Showing papers on "Modal testing published in 2010"
TL;DR: The Natural Excitation Technique (NExT) as mentioned in this paper was developed to overcome these challenges of testing immense structures excited with environmental inputs, and has been used extensively in the field of wind turbine modal testing.
83 citations
TL;DR: In this paper, the purely mathematical modal assurance criterion was enhanced by additional physical information from the numerical model in terms of modal strain energies, and a numerical example and a benchmark study with experimental data are presented to show the advantages of the proposed energy-based criterion.
70 citations
TL;DR: In this article, combined modal testing of footbridges is explored using two case studies: a steel arch footbridge with spans of 75.2 m and 30.3 m and a concrete stress-ribbon footbridge having spans of 30 m and 28 m.
Abstract: In combined vibration testing, an artificial, measured force is used in operational conditions. This requires the identification of a system model that takes both the measured and the operational excitation into account. Advantages with respect to the classical operational modal analysis approach are the possibility of obtaining mass-normalized mode shapes and the increase of the excitation level and its frequency content. An advantage with respect to the classical experimental modal analysis approach, where the ambient excitation is not modeled, but considered as disturbing noise, is the possibility of using excitation levels that are of the same amplitude, or even smaller, than the ambient excitation levels. In this paper, combined modal testing of footbridges is explored using two case studies: a steel arch footbridge with spans of 75.2 m and 30.3 m and a concrete stress-ribbon footbridge with spans of 30 m and 28 m. The comparison of the modal parameters (eigenfrequencies, damping ratios, mode shapes,...
64 citations
TL;DR: In this article, the analytical modeling, modal testing, and finite-element model updating for a two-span masonry arch bridge is presented. But the analysis is performed on the developed 3D finite element model of the bridge to obtain dynamic characteristics.
Abstract: This paper presents the analytical modeling, modal testing, and finite-element model updating for a two-span masonry arch bridge. An Ottoman masonry arch bridge built in the 19th century and located at Camlihemsin, Rize, Turkey is selected as an example. Analytical modal analysis is performed on the developed 3D finite-element model of the bridge to obtain dynamic characteristics. The ambient vibration tests are conducted under natural excitation such as human walking. The operational modal analysis is carried out using peak picking method in the frequency domain and stochastic subspace identification method in the time domain, and dynamic characteristics (natural frequencies, mode shapes, and damping ratios) are determined experimentally. Finite-element model of the bridge is updated to minimize the differences between analytically and experimentally estimated dynamic characteristics by changing boundary conditions. At the end of the study, maximum differences in the natural frequencies are reduced on average from 18 to 7% and a good agreement is found between analytical and experimental dynamic characteristics after finite-element model updating.
59 citations
TL;DR: In this article, the sensitivity of general compound planetary gear natural frequencies and vibration modes to inertia and stiffness parameters is analyzed. But the model admits planetary gears having any combination of stepped-planet, meshed-planet and multiple stage arrangements.
Abstract: This paper studies the sensitivity of general compound planetary gear natural frequencies and vibration modes to inertia and stiffness parameters. The model admits planetary gears having any combination of stepped-planet, meshed-planet, and multiple stage arrangements. Eigensensitivities in terms of eigenvalue and eigenvector derivatives are analytically derived for both tuned (i.e., cyclically symmetric) and mistuned systems. The results are expressed in compact closed-form formulas. The well-defined modal properties of general compound planetary gears simplify the expressions of eigenvalue sensitivities to ones that are proportional to modal strain/kinetic energies. Inspection of the modal strain/kinetic energy distribution plots provides an effective way to quantitatively and qualitatively determine the parameters that have the largest impact on a certain mode. For parameter perturbations that preserve the system symmetry, the structured modal properties imply that the modes of the same type are independent of the same group of system parameters. Parameter mistuning, with a few exceptions, splits a degenerate natural frequency of the unperturbed system into two frequencies; one frequency keeps its original value and retains its well-defined modal properties, while the other frequency changes and its associated mode lose its structured modal properties. DOI: 10.1115/1.4000461
59 citations
TL;DR: In this article, a damage detection method based on an efficient algebraic algorithm of element modal strain energy sensitivity is proposed for detecting damage location and severity in a one-step (non-iterative) procedure.
