An innovative only-output method to identify a structural system

In order to identify time-varying transient modal parameters only from nonstationary vibration response measurement signals for slow linear time-varying (SLTV) structures which are weakly damped, a moving window differential evolution (DE) independent component analysis- (ICA-) based operational modal analysis (OMA) method is proposed in this paper. Firstly, in order to overcome the problems in traditional ICA-based OMA, such as easy to go into local optima and difficult-to-identify high-order modal parameters, we combine DE with ICA and propose a differential evolution independent component analysis- (DEICA-) based OMA method for linear time invariant (LTI) structures. Secondly, we combine the moving widow technique with DEICA and propose a moving window differential evolution independent component analysis- (MWDEICA-) based OMA method for SLTV structures. The MWDEICA-based OMA method has high global searching ability, robustness, and complexity of time and space. The modal identification results in a three-degree-of-freedom structure with slow time-varying mass show that this MWDEICA-based OMA method can identify transient time-varying modal parameters effectively only from nonstationary vibration response measurement signals and has better performances than moving window traditional ICA-based OMA.

SSI-LSTM network for adaptive operational modal analysis of building structures

Conventionally, modal monitoring of Wind Turbine Rotor Blades is primarily based on the evaluation of eigenfrequencies. Beyond this, combining a sensor network with the Operational Modal Analysis (OMA) method, mode shape and parallely a local component are utilized here. In addition it is expected that the damping, which is also determined by the OMA method, will give a lead on damage development at the rotor already at an early stage. Modal monitoring by means of measurement is combined with FEM simulation and with the comparison of results obtained from measurement and simulation. Moreover, this will establish a connection between the engineer and the design data of a rotor blade, which also are based on FEM analyzes. A further significant increase regarding error resolution is possible by combining the global modal methods with locally sensitive monitoring methods, based on guided elastic waves. These assume plate-like structures through which elastic waves propagate in the low-frequency ultrasonic range (10 - 100 kHz) in certain modes. These different wave modes interact distinctively with inner structural damages such as web fractures and delaminations. It is differentiated between piezoelectrically excited waves (acousto ultrasonics), and waves produced by energy released at fractures, delamination etc. (acoustic emission). Applying a moderate number of sensors, the combination of both methods can allow an effective monitoring of the global structure.

Operational modal analysis (OMA) enables the identification of modal characteristics under operational loads and conditions. Traditional frequency-domain methods cannot directly capture modal changes over time, while existing time-frequency representations are not sufficiently interpretable due to spurious modes and implicit parameter design. This paper develops a new OMA method in time-frequency representation based on frequency-domain decomposition (FDD). Short-time FDD and a convolution-based strategy are proposed to obtain singular values and local mode shape similarity, respectively, which are further fused into mode indicators by a fuzzy-based strategy mimicking the modal assurance criterion. The method provides not only a global view of the modal characteristics over time and frequency but also estimates of the modal parameters. It is applicable to strongly nonstationary responses under time-varying loads and conditions. All the parameters explicitly affect the time-frequency representation, and the interpretability is enhanced by including physical information from the user’s prior knowledge in selecting parameters and peak bands. The proposed method is validated based on a study of railway sleepers under train passage. The rigid-body motions and bending modes are identified at frequencies up to 6,500 Hz in laboratory tests and 4,500 Hz in field tests at speeds up to 200 km/h. The identified natural frequencies and mode shapes agree with experimental modal analysis (EMA). The proposed method outperforms EMA in terms of broad frequency range and low measurement cost and can be potentially applied to structural health monitoring under operational conditions.

In order to identify the modal parameters of time invariant three-dimensional engineering structures with damping and small nonlinearity, a novel isometric feature mapping (Isomap)-based three-dimensional operational modal analysis (OMA) method is proposed to extract nonlinear features in this paper. Using this Isomap-based OMA method, a low-dimensional embedding matrix is multiplied by a transformation matrix to obtain the original matrix. We find correspondence relationships between the low-dimensional embedding matrix and the modal coordinate response and between the transformation matrix and the modal shapes. From the low-dimensional embedding matrix, the natural frequencies can be determined using a Fourier transform and the damping ratios can be identified by the random decrement technique or natural excitation technique. The modal shapes can be estimated from the Moore–Penrose matrix inverse of the low-dimensional embedding matrix. We also discuss the effects of different parameters (i.e., number of neighbors and matrix assembly) on the results of modal parameter identification. The modal identification results from numerical simulations of the vibration response signals of a cylindrical shell under white noise excitation demonstrate that the proposed method can identify the modal shapes, natural frequencies, and ratios of three-dimensional structures in operational conditions only from the vibration response signals.

