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Operation conditions such as rub-impact and misalignment usually accumulate and can eventually induce severe faults in rotor systems. These abnormal conditions often make rotor systems behave nonlinearly. Consequently, nonlinear signal and systems analyses have been applied to address rotor system condition monitoring and fault diagnosis problems. Nonlinear frequency response functions (NOFRFs) are a concept proposed to extend the well-known concept of frequency response function to the nonlinear case, and have recently been exploited to solve problems with signal analysis based rotor system condition monitoring and fault diagnosis. However, existing NOFRFs based analyses require conducting offline experiments on rotor systems. This cannot satisfy the industrial need of continuous monitoring of rotor system operational conditions, which is important for online fault diagnosis. In order to resolve this problem, the present article proposes a novel approach that performs the rotor system condition monitoring using the NOFRFs determined directly from the rotor system online operational data which are inherently harmonics. This approach involves the identification of a data-driven NARX (nonlinear auto regressive with eXegenous input) model of rotor systems from harmonic excitations and corresponding responses, evaluation of rotor system NOFRFs using the identified NARX model, and the use of evaluated NOFRFs for rotor system operational condition monitoring and fault diagnosis. The approach, for the first time, enables the NOFRFs to be applied to perform online monitoring of rotor systems. Experimental studies have been conducted to verify the effectiveness and potential applications of the proposed approach as well as its advantages over commonly used signal analysis based techniques. [ABSTRACT FROM AUTHOR]

In this paper, a nonlinear auto regressive with eXegenous input (NARX) model of nonlinear systems, where the physical parameters of interest for the system design appear explicitly as coefficients in the model, is introduced. The model is referred to as the NARX model with parameters of interest for design (NARX-M-for-D). The output frequency response function (OFRF) in terms of these physical parameters is then introduced for the NARX-M-for-D, and an efficient algorithm is derived to determine the OFRF so as to facilitate the design of nonlinear systems in the frequency domain. Moreover, a general procedure for the design of the physical parameters of the NARX-M-for-D in the frequency domain is proposed, which has the potential to be applied to design a wide range of engineering systems and structures. Finally, two case studies are provided to demonstrate the new OFRF-based nonlinear system design and its significance in engineering applications. [ABSTRACT FROM AUTHOR]

OBJECTIVE: To identify long non-coding RNAs (lncRNAs), which may be associated with multifocal and multicentric breast cancer (MMBC) by analyzing the differential expression of non-coding RNA in MMBC and unifocal breast cancers (UBC). PATIENTS AND METHODS: 156 cases of invasive MMBC patients (136 patients with 2 focuses and 20 patients with 3 focuses) and 130 cases of UBC were collected from January 2005 to December 2015 in Yantai Yuhuangding Hospital. The differentially expressed lncRNAs in MMBC and UBC were screened by gene chip. RT-PCR was used to verify the differentially expressed lncRNAs. RESULTS: Significantly different expression was found in 1080 lncRNAs between MMBC and UBC (FC > 4, p < 0.05), among which, 458 were upregulated and 622 were down-regulated. In homologous lncRNAs, the expression levels of four lncRNAs including C19orf33, C3orf52, C15orf48 and C4orf19 in MMBC tissue were significantly different than those in UBC tissues. RT-PCR verified that the expression of C19orf33, C3orf52 and C15orf48 in MMBC tissue was significantly up-regulated and the expression of C4orf19 was significantly down-regulated, which was consistent with that of gene chip. CONCLUSIONS: C19orf33, C3orf52, C15orf48 and C4orf19 may have important functions in MMBC, and may be used as markers for MMBC. [ABSTRACT FROM AUTHOR]

