• Journal of Electrical Engineering and Technology
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• 제10권4호
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• pp.1518-1526
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• 2015

For now, there are some low frequency oscillations in the power system which feature low frequency oscillation with positive damping and cannot be explained by traditional low frequency oscillation mechanisms. Concerning this issue, the dynamic damping effect is put forward on the basis of the power-angle curve and the study of damping torque in this article. That is, in the process of oscillation, damping will dynamically change and will be less than that of the stable operating point especially when the angle of the stable operating point and the oscillation amplitude are large. In a situation with weak damping, the damping may turn negative when the oscillation amplitude increases to a certain extent, which may result in an amplitude-increasing oscillation. Finally, the simulation of the two-machine two-area system verifies the arguments in this paper which may provide new ideas for the analysis and control of some unclear low frequency phenomena.

• Geomechanics and Engineering
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• 제2권1호
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• pp.29-44
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• 2010

Vertical vibration tests were conducted using model footings of different size and mass resting on the surface of finite sand layer with different height to width ratios which was underlain by either rigid concrete base, under both dry and saturated condition. The effect of saturation on the damping ratio of finite sand stratum underlain by a rigid base has been verified and compared with the results obtained for the case of finite dry sand stratum underlain by the rigid base. Comparison of results of the experimental study showed that the damping in both the cases is less than 10%. The damping ratio obtained for finite saturated sand stratum is marginally lower than that obtained on finite dry sand stratum at H/B ratio of 0.5. The difference between the two cases becomes significant when the H/B ratio increases to 3.0, indicating the significant influence of soil moisture on damping ratio of foundation- soil system with increase in the thickness of the finite sand stratum. Comparison of the predicted damping ratio for a homogeneous sand stratum with the experimental damping ratio obtained corresponding to the height to width ratio of 3.0 of the finite sand stratum underlain by the rigid concrete base indicates a significant reduction in damping ratio of the foundation-soil system for both the cases.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1997년도 하계학술대회 논문집 D
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• pp.1121-1123
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• 1997

The control strategy for damping analysis of STATCON is derived based on an energy function of the power system equation. The center-of-angle(COA) coordinated system by a simple equivalent physical system is used and the control law for this system is induced for FACTS devices as TCSC and STATCON. Damping results from the transient performance simulations carried out on the model system.

• Structural Engineering and Mechanics
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• 제14권4호
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• pp.417-434
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• 2002

This paper explores the possibility of using a combination of the empirical mode decomposition (EMD) and the Hilbert transform (HT), termed the Hilbert-Huang transform (HHT) method, to identify the modal damping ratios of the structure with closely spaced modal frequencies. The principle of the HHT method and the procedure of using the HHT method for modal damping ratio identification are briefly introduced first. The dynamic response of a two-degrees-of-freedom (2DOF) system under an impact load is then computed for a wide range of dynamic properties from well-separated modal frequencies to very closely spaced modal frequencies. The natural frequencies and modal damping ratios identified by the HHT method are compared with the theoretical values and those identified using the fast Fourier transform (FFT) method. The results show that the HHT method is superior to the FFT method in the identification of modal damping ratios of the structure with closely spaced modes of vibration. Finally, a 36-storey shear building with a 4-storey light appendage, having closely spaced modal frequencies and subjected to an ambient ground motion, is analyzed. The modal damping ratios identified by the HHT method in conjunction with the random decrement technique (RDT) are much better than those obtained by the FFT method. The HHT method performing in the frequency-time domain seems to be a promising tool for system identification of civil engineering structures.

• 한국안전학회지
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• 제29권2호
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• pp.31-37
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• 2014

For the vibration control of residential buildings, a multiple type slim damper system is developed and dynamic performance test is performed in this study. In conventional damping systems, larger installation space is required in order to achieve acceptable seismic performance, and as a result, it is difficult to determine efficient damping capacity of the device. The proposed damping device is composed of several small slim type dampers and linkage units. It can control damping capacity easily by changing the number of the small damper. To evaluate the proposed damping device, three slim type dampers (single-type, triple-type and penta-type) are designed and manufactured in real scale. Dynamic loading tests are performed by using the three manufactured dampers. From the tests, it is shown that damping coefficient is proportional to the number of the damper combined. Thus, test results validates the practicality of the proposed slim type dampers. applying nonlinear curve fitting technique, numerical model of the dampers are developed and presented.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2002년도 춘계학술대회논문집
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• pp.1032-1038
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• 2002

The Active Constrained Layer Damping(ACLO) combines the simplicity and reliability of passive damping with the low weight and high efficiency of active control to attain high damping characteristics. The proposed ACLD treatment consists of a viscoelastic damping which is sandwiched between an active piezoelectric layer and a host structure. In this manner, the smart ACLD consists of a Passive Constrained Layer Damping(PCLD) which is augmented with an active control in response to the structural vibrations. The Arc type shell model is introduced to describe the interactions between the vibrating host structure, piezoelectric actuator and visco damping, The system is modeled by applying ARMAX model and changing a state-space form through the system identification method. An optimum control law for piezo actuator is obtain by LQR(Linear Quadratic Regulator) Method. The performance of ACLD system is determined and compared with PCLD in order to demonstrate the effectiveness of the ACLD treatment, Also, the actuation capability of a piezo actuator is examined experimentally by using various thickness of Viscoelastic Materials(VEM).

