has proposed an improved variable tap-length algorithm with variable step size for tap-length adaptation γ.
He confirmed the performance of the proposed algorithm by simulations for high and low noises, respectively. The results showed that for both noise conditions, the proposed algorithm obtains both fast convergence rate of the tap-length and small steady-state tap-length fluctuation.
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Jo Sep 30, 2024
Trajectory planning is one of the most important problems in robotics. It refers to generation of position commands, velocity, and acceleration of all degrees of freedom of a robot.
Pick-and-place trajectory planning is finding a smooth and continuous trajectory from the starting position to the desired terminal position within the workspace of a robot.
Many studies have been conducted for effective trajectory planning of a robot. However, previous studies revealed some drawbacks in the aspect of minimization of workspace, especially the equations of velocity and acceleration/deceleration with little information of the path, and therefore, the equations were not directly related to the geometry of the generated path.
To overcome such shortcomings, Choe Myong Song, a researcher at the Faculty of Mechanical Science and Technology, has developed an asymmetric bisect elliptical trajectory planning with minimum workspace suited to the geometry of the real-time path for pick-and-place operation in which both the picking points and placing points are changed randomly in the workspace.
First, he proposed an optimal asymmetric bisect elliptical path with two vertical straight line segments and a method of generating it. Then, he presented a method of generating the optimal motion profile suited to geometry of the elliptical path based on the radius of curvature. Finally, he conducted a simulation to demonstrate the effectiveness of the proposed method.
He drew the following conclusions.
The proposed motion profile based on the radius of curvature of the path is pretty suited to the geometry of the optimal elliptical path, and therefore, its velocity, acceleration and jerk are continuous and very smooth, which enables smoother pick-and-place robot operation. In addition, the proposed trajectory planning approach reduces a cycle period and ensures smoothness of working of robot actuators and real-time transhipping.
For further details, please refer to his paper “An Approach for Elliptical Trajectory Planning with Vertical Straight Line Segments of Pick-and-Place Robot Operation with Height Clearance” in “Mathematical Problems in Engineering” (SCI).
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Jo Sep 27, 2024
Nowadays, global warming is intensifying more and more and fossil fuels are running out all over the world, which leads to increasing numbers of efforts for protecting the environments against green house effects and for using natural energy.
In the production of electric power using natural energy, electrical power fluctuation occurs due to the natural conditions such as luminous intensity, wind speed, tidal current, etc. Therefore, it is necessary to make up for electrical power fluctuation for stabilizing the power supply. Compensation for electrical power fluctuation can be carried out by secondary batteries or a flywheel energy storage system (FESS). Since the FESS using the SMB has longer life time than secondary batteries, it has already been applied in several countries.
The important problems of the FESS are to heighten load capacity, to decrease loss and to raise stability. These problems are ascribable to the improvement of the performance of SMB.
A number of basic experiments were performed to design the SMB and to improve its performance, and the FESSs with SMBs of different load capacities and structures were developed. On the other hand, numerical simulations on SMB were conducted by MATLAB, FORTRAN and COMSOL Multiphysics.
Some researchers presented a way to increase the levitation force using opposite magnetization effect through experiments and simulations, and obtained 170.14N in maximum levitation force. However, they failed to intend optimization on it.
Jo Ju Hak, a researcher at the Faculty of Physical Engineering, has simulated a three surface levitation-superconducting magnetic bearing (TSL-SMB) in two-dimensional axisymmetric system using H-formulation and Taguchi method.
The simulation was conducted by field cooling (FC) method. The effects of geometric parameters on levitation force in TSL-SMB were considered using Analysis of Variance (ANOVA). He concluded that in the optimized TSL-SMB structure the effect of concentric ring PMs with opposite polarization did not play a decisive role. Therefore, he presented four-surface levitation-superconducting magnetic bearing (FSL-SMB) as advanced multi-surface levitation system to improve the performance of SMB.
The simulation results in the FSL-SMB system showed that the gap between the stator and the rotor and levitation force per permanent magnet volume might be increased more than those of TSL-SMB system in the condition of same load capacity.
For more information, you can refer to his paper “Simulation on modified multi-surface levitation structure of superconducting magnetic bearing for flywheel energy storage system by H-formulation and Taguchi method” in “Physica C: Superconductivity and its applications” (SCI).
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