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Introduction
Published in Heung-Fai Lam, Jia-Hua Yang, Vibration Testing and Applications in System Identification of Civil Engineering Structures, 2023
Modal analysis, in general, is the technique used to calculate the modal parameters of a structural system by utilizing the measured dynamic data from vibration tests. If forced vibration tests are conducted (see Section 1.1.2), both the measured excitation and responses are available. Modal testing can be used to identify natural frequencies, mode shapes, and damping ratios of the system. However, both free and forced vibration tests are usually difficult to arrange for civil engineering structures, as discussed earlier. As a result, modal analysis for civil engineering structures is usually done with ambient vibration tests. As it is not necessary to apply external force or introduce disturbance to the target structure, the vibration test and the corresponding modal identification can be carried out under the operational condition of the structure, and therefore, this technique is called “operational modal analysis.” Operational modal analysis is convenient, as only structural responses (system output) are needed to be measured, and its implementation is cost-effective. As the vibration level under ambient excitation is usually very low, the identified modal parameters can only reflect the structural characteristic under low-level vibration. Furthermore, the accelerometer employed must be very sensitive due to the low vibration level. Without using the force information and the relatively low signal-to-noise ratio, the uncertainties associated with the operational modal analysis results are believed to be high.
Experimental Modal Analysis
Published in Jyoti K. Sinha, Industrial Approaches in Vibration-Based Condition Monitoring, 2020
The concept of the modal testing is based on the resonance responses of the objects to identify their modal parameters—natural frequencies, modeshapes and modal damping. The external excitation in the broad frequency band can generates resonance at several natural frequencies of the objects within the frequency band of excitation. Either vibration shaker or the instrumented hammer with force sensor can be used for this external vibration excitation in the objects. The impulsive hammer is the simplest and quickest approach for the modal tests and often useful for the in-situ modal tests for the structures and machines. Hence the instrumented hammer is only discussed in this chapter. The modal test using the instrumented hammer is known as the impulse-response method. In industries, this approach is popularly known as a bump test and often force measurements are done during the test; therefore, this is not a good practice.
Measuring MEMS in Motion by Laser Doppler Vibrometry
Published in Wolfgang Osten, Optical Inspection of Microsystems, 2019
Christian Rembe, Georg Siegmund, Heinrich Steger, Michael Wörtge
Modal testing (performing a modal survey) is usually conducted under controlled stationary (non-time-varying) conditions, using one or more exciters. Furthermore, the excitation forces and their corresponding responses are simultaneously measured. In many cases, especially with operating equipment, the measurement signals may be non-stationary (time varying) and the excitation forces cannot be measured. For these cases, different postprocessing techniques such as operational modal analysis (OMA) are required in order to extract modal parameters.
Identification of Nonlinear Behavior of Bridge Structures Using Time Series Analysis of Vibration Signals
Published in Journal of Earthquake Engineering, 2023
Niusha Navabian, Sherif Beskhyroun
One of the most widely used linear techniques for solving damage identification problems and prediction of the system dynamic behavior is modal testing and analysis. Existence of nonlinearities in a system can alter its dynamic behavior, thus the use of a linear-based approach is improper to model such dynamic systems. In such cases, a linear model is not able to model the structural system, as the basic principle of a linear system is not authentic for a nonlinear system anymore. It means that the superposition, the homogeneity, and the Maxwell reciprocity rules cannot be utilized for simulation of a nonlinear system. In addition, modal models are usually incorrect to simulate the behavior of a nonlinear system (Trendafilova et al. 2001).
Precision of modal analysis to characterise the complex modulus of asphalt concrete
Published in Road Materials and Pavement Design, 2019
Anders Gudmarsson, Jean-Claude Carret, Simon Pouget, Richard Nilsson, Abubeker Ahmed, Hervé Di Benedetto, Cédric Sauzéat
The principle of the modal testing methodology is to determine frequency response functions (FRFs) by measuring the input and output of a system. The input is the frequency content of an applied load used to excite the resonance frequencies of a specimen with free boundary conditions and the output is the frequency response of the excited specimen. Numerically computed FRFs using the finite element method and a suitable viscoelastic model (Havriliak & Negami, 1966; Olard & Di Benedetto, 2003) are used to characterise the complex modulus by curve fitting to the FRF measurements (Gudmarsson et al., 2012).