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Abstract The combination of adaptive feedforward with adaptive feedback disturbance compensation has been suggested since a number of years as a potential solution for improving the performance of active vibration control systems. Unfortunately, as shown in the present paper there is a strong coupling between adaptive feedback and adaptive feedforward which may lead to instabilities and new algorithms which take into account this interaction are proposed in the paper. These algorithms take advantage of the Youla-Kučera parametrization of the feedback controller and feedforward compensator. First, the adaptive feedback algorithm is presented alone. Then, the adaptive feedforward algorithm is designed taking into account the feedback controller's presence in the loop. Stability of the complete scheme is obtained by appropriately filtering the regressor vectors in order to take into account a strictly positive real condition. Experimental results obtained on a relevant test-bench illustrate the performance of this approach to active vibration control. Comparison with adaptive feedback alone, with adaptive feedforward alone and adaptive feedforward + fixed feedback are provided. [ABSTRACT FROM AUTHOR]

This paper presents a time-domain method for multi-exciter random environment tests. Traditional random environment test theory has been formulated in the frequency domain, where an important step is taking the inverse of the frequency response function matrices (FRFMs). The accuracy of this inversion tends to be poor, particularly at frequencies near lightly damped resonances. The currently used control algorithms face difficulties in suppressing abnormal spectral lines caused by this inverse problem. In this paper, traditional formulations of the environment test are reformed, and the time-domain method is adopted; this results in a more precise inverse operation in environment tests. To achieve this, reference spectra are converted into time-domain response signals. The finite long driving signals are derived by the state-space method with estimated state vectors. During the process, the inverse of rank-deficient Toeplitz matrices are stabilized with truncated singular value decomposition (TSVD) to suppress all abnormally high-level components in the driving forces; thus, overall, the spectra lines produced by noise within the frequency band are filtered out. A numerical simulation of a single-axis random vibration test of a cantilever beam is conducted using the traditional frequency-domain procedure (FDP) and the proposed time-domain procedure (TDP). The response spectra generated by both procedures are tested by control algorithms, and the result shows that responses generated by the proposed TDP are more easily controlled. The conditions of stability for both the FDP and the TDP are also determined and introduced in the simulation. Moreover, a multi-axis vibration experiment further validates the effectiveness of the TDP. [ABSTRACT FROM AUTHOR]

The robustness of dynamical systems against external perturbations is crucial in engineering; however, it is often overlooked for the lack of methods for rapidly computing it. This paper proposes a novel algorithm for estimating the robustness of systems subject to time delay. More precisely, the algorithm iteratively estimates the so-called local integrity measure (LIM), that is, the radius of the largest hypersphere centered at the fixed point and located within its basin of attraction. Since time-delayed systems are infinite dimensional, initial conditions are restricted to a constrained type of initial functions. The semi-discretization method is used for rapidly simulating the dynamics of the systems, while trajectories are classified as converging or diverging using a subdivision of the reduced phase space into cells. The algorithm was tested on four different mechanical systems, and in all cases it very quickly provided an accurate estimation of the LIM. Moreover, it enabled the study of LIM trends in a multi-dimensional parameter space, which would have been unfeasible with alternative methods. This breakthrough in computational efficiency has important implications for engineering design, allowing for careful consideration of dynamical integrity and enhancing the safety and reliability of engineered systems, especially in the presence of time delays. • An algorithm for estimating the dynamical integrity of equilibria is developed. • The algorithm works for time-delayed dynamical systems. • The algorithm works iteratively, it is very quick and has no memory issues. • It enables one to consider dynamical integrity during engineering design. • Several case studies demonstrate the effectiveness of the algorithm. [ABSTRACT FROM AUTHOR]

In this paper, a combined numerical and experimental method, called Extended Load Confluence Algorithm, is presented to accurately predict the dynamic response of non-periodic structures when little or no information about the applied loads is available. This approach, which falls into the category of Shape Sensing methods, inputs limited experimental information acquired from sensors to a mapping algorithm that predicts the response at unmeasured locations. The proposed algorithm consists of three major cores: an experimental core for data acquisition, a numerical core based on Finite Element Method for modeling the structure, and a mapping algorithm that improves the numerical model based on a modal approach in the frequency domain. The robustness and precision of the proposed algorithm are verified through numerical and experimental examples. The results of this paper demonstrate that without a precise knowledge of the loads acting on the structure, the dynamic behavior of the system can be predicted in an effective and precise manner after just a few iterations. [ABSTRACT FROM AUTHOR]

While the vibration power for a set of harmonic force and velocity signals is well defined and known, it is not as popular yet for a set of stationary random force and velocity processes, although it can be found in some literatures. In this paper, the definition of the vibration power for a set of non-stationary random force and velocity signals will be derived for the purpose of a time–frequency analysis based on the definitions of the vibration power for the harmonic and stationary random signals. The non-stationary vibration power, defined as the short-time average of the product of the force and velocity over a given frequency range of interest, can be calculated by three methods: the Wigner–Ville distribution, the short-time Fourier transform, and the harmonic wavelet transform. The latter method is selected in this paper because band-pass filtering can be done without phase distortions, and the frequency ranges can be chosen very flexibly for the time–frequency analysis. Three algorithms for the time–frequency analysis of the non-stationary vibration power using the harmonic wavelet transform are discussed. The first is an algorithm for computation according to the full definition, while the others are approximate. Noting that the force and velocity decomposed into frequency ranges of interest by the harmonic wavelet transform are constructed with coefficients and basis functions, for the second algorithm, it is suggested to prepare a table of time integrals of the product of the basis functions in advance, which are independent of the signals under analysis. How to prepare and utilize the integral table are presented. The third algorithm is based on an evolutionary spectrum. Applications of the algorithms to the time–frequency analysis of the vibration power transmitted from an excitation source to a receiver structure in a simple mechanical system consisting of a cantilever beam and a reaction wheel are presented for illustration. [ABSTRACT FROM AUTHOR]