The main problem of using a conventional linear damper on a vibration isolation system is that the reduction of the resonant peak in many cases inevitably results in the degradation of the high-frequency transmissibility. Instead of using active control methods which normally depend on the model of the controlled plant and where unmodelled dynamics may induce stability concerns, recent studies have revealed that optimal vibration isolation over a wide frequency range can be achieved by using nonlinear damping. The present study is concerned with the realization of the ideal nonlinear damping characteristic using a feedback-controlled MR damper. Both simulation and experimental studies are conducted to demonstrate the advantages of the simple but effective vibration control strategy. This research work has significant implications for the effective use of MR dampers in the vibration control of a wide range of engineering systems. [ABSTRACT FROM AUTHOR]

Suppression of vibration transmission from working machineries and other sources is important for the normal operation of a wide range of engineering systems. Traditionally, viscous dampers with approximately linear characteristics are often used to address the issue. However, this solution can have the problem of not being able to reduce the vibration transmission over the whole range of frequencies. In recent studies, the authors have revealed, by both theoretical analysis and experimental test, that nonlinear damping can be applied to resolve the problem. The present study is concerned with the exploitation of this beneficial effect of nonlinear damping to the vibration control of a pitch plane suspension system. A magneto-rheological (MR) damper based implementation of nonlinear damping is applied to provide a novel solution to the pitch plane system vibration control problem. Simulation studies are conducted to demonstrate the effectiveness of the MR damper implementation, and the beneficial effect of nonlinear damping on the pitch plane suspension system vibration control. [ABSTRACT FROM AUTHOR]

The operating conditions of wind turbine components have been monitored by fitting acoustic emission (AE) and vibration sensors inside wind turbines and analysing sensor measurements using appropriate signal processing techniques. However, there is still no systematic approach which can take the operating parameters of wind turbines into account when analysing the sensor measurements for condition monitoring purposes. The operating parameters of wind turbines, such as, for example, wind speed, wind direction and turbine power output etc, often have a significant effect on the measured AE and vibration signals. Therefore, there is a need to develop an effective technique which can systematically consider the effect of these wind turbine operating parameters on the features of measured AE and/or vibration signals. Consequently, any changes in the signal features which are literally produced by an abnormality of the turbine components and/or system can be correctly identified. Motivated by this objective, in the present study we propose a novel baseline model-based technique for condition monitoring of wind turbine components. The idea is to establish a baseline model for an operating wind turbine which represents the relationship between the wind speed, turbine power output, turbine operating parameters and the appropriate features of correspondingly measured vibration and/or AE signals. Given a specific working condition, the model output can be regarded as the baseline features of measured AE and/or vibration signals. Therefore, any significant deviation of practically measured AE and vibration signal features from the baseline can be explained to be due to some kind of abnormality inside the turbine components and/or system. Consequently, the information can be effectively used to achieve the objective of condition monitoring and even fault diagnosis. The application of the proposed method to the real AE, vibration and turbine operating parameter (wind speed and power output) data collected from an operating wind turbine is described in the paper. The results verify the effectiveness of the new approach and demonstrate its potential to address complicated offshore wind turbine condition monitoring problems. [ABSTRACT FROM AUTHOR]

In this paper, a new time–frequency analysis method known as the polynomial chirplet transform (PCT) is developed by extending the conventional chirplet transform (CT). By using a polynomial function instead of the linear chirp kernel in the CT, the PCT can produce a time–frequency distribution with excellent concentration for a wide range of signals with a continuous instantaneous frequency (IF). In addition, an effective IF estimation algorithm is proposed based on the PCT, and the effectiveness of this algorithm is validated by applying it to estimate the IF of a signal with a nonlinear chirp component and seriously contaminated by a Gaussian noise and a vibration signal collected from a rotor test rig. [ABSTRACT FROM AUTHOR]

In this study, an effective method is developed to detect the position of nonlinear components in periodic structures. The detection procedure requires exciting the nonlinear systems only once by using a sinusoidal-like input which has two frequency components. The effectiveness of this method is demonstrated by numerical studies. As the position of a nonlinear component often corresponds to the location of defect in periodic structures, this new method is of great practical significance in fault diagnosis for mechanical and structural systems. [ABSTRACT FROM AUTHOR]

