Showing papers on "Modal testing published in 1994"

Automatic choice of measurement locations for dynamic testing

Abstract: This paper examines the problem of choosing an optimum set of measurement locations for experimental modal testing and suggests criteria whereby the suitability of the chosen locations can be assessed. Two methods of coordinate selection are used: one based on Guyan reduction and the other on the Fisher information matrix. Each begins with a detailed finite element model of the structure being tested. Both procedures reduce this model by one degree of freedom at a time until the number of degrees of freedom in the reduced model equals the number of measurement locations required. The choice of the eliminated coordinates is generally automatic, and the coordinates of the reduced model are those used for modal testing

Second-order structural identification procedure via state-space-based system identification

Abstract: A theory is presented for transforming the system-theory-based realization models into the corresponding physical-coordinate-based structural models The theory has been implemented into a computational procedure and applied to several example problems The results show that the present transformation theory yields an objective model basis possessing a unique set of structural parameters from an infinite set of equivalent system realization models For proportionally damped systems, the transformation directly and systematically yields the normal modes and modal damping When nonproportional damping is present, the relative magnitude and phase of the damped mode shapes are separately characterized, and a corrective transformation is then employed to capture the undamped normal modes and nondiagonal modal damping matrix namics equations has hampered the application (and acceptance) of the system-theory-based structural identification techniques and can eventually curtail the progress of hybrid experimental/analyti- cal modeling and design efforts The present paper offers a theory for transforming the system- theory-based realization models into the corresponding physical- coordinate-based structural models Since a key idea employed in the development of the present theory is an objective common basis normalization, it is designated as a common basis-normal- ized structural identification (CBSI) procedure The resultant model is unique for a given sequence of Markov parameters, which in turn are uniquely determined for a linear structure with given inputs and outputs Therefore, the realized model, after transformation, has a one-to-one correspondence with the physical parameters of the system and can either yield data for finite ele- ment model correlation or alternatively for direct calculation of mass, stiffness, and damping matrices of the original structure Specifically, we begin with the so-called McMillan transformation employed by Longman and Juang9 and show that the McMillan transformation does not in general yield the desired structural nor- mal modes To arrive at an objective transformation basis of gener- alized coordinates, we invoke two invariance properties The first is the output invariance property, viz, the outputs are invariant with respect to any choice of generalized coordinates The second is the normal mode identity, viz, for proportionally damped cases the mode shapes for the displacement, velocity, and acceleration vectors are the same There are several byproducts that the present theory provides, primarily due to the common basis normalization employed in the theory First, the present transformation theory allows the integra- tion of different realized models with varying actuator and sensor locations if they arise from the same structure Second, each sensor and actuator or groups of sensors and actuators can be processed in parallel and combined concurrently or sequentially for the con- struction of a global model Third, it can facilitate a decentralized real-time control implementation From a structural dynamics point of view, the transformation to an objective basis provides a state- space model coinciding with the canonical form of the second- order equations of motion, thus extracting the classical real-valued parameters of interest to modal testing, mass-normalized normal modes, and the general modal damping matrix, while maintaining the system equivalence properties of the state-space form

Modal Testing for Structural Identification and Condition Assessment of Constructed Facilities

Abstract: Condition assessment based on modal testing and structural identification is discussed. This technique was successfully applied to seven highway bridges. Modal flexibility obtained by post-processing the frequencies and massnormalized modal vectors was used as a structural condition index. The reliability of modal flexibility was verified by comparing bridge deflections obtained from modal flexibility to those measured during static truck-load tests. 3D analytical models of the bridges were calibrated by the experimental data, the calibrated models were then used as a basis for condition assessment in the absence of baseline experimental data.

Determination of Modal Parameters from Ambient Vibration Data for Structural Health Monitoring

Abstract: A computationally efficient methodology is presented for· determination of the modal parameters of a structure from its measured ambient vibrations at several instrumented locations. The method, which is an extension of a well established and very efficient algorithm, MODE-ID constitutes the first step of a proposed two-step methodology for continuously monitoring the health of a structure by utilizing its natural ambient vibrations. During this first step, changes in the values of the modal parameters identified. The second step involves the determination of the corresponding loss of stiffness within the structure. The advantage in performing the two steps separately is that the computationally intensive second step need only be performed when the identified changes in modal parameters exceed certain thresholds indicating that structural damage is likely to have occurred. Results of testing the proposed methodology on simulated data and from an ambient vibration survey of Caltech's Millikan Library are presented.

