• Smart Structures and Systems
• /
• 제22권1호
• /
• pp.41-55
• /
• 2018

Damping ratio and frequency have influence on dynamic serviceability or instability such as vortex-induced vibration and displacement amplification due to earthquake and critical flutter velocity, and it is thus important to make determination of damping ratio and frequency accurate. As bridges are getting longer, small scale model test considering similitude law must be conducted to evaluate damping ratio and frequency. Analysis conditions modified by similitude law are applied to experimental test considering different scale ratios. Generally, Nyquist frequency condition based on natural frequency modified by similitude law has been used to determine sampling rate for different scale ratios, and total time length has been determined by users arbitrarily or by considering similitude law with respect to time for different scale ratios. However, Nyquist frequency condition is not suitable for multimode system with noisy signals. In addition, there is no specified criteria for determination of total time length. Those analysis conditions severely affect accuracy of damping ratio. The focus of this study is made on the determination of minimum analysis conditions for different scale ratios. Influence of signal to noise ratio is studied according to the level of noise level. Free initial value problem is proposed to resolve the condition that is difficult to know original initial value for free vibration. Ambient and free vibration tests were used to analyze the dynamic properties of a system using data collected from tests with a two degree-of-freedom section model and performed on full bridge 3D models of cable stayed bridges. The free decay is estimated with the stochastic subspace identification method that uses displacement data to measure damping ratios under noisy conditions, and the iterative least squares method that adopts low pass filtering and fourth order central differencing. Reasonable results were yielded in numerical and experimental tests.

• Journal of Biosystems Engineering
• /
• 제36권6호
• /
• pp.476-483
• /
• 2011

The aerial application using an unmanned helicopter has been already utilized and an attitude controller would be developed to enhance the operational convenience and safety of the operator. For a preliminary study of designing flight controller, a state space model for an RC helicopter would be identified. Frequency sweep flight tests were performed and time history data were acquired in the previous study. In this study, frequency response of the flight test data of a small unmanned helicopter was analyzed by using the CIFER software. The time history flight data consisted of three replications each for collective pitch, aileron, elevator and rudder sweep inputs. A total of 36 frequency responses were obtained for the four control stick inputs and nine outputs including linear velocities and accelerations and angular velocities in 3-axis. The results showed coherence values higher than 0.6 for every primary control inputs and corresponding on-axis outputs for the frequency range from 0.07 to 4 Hz. Also the analysis of conditioned frequency response showed its effectiveness in evaluating cross coupling effects. Based on the results, the dynamic characteristics of the model helicopter can further be analyzed in terms of transfer functions and the undamped natural frequency and damping ratio of each critical mode.

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

• Smart Structures and Systems
• /
• 제11권1호
• /
• pp.19-34
• /
• 2013

The objective of this paper is to develop on-line system parameter estimation and damage detection technique from the response measurements through using the Recursive Covariance-Driven Stochastic Subspace identification (RSSI-COV) approach. To reduce the effect of noise on the results of identification, discussion on the pre-processing of data using recursive singular spectrum analysis (rSSA) is presented to remove the noise contaminant measurements so as to enhance the stability of data analysis. Through the application of rSSA-SSI-COV to the vibration measurement of bridge during scouring experiment, the ability of the proposed algorithm was proved to be robust to the noise perturbations and offers a very good online tracking capability. The accuracy and robustness offered by rSSA-SSI-COV provides a key to obtain the evidence of imminent bridge settlement and a very stable modal frequency tracking which makes it possible for early warning. The peak values of the identified $1^{st}$ mode shape slope ratio has shown to be a good indicator for damage location, meanwhile, the drastic movements of the peak of $2^{nd}$ mode slope ratio could be used as another feature to indicate imminent pier settlement.

