Modal parameter identification is a core issue in the health monitoring and damage detection of hydraulic structures. The parameters are mainly obtained from the measured vibrational response under ...ambient excitation. However, the response signal is mixed with noise and interference signals, which will cover the structure vibration information; therefore, the parameter cannot be identified. This paper proposes an improved filtering method based on an ensemble empirical mode decomposition (EEMD) and wavelet threshold method. A ‘noise index’ is presented to estimate the noise degree of the components decomposed by the EEMD, and this index is related to the wavelet threshold calculation. In addition, the improved filtering method combined with an eigensystem realization algorithm (ERA) and a singular entropy (SE) is applied to an operational modal identification of a roof overflow powerhouse with a bulb tubular unit.
•We propose a ‘noise index’ to estimate the noise degree of the components decomposed by EEMD.•The ''noise index' is used to solve the noise standerd deviation determination problem in wavelet threshold calculation.•The improved filtering method combined with ERA and SE provides an effective approach for the modal parameter indentification of hydraulic structure.
Operational Modal Analysis (OMA) refers to the modal analysis of a structure in its operating state. The advantage of OMA is that only the output vibration signal of a system is used in the analysis ...process. Classic OMA is based on the white noise excitation assumption and many identification methods have been developed in both time domain and frequency domain. But in reality, many environmental excitations are not compliance with the white noise assumption. In this paper, a method of half power bandwidth analysis is applied to power spectrum analysis to deal with the colored noise and trapezoidal spectral excitation. The modal frequencies and modal damping ratios are derived and the error caused by trapezoidal spectral and colored noise excitation are analyzed. It is proved that the OMA algorithm based on the white noise assumption can be extended to the colored noise environments under certain conditions. Finally, a simulation example with a cantilever beam and a vibration test with four kinds of colored noise and trapezoidal spectrum base excitation are carried out and the results support the proposed method.
With the continuous acceleration of the global construction industry, many structural infrastructure structures in China have been put into use for decades. They are very prone to damage due to ...fatigue. The modal parameter identification of civil engineering construction can evaluate the safety status of infrastructure structures. In view of this, an identification system of civil engineering structure modal parameters is proposed based on improved wavelet transform. In the process, the mode shape was chosen as the method of wavelet transform. The data was discretized by selecting the actual data of a high-speed railway station combined with sensors and wavelet transform. Finally, the correct identification of the modal parameters of civil engineering structures is realized. The data shows that under normal conditions where there is only white noise interference, the waveform of the structure is relatively stable, and the amplitude fluctuation is in the -2,3 interval. At the same time, the average amplitude of the structure is in the 2.2, −1.5 interval under normal conditions. In addition, the positive and negative extreme points are 3.7 and −2.3, respectively. This indicates that the structure amplitude fluctuation is in a dynamic and stable state under normal circumstances. The optimized wavelet transform method identifies a total of four orders in the first six natural frequencies. The minimum error is 0.11%, the maximum error is 1.50%, and the average error of the first four natural frequencies is 0.578%. In addition, based on the comparison of theoretical and identification values of longitudinal vibration shapes, the proposed method can successfully detect abnormal values at the 10th and 18th nodes. From the above results, it shows that the wavelet transform method has high accuracy and small error in frequency identification. It meets the requirements of identifying the natural frequency parameters of the structure. From the calculation, the method proposed in the experiment can realize the real-time health detection of the structure, which is of great significance.
High-speed camera measurements are increasingly being used in modal analysis to instantaneously measure full-field structural responses by extracting the displacement information from images using ...digital-image-correlation and other optical-flow methods. High-speed cameras capable of filming full frame at high frame rates can be very expensive and produce image resolutions of only approximately 1 mega pixel, which is why this research aims at measuring and identifying the full-field response using cheaper, still-frame cameras with a higher image and intensity resolution, such as digital single-lens reflex (DSLR) and mirrorless cameras. Using spectral optical flow imaging (SOFI) full-field operational shapes can be acquired using still-frame cameras. This study demonstrates the hybrid modal-parameter identification of full-field mode shapes using an accelerometer and a DSLR camera for responses far above the DSLR camera's frame rate (demonstrated up to 1 kHz).
