Showing papers on "Modal testing published in 2004"

Experimental and Analytical Modal Analysis of Steel Arch Bridge

Abstract: The paper presents the experimental and analytical modal analysis of a steel-girder arch bridge. The field test is carried out by ambient vibration testing under traffic and wind-induced excitations. Both the peak picking method in the frequency domain and the stochastic subspace identification method in the time domain are used for the output-only modal identification. A good agreement in identified frequencies has been found between the two methods. It is further demonstrated that the stochastic subspace identification method provides better mode shapes. The three-dimensional finite element models are constructed and an analytical modal analysis is then performed to generate natural frequencies and mode shapes in the three-orthogonal directions. The finite element models are validated to match the field natural frequencies and mode shapes. It is observed that the finite element Model-2 with the concrete slab provides the greater stiffness in the transverse direction of the bridge. The finite element Model-1 with the lumped masses agrees well with the field tests and can serve as a baseline model of the bridge. DOI: 10.1061/~ASCE!0733-9445~2004!130:7~1022! CE Database subject headings: Modal analysis; Model tests; Experimentation; Vibration; Bridges, steel; Bridges, arch.

Output-only modal parameter identification of civil engineering structures

Abstract: The ambient vibration measurement is a kind of output data-only dynamic testing where the traffics and winds are used as agents responsible for natural or environmental excitation. Therefore an experimental modal analysis procedure for ambient vibration testing will need to base itself on output-only data. The modal analysis involving output-only measurements presents a challenge that requires the use of special modal identification technique, which can deal with very small magnitude of ambient vibration contaminated by noise. Two complementary modal analysis methods are implemented. They are rather simple peak picking (PP) method in frequency domain and more advanced stochastic subspace identification (SSI) method in time domain. This paper presents the application of ambient vibration testing and experimental modal analysis on large civil engineering structures. A 15 storey reinforced concrete shear core building and a concrete filled steel tubular arch bridge have been chosen as two case studies. The results have shown that both techniques can identify the frequencies effectively. The stochastic subspace identification technique can detect frequencies that may possibly be missed by the peak picking method and gives a more reasonable mode shapes in most cases.

An interval finite element approach for the calculation of envelope frequency response functions

Abstract: This paper focusses on the application of the interval finite element method in dynamic analyses. It describes a methodology for calculating frequency response function envelopes from a finite element model containing imprecise parameters defined as interval uncertainties. The resulting envelope functions give a conservative approximation of the possible range of the frequency response function, taking into account that the uncertain parameters in the model can adopt any value in their presumed uncertainty intervals. The methodology is based on the modal superposition principle. It consists of an interval aritmethic algorithm which processes the results of a preliminary global optimization performed on the modal parameters. The algorithm is constructed such that it optimally combines the advantages of both the anti-optimization and the interval arithmetic strategy for general numerical interval calculations. In the first stage of the development, the modal parameter ranges of each individual mode are independently combined in the modal response contributions. This yields the modal rectangle (MR) method. In order to remedy the high conservatism inherent to the MR method, the exact eigenfrequency ranges are added to the analysis. This results in the modal rectangle method with eigenfrequency interval correction (MRE). A second improvement consists of adding extra delimiters to the MRE modal parameter range approximation. This is achieved by performing an extra optimization on the modal response contributions at discrete frequencies. The method is referred to as the locally optimized modal rectangle method with eigenfrequency interval correction (OMRE). Finally, a numerical example illustrates the different algorithms. Copyright © 2004 John Wiley & Sons, Ltd.

Discriminating physical poles from mathematical poles in high order systems: use and automation of the stabilization diagram

Abstract: System identification from measured MIMO data plays a crucial role in structural dynamics and vibro-acoustic system optimization. The most popular modeling approach is based on the i modal analysis concept, leading to an interpretation in terms of visualized eigenmodes. Typically, the number of nodes is very high (often over 100), including modes with high damping and high modal overlap. The paper discusses a key problem of the system identification process: the selection of the correct model order and related to this, the selection of valid system poles. A multi-order approach, followed by a heuristic selection process is outlined. A visual representation of the pole behavior is presented and the possible routes to automation are discussed. The process is illustrated with typical complex datasets, including full-scale industrial tests.