Magnetorheological (MR) damper has recently received considerable attention particularly for vibration isolation applications. However, the relationship between the current and damping force of MR dampers is extremely non-linear. Although it is possible to directly control the current of MR dampers to reduce the vibration of a system, this direct use of MR dampers often cannot achieve a desired system performance. As a result, researchers normally develop a feedback control system to accommodate the non-linearity associated with MR dampers. This technique, however, results in a significant increase in the system cost and complexity. This article applies the concept of the output frequency response function derived recently by the authors to analyze the effects of the damping characteristic parameters and current in an MR damper on the output frequency response of single-degree-of-freedom vibrating system. The results gained from this work are of great significance as they can facilitate the analysis of the effects of MR damper characteristic parameters on output response of the system and potentially allow engineers to directly control the current of a commercially available MR damper to achieve a desired system response without the need to develop a feedback control system, thereby reducing the system cost and complexity. [ABSTRACT FROM AUTHOR]

In the present study, the concept of the output frequency response function, recently proposed by the authors, is applied to theoretically investigate the force transmissibility of single degree of freedom (SDOF) passive vibration isolators with a nonlinear antisymmetric damping characteristic. The results reveal that a nonlinear antisymmetric damping characteristic has almost no effect on the transmissibility of SDOF vibration isolators over the ranges of frequencies, which are much lower or higher than the isolator's resonance frequency. On the other hand, the introduction of a nonlinear antisymmetric damping can significantly reduce the transmissibility of the vibration isolator over the resonance frequency region. The results indicate that nonlinear vibration isolators with an antisymmetric damping characteristic have great potential to overcome the dilemma encountered in the design of passive linear vibration isolators, that is, increasing the level of damping to reduce the transmissibility at the resonance could increase the transmissibility over the range of higher frequencies. These important theoretical conclusions are then verified by simulation studies. [ABSTRACT FROM AUTHOR]

It is well-known that if one or a few components in a structure are of nonlinear properties, the whole structure will behave nonlinearly, and the nonlinear component is often the component where a fault or an abnormal condition occurs. Therefore it is of great significance to detect the position of nonlinear components in structures. Nonlinear output frequency response functions (NOFRFs) are a new concept proposed by the authors for the analysis of nonlinear systems in the frequency domain. The present study is concerned with investigating the NOFRFs of nonlinear one-dimensional chain type systems, which have been widely used to model many real life structures. A series of important properties of the NOFREs of locally nonlinear one-dimensional chain type structures are revealed. These properties clearly describe the relationships between the NOFRFs of different masses in a one-dimensional chain type system, and allow effective methods to be developed for detecting the position of a nonlinear component in the system. The results are an extension of the authors `previous research studies to a more general and practical case, and have considerable significance in fault diagnosis and location in engineering systems and structures. [ABSTRACT FROM AUTHOR]

The analysis of multidegree-of-freedom (MDOF) nonlinear systems is studied using the concept of nonlinear output frequency response functions (NOFRFs), which are a series of one-dimensional functions of frequency recently proposed by the authors to facilitate the analysis of nonlinear systems in the frequency domain. By inspecting the relationships between the NOFRFs of two consecutive masses in the locally nonlinear MDOF systems, a series of properties about the NOFRFs of the nonlinear systems are obtained. These properties clearly reveal how the system linear characteristic parameters govern the distribution of nonlinear effects induced by the system nonlinear component within the whole systems. The results obtained in the present work are of significance to the frequency response analysis for nonlinear systems. One important potential application of these results is the detection and location of damages in engineering structures, which make the structures behave nonlinearly. [ABSTRACT FROM AUTHOR]