Abstract: In this paper, a damage detection method based on an efficient algebraic algorithm of element modal strain energy sensitivity is proposed for detecting damage location and severity. The method adopts a closed form of element modal strain energy sensitivity. The element damage equations have been systematically established and a solution technique incorporated with the singular value decomposition-based regularization algorithm is used to ensure the correct solutions. A number of assumed damage scenarios for numerically simulated beams and two-dimensional frame structure have been used to demonstrate the applicability of proposed method. The noise effect is also presented. It is demonstrated that the proposed method is efficient in a one-step (non-iterative) procedure.
54 citations
TL;DR: In this article, a 3D electromagnetic analysis coupled with a 3-D mechanical analysis was conducted to analyze end-winding vibrations and deformation in an induction machine caused by steady-state magnetic forces on the end winding.
Abstract: We conducted a 3-D electromagnetic analysis coupled with a 3-D mechanical analysis to analyze end-winding vibrations and deformation in an induction machine caused by steady-state magnetic forces on the end winding. Both the analyses were based on the finite-element method. The electromagnetic analysis was used to calculate magnetic forces. During the mechanical analysis, complex support structures in the end region were simplified. We first updated and validated the mechanical model according to a modal model obtained from a modal test, and afterward analyzed deformation, vibrations, and stresses. According to the analysis, the shape of the rotary dynamic deformation of the end winding caused by dynamic forces is similar to the most excitable mode shape though the natural frequency of that mode is much higher than the excitation frequency. The static deformation caused by static forces tends to expand the coil ends outward. Under both types of deformation, the nose portion of the coil ends experiences larger displacement, but von Mises stresses are larger mainly in the knuckle portion.
47 citations
TL;DR: In this article, a procedure to optimize the mass-change strategy is proposed, which uses the modal parameters (natural frequencies and mode shapes) of the original structure as the basic information.
47 citations
TL;DR: In this paper, the performance of SAC305 and Sn37Pb solder interconnects was evaluated using both harmonic and random vibration, and the results showed that the latter is marginally less durable than the former for the harmonic excitation range.
Abstract: In this paper, durability tests were conducted on both SAC305 and Sn37Pb solder interconnects using both harmonic and random vibration. The test specimens consist of daisy-chained printed wiring boards (PWBs) with several different surface-mount component styles. Modal testing was first conducted on a test PWB to determine the natural frequencies and mode shapes. The PWB was then subjected to narrow-band excitation at its first natural frequency. Electrical continuity of the daisy-chain nets was monitored to measure the time-to-failure (and hence cycles-to-failure) of the interconnects. The response history of the PWB was recorded with strain gages located near the components of interest. Finite element analysis (FEA) was conducted for each component type, to estimate the transfer function between the flexural strain of the PWB and the strain in the critical solder joint. The predicted strain transfer function was then combined with the measured PWB strain response history to estimate the strain history in the critical solder joints. The solder strain history was used, in conjunction with the failure history, to estimate lower bounds for the fatigue durability (S-N curves) of the solder interconnects. In the first part of this paper, the results show that the SAC305 interconnects are marginally less durable than Sn37Pb interconnects for the harmonic excitation range used in this paper. The durability model constants are found to be very sensitive to the solder stress-strain curve assumed in the FEA. Since the stress-strain properties reported in the literature for these solder alloys vary significantly, the solder stress-strain curves were parametrically varied in the FEA, to assess the resulting effect on the estimated S-N curves. In the second part of this paper, random-vibration tests were conducted to assess durability under step-stress, broad-band excitation. Conventional cycle counting techniques were used to quantify the random excitation histories in terms of range distribution functions. Using the same time-domain vibration fatigue analysis used earlier for narrow-band excitation, the durability trend for the corresponding SAC305 and Sn37Pb solder interconnects under broad-band excitation was found to be similar to that found earlier under harmonic vibration excitation. Comparison between the durability prediction and test results provides a good understanding of the effect of stress-strain behavior on the fatigue constants of these solder materials. The best set of material properties was then used to verify the durability of leadless chip resistor interconnects under quasi-static mechanical cycling.
