Estimation Of Natural Frequencies Research Articles

Free axisymmetric vibrations of composite annular sandwich plates by higher-order theory using Chebyshev collocation technique

Abstract Free axisymmetric vibrations of composite annular sandwich plates with thick isotropic core and orthotropic facings have been studied using Reddy's higher-order shear deformation theory. Core is assumed to be of uniform thickness while the face sheets are treated as membranes. Equations of motion and natural boundary conditions are developed using Hamilton's principle. Chebyshev collocation technique has been applied to get the frequency equations for clamped-clamped, clamped-simply supported and clamped-free edge conditions. Obtained equations are solved numerically for the lowest three roots and reported as the frequency parameters for the first three modes of vibration. Results obtained from the proposed approach are validated by comparing them numerically and graphically with their counterparts available in the literature. Detailed numerical results are given to analyze the effects of core thickness, thickness of faces and radii ratios on the frequency parameter. It is also shown that the published analysis based on first-order theory is not suitable for estimation of natural frequency of annular sandwich plate with thick core. Three dimensional mode shapes have been plotted for a specified plate for all the three boundary conditions.

Modal parameter identification of structures based on short-time narrow-banded mode decomposition

The accurate estimation of natural frequencies and damping ratios is critical for civil structures. In this article, a method based on short-time narrow-banded mode decomposition is proposed to analyze the modal parameters of civil structures. In this approach, short-time narrow-banded mode decomposition is applied to identify time-varying structures with free vibration responses. On the contrary, by analysis of the weighting factors α and β, short-time narrow-banded mode decomposition is improved to estimate the parameter of time-invariant systems. In the case of enhanced short-time narrow-banded mode decomposition, the original short-time narrow-banded mode decomposition approach is modified in two ways. First, the instantaneous frequency term of the objective function is removed, and one weighting factor remains, that is, α in the objective function. Second, a technique is provided to automatically detect the optimum value of α. Two numerical examples, that is, a three-degree-of-freedom time-variant system and a simple model of the Lysefjord bridge are provided. In addition, an experiment with a real-life pedestrian bridge located at Tufts University, United States, is used to demonstrate the applicability of the proposed method. The analysis results indicate that the proposed method can easily identify high-quality natural frequencies and damping ratios.

Practical Simplified Computing Method of Natural Vibration Frequency of Cable-stayed Bridge during Construction Stage

In this paper, the vibration analysis model of the cable-stayed bridge is established by using double-mass method, and the formula for calculating its natural frequency during construction is deduced. On this basis, the formula is simplified to obtain the estimation formula of the natural vibration frequencies of the cable-stayed bridge during construction. The structure analysis software MIDAS Civil is used to establish the three-dimensional finite element model of the cable-stayed bridge of Jinan Qihe Yellow River Bridge to analyse the vibration characteristics of Qihe Yellow River Bridge during the construction stage, carry on the spot tracking of the modal test of the construction phase, and at the same time calculate the frequency by using the double-mass method and the estimation formula. The double-mass vibration model, the natural frequency estimation formula, the finite element method analysis and the field modal test results are compared and analysed. Both the test results and the calculated results show that the finite element method can accurately simulate the actual force state of the bridge, and the double-mass model and the estimation formula provide a more accurate and efficient method for calculating the natural vibration frequencies, which provides reference for guiding design and construction monitoring work of the cable-stayed bridge.

A signal decomposition method based on repeated extraction of maximum energy component for offshore structures

Contrary to most signal decomposition methods that usually decompose an original signal into a series of components simultaneously, a novel approach based on repeated extraction of Maximum Energy Component (MEC) is proposed. The approach starts from determination of the MEC referring to the estimated Power Spectral Density (PSD) function, and then represents the MEC by employing an exponential function to fit the original signal. By defining a stopping criterion based on two adjacent estimated PSDs, each MEC can be accurately extracted with an improved performance throughout the entire signal decomposition. To verify the proposed method, a single degree-of-freedom system subject to harmonic loads has been examined. Numerical results show that the analytical response can not only be decomposed into four MECs corresponding to the excitation and the system, respectively, but also provide an accurate estimation of natural frequency and damping ratio of the system. Meanwhile, by observing results from the Ensemble Empirical Mode Decomposition (EEMD), Variational Mode Decomposition (VMD) and Prony based on state-space model (Prony-SS), an improved decomposition accuracy has been achieved from the proposed approach. Furthermore, experimental data from the Norwegian Deepwater Programme and two sets of field-test data from one fixed offshore platform and an offshore wind turbine have been used to demonstrate the correctness of the developed signal decomposition method. It is noted that divergence in results by Prony-SS can be observed when a very large model order is used, while the proposed method provides the better decomposition and reconstruction of signals.