Structural Damage Assessment Using Vibration Modal Analysis

Abstract: Vibration techniques have been employed for detecting the presence and monitoring the progression of damage in structures Pinpointing the location of damage is a more complicated and elaborate task This paper presents modal analysis techniques for locating damage in a wooden wall structure by evaluating damage-sensitive parameters such as resonant pole shifts and mode shapes, residue and stiffness changes Artificial damage (simulating termite degradation) was created in one of the walls of a specially constructed room The wall was excited using an impact hammer and its frequency response measured using a laser vibrometer Resonant poles (plotted in the s-plane) were used for identifying modes that are sensitive to damage, since not all modes are equally affected by the presence of damage The damaged region was identified by visual comparison of the deformation mode shapes before and after damage The modal residue and stiffness changes were also quantified for a better representation of the damage lo

The Modal Analysis of a Motorcycle in Straight Running and on a Curve

Abstract: The vibrational modes (generalized) of a two-wheel vehicle are studied in several trim configurations. The modal analysis is carried out on a 3D non-linear mathematical model, developed using the natural coordinates approach. A special procedure for evaluating the steady state solutions in straight running and on a curve is proposed. The paper presents detailed results of the modal analysis for a production sports motorcycle. Furthermore, the influence of speed and lateral (centripetal) acceleration on stability, shape and modal interactions (coupling) is highlighted. Finally, consistency between the first experimental tests and simulation results is shown.

Rotor-Bearing Analysis for Turbomachinery Single- and Dual-Rotor Systems

Abstract: Finite element analysis models were developed to investigate the dynamic characteristics of single- and dual-rotor-bearing turbomachinery systems. When an inertial coordinate system was used, the dynamic models of the rotor-bearing systems included gyroscopic moments, rotary inertias, and bending and shear deformations. The models were analyzed to predict the natural frequencies, to produce critical speed maps, and to estimate the bearing stiffnesses. These rotor-bearing system analyses were then applied to both single-rotor and dual-rotor system applications. In the single-rotor system application, a small turbojet engine and its rotor components were used as a basis for the model. Both theoretical and experimental analyses were used to study this engine rotor-bearing system. Modal testing and a dynamic engine test were used to verify the analytical results, including the predicted critical speed map and the bearing stiffnesses. Very good agreement was found between the analyses and the test data. In the dual-rotor application, the effects of the speed ratio of the high-speed to low-speed shafts of the dual-rotor system on the critical speeds was studied. It was demonstrated that this speed ratio could be used as one of the dual-rotor system design parameters. Finally, it was noted that the interrotor bearing stiffness between the high-speed and the low-speed shafts of the dual-rotor system affected the mode shapes of the shafts within the system, in addition to the rotor system critical speeds.

Improvement of Frequency Domain Output Only Modal Identification from the Application of the Random Decrement Technique

Abstract: This paper explores the idea of estimating the spectral densities as the Fourier transform of the random decrement functions for the application of frequency domain output-only modal identification methods. The gains in relation to the usual procedure of computing the spectral densities directly from the time series, are due to the noise reduction that results from the time averaging procedure of the random decrement technique, and from avoiding leakage in the spectral densities, as long as the random decrement functions are evaluated with sufficient time length to have a complete decay within that length. The idea is applied in the analysis of ambient vibration data collected in a 1⁄4 scale model of a 4-story building. The results show that a considerable improvement is achieved, in terms of noise reduction in the spectral density functions and corresponding quality of the frequency domain modal identification results.

Development of Modal Test Techniques for Validation of a Solar Sail Design

Abstract: §** This paper focuses on the development of modal test techniques for validation of a solar sail gossamer space structure design. The major focus is on validating and comparing the capabilities of various excitation techniques for modal testing solar sail components. One triangular shaped quadrant of a solar sail membrane was tested in a 1 Torr vacuum environment using various excitation techniques including, magnetic excitation, and surfacebonded piezoelectric patch actuators. Results from modal tests performed on the sail using piezoelectric patches at different positions are discussed. The excitation methods were evaluated for their applicability to in-vacuum ground testing and to the development of onorbit flight test techniques. The solar sail membrane was tested in the horizontal configuration at various tension levels to assess the variation in frequency with tension in a vacuum environment. A segment of a solar sail mast prototype was also tested in ambient atmospheric conditions using various excitation techniques, and these methods are also assessed for their ground test capabilities and on-orbit flight testing.

