Jo Nov 17, 2023
Practical application of manipulators needs determination of their dynamic errors to guarantee positioning accuracy.
Many researchers determined kinematic errors of Delta parallel manipulators, but not dynamic errors.
Pang Thae Jin, a researcher at the Robotics Institute, has established a new method for dynamic error determination of a Delta parallel manipulator with three actuators using a deformation model.
He wrote an algorithm based on the kinematic and dynamic models and developed a program for obtaining dynamic errors of a Delta parallel robot, using MATLAB software.
His method can be applied to construction of control systems for error compensation of complicated manipulators of different shapes.
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Jo Nov 14, 2023
Al-Si alloys are widely used in producing precise wear-resistant parts, such as air condition compressors, automobile engine pistons and cylinder bodies in automotive industry and machine-building industry due to the best cast ability among cast aluminum alloys. Especially, hypereutectic Al-Si alloys are rich in silicon. Therefore, they have some advantages such as small coefficient of heat expansion, good resistance to wear, high dimensional stability and lower casting cost.
At the same time, they have some disadvantages of lowering mechanical properties by primary crystal silicon as rough polygon or piece layers unless modified treatment is not enough in crystallization process because of high content of silicon.
Al alloys containing a trace of rare earth metal have high strength, hardness, wear resistance, corrosion resistance and electrical conductivity.
Preceding researches were mainly conducted on the effects of rare earth metal on the mechanical properties of Al-Si alloys.
Ri Hyon Mo, a researcher at the Faculty of Metal Engineering, has investigated the effects of Al-10RE master alloy as grain finer on the tensile strength, elongation percentage and resistance to wear of AiSi19Cu2주. Al-10RE master alloy was used as grain finer of AiSi19Cu2주.
The tensile strength, elongation percentage and resistance to wear of Al-Si hypereutectic cast alloy increase with increase in additive amount of Al-10RE master alloy up to 5%, but they decrease over 5%. After heat treatment, the tensile strength and elongation were 296MPa and 0.84%, respectively, when the additive amount of Al-10RE master alloy was 5%. They increase by 24.4% and 44.8% respectively more than AiSi19Cu2주 without addition of Al-10RE master alloy.
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Jo Nov 10, 2023
Core and Weber set are the most important set-valued solution concepts in TU-games. The core is a set of efficient payoff vectors that satisfy coalitional rationality i.e., a coalition receives at least its own worth, while the Weber set is a convex hull of all marginal vectors. In TU-games, core is always contained in Weber set. Furthermore, a game is convex if and only if its core coincides with its Weber set.
Inclusion of Weber set into core plays an important role in studies of stability of the Shapley value (inclusion of the Shapley value into the core) because the Shapley value is defined as a mean value of all marginal vectors.
O Un Suk, a lecturer at the Faculty of Applied Mathematics, has proved the stability of the extent/intent Shapley value using the inclusion of the extent/intent Weber set into the extent/intent core in games on concept lattices.
In games on extents, she introduced extent Weber set, newly defined strong convexity, and proved the inclusion of the extent Weber set into the extent core under this condition. Similarly, she proved that the intent Weber set is included into the intent core under the weak concavity in games on intents. Finally, she studied the relation between the game on extents and the game on intents, and derived a sufficient condition for stability of two Shapley values.
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Jo Nov 7, 2023
Game theory is a set of algebraic models to investigate behavior conflicting with each other. In practice, event players often have a game in uncertain conditions with lack of opponents’ strategies.
Especially, in such games as two-person zero-sum games, the payoff value of each player’s strategy is imperfect, so high accuracy of player modeling cannot be guaranteed. In general, statistical method processes uncertainty mathematically with lots of observed numerical value data, but observed data usually contain occasional fluctuation caused instantly by various reasons as well as essential information about opponents.