Dynamic identification and model updating of an eight-storey CLT building

So far, few in situtests have been carried out to estimate the modal parameters of multi-storey Cross-Laminated-Timber (CLT) buildings: the understanding of their dynamic behaviour under operational conditions is still an open issue. In this paper, the results of the dynamic identification of an eight-storey CLT building are reported and interpreted in the light of a simplified shear-type analytical model. The structural model is calibrated on the experimental modal parameters, which are assessed using the Stochastic Subspace Identification (SSI). The SSI provides mode shapes, natural frequencies and damping factors of the structures in its first linear response range; The minimum of a modal-based objective function gives an estimation of the unknown parameters of the structural model. Given the results of the dynamic identification, the storey-masses are the chosen optimization parameters: the values of the experimental eigenfrequencies likely reveal a continuum-like behaviour of the building without connections. The identified storey masses are indeed realistic and confirm the negligible role of the connections in the low-amplitude dynamic response. The nonlinear effects of connections over the building response to higher displacements are discussed, providing simplified and practice-oriented correlations for the first natural frequency estimation.

Estimation of natural frequencies of the laboratory truss structure with output only subspace and frequency-domain techniques

Non-destructive methods of structural testing are preferred to identify their current state because they do not damage the structure. Modal analysis is one of these methods that can be used for a comprehensive assessment of structural measurements. Its main advantage is that it can be used for variety applications. The paper presents an application of operational modal analysis to determine modal parameters of a laboratory steel truss structure. Its goal is to make contribution to the still unfinished development of modal analysis in the area of comprehensive application. There is important to try to use it and especially try to derive some basic generally applicable rules. It is an important step for experimental modal analysis before it can be widely used and accepted in engineering applications.

Accurate natural frequency estimation method for myotonometer using a system identification method

Accurate natural frequency estimation method for myotonometer using a system identification method

Evaluation of two-stage scaling for physical modelling of soil−foundation−structure systems

Several factors, such as the payload capacity of a geotechnical centrifuge, performance of an in-flight shake table and a maximum achievable centrifugal acceleration, limit the size of a structure that can be modelled to study soil−foundation−structure interaction (SFSI) effects using centrifuge modelling. The two-stage scaling method is an innovative technique to model larger prototypes in currently available centrifuge facilities. In this study, two sets of dynamic centrifuge experiments were conducted to evaluate the applicability of the method for SFSI studies. Two physical models, representing a target structure placed on dry sandy soil, were designed based on the scaling factors of conventional centrifuge modelling at 50g and the two-stage scaling method at 25g. The models were excited under different realistic earthquake motions. The results showed good agreements for the estimation of flexible-base natural frequencies of the physical models. The performance of the method for estimation of seismic amplification factors, kinematic incoherence parameters and foundation impedance functions was fair. Poor and fair agreements were observed for the estimation of seismic soil settlement recorded beneath the physical models and free field, respectively. Overall, the method can be used for SFSI studies in the centrifuge modelling while considering its strengths and weaknesses.

Research on Structural behavior of Composite Materials on different Cantilever Structures using FSI

The implementation of composite materials in industries like aerospace, automotive, civil, naval, etc., are emerging continuously because of its combined unique effects. The present article deals with a numerical simulation of the natural frequency effect on the different cantilever structure for various composite materials also estimates its structural response to aerodynamic forces using Fluid-Solid Interaction (FSI) in Ansys Workbench environment. This work is intended to analyze and learn the response of various real-time structural parameters like maximum deformation, maximum stress-induced location and modal parameters like modal frequency, mode shapes on different Cantilevered structures. Structures like wind turbines, airplane wings are critical in structural behavior, which are characteristically using the wind to produce power and lift respectively. The present article deals with an estimation of natural frequencies for the prediction of working lifespan and structural parameters variation throughout the cantilever structures to multi-purpose usage. The entire comparative part of this article is executed with an acceptable simulation of the displacement and principal stress for different composite materials such as Kevlar, Glass Fiber Reinforced Plastic (GFRP) and Carbon Fiber Reinforced Plastic (CFRP) by using FSI. Composites Beams are modeled in CATIA and discretized in the Ansys ACP 16.2 tool and Modal analysis of various cross-sectional beams are reported, compared and discussed.