Active vibration control of a smart pultruded fiber-reinforced polymer I-beam

Abstract: Advanced and innovative materials and structures are increasingly used in civil infrastructure applications. By combining the advantages of composites and smart sensors and actuators, active or smart composite structures can be created and be efficiently adopted in practical structural applications. This paper presents results on active vibration control of pultruded fiber-reinforced polymer (FRP) composite thin-walled I-beams using smart sensors and actuators. The FRP I-beams are made of E-glass fibers and polyester resins. The FRP I-beam is in a cantilevered configuration. The PZT (lead zirconate titanate) type of piezoelectric ceramic patches are used as smart sensors and actuators. These patches are surface bonded near the cantilevered end of the I-beam. Utilizing results from modal analyses and experimental modal testing, several active vibration control methods, such as position feedback control, strain rate feedback control and lead compensation, are investigated. Experimental results demonstrate that the proposed methods achieve effective vibration control of FRP I-beams. For instance, the modal damping ratio of the strong direction first bending mode increases by more than 1000% with positive position feedback control.

Identification of Multi-Degree of Freedom Systems With Nonproportional Damping Using the Resonant Decay Method

Abstract: This paper describes an extension of the force appropriation approach which permits the identification of the modal mass, damping and stiffness matrices of nonproportionally damped systems using multiple exciters. Appropriated excitation bursts are applied to the system at each natural frequency, followed by a regression analysis in modal space. The approach is illustrated on a simulated model of a plate with discrete dampers positioned to introduce significant damping nonproportionality. The influence of out-of-band flexible and rigid body modes, imperfect appropriation, measurement noise and impure mode shapes is considered. The method is shown to provide adequate estimates of the modal damping matrix.

Modified modal methods for calculating eigenpair sensitivity of asymmetric damped system

Abstract: Many real systems such as moving vehicles on roads, missiles following trajectories and ships in sea water have asymmetric mass, damping and stiffness matrices. Eigen-sensitivity analysis methods for the symmetric damped system cannot be used in the asymmetric damped case. Therefore, a method for calculating eigenpair sensitivity of the asymmetric damped system is needed. To do this, a modal method employing a modal superposition idea was recently developed. Since the accuracy of the modal method is dependent on the number of modes used in calculation, the modal method needs higher eigenvectors to guarantee the accuracy. In large-scale systems, however, only a few lower modes are generally considered for the dynamic analysis. Hence, if the modal method is used to obtain the eigen-sensitivity of the large-scale system, the significant errors could not be avoided due to the lack of the information of higher modes. In this paper, the modified modal methods for computing the sensitivities of the eigenpairs of asymmetric damped system using a few lowest sets of modes are proposed. Numerical example shows that the proposed methods achieve better calculating efficiency than the previous modal method. Copyright © 2004 John Wiley Sons, Ltd.

Active control vibration isolation using dynamic manifold

Abstract: A robust controller for multi-degree-of-freedom active vibration isolation accounts for plant uncertainties and payload disturbances using frequency-shaped sliding control. Modal decomposition is used to rewrite a multi-degree of freedom vibration control problem as a combination of individual modal control problems in modal coordinates. Modal parameters for decomposition and modelling can be extracted from theoretical or experimental modal analysis. The target frequency-domain performance of isolation, for example a skyhook model, is recast as a frequency-shaped sliding surface. Boundary layer approximation is examined. The ideal skyhook effect can be robustly achieved. The frequency-shaped manifold is also extended to adaptive vibration isolation without model reference, which has been verified by experiments, and demonstrated to be very effective for vibration isolation. Nonlinear target dynamics of the same order as the nominal plant can also be attained. Control can be achieved without measuring the excitation, such as ground motion. Further, performance during the transient stage can be guaranteed. Also, control can be achieved even without knowing the values of entries for mass, stiffness and damping matrices. The methods of controller design can be used to design control for plants subject to disturbances other than only vibration. The plant can be mechanical, electrical, thermal, or any that can be described by system equations that are mathematically of the same character as are those that describe mechanical dynamic systems. The target dynamics need not be skyhook dynamics, but can be any dynamics desired, even non-linear.

Characterization of vibration and acoustic noise in a gradient-coil insert

Abstract: High-speed switching of current in gradient coils within high magnetic field strength magnetic resonance imaging (MRI) scanners results in high acoustic sound pressure levels (SPL) in and around these machines. To characterize the vibration properties as well as the acoustic noise properties of the gradient coil, a finite-element (FE) model was developed using the dimensional design specifications of an available gradient-coil insert and the concentration of the copper windings in the coil. This FE model was then validated using experimentally collected vibration data. A computational acoustic noise model was then developed based on the validated FE model. The validation of the finite-element analysis results was done using experimental modal testing of the same gradient coil in a free-free state (no boundary constraints). Based on the validated FE model, boundary conditions (supports) were added to the model to simulate the operating condition when the gradient-coil insert is in place in an MRI machine. Vibration analysis results from the FE model were again validated through experimental vibration testing with the gradient-coil insert installed in the MRI scanner and excited using swept sinusoidal time waveforms. The simulation results from the computational acoustic noise model were also validated through experimental noise measurement from the gradient-coil insert in the MRI scanner using swept sinusoidal time waveform inputs. Comparisons show that the FE model predicts the vibration properties and the computational acoustic noise model predicts the noise characteristic properties extremely accurately.