Many real-life systems can be modelled as a periodic structure which is constituted by repeating identical systems which are called elements. The periodic structures have drawn extensive attentions from researchers and engineers. In this study, based on the concept non-linear output frequency response function, a novel method is developed to detect the position of non-linear components in periodic structures. The validity of this method is demonstrated by numerical studies. Although the method assumes that the linear stiffness and damping for multi-degree-of-freedom (MDOF) systems under study are known a priori, the results of the numerical studies indicate that, even if only approximate values of the linear stiffness and damping parameters are used, the method can still correctly detect the position of the non-linear component. Since the position of a non-linear component often corresponds to the location of a defect in a periodic structure, this new method is of great practical significance for fault diagnosis in mechanical and structural systems. Moreover, the method does not involve complicated procedure and can be readily applied in practice. [ABSTRACT FROM AUTHOR]

For non-linear Volterra systems which include a non-linear state equation and a general non-linear output function, the system frequency response functions and some related frequency response characteristics are developed and discussed in this study. These new results establish the frequency response functions for this general form of non-linear systems by extending some existing theory, and provide an analytical insight into the relationship between model parameters and frequency response functions, and the relationship between model parameters and the magnitude bound of frequency response functions. Several examples are given to illustrate the new results. [ABSTRACT FROM AUTHOR]

This paper is concerned with extending the energy transfer filters (ETFs) designs to a more general case where the design requirement is to focus signal energy from different input frequency bands into a desired output frequency range. ETFs are a class of non-linear filters recently proposed by the authors to employ non-linear effects to transfer signal energy from one frequency band to a different frequency location. The new ETF designs considered in the present study involve the development of an algorithm for determining output frequencies of non-linear systems subject to an input the frequency components of which are located in two separate frequency intervals and a new procedure for the implementation of the filter design. The results achieved provide an effective approach to the solution to a more general ETF design problem. [ABSTRACT FROM AUTHOR]

The current paper is concerned with the investigation of the relationship between the harmonic balance method (HBM) and the non-linear output frequency response function (NOFRF) approach in the analysis of non-linear systems. Both are applied to the Duffing's oscillator to demonstrate their relationships. The results reveal that, if the Volterra series representation of a non-linear system is convergent, the harmonic components calculated by the NOFRFs are a solution of the HBM. Moreover, the simulation studies show that, in the convergent cases, the NOFRF method can give more accurate results for the higher-harmonic components than the HBM. The relationship investigated in the current study between the two methods should help researchers and engineers to understand the HBM and the NOFRF methods. [ABSTRACT FROM AUTHOR]

In this paper, an algorithm is derived for the determination of the output frequency ranges of nonlinear systems, which extends previous results on the output frequencies of nonlinear systems to a more general situation. The new results are significant for the analysis of the output frequency response of a wide class of non-linear systems. [ABSTRACT FROM AUTHOR]

The concept of non-linear output frequency response functions (NOFRFs) is extended to the non-linear systems that can be described by a multi-input Volterra series model. A new algorithm is also developed to determine the output frequency range of non-linear systems from the frequency range of the inputs. These results allow the concept of NOFRFs to be applied to a wide range of engineering systems. The phenomenon of the energy transfer in a two degree of freedom non-linear system is studied using the new concepts to demonstrate the significance of the new results. [ABSTRACT FROM AUTHOR]

New results about the bound characteristics of both the generalized frequency response functions (GFRFs) and the output frequency response for the NARX (Non-linear AutoRegressive model with eXogenous input) model are established. It is shown that the magnitudes of the GFRFs and the system output spectrum can all be bounded by a polynomial function of the magnitude bound of the first order GFRF, and the coefficients of the polynomial are functions of the NARX model parameters. These new bound characteristics of the NARX model provide an important insight into the relationship between the model parameters and the magnitudes of the system frequency response functions, reveal the effect of the model parameters on the stability of the NARX model to a certain extent, and provide a useful technique for the magnitude based analysis of nonlinear systems in the frequency domain, for example, evaluation of the truncation error in a volterra series expression of non-linear systems and the highest order needed in the volterra series approximation. A numerical example is given to demonstrate the effectiveness of the theoretical results. [ABSTRACT FROM AUTHOR]