Abstract: The key novel element of this work is the introduction of robust multivariate statistical methods into the structural health monitoring (SHM) field through use of the minimum covariance determinant estimator (MCD) and the minimum volume enclosing ellipsoid (MVEE). In this paper, robust outlier statistics are investigated, focussed mainly on a high level estimation of the “masking effect” of inclusive outliers, not only for determining the presence or absence of novelty-something that is of fundamental interest but also to examine the normal condition set under the suspicion that it may already include multiple abnormalities. By identifying and detecting variability at an early stage, the prospects of achieving good generalisation and establishing a correct normal condition classifier may be increased. It is critical to highlight that there is no a priori division between the damaged and the undamaged condition data when the algorithms are implemented, offering a significant advantage over other methodologies. In summary, this paper introduces a new scheme for SHM by exploiting robust multivariate outlier statistics in order to investigate if the selected features are free from multiple outliers before such features can be selected for either supervised or unsupervised analysis. [Copyright &y& Elsevier]

Abstract: Mitigation of the effects of tonal noise and vibration has importance across a wide range of application domains. For low frequency disturbances, active control methodologies have proven to be highly effective when coupled with harmonic algorithms that target specific problematic frequencies. In this paper a novel harmonic control approach is presented that combines a classical exponential forgetting recursive least squares (ExF-RLS) estimator with an optimal gradient descent update law. The paper provides a rigorous proof that with a sinusoidal reference, the ExF-RLS estimator can be approximated by a fixed parameter filter and that the accuracy of the approximation is dependent upon the values of both the forgetting factor and the sampling frequency. With this approximation it is also proven that the complete iterative frequency domain harmonic control algorithm, when used with an instantaneous update, can be accurately represented by a linear time-invariant (LTI) feedback compensator. Such representations have clear importance since the stability, performance, convergence and robustness properties of the adaptive system can thereby be examined using well-established LTI analysis tools. The algorithm and the analytical results described above are illustrated in the paper using an experimental active vibration isolation mount that has been developed for marine systems application. [Copyright &y& Elsevier]

The multiplicative ℓ q -regularization has been recently introduced in structural dynamics for solving force reconstruction problems. Practically, the resolution of this regularization strategy requires the implementation of an iterative procedure. To this end, an Iteratively Reweighted Least-Squares algorithm has been originally implemented. The core idea of this algorithm is to replace the direct resolution of the inverse problem by an equivalent iterative procedure having an explicit and unique solution at each iteration. However, the exploitation of this very general idea allows defining other Iteratively Reweighted schemes. The present paper aims at comparing the overall performances of three particular Iteratively Reweighted algorithms for solving force reconstruction problems derived from a more general and original iterative procedure. The numerical applications proposed in this contribution highlight the ability of the considered algorithms in providing consistent regularized solutions with respect to various parameters such as the measurement noise level or the tolerance chosen to stop the iterative process. • A general Iteratively Reweighted ℓ p algorithm is presented. • General recommendations are given to properly applied the proposed strategy. • The inverse problem is expressed as a multiplicative regularization problem. • The proposed method can be seen as a generalization of the IRLS algorithm. • IR ℓ p algorithm for p ≤ 1 is the better option for sparse excitation fields. [ABSTRACT FROM AUTHOR]

Abstract Extracting impulsive information under strong background noise and harmonic interference is a challenging problem for bearing fault diagnosis. Multi-scale transforms have achieved great success in extracting impulsive feature information, however, how to choose a suitable transform is a difficult problem, especially in the case of strong noise interference. Therefore, dictionary learning methods have attracted more and more attention in recent years. A weighted multi-scale dictionary learning model (WMSDL) is proposed in this paper which integrates the multi-scale transform and fault information into a unified dictionary learning model and it successfully overcomes four disadvantages of traditional dictionary learning algorithms including lacking the multi-scale property; restricting training samples to local patches; being sensitive to strong harmonic interference; suffering from high computational complexity. Moreover, algorithmic derivation, computational complexity and parameter selection are discussed. Finally, The effectiveness of the proposed method is verified by both the numerical simulations and experiments. Comparisons with other state-of-the-art methods further demonstrate the superiority of the proposed method. [ABSTRACT FROM AUTHOR]

Abstract Nonlinear normal modes (NNMs) of a mechanical structure provide a mean to understand nonlinear vibrational phenomena measured experimentally. As they are an interesting extension of the linear normal modes (LNMs), numerical methods are needed to estimate these oscillatory motions, generally including continuation aspects with respect to the circular frequency. In this paper, a continuation method combined to a model reduction based on Proper Generalized Decomposition (PGD) technique is described. This formulation mixes PGD, harmonic balance method (HBM) and basic continuation techniques in order to reach a highly reduced description of the NNMs. This PGD/HBM-based continuation algorithm includes a modal enrichment as the NNM energy grows. The method is applied to two conservative problems including a cubic spring and a unilateral contact. Highlights • An algorithm to compute Nonlinear Normal Modes using few variables is exposed. • A PGD combined to HBM method describes the modes. • Continuation with on the fly enrichment keeps the number of variable very low. • Nonlinear Normal Modes of beams with localised non linearities are computed. [ABSTRACT FROM AUTHOR]

Abstract With the increasing use of composite material and external energy dissipation device in civil structures, the classical Rayleigh damping becomes an inappropriate assumption in structural damage identification. In this paper, a parametric sensitivity-based approach is developed to identify the structural damage and damping defect of non-classically damped shear-type building simultaneously. As the preliminary step, the complex eigenvalues and eigenvectors of damaged structure are obtained by a novel identification technique, which decouples the first-order modal analysis equation under physical coordinate so that the complex modal parameters of each mode can be identified independently. Thereafter, by analysing the sensitivities of complex eigenparameter, non-linear least square method is applied to identify the locations and magnitudes of damage with respect to stiffness reduction and damping defect. The proposed identification approach is illustrated by identifying the damage in a numerical five-storey shear building. The identification results show that the proposed approach can locate and quantify the stiffness reductions satisfactorily even when measurement noise is taken into account. However, in order to quantify the damping defects consistently, their locations need to be known as a priori. Highlights • A novel complex eigenparameter identification approach under state-space coordinate. • Damage identification on non-proportionally damped shear-type building. • Identify the stiffness reduction and damping defect simultaneously. • Identify the design parameters which locate and quantify the damages. • Numerical validation on a five-storey shear building by considering measurement noise. [ABSTRACT FROM AUTHOR]