45 citations
TL;DR: In this paper, a new matrix on the covariance of covariance is formed from the auto/cross-correlation function of acceleration responses of a structure under white noise ambient excitation.
44 citations
TL;DR: In this paper, a real-time active vibration control system for a moving 3-PRR parallel manipulator with three flexible links is presented, and the results demonstrate that the vibration modes of the intermediate links are dynamically coupled and the vibration frequency components are complicated and closely spaced.
Abstract: This paper presents the experimental implementation of active vibration control applied to a moving 3-PRR parallel manipulator with three flexible links. The active vibration control is implemented using three piezoelectric (PZT) transducer pairs applied to one flexible intermediate link based on modal strain rate feedback (MSRF) control. A real-time active vibration control system is developed using two PCs with LabVIEW Real-Time. Modal analyses are conducted, and the results demonstrate that the vibration modes of the intermediate links are dynamically coupled and the vibration frequency components are complicated and closely spaced. Simplified and efficient modal filters are developed to extract modal coordinates in real time, and a second order compensator is used to filter amplified noises and unmodeled high frequency dynamics. A MSRF controller is then designed using an independent mode space control strategy, and is implemented experimentally with the first mode targeted for control. Experimental r...
TL;DR: In this paper, a new approach for expanding incomplete experimental mode shapes is presented which considers the modelling errors in the analytical model and the uncertainties in the vibration modal data measurements, and adopts the perturbed force vector that includes the effect of the discrepancy in mass and stiffness between the finite element model and actual tested dynamic system.
TL;DR: An alternate approach is proposed which utilizes frequency and shape information acquired from modal testing to update reduced order finite element models using exact analytical model improvement techniques.
01 Jan 2010
TL;DR: In this article, a forward Coleman coordinate transformation (also known as multi-blade coordinate transformation) is applied to the data measured on the wind turbine blades, which is then combined with responses measured on turbine tower.
Abstract: Summary Output-only (Operational) Modal Analysis (OMA) is a modern branch of experimental modal analysis; the main advantage of OMA is its ability to extract modal model using only measured responses. This makes OMA extremely attractive for modal analysis of big structures such wind turbines. However, there are issues preventing straightforward application of OMA to operational turbines, e.g. structure invariance during the test. The effect of rotor rotation manifests itself in the equation of motion with time-dependent coefficients. Formulating and solving eigenvalue problem lead to time-dependent eigenvalues and eigenvectors which become meaningless as modal parameters. Fortunately, so-called Coleman coordinate transformation (also known as multi-blade coordinate transformation) allows one to eliminate time dependency of the system matrices, thus converting the original time-varying eigenvalue problem to a time-invariant one. This study extends this approach to experimental modal analysis. Forward Coleman transformation is applied to the data measured on the wind turbine blades, which is then combined with responses measured on the tower. The methods of Operational Modal Analysis are then applied to the transformed data, resulting in modal frequencies, damping and mode shapes. Backward Coleman transformation is finally employed for the mode shapes for their visualization. The study demonstrates the method using simulated vibrational responses of operational 3MW wind turbine. The responses of the tower and blades were obtained from the simulation of operational wind turbine dynamics under realistic wind load using commercial aeroelastic code.
TL;DR: In this paper, a series of simplified rod-fastened rotors, which have different surface contact roughness are manufactured and their modal parameters under different pretightening force, are measured in free-free state.
Abstract: A series of simplified rod-fastened rotors, which have different surface contact roughness are manufactured and their modal parameters under different pretightening force, are measured in free-free state. The concept of surface contact stiffness is introduced to simulate the influence of pretightening force on modal parameters of these simplified rod-fastened rotors using finite element method. The experiment measured results are compared and fitted to the finite element analysis results and the relationship between contact stiffness and contact stress is established in which the contact stress is defined by the pretightening force. The relationship is then applied on the modal analysis of a real gas turbine rotor, and its modal test results and finite element analysis results are consistent with each other, proving that the relationship and the described determination method of contact stiffness based on modal test and finite element analysis are effective.