Being used as a technology of several variables which identifies common characteristic factors in an invariable set, statistical factor analysis uses probability methods for factor analysis. However, these methods include invariable assumptions about probability distribution so the results may not always be right. Also, there are some cases where modeling is not suitable in practice. Though modeling is not correct strictly, we can draw nearly right conclusions from those hypotheses if it is approximately suitable.
Game theoretical decision adoption is a decision-making problem which determines how the best result can be obtained when the result depends both on its own and on the opponent’s actions.
Some decision-making problems like a zero-sum game are inflexible in target weight according to the structure, so they are changed and affected by opponents. Information analysis methods used in the past are not sufficient to deal with these problems.
Kim Ok, a researcher at the Faculty of Information Science and Technology, has proposed an algorithm where a player’s model can be estimated by a small amount of game data in a two-person zero-sum game characterized by a monotonically decreasing function.
The algorithm is based on the analysis of game process profiles. First, she introduced ordered weight operators to the strategic sequence determination of players and defined a compensation function considering monotonically decreasing character of a two-person zero-sum game. Then, she evaluated the game payoff value of the strategy and conducted player modeling.
The proposed algorithm will contribute to enhancing the accuracy of player modeling in two-person zero-sum games.
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Jo Nov 2, 2023
Radiation is an important process of thermal transfer together with conduction and/or convection. Particularly, in high temperature conditions such as burning of pulverized coal, propellants of rockets and thermal plasma, it contributes significantly to overall thermal transfer. However, due to the integro-differential characteristic of a radiative transfer equation for mathematically explaining thermal radiation in absorbing, emitting and scattering media, its algebraic solution is in existence only for extremely limited geometries and conditions. Thus, numerical methods have mainly been utilized to study radiative thermal transfer. Moreover, the advance on performance of modern computers and the interest increased for understanding thermal radiation have promoted the development of numerical methods to solve the radiative transfer equation at a low cost.
Several numerical methods, such as Monte Carlo method, zonal method, discrete ordinates method, finite element method, etc. were applied to model radiative behavior.
The discrete ordinates method (DOM) is a numerical technique proposed first for analyzing radiative thermal transfer in a slab-parallel medium, which has several advantages: low dimensional approximation with sufficient accuracy, modest computational requirement, and applicability into complex geometry.
There were some attempts to use the lattice Boltzmann method (LBM) as a tool to solve the radiative transfer equation, which has developed into an alternative and promising numerical scheme for simulating fluid flows.
Kim Yong Jun, a researcher at the Faculty of Physics Engineering, has developed a discrete ordinate-lattice Boltzmann method (DO-LBM) by combining the advantages of DOM and LBM that can be utilized as a direct tool to solve the radiative transfer equation.
The discrete ordinates scheme is used for angular discretization and the lattice Boltzmann model is applied for spatial discretization.
Introducing Chapmann-Enskog method and dimensionless numbers, he analyzed accuracy of the DO-LBM and non-negativity of the equilibrium distribution function. He compared his method with numerical results by modified discrete ordinates method. The result showed that analyzing radiative transfer in geometry with complex boundaries lowers computational cost more than modified discrete ordinates method often found in literature.
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Jo Oct 30, 2023
Displacement prediction of rock mass surrounding tunnels plays an important role in displacement back analysis for determining geotechnical parameters of rock mass and in-situ stress ratio, as well as safety monitoring and quality control for tunnel construction.
Han Un Chol, a researcher at the Science Engineering Institute, has presented a method to quickly predict the tunnel closure in time-dependent rock mass by using stable deformation rate for tunnels and the non-equidistance grey Verhulst model (NGVM).
Considering preceding studies, he determined the maximum stable deformation rate as 0.05 mm/d.
The proposed method was validated fairly through a case history for a new excavated drift of -600m level in an underground coal mine.
The result showed that NGVM seems to be a more powerful tool compared with non-equidistance grey model (NGM (1, 1)) for displacement prediction of tunnels in time-dependent rock mass.
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