Abstract Finite element (FE) model updating has emerged as a powerful technique for structural health monitoring and damage identification of civil structures. Updating mechanics-based nonlinear FE models allows for a complete and comprehensive damage diagnosis of large and complex structures, but it is computationally demanding. This paper first introduces an Iterated Extended Kalman filter (IEKF) to update mechanics-based nonlinear FE models of civil structures. Different model updating techniques using the Extended Kalman filter (EKF), Unscented Kalman Filter (UKF) and IEKF, are then compared for their performance in terms of convergence, accuracy, robustness, and computational demand. Finally, a non-recursive estimation procedure is presented and its effectiveness in reducing the computational cost, while maintaining accuracy and robustness, is demonstrated. An application example is presented based on numerically simulated response data for a three-dimensional 5-story 2-by-1 bay reinforced concrete (RC) frame building subjected to bi-directional earthquake excitation. Excellent estimation results are obtained with the EKF, UKF, and IEKF used in conjunction with the proposed non-recursive estimation approach. Because of the analytical linearization used in the EKF and IEKF, abrupt and large jumps in the estimates of the model parameters are observed with these filters, which may lead to divergence of the nonlinear FE model solution procedure. The UKF slightly outperforms the EKF and IEKF, but at a higher computational cost. [ABSTRACT FROM AUTHOR]

Abstract: General computational multibody system (MBS) algorithms allow for the linearization of the highly nonlinear equations of motion at different points in time in order to obtain the eigenvalue solution. This eigenvalue solution of the linearized equations is often used to shed light on the system stability at different configurations that correspond to different time points. Different MBS algorithms, however, employ different sets of orientation coordinates, such as Euler angles and Euler parameters, which lead to different forms of the dynamic equations of motion. As a consequence, the forms of the linearized equations and the eigenvalue solution obtained strongly depend on the set of orientation coordinates used. This paper addresses this fundamental issue by examining the effect of the use of different orientation parameters on the linearized equations of a gyroscope. The nonlinear equations of motion of the gyroscope are formulated using two different sets of orientation parameters: Euler angles and Euler parameters. In order to obtain a set of linearized equations that can be used to define the eigenvalue solution, the algebraic equations that describe the MBS constraints are systematically eliminated leading to a nonlinear form of the equations of motion expressed in terms of the system degrees of freedom. Because in MBS applications the generalized forces can be highly nonlinear and can depend on the velocities, a state space formulation is used to solve the eigenvalue problem. It is shown in this paper that the independent state equations formulated using Euler angles and Euler parameters lead to different eigenvalue solutions. This solution is also different from the solution obtained using a form of the Newton–Euler matrix equation expressed in terms of the angular accelerations and angular velocities. A time-domain solution of the linearized equations is also presented in order to compare between the solutions obtained using two different sets of orientation parameters and also to shed light on the important issue of using the eigenvalue analysis in the study of MBS stability. The validity of using the eigenvalue analysis based on the linearization of the nonlinear equations of motion in the study of the stability of railroad vehicle systems, which have known critical speeds, is examined. It is shown that such an eigenvalue analysis can lead to wrong conclusions regarding the stability of nonlinear systems. [Copyright &y& Elsevier]

Abstract: This paper investigates the dispersion relations of axisymmetric wave propagation in initially twisted bi-material compound cylinders, within the scope of the piecewise homogeneous body model, utilizing 3D linearized wave propagation theory in an initially stressed body. It is assumed that the constituents of the compound cylinder are isotropic and homogeneous. The corresponding eigen-value problem is formulated and the solution method for this problem as well as the algorithm for obtaining the numerical results is developed. The numerical results illustrate the influence of the initial twisting of the compound cylinder on the dispersion of the axisymmetric wave propagation as presented and discussed in the present paper. In particular, it is established that as a result of the existence of the initial twisting, at least in one constituent of the considered compound cylinder, the axisymmetric longitudinal and torsional waves cannot propagate separately, i.e. the new type axisymmetric waves, which differ from the axisymmetric torsional and longitudinal waves, must appear. [Copyright &y& Elsevier]

Abstract In the process of vibration suppression of hot galvanized steel strip by using maglev technology, the flexible modal vibration control of steel strip is a challenging problem. Aiming at this problem, this paper proposes a phase compensation control algorithm to control the modal vibration in the maglev steel strip vibration suppression system. The modal vibration suppression process is as follows. First, the model of the steel strip and the mode shape and frequency response curve of the strip are established by using the finite element analysis software. Second, the electromagnet-steel strip model is simplified as a single degree of freedom maglev system. Then, the equivalent damping is obtained according to the theory. And the vibration is suppressed according to the principle of the phase compensator. In addition, the effect of the phase lead angle, compensating field and scale factor on the damping performance of the phase compensator from the zero-pole of the closed-loop system is studied. Finally, the experimental study is carried out on the vibration suppression system of maglev steel strip based on phase compensator. The experimental results show that the phase compensator with appropriate parameters can be selected to reduce the amplitude of the first order bending mode by 89.2%. [ABSTRACT FROM AUTHOR]

Abstract: Structural damage detection methods based on vibration responses are appealing for a variety of reasons such as their potential to observe damage from sensors placed remote from an unknown damage site. Of particular interest to the authors is online damage detection in which changes in the structure can be flagged up in an automated fashion by permanently installed transducers. In a previous paper by the authors, the inner product vector (IPV) was proposed as a damage detection algorithm which uses cross correlation functions between response measurements. Implicitly assumed in the formulation is that the response quantity is that of displacement resulting from white noise excitation. In this paper, the IPV technique is first reviewed and then generalised to address velocity and acceleration response to band pass white noise excitation. It is shown that the IPV is a weighted summation of the mode shapes, and the effect of some particular measurement noise on the IPV can be adaptively eliminated in the calculation of IPV. Then, the damage detection method based on changes in the IPV is proposed. Finally, damage detection experiments of shear frame structure, honeycomb sandwich composite beam and aircraft stiffened panel are presented to illustrate the feasibility and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Abstract: Variable inertia vibration absorbers (VIVA) are previously used for the vibration control of single degree of freedom (dof) primary systems. The performance of such absorbers is studied in many investigations. This paper presents the dynamic modeling and simulation of a proposed modified design of such VIVA’s for the vibration control of two dof primary systems. Lagrange formulation is used to obtain its dynamic model in an analytical form. This model, which is highly nonlinear, is used to develop a computational algorithm to study the absorber performance characteristics. This algorithm is programmed and simulated in Matlab. The obtained results are numerically verified using SAMS2000 software. The effect of mass and stiffness of the proposed VIVA on its performance and tuning is discussed. An optimization algorithm is developed to select the best absorber parameters for vibration suppression of a specific primary system. The obtained results show a good agreement with those obtained using similar techniques. In addition, a linearized model of VIVA dynamics is developed, tested and simulated for the same data used in its nonlinear model. The relative deviation between results of the linear and nonlinear models is less than 1%, which confirms the realistic use of this linearized model. The experimental testing and verification of the simulation results of the proposed VIVA is the subject of another paper. [Copyright &y& Elsevier]