• International Journal of Aeronautical and Space Sciences
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• 제14권3호
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• pp.237-246
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• 2013

It is of great significance to utilize a landing mechanism to explore an asteroid. A landing mechanism named ALISE (Asteroid Landing and In Situ Exploring) for asteroid with soft surface is presented. The landing dynamic in the first turning stage, which represents the landing performance of the landing mechanism, is built by a Lagrange equation. Three key parameters can be found influencing the landing performance: the retro-rocket thrust T, damping element damping $c_1$, and cardan element damping $c_2$. In this paper, the retro-rocket thrust T is solved with considering that the landing mechanism has no overturning in extreme landing conditions. The damping element damping c1 is solved by a simplified dynamic model. After solving the parameters T and $c_1$, the cardan element damping $c_2$ is calculated using the landing dynamic model, which is built by Lagrange equation. The validities of these three key parameters are tested by simulation. The results show a stable landing, when landing with the three estimated parameters T, $c_1$, and $c_2$. Therefore, the landing dynamic model and methods to estimate key parameters are reasonable, and are useful for guiding the design of the landing mechanism.

• Structural Engineering and Mechanics
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• 제24권3호
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• pp.375-393
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• 2006

Under high velocity, pulse type near source earthquakes semi-active control systems are very effective in reducing seismic response base isolated structures. Semi-active control systems can be classified as: 1) independently variable stiffness, 2) independently variable damping, and 3) combined variable stiffness and damping systems. Several researchers have studied the effectiveness of independently varying damping systems for seismic response reduction of base isolated structures. In this study effectiveness of a combined system consisting of a semi-active independently variable stiffness (SAIVS) device and a magnetorheological (MR) damper in reducing seismic response of base isolated structures is analytically investigated. The SAIVS device can vary the stiffness, and hence the period, of the isolation system; whereas, the MR damper enhances the energy dissipation characteristics of the isolation system. Two separate control algorithms, i.e., a nonlinear tangential stiffness moving average control algorithm for smooth switching of the SAIVS device and a Lyapunov based control algorithm for damping variation of MR damper, are developed. Single and multi degree of freedom systems consisting of sliding base isolation system and both the SAIVS device and MR damper are considered. Results are presented in the form of nonlinear response spectra, and effectiveness of combined variable stiffness and variable damping system in reducing seismic response of sliding base isolated structures is evaluated. It is shown that the combined variable stiffness and variable damping system leads to significant response reduction over cases with variable stiffness or variable damping systems acting independently, over a broad period range.

• Structural Engineering and Mechanics
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• 제19권2호
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• pp.141-151
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• 2005

There are several ways of decreasing the vibration energy of structures. One of which is special damping layers made of various viscoelastic materials are widely applied in structures subjected to dynamic loading. In this study, a cantilever beam, partially covered by damping a constraining layers, is investigated by using Finite Element method (FEM). The frequency and system loss factor are evaluated. The effects of different physical and geometrical parameters on the natural frequency and system loss factors are discussed.

• Elastomers and Composites
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• 제20권3호
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• pp.189-204
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• 1985

The purpose of this study is to examine the dynamic properties, especially the damping and fatigue properites, of NR vulcanizates and to find out the vulcanizate which can be used as damping materials in industry. The results of this study can be summarized as follows: 1. In the ODR test, the S-2 vulcanizate was the fastest one in terms of having reached to optimum cure times($t_{90}$) and, with the same formula, when 50phr of HAF carbon black loaded, the shortest optimum cure times has shown. 2. The S - 2 vulcanizate was the best than the others in the physical properties. In aging properties, however, the E - 1 vulcanizate appeared to be better than the other vulcanizates. 3. The results of the RDS test for the NR vulcanization system under the condition of 0.1% strain amplitude and 1 Hz frequency showed no connection between Tg and the the nature of the crosslinking system, but 50phr loading of HAF carbon black increased Tg. The damping values of vulcanizates in the elastic region showed a strong relations the damping values and the crosslinking system. The S - 2 vulcanizate with higher crosslink density had lower damping values than other vulcanizates, and furthermore, the SH - 2 vulcanizates with 50phr loading of HAF carbon black increased the damping values. 4. The Goodrich Flexometer test showed that the heat buildup for the gum NR vulcanizates was less than for those which contained 50phr of HAF carbon black. In particular, the fatigue life of the vulcanizate with lower damping values appeared to be longer than that of the vulcanizates with higher damping values. In the fatigue test, the increasment of the loading or the temperature, applied to the vulcanizates, appeared to be shorten their fatigue life.