• 국제초고층학회논문집
• /
• 제4권1호
• /
• pp.9-25
• /
• 2015

Field measurements of various structures have been conducted for many purposes. Measurement data obtained by field measurement is very useful to determine vibration characteristics including dynamic characteristics such as the damping ratio, natural frequency, and mode shape of a structure. In addition, results of field measurements and modal identification can be used for modal updating of FEM analysis, for checking the efficiency of damping devices and so on. This paper shows some examples of field measurements and modal identification for structural health monitoring. As the first example, changes of dynamic characteristics of a 15-story office building in four construction stages from the foundation stage to completion are described. The dynamic characteristics of each construction stage were modeled as accurately as possible by FEM, and the stiffness of the main structural frame was evaluated and the FEM results were compared with measurements performed on non-load-bearing elements. Simple FEM modal updating was also applied. As the next example, full-scale measurements were also carried out on a high-rise chimney, and the efficiency of the tuned mass damper was investigated by using two kinds of modal identification techniques. Good correspondence was shown with vibration characteristics obtained by the 2DOF-RD technique and the Frequency Domain Decomposition method. As the last example, the wind-induced response using RTK-GPS and the feasibility of hybrid use of FEM analysis and RTK-GPS for confirming the integrity of structures during strong typhoons were shown. The member stresses obtained by hybrid use of FEM analysis and RTK-GPS were close to the member stresses measured by strain gauges.

• 한국지진공학회논문집
• /
• 제13권4호
• /
• pp.47-55
• /
• 2009

본 연구의 목적은 첫째, 비선형성을 포함하는 액체기둥진동흡진기(LCVA)의 감쇠항에 대한 등가선형화된 운동방정식을 바탕으로하여 가진입력인 진동대 가속도와 출력인 제어력의 관계인 전달함수를 해석적인 식으로 규명하는 것이다. 둘째, LCVA의 주요설계변수인 수직기둥과 수평기둥의 단면적비의 변화에 따른 진동특성분석이다. 셋째, 동조의 수단으로 이용되는 수직기둥 액체의 높이를 변화시켜 진동특성을 분석하는 것이다. LCVA를 진동대 위에 설치하고 가진하여 제어력을 측정하여 실험 전달함수를 구하였다. 이것을 해석적인 전달함수와 비교 및 최적화작업을 수행하여 LCVA의 진동특성변수에 영향을 미치는 고유진동수, 감쇠비 및 질량비 등을 파악하였다. 실험결과, 액체 수위 및 단면적비의 변화에 따라 감쇠비 및 참여질량비의 특성이 변화하였다. 수직기둥과 수평기둥이 교차하는 엘보우에서 액체의 흐름 변화로 인하여, LCVA 실험체의 수직기둥 단면적이 작아질수록 감쇠비와 참여질량비가 증가하였다.

• Computers and Concrete
• /
• 제10권3호
• /
• pp.277-294
• /
• 2012

The aim of the study is to determine the modal parameters of a prototype damaged arch dam by operational modal analysis (OMA) method for some damage scenarios. For this purpose, a prototype arch dam-reservoir-foundation model is constructed under laboratory conditions. Ambient vibration tests on the arch dam model are performed to identify the modal parameters such as natural frequency, mode shape and damping ratio. The tests are conducted for four test-case scenarios: an undamaged dam with empty reservoir, two different damaged dams with empty reservoirs, and a damaged dam with full reservoir. Loading simulating random impact effects is applied on the dam to crack. Cracks and fractures occurred at the middle of the upper part of the dams and distributed through the abutments. Sensitivity accelerometers are placed on the dams' crests to collect signals for measurements. Operational modal analysis software processes the signals collected from the ambient vibration tests, and enhanced frequency domain decomposition and stochastic subspace identification techniques are used to estimate modal parameters of the dams. The modal parameters are obtained to establish a basis for comparison of the results of two techniques for each damage case. Results show that approximately 35-40% difference exists between the natural frequencies obtained from Case 1 and Case 4. The natural frequencies of the dam considerably decrease with increasing cracks. However, observation shows that the filled reservoir slightly affected modal parameters of the dam after severe cracking. The mode shapes obtained are symmetrical and anti-symmetrical. Apparently, mode shapes in Case 1 represent the probable responses of arch dams more accurately. Also, damping ratio show an increase when cracking increases.

