• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.29 no.10
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• pp.89-96
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• 2015

Explicit numerical integration methods for power system transient stability simulation require very small time steps to avoid numerical instability. The EXST1 exciter model is a primary source of fast dynamics in power system transients. In case of the EXST1, the required small integration time step for entire system simulation increases the computational demands in terms of running time and storage. This paper presents a practical exciter model reduction approach which allows the increase of the required step size and thus the method can decrease the computational demands. The fast dynamics in the original EXST1 are eliminated in the reduced exciter model. The use of a larger time step improves the computational efficiency. This paper describes the way to eliminate the fast dynamics from the original exciter model based on linear system theory. In order to validate the performance of the proposed method, case studies with the GSO-37 bus system are provided. Comparisons between the original and reduced models are made in simulation accuracy and critical clearing time.

• Advances in concrete construction
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• v.15 no.4
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• pp.215-228
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• 2023

The vibrational behavior of nanoelements is critical in determining how a nanostructure behaves. However, combining vibrational analysis with stability analysis allows for a more comprehensive knowledge of a structure's behavior. As a result, the goal of this research is to characterize the behavior of nonlocal nanocyndrical beams with uniform and nonuniform cross sections. The nonuniformity of the beams is determined by three distinct section functions, namely linear, convex, and exponential functions, with the length and mass of the beams being identical. For completely clamped, fully pinned, and cantilever boundary conditions, Eringen's nonlocal theory is combined with the Timoshenko beam model. The extended differential quadrature technique was used to solve the governing equations in this research. In contrast to the other boundary conditions, the findings of this research reveal that the nonlocal impact has the opposite effect on the frequency of the uniform cantilever nanobeam. Furthermore, since the mass of the materials employed in these nanobeams is designed to remain the same, the findings may be utilized to help improve the frequency and buckling stress of a resonator without requiring additional material, which is a cost-effective benefit.

• Advances in nano research
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• v.14 no.6
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• pp.575-590
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• 2023

Many of small-scale devices should be designed to tolerate high temperature changes. In the present study, the states of buckling and stability of nano-scale cylindrical shell structure integrated with piezoelectric layer under various thermal and electrical external loadings are scrutinized. In this regard, a multi-layer composite shell reinforced with graphene nano-platelets (GNP) having different patterns of layer configurations is modeled. An outer layer of piezoelectric material receiving external voltage is also attached to the cylindrical shell for the aim of observing the effects of voltage on the thermal buckling condition. The cylindrical shell is mathematically modeled with first-order shear deformation theory (FSDT). Linear elasticity relationship with constant thermal expansion coefficient is used to extract the relationship between stress and strain components. Moreover, minimum virtual work, including the work of the piezoelectric layer, is engaged to derive equations of motion. The derived equations are solved using numerical method to find out the effects of temperature and external voltage on the buckling stability of the shell structure. It is revealed that the boundary condition, external voltage and geometrical parameter of the shell structure have notable effects on the temperature rise required for initiating instability in the cylindrical shell structure.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.9 no.1
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• pp.59-69
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• 2001

The rotor blade is modeled using a composite box beam with arbitrary wall. The active constrained damping layers are bonded to the upper and lower surfaces of the box beam to provide active and passive damping. A finite element model, based on a hybrid displacement theory, is used in the structural analysis. The theory is capable of accurately capturing the transverse shear effects in the composite primary structure, the viscoelastic and the piezoelectric layers within the ACLs. A reduced order model is derived based on the Hankel singular value. A linear quadratic Gaussian (LQG) controller is designed based on the reduced order model and the available measurement output. However, the LQG control system fails to stabilize the perturbed system although it shows good control performance at the nominal operating condition. To improve the robust stability of LQG controller, the loop transfer recovery (LTR) method is applied. Numerical results show that the proposed controller significantly improves rotor aeromechanical stability and suppresses rotor response over large variations in rotating speed by increasing lead-lag modal damping in the coupled rotor-body system.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.22 no.3
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• pp.267-275
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• 2009

This paper presents the p-convergent coupling element on the basis of the ESSE(equivalent single layer shell element) and the PLLE(partial-linear layerwise element) to analyze laminated composite plates. The ESSE is formulated by the degenerated shell theory, on the other hand, the assumption of the PLLE is piecewise linear variation of the in-plane displacement and a constant value of lateral displacement across the thickness. The proposed finite element model is based on p-convergence approach. The integrals of Legendre polynomials and Gauss-Lobatto technique are chosen to interpolate displacement fields and to implement numerical quadrature, respectively. This study has been focused on the verification of p-convergent element. For this purpose, various finite element multiple models associated with the combination of ESSE and PLLE elements are tested to show numerical stability. The simple examples such as a cantilever beam subjected vertical load and a plate with tension are adopted to evaluate the performance of proposed element.