Abstract: This paper investigates the effect of unintended high frequency excitations generated by the semi-active sky-hook control algorithm on the isolation properties of a car suspension. Using a quarter car model, an energy transfer from low to high frequency is established. An alternative algorithm is presented in this paper, in order to achieve smooth variations of the control force. Compared with the continuous semi-active sky-hook, the algorithm has been found to provide a similar damping of the body resonance, but also a better isolation at high frequency and global comfort. [Copyright &y& Elsevier]

Abstract: This paper is concerned with decentralised velocity feedback for the control of vibration on a flexible structure. Previous studies have shown that a direct velocity feedback loop with a collocated force actuator produces a damping action. Multiple velocity feedback control loops thus reduce the vibration and sound radiation of structures at low frequency resonances, where the response is controlled by damping. However, if the control gains are too high, so that the response of the structure at the control point is close to zero, the feedback control loops will pin the panel at the control positions and thus no damping action is generated. Therefore, in order to maximise the active damping effect, the feedback gains have optimum values and the loops need to be properly tuned. In this paper, a numerical investigation is performed to investigate the possibility of self-tuning the feedback control gains to maximise the power absorbed by the control loops and hence maximise the active damping. The tuning principle is first examined for a single feedback loop for different excitation signals. The tuning of multiple control loops is then considered and the implementation of a practical tuning algorithm is discussed. [Copyright &y& Elsevier]

This paper discusses the effectiveness of an Active Structural Acoustic Control (ASAC) system for the reduction of repetitive impact noise, radiated by structures with a high modal density in the controlled frequency range. Although there is a significant difference in nature between periodic and transient noise, up till now no specific research on ASAC of transient noise was reported. The development of the ASAC system is divided into two phases: the definition of the control configuration and the design of a suitable control algorithm. The optimal control configuration as well as the implemented control algorithm for the reduction of impact noise differ significantly from the common solutions in periodic noise control. In the first part of the paper, a practical methodology is presented to define a good control arrangement for transient noise control. The second part of the paper focuses on the design of control algorithms, adapted to the specific properties of impact noise. Since many industrial impact noise problems involve successive impacts with a repetitive behaviour, control algorithms with a learning behaviour are discussed. The efficiency of these Iterative Learning Control (ILC) algorithms is extensively demonstrated in this paper. The developed ASAC strategy has been verified on a thick steel plate, which is excited by successive impacts. The obtained results show that ASAC can be a very efficient transient noise control technique in certain industrial applications (e.g. presses, punching machines, etc.). [Copyright &y& Elsevier]

Abstract: This paper presents a neural-based filtered-X least-mean-square algorithm (NFXLMS) to cancel the nonlinear broadband noise in an active noise control (ANC) system. The ways to avoid the premature saturation of backpropagation algorithm and to design the optimal learning rate are also included in the paper to improve the noise reduction performance. Besides, the proposed neural filter can be easily implemented and versatile to the other applications. Several simulation results show that the proposed method can effectively cancel the narrowband and nonlinear broadband noise in an ANC system. [Copyright &y& Elsevier]

Abstract: This paper validates an approach to damage detection and localization based on finite-element model updating (FEMU). The approach has the advantage over other existing methods to FEMU that it simultaneously updates all three finite-element model matrices at the same time preserving their structure (connectivity), symmetry and positive-definiteness. The approach is tested in this paper on an experimental setup consisting of a steel cable, where local mass changes and global change in the tension of the cable are introduced. The new algorithm is applied to identify the size and location of different changes in the structural parameters (mass, stiffness and damping). The obtained results clearly indicate that even small structural changes can be detected and localized with the new method. Additionally, a comparison with many other FEMU-based methods has been performed to show the superiority of the considered method. [Copyright &y& Elsevier]

Abstract: In this paper, an improved damage quantification methodology for a plate structure is presented. The methodology utilizes the relationship between the stiffness loss and the fractional changes of the modal parameters due to damage. To improve the damage quantification performance, the methodology is derived by eliminating erroneous assumptions in the existing modeshape-based methods and adopting additional modal information, i.e., natural frequencies. The basic elements of the approach are summarized in this paper, and the validity of the proposed method is demonstrated using numerical data from a simply-supported plate structure. In the numerical verification, the damage quantification performance of the proposed method is compared with the performance of other existing methods. Also, the effects of incomplete measurement and noisy modal test data on the accuracy of damage quantification are discussed. The result shows the proposed method yields superior damage quantification results to the existing algorithms. [Copyright &y& Elsevier]

Abstract: We report the results of a recent study for the active vibration isolation with whole-spacecraft vibration isolation as an application background into which three parts are divided: (i) energy source control, (ii) nonlinearity and time delay, (iii) implementation and experiment. This paper is the first in this three-part series report, which presents theoretical and experimental investigations into pressure tracking system for energy source control of the isolator. Considering the special environment of the rocket and expected characteristics of actuators, where the isolator will be arranged between the rocket and the spacecraft, pneumatic actuator is proposed to realize the active isolation control. In order to improve the dynamic characteristics of the pneumatic isolator, a cascade control algorithm with double loop structure and predictive control algorithm for pressure tracking control of the inner loop are proposed. In the current paper, a pressure tracking control system using model predictive control (MPC) is studied first. A pneumatic model around pressure work point is built firstly by simplifying the flow equation of valve''s orifices and pressure differential equation of the chambers. With this model, an MPC algorithm in the state space is developed, and problems including control parameter choice and command horizon generator are discussed in detail. In addition, by adding model error correction loop and velocity compensation feedback, effects of model uncertainty and volume variation of chambers are reduced greatly. Thus with this design, the real-time pressure tracking can be guaranteed, and so that the active control system can work at higher frequency range. [Copyright &y& Elsevier]