On-Time Mismatch Based System Identification Technique for Buck Converters

This article introduces a fast and low-complexity natural frequency estimation concept for power converters, enabling their adaptive control. Neither a high-resolution analog-to-digital converter nor complex arithmetic operations, such as multiplications, are required by the proposed approach. Instead, the mismatch between the on -time of the control signal and the effective on -time at the switching node, occurring under light load conditions, is exploited. This mismatch is caused by the parasitic drain–source capacitance and the parasitic body diode of the power MOSFETs. It is not necessary that the value of the parasitic capacitance is known nor is it required to stay constant, e.g., over temperature. Due to its simplicity, the proposed method is well-suited for practical on-chip realizations. The introduced concept is experimentally verified on the example of a buck converter with different output filter sets. A digital controller is adaptively tuned based on the estimated natural frequency, resulting in an improved dynamic performance. For the presented converter, the magnitude of the output voltage transient in response to a load change was reduced by 30%.

Real-time detection of transverse stability changes in fishing vessels

Fishing is a very hazardous activity. Stability failures, caused many times by the crew lacking information related to the level of stability of the vessel, are among the most dangerous accidents affecting this fleet. A possible solution is offered by stability assessment systems that can help the skipper to identify potential risks and support his decision making process.The metacentric height is a key parameter for vessel stability and its real-time monitoring may be beneficial for alerting the crew about changes in stability. The paper proposes the design of a novel stability monitoring system that automatically detects changes in metacentric height based on estimates of the roll natural frequency solely using the measured roll angle. The core of the monitoring system is a combined estimation-detection system that exploits methods in advanced signal processing and statistical change detection to properly address issues of robustness.To analyze the monitoring performance, a nonlinear mathematical model of a stern trawler is used to generate roll motion time series in beam irregular waves of different peak period and significant wave height. Estimation and detection results obtained on the simulated data show a convincing ability of the monitoring system in discerning between safe and unsafe sailing conditions for most of the investigated cases.

On the role played by the openings on the first frequency of historic masonry towers

The manuscript provides a wide database for historic masonry towers collecting their modal, mechanical and geometrical features. This large amount of data was collected according to a literature review, then summing up as many as 56 different case studies. The collected data was then used to identify the main physical parameters influencing the modal behaviour of slender masonry towers, including the openings on the facades. After a critical discussion of the data reported in the database, the existing empirical or semi-empirical formulations available for the estimation of the first natural frequency of such structural typology are first evaluated. Subsequently the effects of the openings, as revealed through the analysis of the experimental results reported in the database, are discussed by comparison with a simple but effective numerical model. Despite the difficulties to quantify the effect of the openings along the height of the tower on the main frequency due to a cross-contribution of mass and stiffness, a simple but effective formulation is proposed which is able to account for this parameter.

Wave spectral analysis for design of a spar floating wind turbine in Mediterranean Sea

Floating offshore wind turbines (FOWTs) represent one promising concept for renewable energy production. However, the quantitative description of their dynamic behavior is extremely challenging, since it depends on the coupled effects of turbine aerodynamics, control system, hull hydrodynamics and mooring dynamics. In this context, hydrodynamic damping is a major source of uncertainty, particularly for relatively slender structures, such as spars. The aim of the present paper is to provide quantitative estimations of hydrodynamic damping ratios, natural frequencies and response spectra of the six rigid body motions of a spar FOWT under a wide range of wave conditions, including wind-generated sea states, swells and mixed seas. To this aim, the results of an open-sea experiment on a 1:30 model of OC3-Hywind spar in parked rotor conditions, carried out at the Natural Ocean Engineering Laboratory (NOEL) of Reggio Calabria (Italy), are analyzed and discussed. The above-mentioned dynamic properties of the model are estimated through the spectral analysis of the rigid body motion time series collected during the experiment, classified depending on the input wave conditions. As a result, quantitative estimations of the dynamic properties of the model are provided, which can be used as input values for numerical models of the full-scale structure. Hydrodynamic damping ratios are estimated, including their variability depending on sea conditions, due to nonlinearity. Also, motion response spectra obtained for wind-generated waves can be easily scaled-up to achieve a reasonable representation of the full-scale structure response under a range of relevant design conditions. Finally, additional information about nonlinear effects, coupling between degrees of freedom and close-to-resonance behavior of the model are drawn and commented.

Distributed-mass payload dynamics and control of dual cranes undergoing planar motions

Abstract Unwanted oscillations caused by the operator-commanded motions lower the capable operating speeds and degrade the safety of dual cranes moving distributed-mass payloads. Mounting numbers of works have been devoted to the dynamics and control of dual cranes. However, few studies have been dedicated to the accurate estimation of natural frequency, which is a critical factor for controlling the dual cranes. This paper develops a new method to predict the natural frequency of dual planar cranes with different cable lengths. Furthermore, a modified extra-insensitive (MEI) input shaper is proposed to suppress the oscillations of the payload swing and pitch in the dual cranes. The MEI shaper has good robustness to large changes in the frequency, and has good performance in the jerk limitation. Simulated and experimental investigations are performed on small-scale dual cranes transporting a slender beam to validate the system dynamic behavior and the effectiveness of the MEI shaper.