• 한방비만학회지
• /
• 제8권2호
• /
• pp.63-72
• /
• 2008

Objectives We aimed to explore obesity pattern among overweight and obese Korean adult women using oriental obesity pattern identification questionnaire. Methods This survey was performed using data of 83 overweight and obese women aged from 20 to 55 yrs (BMI ${\geq}\;23\;kg/m^2$ : n=18, BMI ${\geq}\;25\;kg/m^2$ :ｎ=65) in Seoul, from 2007 to 2008. Subjects were given written consent and this study was performed under the permission of institutional review board of Kyung-Hee East-west Neo Medical Center. Results 1. The distribution of oriental obesity pattern identification did not show any differences between obese and overweight group(p>0.05). 2. The ratio of significantly-scored oriental pattern identification was ordered by Stagnation of the liver Qi(肝欝, 21.7%) > Indigestion(食積, 18.1%) > Spleen deficiency(脾虚, 16.9%) > Yang deficiency(陽虚, 14.5%) (n=83). 3. The frequency of top-scored oriental obesity pattern was ordered by Stagnation of the liver Qi(肝欝, 36.1%) > Indigestion(食積, 24.1%) > Yang deficiency (陽虚, 15.7%) (n=83). 4. The frequency of oriental obesity pattern identification was ordered by Stagnation of the liver Qi(肝欝, 41.7%) > Indigestion(食積, 29.2%) > Yang deficiency(陽虚, 12.5%) > Stagnation of the liver Qi and Yang deficiency(肝欝兼陽虚, 8.3%) (n=24). Conclusions In Korean adult overweight and obese women, Stagnation of the liver Qi(肝欝), Indigestion(食積), and Yang deficiency (陽虚) were found to be the main pathology based on oriental obesity pattern identification questionnaire. It suggests that not only physical status but also general condition and emotional problem should be concerned in treatment of obesity. This study could play a role as a preliminary data of oriental obesity pattern identification.

• 한국전자통신학회논문지
• /
• 제8권6호
• /
• pp.923-931
• /
• 2013

RFID(Radio Frequency Identification)는 버스카드, 출입문 카드, 물류유통, 건축자재 관리 등 일상생활에서 다양하게 사용되는 태그의 일종으로 ID정보를 무선 주파수(RF, Radio Frequency)를 사용하여 인식하는 무선인식장치이다. RFID는 투과성과 인식률, 메모리 크기, 다중태그 인식률, 외부 오염 먼지, 외부 충격 등에 따라 크기와 성능이 달라지고, 이와 같은 요소들을 고려한 RFID 보호를 위한 패키징이 필요하다. 현재 RFID는 건축자재의 효과적인 관리를 위해서도 다양하게 사용되고 있는데, 건축자재에 RFID를 부착하기 위해서는 외부로 부터의 충격에 강건한 건축자재용 RFID 패키징 제작이 요구되고 있다. 본 논문에서는 외부 충격에 강하고, 고장 시 RFID의 교체가 가능하도록 패키징 틀과 본체를 분리하여 설계된 건축자재용 RFID 패키징을 제안한다. 제안된 RFID 패키징을 위한 상세한 설계도를 제시하였으며, 3D 프린터를 사용하여 설계된 패키징을 직접 제작하여 성능 평가를 실시하였다.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2002년도 학술대회지
• /
• pp.427-430
• /
• 2002

The present parer outlines the system identification and vibration control performance of air traffic control tower of Yangyang international airport with tuned mass damper(TMD). From the free vibration test, natural frequency, damping ratio and mode shape of tower are obtained and these values are compared with the values from numerical analysis. In the vibration control test to evaluate the vibration control performance, equivalent damping ratio increased by tuned mass damper are obtained in case the TMD is operated as passive mode. Damping ratio of tower evaluated from free vibration test is about $1.0{\%}$. It is very low value than damping ratio recommended in general code. Damping ratio of passive mode is about $5{\%}$. These equivalent damping ratio increased by TMD is enough to enhance the serviceability of tower structure under wind load.