Showing papers on "Modal testing published in 1988"
TL;DR: In this article, a comparative overview of the disciplines of modal testing used in structural engineering and system identification used in control theory is presented, and a list of representative references from both areas is given.
Abstract: A comparative overview is presented of the disciplines of modal testing used in structural engineering and system identification used in control theory. A list of representative references from both areas is given, and the basic methods are described briefly. Recent progress on the interaction of modal testing and control disciplines is discussed. It is concluded that combined efforts of researchers in both disciplines are required for unification of modal testing and system identification methods for control of flexible structures.
36 citations
TL;DR: In this paper, a method for modal vibration control of elastic systems using additional vibrators called ''wave-trapped horns'' is proposed, and experimental verification of the method has been done for several vibrating tools whose shapes are often adopted in ultrasonic plastic welding.
Abstract: A method for modal vibration control of elastic systems using additional vibrators called `wave-trapped horns' is proposed. Experimental verification of the method has been done for several vibrating tools whose shapes are often adopted in ultrasonic plastic welding. The validity of the method is confirmed.
33 citations
TL;DR: In this article, the influence of modal control using full state-variable feedback is first demonstrated, and then an estimation algorithm is applied to reconstruct the state vector from the minimum number of measurements.
20 citations
01 Jan 1988
TL;DR: In this paper, the authors compare results for a fixed-base and a driven-base modal test of an electronics package using stationary vibration shaker tables to provide a driven base input, and demonstrate the adequacy of using vibration shake tables to excite the fixed-based modes of the system.
Abstract: This paper compares results for a fixed-base and a driven-base modal test of an electronics package A fixed-base modal test uses the common testing procedure of attaching the structure to a large inertial mass which is freely suspended The problem with this approach is that the input levels are typically limited by the size of the shakers and the strength of the attachment An attractive alternative to fixed-base modal testing is to use stationary vibration shaker tables to provide a driven-base input The modes can be obtained at much higher excitation levels than fixed-base excitation modes using portable shakers and flexible attachments Experimental results are presented which demonstrate the adequacy of using vibration shake tables to excite the fixed-based modes of the system A direct comparison of the modes acquired using fixed-base and driven-base excitation for the electronics package shows very close agreement A discussion of the theory for driven-base testing is given, as well as a brief presentation of analytical (finite element) predictions for the structure 4 refs, 11 figs, 1 tab
20 citations
TL;DR: In this article, an active vibration damper system, for bending in two orthogonal directions and torsion, in each of three mutually perpendicular axes is located at the extremities of the flexible appendages of a space platform.
Abstract: An active vibration damper system, for bending in two orthogonal directions and torsion, in each of three mutually perpendicular axes is located at the extremities of the flexible appendages of a space platform. The system components for each axis includes: an accelerometer, filtering and signal processing apparatus, and a DC motor-inertia wheel torquer. The motor torquer, when driven by a voltage proportional to the relative vibration tip velocity, produces a reaction torque for opposing and therefore damping a specific modal velocity of vibration. The relative tip velocity is obtained by integrating the difference between the signal output from the accelerometer located at the end of the appendage with the output of a usually carried accelerometer located on a relatively rigid body portion of the space platform. A selector switch, with sequential stepping logic or highest modal vibration energy logic, steps to another modal tip velocity channel and receives a signal voltage to damp another vibration mode. In this manner, several vibration modes can be damped with a single sensor/actuator pair. When a three axis damper is located on each of the major appendages of the platform, then all of the system vibration modes can be effectively damped.
17 citations
01 Jan 1988
TL;DR: In this article, a free-free configured payload using mass loaded boundary conditions is presented as a means of verifying the coupled loads model of the ASTRO-1 flight payload, which allows evaluation of the influence of local load paths into the frequency range of the free free test.
Abstract: Traditionally, a fixed base modal test has been performed as a means of verifying the coupled loads math model for Space Shuttle flight payloads. An alternate method, a free-free configured payload using mass loaded boundary conditions, is presented as a means of verifying the coupled loads model of the ASTRO-1 flight payload. This method allows evaluation of the influence of local load paths into the frequency range of the free-free test. The method is cost effective and does not contaminate the modal test results with fixture coupled modes or boundary condition uncertainties. This paper describes the mass additive modal test technique as applied to the Space Shuttle ASTRO-1 flight payload.
16 citations
01 Jan 1988
TL;DR: In this paper, step-relaxation and wind were used to excite a wind turbine with four modal frequencies below 1.0 Hz for a modal test of a 110 m tall EOLE wind turbine.
