• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1488-1493
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• 2004

The purpose of the present work is to develop a compressed air discharging system to eject a projectile from the underwater. For the flow analysis of compressed air tank, projectile ejection tube, and pipe system, the air is assumed as an ideal gas, undergoing 1-dimensional axisymmetric, compressible flow, the Fanno flow analysis was applied. The commercial Fluent code was used to solve 3-D Navier-Stokes equation of the internal flow within the valve. The dynamics of the projectile within the ejection tube was assumed 1-degree of freedom. The calculations were performed to four cases of valve opening area ratio, i.e., 25%, 50%, 75%, and 100% opening area, at both depths of 10m and 50m. The results were shown as the figures of time variation of pressure of the compressed air tank and projectile ejection tube. The velocity and distance of the projectile were also predicted.

• 제어로봇시스템학회:학술대회논문집
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• 2004.08a
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• pp.1424-1429
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• 2004

In this paper a mathematical framework for deriving acceleration bounds from given joint torque limits of multiple cooperating robots are described. Especially when the different frictional contacts for every contact are assumed and the torque limits are given in 2-norm sense, we show that the resultant geometrical configuration for the acceleration is composed of corresponding parts of ellipsoids. Since the frictional forces at the contacts are proportional to the normal squeezing forces, the key points of the work includes how to determine internal forces exerted by each robot in order not to cause slip at the contacts while the object is carried by external forces. A set of examples composed of two robot systems are shown with point-contact-with-friction model and insufficient or proper degree of freedom robots.

• Structural Engineering and Mechanics
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• v.2 no.3
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• pp.245-256
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• 1994

A new axisymmetric crack model is proposed on the basis of p-version of the finite element method limited to theory of small scale yielding. To this end, axisymmetric stress element is formulated by integrals of Legendre polynomial which has hierarchical nature and orthogonality relationship. The virtual crack extension method has been adopted to calculate the stress intensity factors for 3-D axisymmetric cracked bodies where the potential energy change as a function of position along the crack front is calculated. The sensitivity with respect to the aspect ratio and Poisson locking has been tested to ascertain the robustness of p-version axisymmetric element. Also, the limit value that is an exact solution obtained by FEM when degree of freedom is infinite can be estimated using the extrapolation equation based on error prediction in energy norm. Numerical examples of thick-walled cylinder, axisymmetric crack in a round bar and internal part-thorough cracked pipes are tested with high precision.

• Journal of Institute of Control, Robotics and Systems
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• v.10 no.10
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• pp.887-898
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• 2004

In this paper a mathematical framework fur deriving acceleration bounds from given joint torque limits of multiple cooperating robots are described. Especially when the different frictional contacts for every contact are assumed and the torque limits are given in 2-norm sense, we show that the resultant geometrical configuration for the acceleration is composed of corresponding parts of ellipsoids. Since the frictional forces at the contacts are proportional to the normal squeezing forces, the key points of the work includes how to determine internal forces exerted by each robot in order not to cause slip at the contacts while the object is carried by external forces. A set of examples composed of two robot systems are shown with point-contact-with-friction model and insufficient or proper degree of freedom robots.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 1997.04a
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• pp.225-232
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• 1997

A study of effects of design parameters on the seismic responses of base isolated structure is performed to reduce the seismic responses using a linear tw0-degree of freedom system and a lumped-mass model of a nuclear power p;ant(NPP). From the simplified 2 DOF system the optimal isolation frequency being less than 1/10th of the fundamental frequency of superstructure is obtained, and the isolator damping minimizing the peak acceleration depends on superstructure frequency. From the time history analyses for lumped mass model of NPP the optimal damping is calculated as 40% in containment building and 65% in reactor internal structure. Similar results are obtained in 2 DOF system

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2001.06a
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• pp.265-270
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• 2001

Hip joint simulator which Is an essential device for evaluating the wear of biomaterials (ultrahigh molecular weight polyethylene, Co-Cr alloy, alumina, etc.) used in total hip joint replacement was developed. This hip joint simulator mimics the joint motion and joint loading of human gait by adapting the 4 degree of freedom in kinematic motion (flexing/extension, adduction/abduction, Internal rotation/external rotation) and axial loading, Four stations are operated by 8 servo-motors and harmony drives. Joint leading was imposed by displacement control from a ball screw, LM guide, and spring system. Each kinematic link system operates separately or coupled modes. A heater and a thermocouple were installed for keeping the body temperature in each station.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1994.10a
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• pp.35-39
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• 1994