Interaction of structure vibration and piezoelectric actuation

Abstract: A stepped beam model is developed to predict analytically the natural frequencies and mode shapes at different piezoelectric sensor/actuator locations The modal solution obtained by including the inertia and stiffness of the surface-bonded piezoelectric materials is applied to investigate the interaction of structure vibration and piezoelectric actuation under closed circuit condition It is shown that a generalized stiffness from electromechanical coupling is induced by the interaction The generalized stiffness is a function of structure mode shapes and piezoelectric sensor/actuator locations, thus either softening or stiffening system stiffness is achievable in closed circuit condition All analytical predictions are validated by modal testing

Structural modal analysis using collocated piezoelectric actuator/sensors: an electromechanical approach

Abstract: This paper presents a modal analysis technique based on the measurement of electric admittance of collocated actuator/sensors. The technique utilizes thin piezoelectric patches bonded on structures as both sensor and actuator. A commercial electrical impedance analyzer is used to measure the electrical admittance of the PZT patch. An SDOF model governing the electromechanical interaction is derived and then used to extract the mechanical impedance of the structures from the measured electrical admittance. Two corresponding algorithms, revised inverse Nyquist plane curve fitting and admittance matching-half power bandwidth approaches are presented for the extraction of modal parameters. Both approaches exclude the stiffening effect of PZT on structure yielding better estimations of extracted structure natural frequencies. The placement of PZT on structure is also studied. An experimental example is given on a small flexible beam. The results show the advantages of this technique in modal test of lightweight and flexible structures whose modal parameters are extremely sensitive to the stiffening of the transducers and shaker.

Characteristics of Modal Coupling in Nonclassically Damped Systems Under Harmonic Excitation

Abstract: The normal coordinates of a nonclassically damped system are coupled by nonzero off-diagonal elements of the modal damping matrix. The purpose of this paper is to study the characteristics of modal coupling, which is amenable to a complex representation. An analytical formulation is developed to facilitate the evaluation of modal coupling. Contrary to widely accepted beliefs, it is shown that enhancing the diagonal dominance of the modal damping matrix or increasing the frequency separation of the natural modes need not diminish the effect of modal coupling. The effect of modal coupling may even increase. It is demonstrated that, within the practical range of engineering applications, neither diagonal dominance of the modal damping matrix nor frequency separation of the natural modes would be sufficient for neglecting modal coupling.

Experiments in piezostructure modal analysis for MIMO feedback control

Abstract: An approximate method for modal analysis of a piezostructure testbed is used to generate a dynamic model for closed-loop, multiple-input-multiple-output (MIMO) feedback controller design. An innovative pole-residue system model for structures instrumented with piezoelectric sensor and actuators is developed which is compatible with existing modal curve-fitting algorithms. The authors examine the use of the new pole-residue model in the absence of truly collocated response information. It is shown that nearly-collocated measurements may be used to estimate a structure's modal parameters; high-precision signal conditioning electronics required for exact drive-point response measurements are thereby avoided. A simply-supported plate is used to demonstrate the approximate piezostructure modal test approach. The test model is then used to design up to a four input, sixteen channel output MIMO feedback control experiment. Closed-loop results are presented which show that more than 10 dB of suppression is achieved near structural resonances within the control bandwidth (10-250 Hz).

Vibration of Circular Rings With Local Deviation

Abstract: The approach is based on the Laplace transformation method and the expression of local deviation as the variation of heaviside unit step function. The effect of local deviation on the natural frequencies and mode shapes of the rings are predicted for the proposed nondimensional mass and stiffness parameters. The validity of the approach was examined through modal testing and/or finite element computation

Determination of Stiffness Changes from Modal Parameter Changes for Structural Health Monitoring

Abstract: A Bayesian probabilistic methodology is presented for identification of structural stiffness loss from changes in estimated modal parameters. The method constitutes the second step of a two-step procedure for continuously monitoring the health of a structure by utilizing its natural ambient vibrations. The version presented here assumes that any damage that occurs is localized to one part of the structure. Numerical testing of the proposed scheme shows that under specified conditions, the method successfully determines the existence, location, and degree of stiffness loss, even in the presence of levels of uncertainty in the estimated modal parameters that are expected in real applications.