Abstract: Modal Testing of immense and very flexible structures poses a number of problems. It requires innovative excitation techniques since the modal frequencies of these stuctures can be quite low. Also, substantial energy must be input to the structure to obtain reasonable levels of response. In this paper, results are presented from a modal test of the 110 m tall EOLE wind turbine which had four modal frequencies below 1.0 Hz. Step-relaxation and wind were used to excite the structure. 5 refs., 14 figs., 2 tabs.
15 citations
TL;DR: In this article, the theoretical basis for the approximation of a modified structure eigensystem, having only an incomplete set of modes and frequencies of the original model (modal testing results), and the amount of modification in the mass and stiffness matrices, is discussed.
13 citations
01 Apr 1988
TL;DR: In this paper, a turbine was tested and two modal frequencies were found to be very close to integral multiples of the operating speed of the turbine, which caused a resonant condition.
Abstract: This paper reviews several techniques which have been used to successfully measure modal parameters for wind turbines. Due to problems in providing low frequency excitation (0.1 to 5.0 Hz), modal testing of moderate-size turbines can be difficult. Several techniques of low frequency excitation have been explored, including impact, wind, step-relaxation, and human input. As one application of these techniques, a prototype turbine was tested and two modal frequencies were found to be very close to integral multiples of the operating speed, which caused a resonant condition. The design was modified to shift these frequencies, and the turbine was retested to confirm the expected changes in the modal frequencies. 8 refs., 16 figs., 1 tab.
12 citations
TL;DR: The correlated noise is appropriately and automatically modelled by the DDS, the modal parameters are shown to be estimated very accurately and hence no preprocessing of the data is needed, and complex mode shapes and non-classical damping are as easily analyzed as the classical normal mode analysis.
11 citations
TL;DR: In this article, a basic asymptotic theory for structural wall/acoustic cavity interaction is presented, and the analysis is illustrated with examples of the acoustic cavity response to a prescribed wall motion.
Abstract: A basic asymptotic theory for structural wall/acoustic cavity interaction is presented, and the analysis is illustrated with examples of the acoustic cavity response to a prescribed wall motion. Although, when spatially averaged, the classical modal analysis (CMA) response approaches the asymptotic modal analysis (AMA) response more rapidly as the number of modes increases, it is shown that information about local response intensification is lost in the averaging process. A larger bandwidth at a given center frequency is found to contain more excited modes than a smaller bandwidth; however, the AMA is slightly more accurate in the smaller bandwidth. All AMA asymptotes were shown to be approached from below by a CMA with fixed bandwidth and increasing center frequency.
TL;DR: The power penalty due to modal noise has been quantified and experimentally verified for single-mode fiber systems operating above their cutoff frequency and how the modal power distribution evolves from one connector/splice to the next and affects the degree of modal Noise.
Abstract: The power penalty due to modal noise has been quantified and experimentally verified for single-mode fiber systems operating above their cutoff frequency. It is shown how the modal power distribution evolves from one connector/splice to the next and affects the degree of modal noise.
TL;DR: In this article, a dynamic modeling of structures with bolted joint and bearing joints was investigated by using initial analysis, modal testing, condensation method of FEM and sensitivity analysis.
TL;DR: The first ten chapters of The Theory of Sound constitute an unprecedented exposition of vibration theory as mentioned in this paper, which set the mold for the teaching of vibration for more than a century and has served us well for a century but it appears that we are now on the threshold of major changes in the way vibration theory is taught.
Abstract: The first ten chapters of The Theory of Sound constitute an unprecedented exposition of vibration theory. This tour de force set the mold for the teaching of vibration for more than a century. The central core is linear modal analysis applied to lumped parameter systems and to strings, bars, beams, membranes, plates, and shells. In addition, there are preliminary investigations of nonlinear vibrations, parametrically excited vibrations, and random vibrations. Most of the textbooks on vibrations for engineers that have appeared in the past 50 years may be viewed as attempts to explain Rayleigh's idea to university students. Rayleigh's paradigm has served us well for a century but it appears that we are now on the threshold of major changes in the way vibration theory is taught. These changes are being driven by the tremendous advances in instrumentation and data processing facilities. The second coming of modal analysis (modal testing, really) is largely based on the concept of transfer functions. Although Rayleigh occasionally used the complex exponential and even computed a few transfer functions in passing, he did not attach any special significance thereto. It is interesting to speculate what Rayleigh would have done had he envisioned today's multichannel spectral analyzers. Other advances, unavailable to Rayleigh, which are currently entering vibration courses, are discussed.