Abstract: The basic principle of feedforward noise cancellation for broadband active noise reduction is based on the availability of a reliable measurement of the noise source. Reliability is compromised in case the noise measurement is acoustically coupled to the actual active noise cancellation (ANC), as stability and performance of the feedforward compensation becomes ambiguous. This paper presents a framework to recursively estimate a feedforward filter in the presence of acoustic coupling, addressing both stability and performance of the active feedforward noise cancellation algorithm. The framework is based on fractional model representations in which a feedforward filter is parameterized by coprime factorization. Conditions on the parameterization of the coprime factorization formulated by the existence of a stable perturbation enables stability in the presence of acoustic coupling. In addition, the paper shows how the stable perturbation can be estimated on-line via a recursive least-squares estimation of a generalized FIR filter to improve the performance of the feedforward filter for ANC. [Copyright &y& Elsevier]

Abstract: Isolation from disturbances, particularly from foundations of high precision instruments, is achieved through either passive or active vibration control systems. Although a passive isolation system offers a simple and reliable means of protecting precision equipment from a vibration environment, it has performance limitations since its controllable frequency range is limited. An effective method for reducing an oscillation is by using an active vibration isolation system, which allows control of the dynamic rigidity of shock absorbers. In this paper, by considering the characteristics of the disturbing influences acting upon vibro-isolated objects, the dynamic characteristics of the AVIS device and control restriction, new optimal and quasi-optimal control algorithms are proposed. The characteristics of the new quasi-optimal active vibration isolation system proposed in the paper are investigated via experiments. It is shown that the adopted quasi-optimal active vibration isolation system can improve performance using in situ measurements. [Copyright &y& Elsevier]

The purpose of this paper is to present the results of a comparative study of various formulations of a frequency-domain least-squares (LS) estimator. This study was the basis for the optimization of the LS algorithm for modal parameter estimation. Nowadays, modal analysis studies the dynamical behaviour of very complex structures, which requires both robustness for high model orders and efficient numerical properties to analyse extensive data sets that are obtained during high channel-count modal testing. This paper focuses on the optimal choice between various transfer function models and a weighted formulation of the LS problem by taking also a measure for the noise on the data into account. The evaluation is based on a practical case study, discussing the dynamical modelling of slat tracks of an Airbus A320 commercial aircraft. This safety critical component is characterized by a complex modal behaviour requiring both high model orders and a high spatial resolution in order to achieve an accurate model. [Copyright &y& Elsevier]

The present paper describes a procedure to consider initial conditions in the frequency-domain analysis of continuous media discretized by the FEM. The frequency-domain formulation presented here is based on a standard DFT procedure, the FFT algorithm being employed to transform from time to the frequency domain and vice versa. The standard Galerkin finite element method (displacement model) is used to replace the original differential governing equation by an integral equation amenable to numerical solution. The scalar wave equation (one- and two-dimensional) is used to illustrate the proposed approach. At the end of the paper, examples of wave propagation for a taut string, a one-dimensional rod and a membrane are presented to illustrate the robustness of the formulation presented here. [Copyright &y& Elsevier]

This paper deals with the generalization of the field method to weakly non-linear vibrational systems with one degree of freedom. The field co-ordinate and field momentum approaches are combined with the method of multiple time scales in order to obtain the amplitude and phase of oscillations in the first approximation. Apart from the fact that the algorithm of obtaining motion is condensed for the cases of a free vibrational system and system with slowly varying parameters, in this paper the field method is extended to a parametrically excited system and system with external excitation. [Copyright &y& Elsevier]

The performance of the independent modal space control (IMSC) algorithm for structural vibration control is examined in this paper. Both the theoretical analysis and numerical simulation show that, for a multi-degree-of-freedom system, the modal control forces may increase the contributions of the vibration of higher modes (uncontrolled modes) to the system response if the IMSC algorithm is used to design a structural control system. Therefore, the responses of the controlled structure may be underestimated if the effects of control forces on the higher modes are not considered in the response analysis. A new control algorithm—modified independent modal space control (MIMSC) algorithm is proposed in this paper for eliminating the effect of modal control force on the uncontrolled modes. Numerical example shows that the structural responses can be effectively reduced when control system design is carried out based on the proposed algorithm. By comparing the simulated results obtained by the IMSC and MIMSC algorithms, it is found that, in order to achieve the same control objective, the proposed algorithm is more effective than IMSC since the modal control forces do not have any effect on the uncontrolled modes. In order to verify the effectiveness of the proposed algorithm, a practical example—active control design of UCLA Math-Science Building is presented and discussed. [Copyright &y& Elsevier]

Blind deconvolution algorithms prove to be effective tools for fault identification, being able to extract excitation sources from noisy observations only. In this scenario, the present paper introduces a novel blind deconvolution method based on the generalized Rayleigh quotient and solved by means of an iterative eigenvalue decomposition algorithm. This approach not only is characterized by a weighting matrix that drives the deconvolution process, but can also be easily adapted to arbitrary criteria. Based on this framework, a novel criterion rooted on the maximization of the cyclostationarity of the excitation – as typically encountered with machine faults – is proposed and compared with other blind deconvolution methods existing in the literature. The comparisons involve both synthesized and real vibration signals, taking into account a gear tooth spall and an outer race bearing fault. The results reveal superior capability to recover impulsive cyclostationary sources with respect to other blind deconvolution methods, even in the presence of impulsive noise or under non-constant speed. [ABSTRACT FROM AUTHOR]

