Output-only analysis of structures with closely spaced poles

In the framework of the operational modal analysis, several approaches have been developed for estimating the modal parameters, i.e., natural frequencies, damping ratios, and mode shapes. Specifically, a technique capable to evaluate the biased (i.e., unscaled by a constant or an almost constant function) frequency response functions, FRFs, has been proposed. Assuming that only the responses of the structure are disposable, the technique allows one to estimate biased FRFs starting from the power spectral densities, PSDs, and applying the Hilbert transform. This paper deals with the estimates of the modal analysis parameters mentioned above. It is possible to obtain each single mode shape, from the singular vectors achieved by applying the singular value decomposition to the FRF matrix evaluated at the spectral line corresponding to the selected natural frequency. A special attention will be devoted to structures with coupled modes, i.e., closely spaced modes. Once the FRFs have been obtained, the natural frequencies and damping ratios could be achieved either in the frequency domain or in the time domain. Experimental tests, carried out on beams, plates and on the AB-204 helicopter blade, will be presented.     

IDENTIFICATION OF TIME-VARYING MODAL PARAMETERS USING LINEAR TIME-FREQUENCY REPRESENTATION

A new method of parameter identification based on linear time-frequency representation andHilbert transform is proposed to identify modal parameters of linear time-varying systems frommeasured vibration responses. Using Gabor expansion and synthesis theory measured responses arerepresented in the time-frequency domain and modal components are reconstructed by time-frequencyfiltering. The Hilbert transform is applied to obtain time histories of the amplitude and phase angle ofeach modal component, from which time-varying frequencies and damping ratios are identified. The     

Identification of the beat characteristics and damping ratios of the Hwacheon World Peace Bell using wavelet transform

In this paper, a new measurement technique is proposed to identify the beat characteristics and modal damping ratios of a Korean bell in the casting field. The beat response caused by the mutual interference of mode pair with very close doublet frequency is unique feature of the Korean bell and should be accurately measured in order to quantitatively estimate the bell sound. However, the conventional method based on a filtering concept such as the Fourier transform has difficulty in extracting the beat frequencies and modal damping ratios because the method should individually decompose the measured signal into each mode. The aim of this paper is to propose an effective measurement method to identify the beat frequencies, mode pairs and modal damping ratios using the continuous wavelet transform in a real striking condition. The proposed method is verified with the Hwacheon World Peace Bell cast in the year 2008, which is the largest bell in Korea. In the future, the proposed method can be applicable to the casting field of the Korean bell to effectively estimate its beat characteristics and damping characteristics.     

振动模态固有频率和阻尼比的EMD识别方法

针对机械系统固有频率和阻尼比的识别问题,提出了基于经验模式分解(EMD)的模态参数识别方法.该方法首先对脉冲激励下机械系统的位移响应进行了EMD分解,确定与该系统的各阶模态对应的固有模式函数(IMF),分别对各阶IMF进行希尔伯特变换以得到各自的瞬时幅值和瞬时相位曲线,并对所得曲线进行线性拟合,最后根据拟合曲线的参数来...     

Unsupervised identification of arbitrarily-damped structures using time-scale independent component analysis: Part I

In this study, a new method is proposed to identify the dynamic parameters of structures with higher accuracy compared to current methods. First, the wavelet-transformed representation of system responses is extracted from measured responses, and then the independent component analysis is used to achieve the modal characteristics. The simulation results of a multi-degree-of-freedom system illustrate that this method is capable of accurately identifying the modal information of lightly- and highly-damped structures. It is represented that continuous wavelet transform, due to its adaptive time-frequency resolution, is more efficient to be incorporated into independent component analysis compared to Short time Fourier transform (STFT). The latter is unable to accurately determine the modal response, especially at higher frequencies, while the proposed method can identify the system with marked accuracy. The efficiency of proposed method is also investigated under additive noise. Results shown that for highly- and lightly- damped system, the proposed method is able to capture the modal parameters especially in higher frequencies of vibration, along with the modal assurance criterion values with satisfactory accuracy, which indicates the robustness of the procedure compared to other available methodologies.     

Measurements of dynamic properties of a medium span suspension bridge by using the wavelet transforms

The aim of this paper is to show the capabilities of the real-time kinematic (RTK) global positioning network system (GPS) to measure the low-frequency vibration of a medium span suspension bridge. In particular, this paper presents the results of studies conducted on the identification of modal parameters including natural frequencies, damping coefficients and mode shapes of a suspension bridge using ambient excitation loads. A real-time kinematic (RTK) global positioning system (GPS) was designed and installed on the Nottingham Wilford Bridge to provide long-term and real-time measurement of bridge deck movement. An approach to estimate modal parameters, from only output data in the time domain using the wavelet transform, is presented. Displacements responses of the bridge are used in the wavelet transform to identify its dynamic characteristics. The modal properties were extracted using a two-step methodology. In the first step, the random decrement method was used to transform random signals in free vibration responses. Secondly, a wavelets-based technique was used to extract natural frequencies and to determine the mode shapes of the structure. This method was compared with the well-established techniques eigensystem realisation algorithm showing a difference of 1% in the estimated first natural frequency.The efficiency of RTK–GPS was demonstrated in the full-scale measurement. In particular, the results showed that the RTK–GPS data can be used for extracting modal properties from in-service-loads induced low-frequency vibration (<5 Hz) by processing the signal with the wavelets transform.     