Seismic Evaluation of a Long Span Bridge by Modal Testing

Abstract: This paper descibes the results of experimental and analytical vibration studies on the Queensborough Bridge over the Fraser River in Vancouver, one of the densely populated areas of significant seismic hazard in Canada. To evaluate the bridge’s dynamic response, ambient vibration measurements were conducted to determine the hmdamental mode shapes and frequencies. The vertical and torsional modes of the Main Structure were used to veri@ the result.9 from a linear elastic free vibration analysis.

Damage detection and health monitoring of operational structures

Abstract: Initial damage detection/health monitoring experiments have been performed on three different operational structures: a fracture critical bridge, a composite wind turbine blade, and an aging aircraft. An induced damage test was performed on the Rio Grande/I40 bridge before its demolition. The composite wind turbine test was fatgued to failure with periodic modal testing performed throughout the testing. The front fuselage of a DC-9 aircraft was used as the testbed for an induced damage test. These tests have yielded important insights into techniques for experimental damage detection on real structures. Additionally, the data are currently being used with current damage detection algorithms to further develop the numerical technology. State of the art testing technologies such as, high density modal testing, scanning laser vibrometry and natural excitation testing have also been utilized for these tests.

Active modal control with piezo-electric layers optimization

Abstract: Modal filter concept is introduced for structural vibration analysis and control. The modal filter is a means to extract the modal coordinates of each mode from the system outputs by mapping the response vector from the physical space to the modal space. This concept was used originally to deal with the spill-over problem in the control of a continuous system. We have chosen to optimize some semi-distributed piezo-electric layers by minimizing the influence of unwanted modes on the sensors and actuators. Confirmation of our computation was obtained by an experimental set-up of optimal active modal control on a thin composite plate. Thus, experimental optimal control in the independent modal space of this plate has been possible without any effect of spill-over. The efficiency and the robustness of this procedure was demonstrated by our experimental analysis.

Active Vibration Control Using the Modified Independent Modal Space Control (M.I.M.S.C.) Algorithm and Neural Networks as State Estimators

Abstract: This article presents a numerical study related to the active control of the vibrations of a cantilevered beam with piezoelectric actuators using the Modified Independent Modal Space Control (M.I.M.S.C.) algorithm coupled with a neural network for state estimation. Among other control strategies, the M.I.M.S.C. algorithm has been shown to have excellent closed-loop structural damping. Such an algorithm requires as input data the modal displacements and velocities of the vibrating beam. A Neural Network is proposed in this paper as an alternative approach to classic estimation structures for the on-line estimation of the modal parameters. The results of a numerical case study are presented and discussed.

Identification of modal quantities from two earthquake responses

Abstract: The work deals with the identification of modal parameters of a structure from earthquake records when the input ground motion is unknown. This may occur, for example, owing to instrumental malfunctions. The procedure is based on the assumption that at least two responses are available and consists of two main steps. In the first one, modal frequencies are estimated by searching relative minima of a function that involves the ratio of the Fourier amplitudes of the two records, while the second phase is devoted to the identification of other modal quantities (i.e. effective participation factors and modal dampings). Once the identification process has been completed, an estimate of the unknown base input may be performed by means of the IFFT algorithm. The proposed approach has been checked against both finite element simulations of simple structures and field measurements on real buildings.

Vibration of a master plate with attached masses using modal sampling method

Abstract: At medium frequencies, several problems deal with systems having a large number of modes where classical modal methods must cope with the solution of large linear systems. The objective of the modal sampling method is to reduce these systems of equations in order to obtain a simplified model. The vibration response of a structure is reconstructed using a limited number of modes, called a sample, to interpolate the response of other modes. The accuracy of the prediction can be improved by increasing the size of the sample. The method, which has been developed in the case of a long beam, is presented here in the case of a nonhomogeneous structure such as a large, thin plate where point masses have been added. The structure is excited by a harmonic point force or a planar acoustic wave. A study of the convergence of the method shows that global quantities such as space—and/or frequency—averaged energy converge quickly. Thus predictions can be obtained with models having few degrees‐of‐freedom. Moreover, results show the ability of the method to take into account a master plate with attached point mass heterogeneities.