Many researchers have explored the use of active bearings, such as non-contact Active Magnetic Bearings (AMB), to control imbalance vibration in rotor systems. Meanwhile, the advantages of a passive Auto-balancer device (ABD) eliminating the imbalance effect of rotor without using other active means have been recently studied. This paper develops a new hybrid imbalance vibration control approach for an ABD-rotor system supported by a normal passive bearing in augmented with an AMB to enhance the balancing and vibration isolation capabilities. Essentially, an ABD consists of several freely moving eccentric balancing masses mounted on the rotor, which, at supercritical operating speeds, act to cancel the rotor's imbalance at steady-state. However, due to the inherent nonlinearity of the ABD, the potential for other, non-synchronous limit-cycle behavior exists resulting in increased rotor vibration. To address this, the algorithm of proposed hybrid control is designed to guarantee globally asymptotic stability of the synchronous balanced condition. This algorithm also incorporates with a “Luenberger-like” observer that continuously estimates the states of a balancer ball circulating around within ABD. In particular, it is shown that the balanced equilibrium can be made globally attractive under the hybrid control strategy, and that the control power levels of AMB are significantly reduced via the addition of the ABD because the control is designed such that it is only switched on for the abnormal operation of ABD and will be disengaged otherwise. Moreover, unlike other imbalance vibration control applications based upon ABD such as rotor speed regulator [21,22], this approach enables the controller to achieve the desirable performance without altering rotor speed once the rotor initially reaches the target speed. These applications are relevant to limited power applications such as in satellite reaction wheels, flywheel energy storage batteries or CD-ROM application. [ABSTRACT FROM AUTHOR]

Nowadays phased microphone arrays have become a standard technique for acoustic source mapping. Compared with the conventional delay-and-sum beamforming method, deconvolution approaches such as DAMAS successfully improve the spatial resolution, however require high computational effort. Without optimizing DAMAS algorithm, recently DAMAS with wavelet compression computational grid (denoted by DAMAS-CG1) has reduced significantly computational run time of DAMAS in applications (Ma & Liu, J. Sound Vib., 395, 2017), however DAMAS-CG1 has an inevitable deficiency that the occurrence probability of aliasing increases slightly for complicated sound source. This paper proposes a novel algorithm that DAMAS with computational grid compressed by discarding grid points with non-positive beamforming (denoted by DAMAS-CG2). Application simulations and an airfoil trailing edge noise experiment show that DAMAS-CG2 not only reduces significantly the computational run time but also retains the spatial resolution of DAMAS on original grid. Moreover DAMAS-CG2 overcomes the inevitable deficiency of DAMAS-CG1, and is simpler and more practical than DAMAS-CG1, although DAMAS-CG2 is usually less effective than DAMAS-CG1. [ABSTRACT FROM AUTHOR]

The order tracking method based on time-frequency representation is regarded as an effective tool for fault detection of bearings with varying rotating speeds. In the traditional order tracking methods, a tachometer is required to obtain the instantaneous speed which is hardly satisfied in practice due to the technical and economical limitations. Some tacholess order tracking methods have been developed in recent years. In these methods, the instantaneous frequency ridge extraction is one of the most important parts. However, the current ridge extraction methods are sensitive to noise and may easily get trapped in a local optimum. Due to the presence of noise and other unrelated components of the signal, bearing fault features are difficult to be detected from the envelope spectrum or envelope order spectrum. To overcome the abovementioned drawbacks, an adaptive and tacholess order analysis method is proposed in this paper. In this method, a novel ridge extraction algorithm based on dynamic path optimization is adopted to estimate the instantaneous frequency. This algorithm can overcome the shortcomings of the current ridge extraction algorithms. Meanwhile, the enhanced empirical wavelet transform (EEWT) algorithm is applied to extract the bearing fault features. Both simulated and experimental results demonstrate that the proposed method is robust to noise and effective for bearing fault detection under variable speed conditions. [ABSTRACT FROM AUTHOR]

The spectral decomposition algorithm presented in the paper may be applied to selected parts of the SPL spectrum, i.e. to specific noise generating mechanisms. It yields the propagation and the generation functions, and indeed the Mach number scaling exponent associated with each mechanism as a function of the Strouhal number. The input data are SPL spectra obtained from measurements taken during speed ramps. Firstly, the basic theory and the implemented algorithm are described. Then, the behaviour of the new method is analysed with reference to numerically generated spectral data and the results are compared with the ones of an existing method based on the assumption that the scaling exponent is constant. Guidelines for the employment of both methods are provided. Finally, the method is applied to measurements taken on a cooling fan mounted on a test plenum designed following the ISO 10302 standards. The most common noise generating mechanisms are present and attention is focused on the low-frequency part of the spectrum, where the mechanisms are superposed. Generally, both propagation and generation functions are determined with better accuracy than the scaling exponent, whose values are usually consistent with expectations based on coherence and compactness of the acoustic sources. For periodic noise, the computed exponent is less accurate, as the related SPL data set has usually a limited size. The scaling exponent is very sensitive to the details of the experimental data, e.g. to slight inconsistencies or random errors. [ABSTRACT FROM AUTHOR]

This paper presents the recent advances in reduction of multifrequency noise inside helicopter cabin using an active structural acoustic control system, which is based on active gearbox struts technical approach. To attenuate the multifrequency gearbox vibrations and resulting noise, a new scheme of discrete model predictive sliding mode control has been proposed based on controlled auto-regressive moving average model. Its implementation only needs input/output data, hence a broader frequency range of controlled system is modelled and the burden on the state observer design is released. Furthermore, a new iteration form of the algorithm is designed, improving the developing efficiency and run speed. To verify the algorithm's effectiveness and self-adaptability, experiments of real-time active control are performed on a newly developed helicopter model system. The helicopter model can generate gear meshing vibration/noise similar to a real helicopter with specially designed gearbox and active struts. The algorithm's control abilities are sufficiently checked by single-input single-output and multiple-input multiple-output experiments via different feedback strategies progressively: (1) control gear meshing noise through attenuating vibrations at the key points on the transmission path, (2) directly control the gear meshing noise in the cabin using the actuators. Results confirm that the active control system is practical for cancelling multifrequency helicopter interior noise, which also weakens the frequency-modulation of the tones. For many cases, the attenuations of the measured noise exceed the level of 15 dB, with maximum reduction reaching 31 dB. Also, the control process is demonstrated to be smoother and faster. [ABSTRACT FROM AUTHOR]

Microphone array methods aim at the characterization of multiple simultaneously operating sound sources. However, existing data processing algorithms have been shown to yield different results when applied to the same input data. The present paper introduces a method for estimating the reliability of such algorithms. Using Monte Carlo simulations, data sets with random variation of selected parameters are generated. Four different microphone array methods are applied to analyze the simulated data sets. The calculated results are compared with the expected outcome, and the dependency of the reliability on several parameters is quantified. It is shown not only that the performance of a method depends on the given source distribution, but also that the methods differ in terms of their sensitivity to imperfect input data. [ABSTRACT FROM AUTHOR]