Direct identification of non-proportional modal damping matrix for lumped mass system using modal parameters

Several damping materials have been employed to reduce the vibration of marine structures. In this paper, a new method of identifying system matrices for non-proportional damping structures using modal parameters is proposed. This method has two advantages. First, the mass and stiffness matrices do not need to be calculated using the FEM, so errors due to the inaccuracy of these matrices can be reduced. Second, various indirect methods can be used to identify modal parameters such as natural frequencies, modal damping ratios and mode shapes. Three case studies of lumped mass systems with non-proportional damping are carried out to verify the performance of the proposed method in this study.     

桥梁工作模态分析中阻尼比识别的离散性研究

准确识别阻尼比一直是桥梁结构模态参数识别的难题。为研究工作模态分析中识别的阻尼比离散性问题,总结了现有的代表性的频域、时域和时频分析的阻尼比识别方法,指出了各种方法导致识别结果不准确的原因。以一个预应力混凝土连续梁桥的工作模态分析为例,分析了阻尼比识别的结果,研究了减小识别的阻尼比离散性的方法。结果表明:相对频率而言,阻尼比识别结果离散程度较高;在混合自由振动响应的情况下,通过增加采样时间,能改善阻尼比识别离散较大的问题,提高识别精度;利用振动水平较低的随机振动响应识别的阻尼比离散性较小。     

Improved independent component analysis based modal identification of higher damping structures

Structural modal parameter identification under ambient excitation has strong engineering value and theoretical significance. As the most popular tool for solving Blind Source Separation (BSS) problems, Independent Component Analysis (ICA) is able to directly extract the time-domain modal parameters, including frequencies, damping ratios and modal shapes. ICA, however, has a fatal flaw of failing to identify structures with higher damping. To overcome the flaw above, the paper proposes a new method named “ICA + IDT”. Firstly, free vibration response of a structure is obtained from structural outputs under ambient excitation. Inverse damping transfer (IDT) is employed to turn a highly damped signal into a low damping response signal without changing of frequencies and mode shapes. Then, structural modal parameters are extracted from the low damping response signal by ICA. Finally, the identified damping ratios are adjusted to eliminate the impact of IDT. To verify the effectiveness and applicability of IDT + ICA proposed herein, two numerical simulations—mass-spring model and simply supported concrete beam—and an experiment model of three-story steel frame are built, and the analysis results reveal that presented method can identify structures with higher damping effectively.     

基于递归理论的泵站压力管道运行状态监测

为了准确地识别建筑结构的模态参数,提出了一种基于多重信号分类算法（multiple signal classification,简称MUSIC）、经验小波变换（empirical wavelet transform,简称EWT）和同步提取小波变换（synchroextracting transform ,简称SET）的结构模态参数识别方法。首先,通过MUSIC-EWT对实测振动信号进行分解;其次,使用SET对单模态信号进行去噪处理;然后,采用自然环境激励技术（natural excitation technique,简称NExT）得到单模态信号的自由衰减响应;最后,利用Hilbert变换（hilbert transform,简称HT）和曲线拟合获得结构的自振频率和阻尼比。通过三层框架结构的数值模拟验证了该方法的准确性和鲁棒性。利用该方法对台风“达维”作用下广州中信广场的实测加速度数据进行分析,并将估计的结构模态参数和其他识别方法的分析结果进行对比,进一步证明了该方法的准确性和鲁棒性。     

基于递归理论的泵站压力管道运行状态监测

针对不利因素导致的管道运行异常问题,提出一种基于递归理论的泵站管道运行状态监测方法。首先,通过振动传感器提取压力管道关键部位的实测信息,并将同一位置不同方向的数据信息进行融合,得到一组反映结构整体动力特性的综合数据;其次,利用伪近临法与互信息法分别选取相空间重构参数m和τ;最后,绘制并计算代表管道动力特性的递归图及递归量化指标。将该方法应用于景泰川工程二期七泵站管道运行监测,通过设置不同的运行工况进行验证,结果表明：机组开关瞬间与稳定运行工况下,管道结构振动信号的递归图呈现不同模式,递归量化指标-确定性、对角线平均长度L、递归率及递归熵也呈现明显差异,能有效区分管道振动状态。该方法为压力管道的无损动态监测提供了新思路。     

Ambient vibration test of building base slab for different ground conditions

It is very important to know the dynamic behavior of base slabs of buildings in earthquake prone areas. The slabs are generally assumed as a rigid diaphragm in structural analysis and design. But their response is considerably affected from ground condition. In this study, the dynamic characteristics of a scaled base slab are determined for different ground conditions. The dimensions of the base slab model are 150 cm long, 100 cm wide and 5 cm thick. This model is designed to be smaller than 20 times from an actual building base. The base slab model is tested for the ground conditions of sand, gravel and clay–silt mixture by ambient vibration test. The dynamic characteristics, such as natural frequencies, modal damping ratios, mode shapes, are identified from collected signals by Operational Modal Analysis method. The first five modes are taken into consideration to compare the effect of ground conditions on the dynamic characteristics. It is observed that the natural frequencies, mode shapes and modal damping ratios are considerably affected from ground conditions. The highest natural frequency is obtained for the gravel ground case. The lowest natural frequency is identified for the clay–silt mixture ground case. The mode shapes changed randomly in each case. But the main modal behaviors are vertical mode and bending modes.     