Safety assessment of structures for near-field blast-induced ground excitations using operational modal analysis

Operational modal analysis (OMA) is a procedure that allows the modal parameters of a structure to be extracted from the measured response to an unknown excitation generated during operation. Nonlinearity is inevitably and frequently encountered in OMA. The problem: The traditional OMA method based on linear modal theory cannot be applied to a nonlinear oscillation system. The solution: This paper aims to propose a nonlinear OMA method for nonlinear oscillation systems. The new OMA method is based on the following: (1) a self-excitation phenomenon is caused by nonlinear components; (2) the nonlinear normal modes (NNMs) of the system appear under a single-frequency harmonic excitation; and (3) using forced response data, the symbolic regression method (SR) can be used to automatically search for the NNMs of the system, whose modal parameters are implicit in the expression structure expressing each NNM. The simulation result of a three-degree-of-freedom (3-DOF) nonlinear system verifies the correctness of the proposed OMA method. Then, a disc-rod rotor model is considered, and the proposed OMA method’s capability is further evaluated.

This document addresses enduring challenges encountered in modern experimental modal analysis as well as earlier “resonance testing” associated with measured mode orthogonality requirements. To this end, the document’s goals are (1) retirement of false damping and real mode notions (dating back to Lord Rayleigh, 1877), (2) achievement a community consensus on 85+ years of resonance and modal testing, and (3) establishment of an experimental modal analysis process that is independent of a-priori predictive mathematical models. The above goals are satisfied by addressing six key objectives, namely (a) validation of measured complex experimental modes, (b) empirical isolation of experimental modes (the “Holy Grail” of resonance testing), (c) definition of a state-space orthogonality standard, independent of an approximate reduced-order mass matrix, (d) exploitation of state-space, left-hand eigenvectors and their relationship to modal momentum and kinetic energy distributions, (e) establishment of a universal practice for a broad family of modal testing methods, and (f) application of the above six objectives to ambient (operational modal analysis) methods.

A new operational modal analysis (OMA) method is developed for estimation of modal parameters (MPs) of a rotating structure (RS) subject to random excitation using a nonuniform rotating beam model, an image processing method, and an improved demodulation method. The solution to the governing equation of a nonuniform rotating beam is derived, which can be considered as the response of the beam measured by a continuously scanning laser Doppler vibrometer (CSLDV) system. A recently developed tracking CSLDV system can track and scan the RS. The image processing method determines the angular position of the RS so that the tracking CSLDV system can sweep its laser spot along a time-varying path on it. The improved demodulation method obtains undamped mode shapes (UMSs) of the RS by multiplying its measured response by sinusoids whose frequencies are its damped natural frequencies (DNFs) that are obtained from the fast Fourier transform of the measured response. Experimental investigation of the OMA method using the tracking CSLDV system is conducted, and MPs of a rotating fan blade (RFB), including DNFs and UMSs, with different constant speeds and its instantaneous MPs with a non-constant speed are estimated. Estimated first DNFs and UMSs of the stationary fan blade and RFB are compared with those from the lifting method that was previously developed by the authors.

This paper investigates the use of conventional operational modal analysis (OMA) techniques for modal parameters identification of a rotating system supported by roller bearings. Although largely applied in civil engineering, in-depth studies on different types of systems are still limited in the literature. The novel of the paper is to address such issue by applying conventional OMA methods, such as enhanced frequency domain decomposition (EFDD) and stochastic subspace identification (SSI-data), to identify the modal parameters of a rotating machine, investigating the challenging particularities of these systems due to their inherent operating conditions, especially regarding the presence of harmonic forces, eventually closed-spaced modes, and non-proportional damping due to the bearings. The results presented in the experimental tests showed that, with the use of specific tools, in comparison with traditional experimental modal analysis (EMA), the used OMA methods have managed to successfully identify the modal parameters of a roller bearing supported rotor.