Modal analysis of a DAM–Foundation system based on an FE–BE method in the time domain

Abstract: A modal analysis procedure based on an FE–BE method in the time domain is first formulated and then applied to a dam–foundation system. In the application, horizontal and vertical impulsive responses are calculated for the system having six different impedance ratios. Modal characteristics such as natural frequencies, damping ratios and mode shapes are evaluated from the Fourier spectra of the responses. The proposed procedure allows analysis of not only the underdamped but also the overdamped modes. According to the analysis, the radiation damping pertinent to the vertical vibration is half of that pertinent to the horizontal vibration and the interaction effect on the modes is not negligibly small when the impedance ratio exceeds 0·3.

Modal Test Technology as Non-Destructive Evaluation of Space Shuttle Structures

Abstract: Modal test and analysis Is being used for nondestructive evaluation of Space Shuttle structures. The purpose of modal testing is to measure the dynamic characteristics of a structure to extract its resonance frequencies, damping, and mode shapes. These characteristics are later compared to subsequently acquired characteristics. Changes in the modal characteristics indicate damage in the structure. Use of modal test technology as a damage detection tool was developed at JSC during the Shuttle acoustic certification program and subsequent test programs. The Shuttle Modal Inspection System was created in order to inspect areas that are impossible or impractical to inspect with conventional methods. Areas on which this technique has been applied include control surfaces, which are covered with thermal protection tiles, and the Forward Reaction Control Module, which is a frame structure that supports various tanks, thrusters, and fluid lines, which requires major disassembly to inspect. This paper traces the development of the technology, gives a status of its implementation on the Shuttle, explains challenges involved in implementing this type of inspection program, and suggests future improvements in data analysis and interpretation. Dual-use applications of the technology include inspections of bridges, oil-platforms, and aircraft.

Damage assessment of smart composite plates using piezoceramic and acoustic emission sensors

Abstract: An experimental study is conducted to develop a technique for detecting and assessing damage in laminated composite plates using piezoceramic (PZT) and acoustic emission (AE) sensors. Test specimens of glass/epoxy and graphite/epoxy composite plates are fabricated using a hot press cure technique. PZT patches and AE sensors are surface mounted on the composite plates to serve as sensors. Low velocity impact tests of plates with all four edges clamped are conducted using a drop weight testing frame with the composites in an undamaged state and again after the composites are damaged. Two test methods are used to assess the damage of the composite plates. The piezoceramic sensor output and the acoustic emission sensor signals are measured during the impact tests. Modal testing is performed to determine the frequencies and dampings of the structures from the frequency response function. The plate is then damaged by a high velocity impact and the results are correlated with the undamaged plate data. It is found that the piezoceramic sensor output is very sensitive to the composite damage and change in modal frequencies and dampings is observed.

Modal Analysis of a Suspension Assembly

Abstract: The dynamic characteristics of a suspension assembly are examined using new numerical and experimental techniques. The p-type finite element method is used to construct a numerical model of the suspension. There are significant advantages in using this approach to analyze these types of structures. The model is verified by an experimental modal analysis system, which has been shown to be effective in the study of small structures. The modelled modal parameters agree within 4.5 percent with the experimental results for 14 modes. Since the experimental system uses an electromagnetic exciter, a ferromagnetic target must be attached to the nonferrous suspension so that it can be excited. Innovative techniques are investigated to improve the attachment of this ferromagnetic target. Furthermore, the finite element model is utilized to evaluate the sensitivity of the modal parameters of the suspension to changes in its geometrical features.

Modal Identification and Determination of Effective Mass Using Environmental Vibration Testing on an Electro-Dynamic Shaker

Abstract: An experimental determination of effective modal masses of a mechanical structure is proposed using environmental vibration tests on an electro-dynamic shaker. The method is based on the identification of electro-mechanical parameters of the electro-dynamic model of the shaker. An experimental example is presented to illustrate the proposed procedure.

A vibrational model using modal shapes and magnetic forces: experimental results for a permanent magnet machine

Abstract: The frequency inverter current spectrum generates a complex magnetic field in the airgap producing attracting forces having a wide spectrum and leads to audible noise. A mathematical model is presented, predicting vibrations of a permanent magnet machine, based on the modal analysis technique including mechanical damping. Modal forces produced by the magnetic field, are related to the supply currents using a finite element calculation. Experimental results are presented.