One of the important characteristics of the response of nonlinear systems is the existence of subharmonic resonances. When some conditions in parameter space are satisfied. It is possible even in the presence of damping for a periodically excited nonlinear system to possess a response which is the combination of a contribution at the excitation frequency and a component at the system natural frequency. The system natural frequency being a submultiple of the excitation frequency implies that the resulting response is a subharmonic oscillation. In general, there also co-exists, for the system, a response at the excitation frequency, and initial conditions determine which of the steady-state responses is achieved in an experiment or a numerical simulation. In single-degree-of-freedom systems with harmonic excitation, depending on the type of the nonlinearity, e.g., cubic or quadratic the frequency of subharmonic response is respectively, one-third or one-half of that of the excitation frequency. Although subharmonic resonance is one of the principal characteristics of a nonlinear system the subharmonic responses of structures in the presence of internal resonances have been studied very rarely. In this work, we consider subharmonic responses in the two-mode approximation of the plate equations. It is assumed that the two modes are in one-to-one internal resonance. Constant and periodic steady-state solutions of the averaged equations are studied. Finally, the results of direct time integration of the original equations of motion are presented and compared with those obtained from the averaged equations.

• Wind and Structures
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• v.16 no.1
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• pp.93-115
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• 2013

This paper describes a study of the influence of a dynamically flexible building structure on pressures inside and net pressures on the roof of low-rise buildings with a dominant opening. It is shown that dynamic interaction between the flexible roof and the internal pressure results in a coupled system that is similar to a two-degree-of-freedom mechanical system consisting of two mass-spring-damper systems with excitation forces acting on both the masses. Two resonant modes are present, the natural frequencies of which can readily be obtained from the model. As observed with quasi-static building flexibility, the effect of increased dynamic flexibility is to reduce the first natural frequency as well as the corresponding peak value of the admittance, the latter being the result of increased damping effects. Consequently, it is found that the internal and net roof pressure fluctuations (RMS coefficients) are also reduced with dynamic flexibility. This model has been validated from experiments conducted using a cylindrical model with a leeward end flexible diaphragm, whereby good match between predicted and measured natural frequencies, and trends in peak admittances and RMS responses with flexibility, were obtained. Furthermore, since significant differences exist between internal and net roof pressure responses obtained from the dynamic flexibility model and those obtained from the quasi-static flexibility model, it is concluded that the quasi-static flexibility assumption may not be applicable to dynamically flexible buildings. Additionally, since sensitivity analyses reveal that the responses are sensitive to both the opening loss coefficient and the roof damping ratio, careful estimates should therefore be made to these parameters first, if predictions from such models are to have significance to real buildings.

• Coupled systems mechanics
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• v.7 no.6
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• pp.649-668
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• 2018

In this paper, we present a numerical model for fluid-structure interaction between structure built of porous media and acoustic fluid, which provides both pore pressure inside porous media and hydrodynamic pressures and hydrodynamic forces exerted on the upstream face of the structure in an unified manner and simplifies fluid-structure interaction problems. The first original feature of the proposed model concerns the structure built of saturated porous medium whose response is obtained with coupled discrete beam lattice model, which is based on Voronoi cell representation with cohesive links as linear elastic Timoshenko beam finite elements. The motion of the pore fluid is governed by Darcy's law, and the coupling between the solid phase and the pore fluid is introduced in the model through Biot's porous media theory. The pore pressure field is discretized with CST (Constant Strain Triangle) finite elements, which coincide with Delaunay triangles. By exploiting Hammer quadrature rule for numerical integration on CST elements, and duality property between Voronoi diagram and Delaunay triangulation, the numerical implementation of the coupling results with an additional pore pressure degree of freedom placed at each node of a Timoshenko beam finite element. The second original point of the model concerns the motion of the outside fluid which is modeled with mixed displacement/pressure based formulation. The chosen finite element representations of the structure response and the outside fluid motion ensures for the structure and fluid finite elements to be connected directly at the common nodes at the fluid-structure interface, because they share both the displacement and the pressure degrees of freedom. Numerical simulations presented in this paper show an excellent agreement between the numerically obtained results and the analytical solutions.

• Journal of Institute of Control, Robotics and Systems
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• v.6 no.7
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• pp.559-567
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• 2000

This paper proposes simple and practical methods in order to overcome complex formulation and heavy computational burden of the forward kinematics of 6 앨 3-6 type parallel manipulators. Three ap-proaches for the forward kinematics are presented : one extra sensor a modified structure and novel numerical method. The proposed methods are applied to the forward kinematics of a new 6 앨 parallel manipulator with special geometry that has three internal links three external links and a moving platform of a cone shape. The proposed methods use three tetrahedrons for finding the position and orientation vector of the moving platform. The main advantages of the appraches using tetrahedrons are to abbreviate the formulation to easily find so-lutions of the forward kinematics and to be able to practically control of the manipulator in real time.