• Journal of Power System Engineering
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• v.18 no.5
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• pp.129-136
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• 2014

This paper proposes improvement on sensorless vector control performance of a permanent magnet synchronous motor (PMSM) with sliding mode observer. An adaptive observer gain and second order cascade low-pass filter (LPF) were used to improve the estimation accuracy of the rotor position and speed. The adaptive observer gain was applied to suppress the chattering intensity and obtained by using the Lyapunov's stability criterion. The second order cascade LPF was designed for the system to escalate the filtering performance of the back-emf estimation. Furthermore, genetic algorithm was used to optimize the system PI controller's performance. Simulation results showed the effectiveness of the suggested improvement strategy. Moreover, the strategy was useful for the sensorless vector control of PMSM to operate on the low-speed area.

• The Transactions of the Korean Institute of Electrical Engineers D
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• v.51 no.6
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• pp.235-240
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• 2002

This paper presents a new method for estimating the block pulse series coefficients by using the Lagrange's second order interpolation polynomial. Block pulse functions have been used in a variety of fields such as the analysis and controller design of the systems. When the block pulse functions are used, it is necessary to find the more exact value of the block pulse series coefficients. But these coefficients have been estimated by the mean of the adjacent discrete values, and the result is not sufficient when the values are changing extremely. In this paper, the method for improving the accuracy of the block pulse series coefficients by using the Lagrange's second order interpolation polynomial is presented.

• The Pure and Applied Mathematics
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• v.17 no.4
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• pp.289-298
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• 2010

In this paper, we propose a second-order prediction/correction (SPC) domain decomposition method for solving one dimensional linear hyperbolic partial differential equation $u_{tt}+a(x,t)u_t+b(x,t)u=c(x,t)u_{xx}+{\int}(x,t)$. The method can be applied to variable coefficients problems and singular problems. Unconditional stability and error analysis of the method have been carried out. Numerical results support stability and efficiency of the method.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2684-2689
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• 2007

Non-equilibrium first order extrapolation boundary condition proposed by Guo et $al.^{(9)}$ proposed has a good application for complex geometries, a second order accuracy and a treatment on non-slip wall boundary condition easily. However it has a lack of the numerical stability from high Reynolds number. Guo et $al.^{(9)}$ substituted the density value of adjacent nodes for the density of boundary nodes. This procedure causes the numerical instability on the boundary. In this paper, we derived a procedure of density extrapolation and compared to previous results.

• Publications of The Korean Astronomical Society
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• v.13 no.1 s.14
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• pp.167-180
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• 1998

We have carried out high precision orbit calculation, by using various numerical techniques with accuracy of higher than fourth order, in order for exact prediction on position and velocity of celestial bodies and artificial satellites. General second order ordinary differential equation has been solved numerically to test the performance for each of numerical methods. We have compared computed values with exact solution obtained by using universal variables for two body problem and discussed overall results of numerical methods used in our calculation. As a result, it is found that high order difference table method called as Gauss-Jackson method is best one with easiness and efficiency in the increase of accuracy by number of initial values.

• 한국전산유체공학회:학술대회논문집
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• 1999.11a
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• pp.95-107
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• 1999

The spatial error due to the non-conservative interpolation become first-order when second-order conservative schemes are used, discontinuities are located away from the overlapped regions, and if the length of the overlapped region is not proportional to the grid spacing. Therefore, the solution accuracy is ensured if two domains overlap each other with a fixed grid point and the interpolation is occurred in smooth flow regions. To validate the spatial and temporal accuracy due to the non-conservative interpolation, inviscid and viscous problems are tested.

• Convergence Security Journal
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• v.5 no.3
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• pp.93-97
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• 2005

MacCormack's method is an explicit, second order finite difference scheme that is widely used in the solution of hyperbolic partial differential equations. Apparently, however, it has shown entropy violations under small discontinuity. This non-physical shock grows fast and eventually all the meaningful information of the solution disappears. Some modifications of MacCormack's methods follow ideas of central schemes with an advantage of second order accuracy for space and conserve the high order accuracy for time step also. Numerical results are shown to perform well for the one-dimensional Burgers' equation and Euler equations gas dynamic.

• Journal of applied mathematics & informatics
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• v.36 no.1_2
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• pp.135-154
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• 2018

In this paper, an almost second order overlapping Schwarz method for singularly perturbed third order convection-diffusion type problem is constructed. The method splits the original domain into two overlapping subdomains. A hybrid difference scheme is proposed in which on the boundary layer region we use the combination of classical finite difference scheme and central finite difference scheme on a uniform mesh while on the non-layer region we use the midpoint difference scheme on a uniform mesh. It is shown that the numerical approximations which converge in the maximum norm to the exact solution. We proved that, when appropriate subdomains are used, the method produces convergence of second order. Furthermore, it is shown that, two iterations are sufficient to achieve the expected accuracy. Numerical examples are presented to support the theoretical results. The main advantages of this method used with the proposed scheme are it reduce iteration counts very much and easily identifies in which iteration the Schwarz iterate terminates.

• Speech Sciences
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• v.12 no.3
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• pp.139-151
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• 2005

Previous studies have shown that Korean students have difficulty identifying some English consonants which are not in the Korean sound inventory. The aim of this study was to examine the accuracy rate of English consonants correctly identified by 130 college students in order to find out which English consonants were difficult for the students to perceive. The subject's task was to identify one of the minimal pairs played in a quiet laboratory classroom. 100 minimal pairs consisted of syllables with various onsets or codas: stops, fricatives, affricates, liquids and nasals. Results were as follows: First, the average score of the English major group was significantly higher than that of the non-English major group. Second, there was a similar distribution in the rank order of minimal pairs sorted by the accuracy rate between the two groups. Third, the accuracy rate systematically decreased as each score range decreased. Fourth, the students showed higher accuracy in the perception of liquids than that of the stop-fricative contrast. Fifth, the accuracy score in onset position was higher than in coda position. Finally, the students still had problem telling voiced consonants from voiceless ones, especially in coda position. It would be desirable to extend the present research to middle or high school students to fundamentally resolve those listening problems.

• Journal of the Korea Safety Management & Science
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• v.12 no.1
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• pp.127-136
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• 2010

In this paper, the author proposed following two methods to improve the accuracy of the recommender system. First, in order to classify the users more accurately, the author used a EMC(Expanded Moving Center) heuristic algorithm which improved clustering accuracy. Second, the author proposed the Neighborhood-oriented preference prediction method that improved the conventional preference prediction methods, so the accuracy of the recommender system is improved. The test result of the recommender system which adapted the above two methods suggested in this paper was improved the accuracy than the conventional recommendation methods.