Torsional natural frequencies by Holzer method

In this work is presented the estimation of the torsional natural frequencies, eigenvalues of the equation of motion, as a result of the modal analysis of a mechanical system, by the iterative method Holzer. A numerical model in Workbench ANSYS 14 is used to evaluate the effectiveness of the Holzer method in detecting the torsional natural frequencies. The system consists of a motor coupled to a flywheel by means of a rigid coupling, which has been simplified in a torsional equivalent system of inertial discs and torsional flexible shaft sections. The results obtained by the Holzer method are in good agreement with the results for the numerical case implemented in ANSYS.

Learning to See the Vibration: A Neural Network for Vibration Frequency Prediction.

Vibration measurement serves as the basis for various engineering practices such as natural frequency or resonant frequency estimation. As image acquisition devices become cheaper and faster, vibration measurement and frequency estimation through image sequence analysis continue to receive increasing attention. In the conventional photogrammetry and optical methods of frequency measurement, vibration signals are first extracted before implementing the vibration frequency analysis algorithm. In this work, we demonstrate that frequency prediction can be achieved using a single feed-forward convolutional neural network. The proposed method is verified using a vibration signal generator and excitation system, and the result compared with that of an industrial contact vibrometer in a real application. Our experimental results demonstrate that the proposed method can achieve acceptable prediction accuracy even in unfavorable field conditions.

Оценка собственных частот крутильных колебаний композиционного нелинейно вязкоупругого вала

Оценка собственных частот крутильных колебаний композиционного нелинейно вязкоупругого вала

A method for an accurate estimation of natural frequencies using swept-sine acoustic excitation

Abstract Early detection of cracks in engineering structures by vibration-based methods requires monitoring systems that can evaluate natural frequencies with high accuracy. This paper introduces a method for estimating the natural frequencies using features extracted from the vibrational response of structures excited to cross through the resonance. To this aim, an open-loop acoustic system able to ensure bidirectional excitation, in swept-sine or short-time mode, with controllable parameters was developed. Analyzing the structural response at the crossing area through the resonance, two particularities on which the method is based have been revealed. The first refers to the symmetry of the frequency shift at peak amplitude from the natural frequency, if positive and negative sweep rates are applied. Therefore, the natural frequency can be found as the average of the frequency values at the resonance. Also, the necessary time to achieve the peak amplitude after passing through the natural frequency is similar, irrespective to the sign of the sweep rate. This accomplishment allows finding the natural frequency from a graphical representation. Frequency estimation made for a test structure by employing the two procedures lead to convergent results. Comparing these results to those achieved by impulsive excitation, a better precision was remarked at the swept-sine excitation. With regard the simplicity, rapidity and stability of the method, the swept-sine excitation has been proved as the most effective.

Yule Series Model Application for Blade Tip Timing Readouts Processing Resulting in Blade Oscillation Characteristics Estimation

This paper deals with an application of the Yule series model for contactless (blade tip timing, BTT) measurements. The approach creates a straightforward processing procedure from the experimental data to obtain estimates of blade logarithmic decrement and natural frequency. The existing analytical expressions for confidence intervals enable a priori evaluation for the accuracy of blade oscillation characteristics estimates. Proposed sensors allocation scheme ensures oscillation sampling rates providing efficient estimates of oscillation characteristics. Real-time measurements of blade logarithmic decrement and natural frequency give a chance of this method implementation for research and health monitoring purposes.

High performance estimations of natural frequency of basalt FRP laminated plates with intermediate elastic support using response surfaces method

Studying the changes of the natural frequencies due to intermediate elastic support of laminated composites is usually need a lot of computational processes or difficult to estimate. The present study employs a new high performance method for natural frequency estimating in basalt fiber reinforced polymer (FRP) laminated, variable thickness plates with intermediate elastic support based on the finite strip transition matrix (FSTM) with response surfaces (RS) method. Author has found that the FSTM method is very effective. However, a large error of estimation remains for estimation of natural frequency due to the large number of an iteration implemented in FSTM algorithm to estimate the natural frequency. In the present study, a new data processing procedure is proposed to improve performance of estimations of natural frequency. The estimation responses for four of classical boundary conditions at the plate ends with different elastic restraint coefficients (KT) are computed to obtain the first six frequency parameters (Ω). As a result, the method reveals excellent performance of estimations of natural frequencies.