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•Spectral Optical Flow Imaging (SOFI) is used to measure full-field dynamic responses.•Using SOFI, responses up to 1000 Hz were measured with a consumer DSLR camera.•Apparent motion between frames was discovered, likely caused by the shutter.•Due to the mechanical shutter operation, video was shot instead of separate frames.•A hybrid modal parameter identification combining SOFI and a point sensor is introduced.
The end-operation accuracy of the satellite-borne robotic arm is closely related to the satellite attitude control accuracy, and the influence of the vibration of the satellite's flexural structure ...on the satellite attitude control is not negligible. Therefore, a stable and reliable vibration frequency identification method of the satellite flexural structure is needed. Different from the traditional non-contact measurement and identification methods of large flexible space structures based on marker points or edge corner points, the condition of non-marker points relying on texture features can identify more feature points, but there are problems such as low recognition and poor matching of features. Given this, the concept of 'the comprehensive matching parameter' of scenes is proposed to describe the scene characteristics of non-contact optical measurement from the two dimensions of recognition and matching. The basic connotation and evaluation index of the concept are also given in the paper. Guided by this theory, the recognition accuracy and matching uniqueness of features can be improved by means of equivalent spatial transformation and novel relative position relationship descriptor. The above problems in non-contact measurement technology can be solved only through algorithm improvement without adding hardware devices. On this basis, the Eigensystem Realization Algorithm (ERA) method is used to obtain the modal parameters of the large flexible space structure. Finally, the effectiveness and superiority of the proposed method are verified by mathematical simulation and ground testing.
Based on two machine learning algorithms and eight evaluation indexes, this paper proposes a novel stochastic subspace identification (SSI) approach for the operational modal analysis of civil ...structures. The detailed procedure of the proposed approach is first demonstrated via a numerical example of a five-degree-of-freedom (5DOF) simulation model, and the results show that the proposed approach can automatedly and effectively distinguish the physical modes from non-physical ones for the SSI. The modal parameters of the 5DOF model estimated by the proposed approach agree well with their theoretical values, verifying the accuracy and effectiveness of the proposed approach. Furthermore, the proposed approach is applied to field measurements on a 195-m-tall building under its operating condition, and the modal parameters of the building are well identified in an automated manner. Through the statistical analysis, the probability distributions and amplitude-dependent features of the natural frequencies and damping ratios of the 195-m-tall building are further revealed. The objective of this study is to provide an efficient tool for the operational modal analysis of civil structures.
•Propose a machine learning-based stochastic subspace approach for operational modal analysis (OMA) of civil structures.•Demonstrate the accuracy and efficiency of the proposed method through numerical simulation study.•Verify the applicability and effectiveness of the proposed approach for OMA via field measurements on a 195-m-tall building.•Investigate the structural dynamic properties of the monitored 195-m-tall building under its operating condition.
Operational modal analysis refers to the modal analysis of a structure in its operating state. The advantage of operational modal analysis is that only the output vibration signal of a system is ...used. The classical operational modal analysis algorithm is based on the white noise excitation assumption, and it is considered that there is no correlation between the excitations; several identification methods have been developed in time and frequency domains. But excitations are not completely independent with each other and not pure white. In this article, the matrix theory is used to prove that the operational modal analysis algorithm can still be used to identify modal parameters when the excitation is correlated. In the simulation, five kinds of colored noise excitations are applied to the cantilever beam with correlated excitations, which shows that the idea proposed in this article is rational. In the experiment, the foundation excitation of colored noise is added to the cantilever beam, which can be regarded as applying several related excitations. It also shows the rationality of this idea.