Scaling the Mode Shapes of a Building Model by Mass Changes

Abstract: It is well known, that when using natural input modal analysis, the loads are not known, and thus, the mode scaling factor that relates the magnitude of the loading to the magnitude of the response cannot be estimated. However It has been pointed out by several theoretical papers that mode shapes can be scaled by performing several natural input modal analysis tests with different mass changes, observe the frequency shift introduced by the mass changes and then follow an estimation scheme that allows the user to estimate the scaling factor modeby-mode, i.e. only information of the particular mode of interest is used to obtain the scaling factor for that mode. The procedure is of high practical interest also in mechanical engineering since it is well known that the traditionally estimated scaling factor is often suffering from large uncertainties. In this paper it is shown how the mass change technique can be used on a 1⁄4 scale model of a 4-storey building. The uncertainties on the estimated scaling factors are illustrated by repeating the estimation using different mass changes. INTRODUCTION Output-only modal testing and analysis – or as it may be should be called: Natural input modal testing and analysis is becoming more and more popular due to the clear advantages of the technology: The testing is easier, the technology is applicable to a wider a range of the structures and it has a wider range of potential applications since the actual responses are stored and can be used for instance fatigue analysis and vibration level estimation. Also the technology gives better and more reliable results in cases where the actual loading conditions and operating conditions are important for the structural response. However the technology is still in the early stages of development and many problems still remain unsolved or partly unsolved. One of the important remaining problems is the problem of mode shape scaling. If the identified modal model is going to be used for structural response simulation or for structural modification, then the scaling factors of the mode shapes must be known. Also in health monitoring applications and in cases where damage is to be identified, the scaling factors might be useful. Recently some suggestion has been given in the literature for solving this problem. One solution has been suggested by Bernal and Gunes [1] based on the assumption that partition of the inverse of the mass matrix associated with the measured coordinates is diagonal. However, the approach gives exact answers only when there is a full set of modes, and robustness for a truncated modal space has not been demonstrated. Recently Parloo et al. [2] have published a new approach based on a more extensive testing procedure that involves repeated testing where mass changes are introduced in the points of the structure where the mode shape is known. This approach seems more appealing, since to scale a certain mode, only that particular mode has to be known. Brincker et al. [3], [4] have published an improved procedure following the ideas of Parloo et al. and also performed an uncertainty analysis that clearly indicates a lower uncertainty on the mode shape when following this estimation procedure for the scaling factor than what is know to be the case using traditional modal analysis. In Brincker et al. [3], [4] it is shown that if one arbitrary mode shape φ estimated by natural input modal analysis is scaled to unity, i.e. so that 1 = φ φT and the corresponding mode shape ψ scaled as usual so that 1 = Mψ ψT , then the scaling factor α relating the two mode shape estimates α = ψ φ can be found by introducing a mass change and obtaining the estimate as 2 2 1 2 2 2 ω ω α ω − = T φ ∆Mφ (1) where ∆M is the mass change matrix and 1 ω , 2 ω are the natural frequency of the considered mode before and after application of the mass change. If the mass change matrix is proportional to the initial mass matrix, then the mode shapes will not change, and equation (1) is then an exact linear elastic solution for the relationship between mass changes, frequency shift and mode shape scaling factor. In the case where the mass change matrix is not proportional to the initial mass matrix, equation (1) becomes approximate. Assuming that the uncertainty α σ on α is controlled by the uncertainty ω σ ∆ on the frequency shift ω ∆ and by the uncertainty φ σ on the mode shape coordinates, then, if the mass change matrix can be written D ∆M m ∆ = , where the matrix D is a diagonal matrix with only unity values and zeroes in the diagonal, then the uncertainty on the scaling factor can be estimated from

Modal analysis of industrial system harmonics using the s-domain approach

Abstract: This paper describes the use of modal analysis of s-domain network models in the solution of harmonic distortion problems of industrial systems. Modal analysis involves the computation of the system poles, transfer function zeros as well as their sensitivities to changes in network parameters. A new modal sensitivity coefficient and a new modal observability index are introduced. These new coefficient and index are used to solve harmonic problems in a practical industrial system having multiple harmonic sources. The results obtained are believed to demonstrate the practical value of the proposed method.