随机减量法在斜拉桥拉索模态参数识别中的应用

斜拉桥拉索模态参数(固有频率、阻尼比)在索力测试、拉索减振、实时控制等方面起着重要作用。文中利用随机减量技术从岳阳洞庭桥环境激励下拾取的拉索的加速度响应中分离出自由衰减振动加速度响应信号，将获取的加速度响应信号表达为一理论形式，综合运用参数识别、最优估计理论，识别出拉索的模态参数，与理论值吻合良好。本文方法简单，试验容易实施，具有工程实用价值。数字仿真与工程测试结果表明了方法的有效性和可行性。     

Output-only modal analysis of wind turbine tower based on vibration response under emergency stop

The identification technique of output-only modal parameters is proposed for the large wind turbine tower under emergency stop. Compared with the response of regular operating conditions, the immediate tower structural response under emergency stop much more resembles a state of free vibration, which is more appropriate for the modal identification of the wind turbine tower. The vibration response is measured in the nacelle, which is easy to perform in the field modal test. The variational mode decomposition (VMD) is applied to decompose the vibration response into several band-limited intrinsic mode functions. The free responses of decomposed functions are extracted by applying the random decrement technique (RDT). Finally, the modal damping ratio and natural frequency are identified from each free modal response by using the Hilbert transform method. Simulations and a 1.5 MW wind turbine field modal test results verify the effectiveness of the proposed identification method. The main modal parameters of wind turbine, including weak modes, are effectively extracted by using output-only vibration responses under emergency stop. The modal parameter identification method is provided for the large wind turbine structure under the engineering condition.     

基于小波变换方法的材料小阻尼识别

基于连续小波变换实现了对振动系统的小阻尼识别。该方法基本原理为通过对系统自由衰减响应的小波系数取极大获取小波脊,借助小波脊理论确定系统的各阶模态频率与指数衰减系数iζωni,采取求中值的方法获取衰减稳定段的参数,进而识别出各阶模态阻尼比。与HT法相比,该方法具有较高的稳定性和识别精度,仿真结果表明,无噪声时识别误差为0.023%,30%噪声时识别的最大误差为3.49%,而且研究中发现阻尼识别的误差与信号采样频率相关。进行了LY12材料的阻尼测试试验,根据试验测试数据对阻尼识别的结果,证明了本文方法的有效性。     

相机结构系统的试验模态分析

介绍了锤击法试验模态分析技术在某卫星相机光机结构系统动态特性研究中的应用;阐述了多自由度系统模态分析的理论基础和模态参数识别原理;测量分析得到了相机整机结构和关键部件的固有频率,以及与之相对应的模态振型和模态阻尼,并对模态数据及其可能对相机工作的影响作了分析,在此基础上,给出了相机强度和振动试验的载荷谱,保证了试验的安全顺利进行;最后就相机结构设计问题作了讨论。     

Identification of modal parameters using the wavelet transform

The wavelet transform is used as a time-frequency representation for the determination of modal parameters such as natural frequencies, damping ratios and mode shapes of a vibrating system. It is shown that using a particular form of the son wavelet function, results are improved compared to those obtained with the traditionally Morlet wavelet function. The accuracy of this new technique is confirmed by applying it to a numerical example and to ambient vibration measurements of a tower excited by wind.     

Smoothing of power spectral densities by successive autocorrelation: An application to flight flutter tests

A simple modal parameters mxtraction method (natural frequencies and damping ratios) from response power spectral densities (PSD) due to random excitations is presented. Firstly, the noisy PSD are smoothed using the ‘successive autocorrelation method’. The inverse of the resulting smoothed displacement-PSD is then fitted in the frequency domain using a hysteretically damped sdof receptance model.The validity and efficiency of the method is shown using a small plate. The method is then applied to the delicate flight flutter testing problem.     

Modal identification of linear non-proportionally damped systems by wavelet transform

A time-frequency identification technique based on wavelet transform is formulated and applied to free-decay responses of linear systems with non-proportional viscous damping. The Cauchy mother wavelet is used. Frequencies, modal damping ratios and complex mode shapes are identified from output-only free vibration signals. This identification technique has also shown to be effective when the (non-proportional) damping is significant.     

Effects of Joint Controller on Analytical Modal Analysis of Rotational Flexible Manipulator

Modal analysis is a fundamental and important task for modeling and control of the flexible manipulator. However, almost all of the traditional modal analysis methods view the flexible manipulator as a...