TL;DR: In this article, the authors compared the broadband excitation and sine-divergence modal test methods for large, complex spacecraft structural systems, and the usefulness or shortcoming of the methods were given from a practicing engineer's view point.
Abstract: For large, complex spacecraft structural systems, the objectives of the modal test are outlined. Based on these objectives, the comparison criteria for the modal test methods, namely, the broadband excitation and the sine dwell methods are established. Using the Galileo spacecraft modal test and the Centaur G Prime upper stage vehicle modal test as examples, the relative advantages or disadvantages of each method are examined. The usefulness or shortcoming of the methods are given from a practicing engineer's view point.
TL;DR: In this article, the effects of support flexibility on the modal damping of flexural oscillation of cables are examined in relation to the dynamic characteristics of cables and it is shown that the energy dissipation from support is one of the major sources of damping and that this must be further studied.
Abstract: Effects of support flexibility on the modal damping of flexural oscillation of cables are examined in relation to the dynamic characteristics of cables. The modal damping is experimentally measured using elastically-supported cable model. Finite element method is employed in order to calculate natural frequencies, normal modes and additional dynamic strains. It is found that the flexibility of support has significant effects upon the modal damping of only symmetric modes in the region of modal transition. The experiment indicates that the energy dissipation from support is one of the major sources of damping and that this must be further studied.
TL;DR: In this article, the modal structure of a multimode fiber was found from the far-field radiation pattern using a polarizer, and then deduced the modality coefficients from the result.
Abstract: It is shown how and to what extent the modal structure of a multimode fiber may be found from the far-field radiation pattern. Although specific inverse integral coefficient type formulas are not obtained for each modal weight, the analysis shows how to mathematically isolate the L from m modes using a polarizer, and then deduce the modal coefficients from the result. >
01 Jan 1988
TL;DR: In this paper, the authors present their recent experiences in the use of MSC/pal to model the dynamics of a structure, and in use of the SMS STAR system to perform a modal test of the same structure.
Abstract: This paper presents our recent experiences in the use of MSC/pal to model the dynamics of a structure, and in the use of the SMS STAR system to perform a modal test of the same structure. Both of these complementary engineering tools run on an IBM-PC AT type of computer, and both yield a set of modal parameters which define the linear dynamic properties of a structure. The paper points out the advantages of finite element modeling, modal testing, and Structural Dynamics Modification (SDM), and eigenvalue modification technique which can be used with either analytical or experiment al modal data. It then describes how combined testing and analysis was used on a substructuring problem. In the case presented here, two different flat plate structures were tested separately using STAR, and then “attached together” using the SDM capabilities of the STAR system. Then, these two substructures were also modeled using MSC/pal, first as “unattached” structures, and then attached together. Finally, the combined substructures were tested, and the modal properties resulting from SDM, the finite element model, and the test were compared. Some comments regarding the amounts of time required to perform the modal test, perform SDM calculations, and build the finite element model are also included.
TL;DR: In this article, a quasi-modal control of a flexible rotor supported by linear actuators through bearing housings is designed based on an experimental modal analysis instead of a mathematical model.
Abstract: Vibration of a flexible rotor is reduced by a quasi-modal control method with only velocity feedback. If a mathematical model includes much error of parameter identification, much cumulated error in feed back gains result from an incorrect equation of motion. In these cases, the responses with control are not improved as we expected, or the control system becomes unstable for the worst case. Therefore, in this paper, the control system is designed based on an experimental modal analysis instead of a mathematical model. The first mode to the fifth mode of the flexible rotor supported by linear actuators through bearing housings are controlled by only two sensors and two actuators in the experiments. The maximum amplitudes of unbalance responses are reduced, and the senf-excited vibrations caused by dry friction are prevented by the quasi-modal control.
TL;DR: In this paper, the modal curves of normal mode vibrations are investigated investimgated by the harmonic balance method, provided that the fundamental harmonics are dominant when the normal modes are expanded in Fourier series in time domain.
Abstract: In a nonlinear holonomic conservative system having two-degree-of-freedom, the modal curves of normal mode vibrations are investigated investimgated by the harmonic balance method. The general procedure to compute the modal curve is suggested. Even if the linearized frequencies of the system are satisfied with the commensurability condition under which the approaches using the perturbation method have the problem of small divisor, the modal curve can be obtained by this method, provided that the fundamental harmonics are dominant when the normal modes are expanded in Fourier series in time domain. As an example, in a system with cubic nonlinearity the modal curves are computed analytically and numerically to compare both results.