• Steel and Composite Structures
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• v.25 no.2
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• pp.157-175
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• 2017

The novelty of this work is the use of a new displacement field that includes undetermined integral terms for analyzing thermal buckling response of functionally graded (FG) sandwich plates. The proposed kinematic uses only four variables, which is even less than the first shear deformation theory (FSDT) and the conventional higher shear deformation theories (HSDTs). The theory considers a trigonometric variation of transverse shear stress and verifies the traction free boundary conditions without employing the shear correction factors. Material properties of the sandwich plate faces are considered to be graded in the thickness direction according to a simple power-law variation in terms of the volume fractions of the constituents. The core layer is still homogeneous and made of an isotropic material. The thermal loads are assumed as uniform, linear and non-linear temperature rises within the thickness direction. An energy based variational principle is employed to derive the governing equations as an eigenvalue problem. The validation of the present work is checked by comparing the obtained results the available ones in the literature. The influences of aspect and thickness ratios, material index, loading type, and sandwich plate type on the critical buckling are all discussed.

• International Journal of Control, Automation, and Systems
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• v.1 no.2
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• pp.243-251
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• 2003

In this paper, a new model for mobile soccer robot, a type of linear system, is presented. A controller, consisting of two loops the one of which is the inner state feedback loop designed for stability and plant be well conditioned and the outer loop is a well-known PI controller designed for tracking the reference input, is suggested. Because the plant, the soccer robot, is parameter dependent, it requires the controller to be insensitive to the parameter variation. To achieve this objective, the pole-sensitivity as a pole-variation with respect to the parameter variation is defined and design algorithms for state-feedback controllers are suggested, consisting of two matrices one of which is for general pole-placement and other for parameter insensitive. This paper shows that the PI controller is equivalent to the state feedback and the cost function for reference tracking is equivalent to the LQ cost. By using these properties, we suggest a tuning procedure for the PI controller. We that the control algorithm in this paper, based on the linear system theory, is well work by simulation, and the LPD system modeling and control are more easy treatment for soccer robot.

• Journal of Advanced Navigation Technology
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• v.19 no.6
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• pp.574-580
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• 2015

The stability condition of linear discrete interval systems with a time-varying delay time is considered. The considered system has interval system matrices for both non-delayed and delayed states with time-varying delay time within given interval values. The proposed condition is derived by using Lyapunov stability theory and expressed by very simple inequality. Compared to previous results, the stability issue on the interval systems is expanded to time-varying delay. Furthermore, the new condition can imply the existing results on the time-invariant case and show the relation between interval time-varying delay time and stability of the system. The proposed condition can be applied to find the stability bound of the discrete interval system. Some numerical examples are given to show the effectiveness of the new condition and comparisons with the previously reported results are also presented.

• Journal of Advanced Navigation Technology
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• v.23 no.6
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• pp.577-583
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• 2019

A dynamic system is called positive if any trajectory of the system starting from non-negative initial states remains forever non-negative for non-negative controls. In this paper, we consider the new stability condition for the positive time-varying linear discrete interval systems with time-varying delay and unstructured uncertainty. The delay time is considered as time-varying within certain interval having minimum and maximum values and the system is subjected to nonlinear unstructured uncertainty which only gives information on uncertainty magnitude. The proposed stability condition is an improvement of the previous results which can be applied only to time-invariant systems or had no consideration of uncertainty, and they can be expressed in the form of a very simple inequality. The stability conditions are derived using the Lyapunov stability theory and have many advantages over previous results using the upper solution bound of the Lyapunov equation. Through numerical example, the proposed stability conditions are proven to be effective and can include the existing results.

• Steel and Composite Structures
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• v.18 no.2
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• pp.443-465
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• 2015

In this paper, an efficient and simple trigonometric shear deformation theory is presented for thermal buckling analysis of functionally graded plates. It is assumed that the plate is in contact with elastic foundation during deformation. The theory accounts for sinusoidal distribution of transverse shear stress, and satisfies the free transverse shear stress conditions on the top and bottom surfaces of the plate without using shear correction factor. Unlike the conventional trigonometric shear deformation theory, the proposed sinusoidal shear deformation theory contains only four unknowns. It is assumed that the mechanical and thermal non-homogeneous properties of functionally graded plate vary smoothly by distribution of power law across the plate thickness. Using the non-linear strain-displacement relations, the equilibrium and stability equations of plates made of functionally graded materials are derived. The boundary conditions for the plate are assumed to be simply supported on all edges. The elastic foundation is modelled by two-parameters Pasternak model, which is obtained by adding a shear layer to the Winkler model. The effects of thermal loading types and variations of power of functionally graded material, aspect ratio, and thickness ratio on the critical buckling temperature of functionally graded plates are investigated and discussed.