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 6, 2024
Kkaktugi is a kind of kimchi that Korean people usually make for winter.
It is a mixture of cubed radish or cucumber with seasonings like pepper or garlic. The name kkaktugi has been said to imitate the sound heard when radish is being cubed.
Old records say that Kuryo people regarded radish as a main ingredient together with bean paste and kelp and that foodstuff preservation technique was on a high level in those days, which demonstrates that Korean ancestors processed radish and other vegetables in several ways from olden times.
A lot of seasonings added to kkaktugi make it look delicious and taste strong, which improves appetite even more.
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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 Oct 2, 2024
Proverbs are known to have been widely used in Korea since the period of Three Kingdoms.
The proverb found in “Samgukyusa” (“Chronicles of the Three Kingdoms”) is the first one in all the existing historical documents. At that time it was called “rion”. After that it was given the name “sangon” and it has been called “soktam” (Korean pronunciation of “proverb”) since the middle years of the feudal Joson dynasty.
The first book of proverbs in Korea is “Sunoji” (by Hong Man Jong) published in the seventeenth century. It has a collection of more than 130 proverbs that were popular in those days. Later, “Paekonhae” (by Ri Ik), “Yangyopki” (by Ri Tok Mu), “Idamsokchan” (by Jong Yak Yong), etc. were compiled.
The first proverb dictionary written in Korean is “Joson Rion” compiled by Choe Won Sik in 1913, which has over 900 proverbs. Later, several proverb books were compiled.
After liberation, “Book of Korean Idioms and Proverbs”, “Book of Korean Proverbs”, “Korean Proverbs”, “Dictionary of Korean Idioms and Proverbs”, etc. were compiled and published.
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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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