• 대한기계학회논문집A
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• 제26권4호
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• pp.764-774
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• 2002

Dynamic behaviors are analyzed for an automatic ball balancer (ABB) with triple races, which is a device to reduce the unbalanced mass of optical disk drives (ODD) such as CD-ROM or DVD drives. The nonlinear equations of motion are derived by using Lagrange's equations with the polar coordinate system. It is shown that the polar coordinate system provides the complete stability analysis while the rectangular coordinate system used in other previous studies has limitations on the stability analysis. For the stability analysis, the equilibrium positions and the linearized perturbation equations are obtained by the perturbation method. Based on the linearized equations, the stability of the system is analyzed around the equilibrium positions; furthermore, to confirm the stability, the time responses for the nonlinear equations of motion are computed by using a time integration method and experimental analyses are performed. Theoretical and experimental results show a superiority of the ABB with triple races.

• 한국의학물리학회지:의학물리
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• 제13권3호
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• pp.109-113
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• 2002

본 연구에서는 의료용선형가속기의 기기점검을 위한 일목요연하고 장기간의 통계처리가 가능한 새로운 정도관리 양식을 개발하였다. 개발된 양식을 2001년 8월부터 12월까지 국내의 6개의 기관에 적용하여 그 양식의 효율성을 평가하였다. 기존의 점검 양식은 단순히 선형가속기에 숫자로 표시되는 값들(digital data)과 측정장치를 이용하여 관찰된 값들(mechanical data)의 비교에 목적을 두고 있고, 각 항목들이 서술적으로 나열되어 있어 공간적으로 문제점을 파악하기 어렵다. 더욱이 기기의 장기간에 걸친 오차의 변화를 일목요연하게 알 수가 없다. 그러나 새로 개발된 점검 양식을 사용한다면, 디지털 데이터와 기계적 데이터의 비교는 물론 기기의 오차를 공간적으로 파악할 수 있게 될 것이다. 또한 기기의 장기간에 걸친 오차의 범위를 확인하게 되어 기기의 장기적 안정성을 알 수 있게 될 것이다. 뿐만 아니라 현실에 맞는 항목을 선택함으로써 기계적 정도관리(mechanical QA)를 수행하는 시간을 단축함과 동시에, 각 기관이 보유하고 있는 선형가속기의 장기간에 걸친 정도관리를 통해 얻은 데이터를 이용하여 치료장비의 오차에 대한 통계적, 물리적 데이터를 정도관리 직후 분석할 수 있게 되어 방사선 치료에 있어서 매우 중요한 요소인 방사선 치료장비의 기기적 정확성을 높일 수 있게 될 것이다.

• Structural Engineering and Mechanics
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• 제66권5호
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• pp.583-594
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• 2018

A theoretical formulation based on the linearized potential theory, the Descartes' rule and the extremum optimization method is presented to calculate the critical distance of lifting points of the fully balanced hoist vertical ship lift, and to study pitching stability of the ship lift. The overturning torque of the ship chamber is proposed based on the Housner theory. A seven-free-degree dynamic model of the ship lift based on the Lagrange equation of the second kind is then established, including the ship chamber, the wire rope, the gravity counterweights and the liquid in the ship chamber. Subsequently, an eigenvalue equation is obtained with the coefficient matrix of the dynamic equations, and a key coefficient is analyzed by innovative use of the minimum optimization method for a stability criterion. Also, an extensive influence of the structural parameters contains the gravity counterweight wire rope stiffness, synchronous shaft stiffness, lifting height and hoists radius on the critical distance of lifting points is numerically analyzed. With the Runge-Kutta method, the four primary dynamical responses of the ship lift are investigated to demonstrate the accuracy/reliability of the result from the theoretical formulation. It is revealed that the critical distance of lifting points decreases with increasing the synchronous shaft stiffness, while increases with rising the other three structural parameters. Moreover, the theoretical formulation is more applicable than the previous criterions to design the layout of the fully balanced hoist vertical ship lift for the ensuring of the stability.

• 대한기계학회논문집
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• 제19권12호
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• pp.3344-3351
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• 1995

The stability of diffusion flame formed behind a bluff body with fuel injection slits was experimentally investigated in various fuel injection angles, fuel injection ratios, grids and extension ducts. The flame stability limits, temperature distributions and length of recirculation zones, direct photographs of flames were measured in order to discuss the stabilization mechanism of the diffusion flame. The results from this study are as follows. The fuel injection angle is an important factor in determining the flame stability. Stability limits can be improved by variety of the fuel injection ratio. When the grid and extension duct are set, stability characteristics are varied with the blockage ratios, grid intervals, and grid numbers. The recirculation zone not only serves as a steady ignition source of combustion stream but also governs the stabilization mechanism.