This paper is concerned with the development of a new method for the design of energy transfer filters (ETFs). ETFs are a new class of non-linear filters recently proposed by the authors, which employ non-linear effects to transfer signal energy from one frequency band to a different frequency location. The new method uses the powerful orthogonal least squares (OLS) algorithm to solve the least squares problem associated with the design and compared with previous methods achieves much better filtering performance. [ABSTRACT FROM AUTHOR]

In Part 1 of this paper the concepts of input and output frequency subdomains were introduced to give insight into the relation between one dimensional and multi-dimensional frequency spaces. The visualisation of both magnitude and phase responses of third order generalized frequency response functions was also presented. In this, the second part, symbolic expansion techniques are introduced and together with the results achieved in Part 1, yield new methods for analysing the properties of generalized frequency response functions. Case studies are included to illustrate the application of the new methods. [ABSTRACT FROM AUTHOR]

The characteristics of generalized frequency response functions (GFRFs) of non-linear systems in higher dimensional space are investigated using a combination of graphical and symbolic decomposition techniques. It is shown how a systematic analysis can be achieved for a wide class of non-linear systems in the frequency domain using the proposed methods. The paper is divided into two parts. In Part 1, the concepts of input and output frequency subdomains are introduced to give insight into the relationship between one dimensional and multi-dimensional frequency spaces. The visualization of both magnitude and phase responses of third order generalized frequency response functions is presented for the first time. In Part 2 symbolic expansion techniques are introduced and new methods are developed to analyse the properties of generalized frequency response functions of non-linear systems described by the NARMAX class of models. Case studies are included in Part 2 to illustrate the application of the new methods. [ABSTRACT FROM AUTHOR]

In this paper, an analysis of the energy transfer properties of non-linear systems in the frequency domain is studied based on a new concept known as non-linear output frequency response functions (NOFRFs). The new concept allows the analysis to be implemented in a manner similar to the analysis of linear systems in the frequency domain, and provides great insight into the mechanisms which dominate the non-linear behaviour. The new analysis is also helpful for the design of non-linear systems in the frequency domain. [ABSTRACT FROM AUTHOR]

A correction to the article "New bound characteristics of NARX model in the frequency domain," that was published in the previous issue is presented.

High-cycle fatigue failures of fan blade systems due to vibrational loads are of great concern in the design of aeroengines, where energy dissipation by the relative frictional motion in the dovetail joints provides the main damping to mitigate the vibrations. The performance of such a frictional damping can be enhanced by suitable coatings. However, the analysis and design of coated joint roots of gas turbine fan blades are computationally expensive due to strong contact friction nonlinearities and also complex physics involved in the dovetail. In this study, a data-driven surrogate model, known as the Nonlinear in Parameter AutoRegressive with eXegenous input (NP-ARX) model, is introduced to circumvent the difficulties in the analysis and design of fan systems. The NP-ARX model is a linear input-output model, where the model coefficients are nonlinear functions of the design parameters of interest, such that the Frequency Response Function (FRF) can be directly obtained and used in the system analysis and design. A simplified fan-bladed disc system is considered as the test case. The results show that using the data-driven surrogate model, an efficient and accurate design of aeroengine fan systems can be achieved. The approach is expected to be extended to solve the analysis and design problems of many other complex systems. [ABSTRACT FROM AUTHOR]

In this paper a new concept, spatio-temporal generalized frequency response functions (STGFRF), is introduced for the first time to characterize nonlinear spatio-temporal dynamical systems in the frequency domain. A probing method is developed to calculate the STGFRFs recursively for both continuous and discrete spatio-temporal systems. The algorithm is computationally compact and exposes the explicit relationship between the continuous and discrete models and the elements of the generalized frequency response functions. [ABSTRACT FROM AUTHOR]