Identification of torsional receptances Tsai, Sung-Han; Ouyang, Huajiang; Chang, Jen-Yuan
Mechanical systems and signal processing,
07/2019, Volume:
126
Journal Article
Peer reviewed
•Developed two receptance-based methods for torsional receptance measurement.•Only a few receptances of the T-block and the coupled structure are required.•Successfully obtained high-quality ...torsional FRFs on a two-disc rotor-bearing rig.•Eliminated the appearance of bending resonances in the estimated torsional FRFs.•Incorporating the rotational accelerometer enhances the method’s robustness.
Identification of torsional receptances of shaft structures has been a challenge. This paper presents a method for measuring or estimating high-quality torsional receptances using two different methods based on the Receptance Decoupling Technique. In both methods, a T-block needs to be attached to facilitate the generation of torsion, and only the numerical receptance data of the T-block cast in a simple theoretical model and a few measured receptance data of the assembled system are required. Both methods are studied and assessed in numerical simulation, and the more robust method is further validated in experiments. The usage of rotational accelerometer is shown to significantly improve the quality of the estimation when the noise level is high. It is demonstrated that high-quality torsional receptances can be indirectly measured with high repeatability using the proposed method and thus can be used subsequently to identify torsional modal parameters, update finite element models, make structural modifications, or implement active torsional vibration control. Due to ubiquitous use of rotating machines, this novel method has significant applications.
Offshore platforms are constantly exposed to dynamic forces by winds, waves, currents and earthquakes, which can easily lead to damage to the platforms during their serviceable lifetime. The ...real-time and continuous dynamic monitoring of offshore platforms in service plays an important role in reducing the risk of structural failure, instability and even destruction. In this study, a two-step approach was presented to capture the dynamic response of an offshore jacket platform based on the real-time kinematic global navigation satellite system (RTK-GNSS), which combines a Chebyshev filter and complementary ensemble empirical mode decomposition with adaptive noise (hereinafter referred to as CF-CEEMDAN). In the first step, the RTK-GNSS technique was employed to measure the dynamic displacements of the offshore platform under ambient excitations. Different from traditional accelerators, this technique has no unavoidable errors caused by double integration and directly measures the real-time and continuous dynamic displacements of the offshore platform at site. Next, a CF-CEEMDAN filter was developed to weaken the influence of the background noise in the RTK-GNSS signals. In the second step, the natural excitation technique and eigensystem realization algorithm (NExT-ERA) was applied to estimate the modal parameter (i.e. the first natural frequency) of the platform with filtered RTK-GNSS signals. In order to evaluate the efficiency of the proposed approach, a nonlinear signal with additive noise was firstly introduced to assess the effect of the CF-CEEMDAN method, and a finite element model of the platform was established to predict the natural frequency for comparison. The results prove that the CF-CEEMDAN filter not only has a better noise reduction effect than single CF or CEEMDAN method but also effectively improve the accuracy of RTK-GNSS technique to monitor the dynamic displacements of offshore platforms. Furthermore, the natural frequency obtained by experiment and NExT-ERA analysis is consistent with the predicted value based on finite element model.
This paper presents an experimental study implementing the input shaping control of the first two modes of the vibration of a non-uniform flexible cantilever beam having a translating base. Examples ...of a moving cantilever beam appear in many industrial systems. Vibration suppression of the beam has important implications for improving the effectiveness of such systems. The equations of motion of the cantilever beam, including the moving base, are developed using the extended Hamilton principle. The partial differential equation representing the beam’s dynamics is then transformed into a finite-dimensional model using the Galerkin method. Accordingly, the modal parameter identification procedure is established based on experimental modal analysis. Under the estimated modal parameters, including the natural frequency and damping ratio, single- and two-mode input shaping controllers of three different types (zero vibration, zero vibration derivative, and zero vibration derivative-derivative) are designed for vibration suppression of the beam. Experimental results are discussed, reporting that the two-mode shaper’s vibration suppression was superior to the single-mode shaper. In contrast, the two-mode shaper’s settling time has slightly increased compared to that of the single-mode shaper.
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