TL;DR: In this article, the results of the modal testing conducted on an office building floor and analysis of the collected vibration measurements were compared with the structural response using computer analyses, and a sensitivity study was conducted to assess the importance of various structural parameters on the floor dynamic response.
Abstract: Buildings with large column-free floors or long-cantilevered structures can be susceptible to annoying vibrations due to everyday occupants’ activities such as walking. Computer modeling and analytical representation of building structural properties to predict the floor response subjected to excitations due to human activities are important issues that require further studies. Vibration testing and analysis of built structures can assist in more accurate estimation of structure dynamic properties. This paper presents the results of the modal testing conducted on an office building floor and analysis of the collected vibration measurements. It compares these results with the structural response using computer analyses. It also presents a sensitivity study to assess the importance of various structural parameters on the floor dynamic response. From the results presented, it is concluded that for the structure used in this study the raised flooring and nonstructural elements acted mainly as added mass and d...
TL;DR: In this paper, a modal filter is used to extract the modal coordinates of each individual mode from a system's output, which is achieved by mapping the response vector from the physical space to modal space.
Abstract: A modal filter is a tool used to extract the modal coordinates of each individual mode from a system\'s output. This is achieved by mapping the response vector from the physical space to the modal space. It decomposes the system\'s responses into modal coordinates, and thus, on the output of the filter, the frequency response with only one peak corresponding to the natural frequency to which the filter was tuned can be obtained. As was shown in the paper (Deraemecker and Preumont 2006), structural modification (e.g. a drop in stiffness or mass due to damage) causes the appearance of spurious peaks on the output of the modal filter. A modal filter is, therefore, a great indicator of damage detection, with such advantages as low computational effort due to data reduction, ease of automation and lack of sensitivity to environmental changes. This paper presents the application of modal filters for the detection of stiffness changes. Two experiments were conducted: the first one using the simulation data obtained from the numerical 7DOF model, and the second one on the experimental data from a laboratory stand in 4 states of damage.
TL;DR: Comparison of the results indicates that BA is a simple and robust approach that could be effectively applied to the FE model updating problems.
Abstract: In this paper the application of bees algorithm (BA) in the finite element (FE) model updating of structures is investigated. BA is an optimization algorithm inspired by the natural foraging behavior of honeybees to find food sources. The weighted sum of the squared error between the measured and computed modal parameters is used as the objective function. To demonstrate the effectiveness of the proposed method, BA is applied on a piping system to update several physical parameters of its FE model. To this end, the modal parameters of the numerical model are compared with the experimental ones obtained through modal testing. Moreover, to verify the performance of BA, it is compared with the genetic algorithm, the particle swarm optimization and the inverse eigensensitivity method. Comparison of the results indicates that BA is a simple and robust approach that could be effectively applied to the FE model updating problems.
TL;DR: In this article, the autoregressive moving average vector (ARMAV) method and the state-space method were used for modal parameter identification of vibrating systems using only the singular value decomposition of a block Hankel matrix.
Abstract: An accurate prediction for the response of civil and mechanical engineering structures subject to ambient excitation requires the information of dynamic properties of these structures including natural frequencies, damping ratios and mode shapes. Since the excitation force is not available as a measured signal, we need to develop techniques which are capable of accurately extracting the modal parameters from output-only data. This article presents the results of modal parameter identification using two time-domain methods as follows: the autoregressive moving average vector (ARMAV) method and the state–space method. These methods directly work with the recorded time signals and allow the analysis of structures where only the output is measured, while the input is unmeasured and unknown. The equivalence between ARMAV and state–space approaches for the problem of modal parameter identification of vibrating systems is shown in the article. Using only the singular value decomposition of a block Hankel matrix of sample covariances, it is shown that these two approaches give identical modal parameters in the case where the block Hankel matrix has full row rank. The time-domain modal identification algorithms have a serious problem of model order determination: when extracting structural modes these algorithms always generate spurious modes. A modal indicator to differentiate spurious and structural modes is presented. Numerical and experimental examples are given to show the effectiveness of the ARMAV or state–space approaches in modal parameter identification using response data only.