In this study, 4 different pieces of automotive rear lamp lenses which are made of Polymethylmethacrylate (PMMA) and have different geometries have been inspected. Free-free natural frequencies and mode shapes of lenses have been estimated by the help of ANSYS WB® Finite Element Analysis (FEA) software. Meanwhile, by the help of Experimental Modal Analysis (EMA) methods; natural frequencies, damping ratios and mode shapes of the lenses have been determined. Finally, mode shapes calculated by FE and measured by EMA were compared and Modal Assurance Criteria matrix (MAC) was determined. After experimental modal tests, FRF’s were calculated by DEWESoft® X3 software then natural frequencies, damping ratios and mode shapes were calculated by the use of ME’scope® VESVT-570 Visual Modal software. In addition, MAC was calculated by the help of MEScope® software. The main reason for the use of such experimental methods was to understand if the assumptions made before theoretical analysis were satisfied in reality or not.

The phenomenon of track rigidity in desert areas due to sand dune movement and ballast contamination is one of the most important causes of ballasted railway track degradation and consequently increase in train-induced vibrations. In addition to looking at the existing methods for reducing the track rigidity such as using ballast mat and installing the under sleeper pad, this paper focuses on evaluating the dynamic behavior of railway track including ballast mixed with tire-derived aggregate in a field study. Based on prior laboratory tests done by the authors and latest researches, 10 volume percentages of tire-derived aggregate were adopted as maximum suitable amount to be mixed with ballast for decreasing the track rigidity and increasing its damping property maintaining the basic ballasted track capabilities. In this regard, in a test track located in central desert of Iran in Bafgh-Jandagh railway route, the ballast was mixed with 0, 5 and 10% of tire-derived aggregate by tamping machine in three different sections with 20 m lengths. Moreover, another test section in the same route with fully fouled ballast was considered as a critical bench mark. Many tests were carried out on the mentioned sections using impact test method. In the next stage, the operational modal analysis method was utilized to achieve the tested sections’ mode shapes, natural frequencies and damping ratios as well as geometrical attenuation from the recorded responses. By knowing the modal properties, numerical models of tracks were developed, and the effect of tire-derived aggregate as well as fouling on track stiffness was evaluated.

Laminated rubber bearing is a significant device found in structures such as bridges and buildings. It is used to isolate the foundation and the superstructure from seismic loads. Since it is made up from a combination of rubbers and steel plates in alternate layers which make the laminated rubber bearing a complex material, the measurements of damping of the laminated rubber bearing is difficult in practice. Damping is a dissipation of energy or energy losses in the vibration of the structure when the structures are excited with external dynamic loading. An accurate value of damping is very important as damping plays a crucial role in fixing the borderline between stability and instability in many structural systems. Therefore, it is essential to determine the accurate value of damping in structural analysis. Finite element and experimental modal analysis is one of the methods that can be used to determine dynamic properties including damping in any structures. Hence, the main objectives of this research are to determine the natural frequencies, mode shapes, Rayleigh’s damping coefficients, α and β and to evaluate the performance of the laminated rubber bearing components i.e. steel and rubber plates using finite element and experimental modal analysis. Based on the finding, the finite element modal analysis with the added Rayleigh’s damping coefficients α and β shows a good agreement with the experimental modal analysis in term of natural frequencies and mode shapes. The value of natural frequencies reduces after the Rayleigh’s damping coefficients are added into the finite element modal analysis and the minimum and maximum displacement are found to be unaffected by the Rayleigh’s damping coefficients for rubber plate. It can be concluded that modal analysis method can be used to estimate the accurate values of damping ratio and to determine the Rayleigh’s damping coefficients α and β as well.

In this research, we computationally analyzed the structural modes of an HSK63E type tool system and verified the results with experimental modal analysis (EMA). The computation was implemented using the finite element method (FEM) based on a linear-elastic theory, with which we computed the natural frequency and obtained the modal shapes under both free and operating conditions. With the method of experimental modal analysis, we also obtained structural modal parameters including the natural frequency, modal shapes and damping ratio under the same free and operating conditions. The computational data were compared with the experimental findings. The reasonable agreement between the two data sets indicated the validity and provided a theoretical basis for using computational methods for determining the natural characteristics and influencing factors of HSK tool system. With this validated FEM model, further analyses were performed to evaluate the unique dynamic performance of the HSK tool system, and to apply dynamic optimum design to other types of HSK tool system.

Abstract. In the present study we investigate development of operational modal analysis (OMA), different OMA techniques as well as relevant issues and experimental studies. Furthermore, we classify previously performed studies have been conducted so far and consider the most important one. So, first we review the fundamental concepts. Then, the history and application of the OMA method will be examined comprehensively to observe their role in the development of the technique. Three basic methods of FDD, SSI, and Next will be discussed and the research works associated to them will be analyzed.