The effectiveness of common algorithms for feedforward compensation of narrowband disturbance depends mainly on the model quality. To avoid this dependency several direct adaptive control algorithms without explicitly identified secondary path models have been developed over the last years. However an overview of their properties and a comparison of their performances in a standardized benchmark is still lacking. In this paper the three most promising algorithms are modified for narrowband feedforward vibration control for the use in rotating machinery. As in this application the reference signal is generated using the frequency measurement from a speed sensor it can be assumed that there is no coupling between reference measurement and the secondary path. First the algorithms are tested in simulation, then they are implemented on a test rig for active vibration control of unbalance induced rotor vibration. In simulation as well as for the test rig the performances of the algorithms are compared to each other. Advantages and drawbacks of the algorithms are discussed and practical instructions for implementation are given. The work is intended to serve as starting point and motivation for future research in this field of study. [ABSTRACT FROM AUTHOR]

This paper proposes a new methodology for moving force identification (MFI) from the responses of bridge deck. Based on the existing time domain method (TDM), the MFI problem eventually becomes solving the linear algebraic equation in the form Ax = b . The vector b is usually contaminated by an unknown error e generating from measurement error, which often called the vector e as ‘‘noise’’. With the ill-posed problems that exist in the inverse problem, the identification force would be sensitive to the noise e . The proposed truncated generalized singular value decomposition method (TGSVD) aims at obtaining an acceptable solution and making the noise to be less sensitive to perturbations with the ill-posed problems. The illustrated results show that the TGSVD has many advantages such as higher precision, better adaptability and noise immunity compared with TDM. In addition, choosing a proper regularization matrix L and a truncation parameter k are very useful to improve the identification accuracy and to solve ill-posed problems when it is used to identify the moving force on bridge. [ABSTRACT FROM AUTHOR]

In this paper, the vibration model of swing frame of two-plane balancing machine is established to calculate the vibration center position of swing frame first. The torsional stiffness formula of spring plate twisting around the vibration center is then deduced by using superposition principle. Finally, the dynamic balancing experiments prove the irrationality of A-B-C algorithm which ignores the torsion effect, and show that the torsional stiffness deduced by experiments is consistent with the torsional stiffness calculated by theory. The experimental datas show the influence of the torsion effect of swing frame on the separation ratio of sided balancing machines, which reveals the sources of measurement error and assesses the application scope of A-B-C algorithm. [ABSTRACT FROM AUTHOR]

This paper presents an explicit time-domain method for sensitivity analysis of structural responses under non-stationary random excitations. Based on time-domain explicit expressions of dynamic responses, a new and more concise time-domain explicit expression of response sensitivity is derived using the direct differentiation method (DDM). Then a more efficient algorithm for direct construction of the explicit expression of response sensitivity is developed based on the physical meanings of the coefficient matrices in the formulation. The adjoint variable method (AVM) is further used to establish the explicit expression of the sensitivity of an arbitrary response. Finally, based on the time-domain explicit expressions for both dynamic response and its sensitivity, an efficient time-domain approach is proposed to calculate the sensitivity of variance responses of a structure subjected to non-stationary random excitations. Numerical examples of different structural systems under non-stationary random excitations are presented to demonstrate the accuracy and efficiency of the proposed method. [ABSTRACT FROM AUTHOR]

An envelope synchronous average scheme for multi-axis gear faults detection is proposed in this paper. In the proposed approach, the vibration from a gearbox is picked up at first. Then, the complex envelope of the vibration is extracted by the fast kurtogram algorithm, which can determine an optimal resonant zone for amplitude demodulation. Subsequently, the envelope is re-sampled by the equal-angular increment sampling scheme to eliminate the possible frequency blur phenomena caused by the speed fluctuation. Moreover, a multi-axis synchronous average strategy is employed to remove the random components in the envelope. Finally, the vibration features of gear faults can be clearly exposed by the envelope order spectra. Compared with the conventional synchronous averaging scheme or the envelope analysis approach, the proposed method is robust for the incipient local fault detection of gears. Experiments are conducted to verify the validity of the proposed method. [ABSTRACT FROM AUTHOR]

This paper reviews the recent advances in computational methods for nonlinear normal modes (NNMs). Different algorithms for the computation of undamped and damped NNMs are presented, and their respective advantages and limitations are discussed. The methods are illustrated using various applications ranging from low-dimensional weakly nonlinear systems to strongly nonlinear industrial structures. [ABSTRACT FROM AUTHOR]

This paper outlines and compares four beamforming algorithms for accurately localising acoustic dipole sources in a three-dimensional domain, such as noise sources produced by flow–body interaction. These algorithms include conventional cross-spectral beamforming, conventional beamforming with deconvolution via CLEAN-SC, ‘multiplicative’ cross-spectral beamforming and multiplicative beamforming with CLEAN-SC. The latter two algorithms are novel to the field of aeroacoustics and rely on the mutual cancellation of spatially incoherent sources between orthogonally aligned microphone arrays to improve the quality of the source map. The algorithms were used on both synthetic and experimental data. By comparing the performance of each algorithm in terms of source localisation accuracy, source strength estimation and resolution, it was found that conventional beamforming with CLEAN-SC is the preferred method for beamforming aeroacoustic sources in three dimensions, albeit at a higher computational cost than the other three. The results also showed that multiplicative beamforming methods give source maps that are more interpretable than conventional cross-spectral beamforming methods at no extra computational expense. [ABSTRACT FROM AUTHOR]

This paper investigates the use of magnetorheological (MR) dampers to semi-actively control wind induced vibrations of a 1/20 scaled wind tower model. The idea consists in realizing a variable restraint at the base of the model, able to modify in real time its mechanical properties according to the instantaneous response of the tower. The restraint is made up of a cylindrical hinge, two springs and two prototype MR devices, the behavior of the latter being driven by control algorithms aiming to reduce the structural demand imposed to the steel tower. The demand is expressed in terms of base bending stress and top displacement. The model has been tested on a shaking table facility at the Denmark Technical University in Copenhagen imposing two different types of accelerograms at the base, equivalent to an extreme short operating gust and a longer high wind speed excitation, respectively. Two distinct control algorithms, each designed according to different philosophies and goals have been adopted to drive the semi-active devices. The results show the effectiveness of the proposed control technique in reducing the stress demand at the base, this at the cost, in the worst case, of a slight increase of top displacement. They encourage investigating further in this direction in order to promote the use of such strategy for a cost effective design of high wind towers. [ABSTRACT FROM AUTHOR]