01 Jan 1988
TL;DR: In this article, the authors used the finite element method to model the turbogenerator shaft and solved the equations of motion for the transient vibrations with the modal analysis technique.
Abstract: Large steam-turbine generators in operation may be stimulated to torsional vibrations by dynamic moments at the generator due to electrical system transients. To solve the torsional vibration problem the turbogenerator shaft is modelled by the finite element method. The equations of motion are solved for the transient vibrations with the modal analysis technique — ‘Time-History-Method’. If the designer is interested in an approximation for the maximum response the ’Response-Spectrum-Method’ proves to work very effectively. This paper discusses ‘Time-History-’ and ’Response-SpectrumMethod’ and presents the results for a 600 MW and a 722 MW turbo set.
TL;DR: In this paper, a new quasi-modal control method with only velocity feedback for the control system design of an active control bearing system of a flexible rotor is presented. But the method is not suitable for the case where the number of controlled modes is more than the total number of sensors and actuators.
Abstract: This study provides a new quasi-modal control method with only velocity feedback for the control system design of an active control bearing system of a flexible rotor. The velocity feedback coefficients are determined on the basis of a modal analysis. This quasi-modal control method becomes effective for cases where the number of controlled modes is more than the number of sensors and actuators. Particularly, this paper provides a method for optimizing the locations of sensors. After ideal modal damping forces are estimated, the actual modal damping forces, using a quasi-modal control method, are compared for each mode. The optimum locations of sensors minimizes the difference of the two. Moreover, the control efficiency of the quasi-modal control method is compared with that of an optimal control method with state feedback.
TL;DR: In this paper, an attempt is made to reduce modeling and analysis costs in the design process of an engine by using a simpler finite element modeling technique using only 8 node solid elements for both dynamic and static analyses.
Abstract: Analytical and experimental investigations of a diesel engine cylinder block are performed. An attempt is made to reduce modeling and analysis costs in the design process of an engine. In this paper, a simpler finite element modeling technique using only 8 node solid elements for both dynamic and static analyses is presented. Based on this integrated modeling technique of finite elements, eigenvalues are calculated and compared with the experimental data obtained from modal testing of an actual engine cylinder block
TL;DR: In this article, a pure time domain technique is proposed to reduce the conversion error caused by the FFT calculation, where the vibrating modes are determined so as to fit the response with the linear summation of the normalized responses of the identified poles.
Abstract: Experimental modal analysis has been widely used to identify the modal parameters of structures and reduce vibration. Mainly, it is based on frequency domain identification. Recently, a time domain technique has been proposed. This technique usually utilizes the FFT analyzer to condense the data, which is then inversely transformed to obtain the impulse responses. The main defect is the conversion error caused by the FFT calculation. This paper introduces a pure time domain technique. Impulse responses are calculated from the consecutive hammering force and response, and are then averaged to reduce experimental noise and error. The characteristic poles in the z-domain are calculated from these impulse responses and converted into the s-domain. The vibrating modes are determined so as to fit the response with the linear summation of the normalized responses of the identified poles. The proposed technique is applied and its accuracy is tested.
01 Feb 1988
TL;DR: An overview of the current state of the art multiple-input, multiple-output (MIMO) modal testing technology is discussed in this paper, with an emphasis on sine testing.
Abstract: An overview of the current state of the art multiple-input, multiple-output modal testing technology is discussed. A very brief review of the current time domain methods is given. A detailed review of frequency and spatial domain methods is presented with an emphasis on sine testing.
01 Jan 1988
TL;DR: In this paper, the authors present a global view of the significance of nonlinearity and the manner in which this is considered in relation to modal testing and analysis, and show why non-linearity is important.
Abstract: As a starting point it is worthwhile presenting a global view of the significance of non-linearity and the manner in which this is considered in relation to modal testing and analysis. Figures la and lb show why non-linearity is important and how modal testing inter-relates with nonlinearity.
01 Jan 1988
TL;DR: In this paper, the relevent mathematics required for structural system identification is presented in terms of linear structural systems with respect to linear matrix systems and the modal information with measured modal characteristics.
Abstract: Advances in modal testing and in partial eigenvalue and eigenvector calculation of large matrix systems and in eigensensitivity make it feasible to estimate actual physical parameters of finite element or frame analysis models by comparing calculated modal information with measured modal characteristics. The relevent mathematics required for structural system identification is presented in this paper with regards to linear structural systems.