TL;DR: In this paper, an auto-regressive model with exogenous input (ARX) of second-order including a forgetting coefficient as a weighting coefficient is used for the estimation of modal parameters.
Abstract: Temporal variation of dynamical modal properties of a base-isolated building is investigated using earthquake records in the building. A batch processing least-squares estimation method is applied to segment-wise time-series data. To construct an input-output system, an auto-regressive model with exogenous input (ARX) of second-order including a forgetting coefficient as a weighting coefficient is used for the estimation of modal parameters. The fundamental and second natural frequencies and the damping ratios of the fundamental and second natural modes of the base-isolated building are identified in the time domain. The identified results are consistent with the results obtained from the micro-tremor vibration data, forced-vibration test data and earthquake records in the present base-isolated building in the case of taking into account the amplitude-dependency of the isolators and viscous dampers. It is finally pointed out that several factors, e.g., amplitude dependency of the isolator and damper system and special characteristics of the series-type viscous damper system, may be related complicatedly with the temporal variation in modal properties of the above-mentioned system.
TL;DR: In this article, the effects of multi-site damage on the vibration response of honeycomb sandwich beams, damaged by two different ways i.e., impact damage and core-only damage simulating damage due to bird or stone impact or due to mishandling during assembly and maintenance are studied.
TL;DR: In this paper, the shape of a piezoelectric modal sensor for a beam with arbitrary boundary conditions is proposed, in which the output signal of the designed sensor is proportional to the response of the target mode.
15 Sep 2010
TL;DR: In this paper, modal damping is used for transient simulation of full finite element models, where the viscous matrix is a linear combination of the mass and stiffness, or piece-wise Rayleigh damping.
Abstract: Experimentally, modal damping is known to allow a relatively accurate representation of damping for wide frequency ranges. For transient simulation of full finite element models, viscous damping is very often the only formulation that is associated with an acceptable computation time. In the absence of a proper material damping model, it is common to assume Rayleigh damping, where the viscous matrix is a linear combination of the mass and stiffness, or piece-wise Rayleigh damping. This representation very often results in modal damping ratios that do not correspond to the physical reality that can be tested. One thus introduces an implicit representation of the viscous matrix that allows transient time simulation with minor time penalty, while using a much more appropriate damping representation. Modal amplitudes are also considered to postprocess time simulations and analyze damping levels. When studying vibrations induced by the passage of pantographs under a catenary, the high modal density of catenaries and the load moving over a large part of the model is a strong motivation to use full model transient analysis. Modal damping is shown to be a practical tool to analyze the properties of this complex system.
TL;DR: In this paper, a method for seismic design of plane steel moment resisting frames based on the use of equivalent modal damping ratios is developed, which determines the design base shear of the structure through spectrum analysis using rationally obtained equivalent damping ratio instead of the crude strength reduction (behavior) factor.
01 Sep 2010
TL;DR: Experimental validation on a full body-in-white proves industrial applicability of the proposed technique to overcome the downsides of the classic static test by determining both global static, as well as dynamic stiffness based on a single modal analysis test.
Abstract: Global car body stiffness is an important design attribute in vehicle design. Therefore accurate characterization of this stiffness is needed. The current industrial method for static stiffness determination has several downsides, amongst others its time consuming set-up preparation. Dynamic characterization by means of modal analysis on the other hand has high repeatability and a less time consuming set-up preparation. This paper describes and experimentally validates the industrial implementation of a method to overcome the downsides of the classic static test by determining both global static, as well as dynamic stiffness based on a single modal analysis test. The method combines several techniques available in literature. Theory behind the method is elaborated, simulation tests show the potential of the method and finally experimental validation on a full body-in-white proves industrial applicability of the proposed technique.
TL;DR: In this paper, the authors present multi-mode vibration control and analysis of moving platform position errors of a planar 3-PRR parallel manipulator with three flexible intermediate links using PZT transducers.