• 대한기계학회논문집A
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• 제26권6호
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• pp.1035-1046
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• 2002

Time Delay Control(TDC) is a robust nonlinear control scheme using Time Delay Estimation(TDE) and also has a simple structure. To apply TDC to a real system, we must design Time Delay Controller to guarantee stability. The earlier research stated sufficient stability condition of TDC for general plants. In that research, it was assumed that time delay is infinitely small. But, it is impossible to implement infinitely small time delay in a real system. So, in this research we propose a new sufficient stability condition of TDC for general plants with finite time delay. And the simulation results indicate that the previous sufficient stability condition does not work even for small time delay, while our proposed condition works well.

• Tribology and Lubricants
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• 제13권1호
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• pp.28-33
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• 1997

Results of theoretical investigations of the stability characteristics of externally pressurized air journal bearing, of which bearing is synchronously vibrate with respect to rotor, are presented. Linearized perturbation method is used to get the dynamic coefficients of air bearing, and the Routh-Hurwitz criterion is used to obtain stability map. The stability characteristics operating at zero steady-state eccentricity is investigated of various phase difference of bearing to rotor. It is shown that stability of air bearing is greatly influenced by synchronous motion of bearing, there exists optimum phase difference which gives maximum stability threshold.

• 대한기계학회논문집
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• 제18권9호
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• pp.2284-2296
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• 1994

Force control of robotic mechanisms continues to be a challenging area. Previous implementation have seldom produced satisfactory results, and researchers in the past have experienced significant instability problems associated with their force controllers. In this study, a new stability factor in force control will be pointed out. When a manipulator is constrained to an environment(force-controlled), geometric instability due to the relationship between the manipulator configuration and the force-controlled direction is shown to be a significant factor in overall system stability. This exploratory study points out a rather intuitive, geometrically based stability factor in terms of an effective system stiffness and analyzes the phenomenon both analytically and graphically. Also, a stiffness control algorithm using the kinematic redundancy of a kinematically redundant manipulator is proposed to improve the overall stability in force control.

• 대한기계학회논문집
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• 제19권3호
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• pp.820-833
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• 1995

An analysis is performed to examine the equilibrium state and the stability of modeled Reynolds stress equations for homogeneous turbulent shear flows. The system of the governing equations consists of four coupled ordinary differential equations. The equilibrium states are found by the steady state solution of the governing equations. In order to investigate the stability of the system about its state in equilibrium, and eigenvalue problem is constructed. As a result, constraints for the coeffieients in the model equations are obtained by the stability condition of the equilibrium state as well as by their physically realizable bounds. It is observed that the models with pressure-strain rate correlation that are linear in the anisotropy tensor are stable and produce reasonable equilibrium tensor do not behave properly. Stability considerations about three most commonly used models are given in detail in the final section.

• 대한기계학회논문집A
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• 제24권3호
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• pp.543-549
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• 2000

Results of theoretical investigations of the stability characteristics of an actively controlled externally pressurized air journal bearing are presented. Proportional control and derivative control are used for the control algorithm of active air bearing. The stability characteristics of the actively controlled bearing operating at zero steady-state eccentricity is investigated with the step jump method. The speed at onset of instability is raised for both proportional control and derivative control of bearing. Proportional control increased the stability threshold without affecting the whirl ratio. But for derivative control of bearing, stability threshold increase is accompanied by a parallel reduction of the whirl ratio. Results show active control of bearing can be adopted for the stability improvement of air journal bearing.

• 대한기계학회논문집A
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• 제28권2호
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• pp.143-148
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• 2004

This paper discussed on the stability of elastic material subjected to dry friction force for low boundary conditions: clamped free, clamped-simply supported, simply supported-simply supported, clamped-clamped. It is assumed in this paper that the dry frictional force between a tool stand and an elastic material can be modeled as a distributed follower force. The friction material is modeled for simplicity into a Winkler-type elastic foundation. The stability of beams on the elastic foundation subjected to distribute follower force is formulated by using finite element method to have a standard eigenvalue problem. It is found that the clamped-free beam loses its stability in the flutter type instability, the simply supported-simply supported beam loses its stability in the divergence type instability and the other two boundary conditions the beams lose their stability in the divergence-flutter type instability.