In this paper, a feedback control algorithm to regulate oscillation amplitude of a microelectromechanical (MEMS) cantilever beam operated at parametric resonances is developed. The control objective is to drive the oscillation amplitude of the micro-beam, which is amplified using parametric excitation, to the desired values. The principle of the control algorithm is to establish an output tracking control based on the nonlinear dynamic model of the micro-beam, where the supply voltage is considered as a control input. The tracking control algorithm is designed to solve the singularities resulting from the zero-deflection state of the micro-beam. The Galerkin method is applied in order to reduce the partial differential equation describing the dynamics of the beam into a set of ordinary differential equations (ODEs). Uniformly ultimate boundedness stability of the control system is proved using Lyapunov method. The effectiveness of the proposed control algorithm is illustrated via numerical simulations. [ABSTRACT FROM AUTHOR]

• A novel two-stage sparse algorithm is proposed to localize and characterize the local nonlinearities in structures. • The Duhamel's integral is adopted to avoid performing the second-order derivative in the identification process. • A nonparametric de-noising method is developed to reduce the effect of noise. • The proposed algorithm for identification of local nonlinearities can be used to localize and characterize local damages in structures. This paper considers the localization and characterization of nonlinear components in local nonlinear structures with noisy measurements, and a novel two-stage sparse identification method is proposed. In this algorithm, one set of candidate nonlinear basis functions are leveraged to characterize system nonlinearities. To effectively localize and characterize nonlinear components, the algorithm introduces two sparse stages. At the first sparse stage, the adaptive group Lasso is adopted to localize nonlinear components in structures. At the second sparse stage, the adaptive Lasso is used to further characterize the contributing nonlinear basis functions in the identified nonlinear positions. Since the first sparse stage can greatly decrease the number of candidate nonlinear basis functions, the contributing functions can be effectively selected and characterized at the second sparse stage. Compared with the one-stage sparse algorithms, the proposed algorithm is more robust to noise, i.e., the new method can correctly identify the local nonlinear structures with a lower Signal to Noise Ratio (SNR). In addition, many existed nonlinear identification methods, simultaneously require the synchronous data of vibration displacements, velocities and accelerations. If only vibration displacements are measured, the first and second order derivatives of displacement are conducted to estimate velocities and accelerations, respectively. Nevertheless, the noise mixed into the displacement measurement will be severely amplified in the process of derivative calculation, which will make the estimation of velocities and accelerations deviating from the actual value and reduce the identification accuracy. To avoid performing the second order derivative, the Duhamel's integral is adopted to rewrite the dynamic relationship between the system input and output. In addition, a non-parametric de-noising method based on reproducing kernel Hilbert space (RKHS) is further developed to reduce the effect of noise in vibration displacements and obtain vibration velocities, which can effectively reduce the effect of noise and improve the identification accuracy. The numerical and experimental studies verify the potential and effectiveness of the proposed algorithm in localizing and characterizing nonlinear components in local nonlinear structures. [ABSTRACT FROM AUTHOR]

This paper presents a multi-objective control method with wheelbase preview for active vehicle suspension. A four-degree-of-freedom half-car model with active suspension is considered in this study. H∞ norm and generalized H2 norm are used to improve ride quality and ensure that hard constraints are satisfied. Disturbances at the front wheel are obtained as preview information for the rear wheel. Static output-feedback is utilized in designing controllers, the solution is derived by iterative linear matrix inequality (ILMI) and cone complementarity linearization (CCL) algorithms. Simulation results confirm that multi-objective control with wheelbase preview achieves a significant improvement of ride quality (a maximum 27 percent and 60 percent improvement on vertical and angular acceleration, respectively) comparing with that of control without preview, while suspension deflections, tyre deflections and actuator forces remaining within given bounds. The extent of the improvement on the ride quality for different amount of preview information used is also illustrated. [ABSTRACT FROM AUTHOR]

Abstract: This paper addresses the evaluation of the exact natural frequencies and vibration modes of structures obtained by assemblage of plane circular arched Timoshenko beams. The exact dynamic stiffness matrix of the single circular arch, in which both the in-plane and out-of-plane motions are taken into account, is derived in an useful dimensionless form by revisiting the mathematical approach already adopted by Howson and Jemah (1999 [18]), for the in plane and the out-of-plan natural frequencies of curved Timoshenko beams. The knowledge of the exact dynamic stiffness matrix of the single arch makes the direct evaluation of the exact global dynamic stiffness matrix of spatial arch structures possible. Furthermore, it allows the exact evaluation of the frequencies and the corresponding vibration modes, for the distributed parameter model, through the application of the Wittrick and Williams algorithm. Consistently with the dimensionless form proposed in the derivation of the equations of motion and the dynamic stiffness matrix, an original and extensive parametric analysis on the in-plane and out-of-plane dynamic behaviour of the single arch, for a wide range of structural and geometrical dimensionless parameters, has been performed. Moreover, some numerical applications, relative to the evaluation of exact frequencies and the corresponding mode shapes in spatial arched structures, are reported. The exact solution has been numerically validated by comparing the results with those obtained by a refined finite element simulation. [Copyright &y& Elsevier]

Abstract: In this paper a model updating algorithm is presented to estimate structural parameters at the element level utilizing frequency domain representation of the strain data. Sensitivity equations for mass and stiffness parameters estimation are derived using decomposed form of the strain-based transfer functions. The rate of changes of eigenvectors and a subset of measured natural frequencies are used to assemble the sensitivity equation of the strain-based transfer function. Solution of the derived sensitivity equations through the least square method resulted in a robust parameters estimation method. Numerical examples using simulated noise polluted data of 2D truss and frame models confirm that the proposed method is able to successfully update structural models even in the presence of mass modeling errors. [Copyright &y& Elsevier]