Abstract: This paper presents multi-mode vibration control and analysis of moving platform position errors of a planar 3-PRR parallel manipulator with three flexible intermediate links using PZT transducers....
TL;DR: In this paper, an effective technique for the extraction of modal parameters is proposed to get over the limitation of the available time samples for the analysis of high-speed trains passing through railway bridges.
TL;DR: In this article, a model-based delamination detection methodology is presented for laminated composite plates and its performance is studied both numerically and experimentally, where a modal analysis of an undamaged baseline finite element (FE) model and experimental modal testing of panels with delamination damage at single or multiple locations are performed.
Abstract: In this paper, a new model-based delamination detection methodology is presented for laminated composite plates and its performance is studied both numerically and experimentally. This methodology consists of two main parts: (1) modal analysis of an undamaged baseline finite element (FE) model and experimental modal testing of panels with delamination damage at single or multiple locations and (2) a sensitivity based subset selection technique for single or multiple delamination damage localizations. As an identification model, a higher-order finite element model is combined with a rational micromechanics-based CDM model which defines the delamination damage parameter as a ratio of delaminated area to entire area. The subset selection technique based on sensitivity of the dynamic residual force has been known to be capable of detecting multiple damage locations. However, there has been no experimental study specifically for the applications in laminated composite structures. To implement the methodology, a sensitivity matrix for the laminated composite plate model has been derived. Applications of the proposed methodology to an E-glass/epoxy symmetric composite panel composed of 16 plies [CSM/UM1208/3 layers of C1800]s = [CSM/0/(90/0)3]s with delamination damage are demonstrated both numerically and experimentally. A non-contact scanning laser vibrometer (SLV), a lead zirconate titanate (PZT) actuator and a polyvinylidene fluoride (PVDF) sensor are used to conduct experimental modal testing. From the experimental example, capabilities of the proposed methodology for damage identification are successfully demonstrated for a 2D laminated composite panel. Furthermore, various damage scenarios are considered to show its performance and detailed results are discussed for future improvements.
TL;DR: In this article, a 1-kg tuned mass dampers (TMDs) were used to damp the first two FSS flexural modes, which are the main sources of jitter.
Abstract: The Gemini Planet Imager (GPi) is an instrument that will mount to either of two nominally identical Telescopes,
Gemini North in Hawaii and Gemini South in Chile, to perform direct imaging and spectroscopy of extra-solar planets.
This 2,000-kg instrument has stringent mass, center-of-gravity, flexure, and power constraints. The Flexure Sensitive
Structure (FSS) supports the main opto-mechanical sub-systems of the GPi which work in series to process and analyse
the telescope optical beam.
The opto-mechanical sub-systems within the FSS are sensitive to mechanical vibrations, and passive damping strategies
were considered to mitigate image jitter. Based on analysis with the system finite element model (FEM) of the GPi, an
array of 1-kg tuned mass dampers (TMDs) was identified as an efficient approach to damp the first two FSS flexural
modes which are the main sources of jitter. It is estimated that 5% of critical damping can be added to each of these
modes with the addition of 23 kg of TMD mass. This estimate is based on installing TMD units on the FSS structural
members. TMD mass can be reduced by nearly 50% if the units can be installed on the opto-mechanical sub-systems
within the FSS with the highest modal displacements.
This paper describes the structural design and vibration response of the FSS, modal test results, and plans for
implementation of the TMDs. Modal measurements of the FSS structure were made to validate the FEM and to assess
the viability of TMDs for reducing jitter. The test configuration differed from the operational one because some
payloads were not present and the structure was mounted to a flexible base. However, this test was valuable for
understanding the primary modes that will be addressed with the TMDs and measuring the effective mass of these
modes.
Abstract: In this paper the authors propose a modification to the known force identification procedure based on modal filtration. The modification consists of replacing the modal vectors with the Ritz vectors. The latter seem to be more accurate because they take into account the static deformation of the structure and are less sensitive to truncation error. After the main idea- the algorithm modification- is presented, it is verified and then compared with the original solution. Two sets of data are used for this purpose, firstly simulation data from the numerical model, and then physical data recorded during a laboratory experiment.