modal analysis research articles

Modal analysis and its application

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Gordon Ebbitt , Hideo Suzuki , Lou Abbagnaro; Modal analysis and its application. J. Acoust. Soc. Am. 1 November 1982; 72 (S1): S83. https://doi.org/10.1121/1.2020102

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Modern modal analysis techniques provide a method for the rapid determination of structural mode shapes and frequencies. This analysis utilizes the impulse measurement of the transfer inertance (acceleration/force) between a reference point and points on the surface of a structure. The procedure involved in the analysis will be outlined and potential problems in its implementation will be discussed. These potential problems include the improper location of the reference position, the number of measurement points chosen, the resolution of the processing system, and the spectral content of the impulse. The application of modal analysis to musical instruments will be considered and some results will be presented.

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Flight Vibration Test – Real-Time Data Analysis using Artificial Intelligence

Aircraft structures are prone to vibrations due to their lightweight construction. In most cases, these vibrations are not critical. They decay due to damping effects from aerodynamic forces and damping forces from the structure itself. Varying flight conditions, like airspeed, altitude and temperature, have a large impact on the damping behavior. During flight vibration tests the complex aeroelastic behavior of an aircraft is experimentally identified, to be sure that no critical vibrations can occur within the flight envelope. The DLR Institute of Aeroelasticity has enhanced its flight-testing methods by the use of artificial intelligence to get faster and even more reliable data.

The flutter phenomenon

A well-known vibration phenomenon on aircraft wings is "flutter". In case of aircraft flutter, eigenmodes can couple with each other. This happens when their frequencies shift, in particular also due to aerodynamic influences. Consequently, the damping of one mode becomes zero or negative. Only an initial small disturbance is required to cause instabilities with fast increasing vibration amplitudes. The permanent excitation with external forces is not required to produce large vibrations. This is referred to as "self-excited vibrations".

System identification on ground and in flight

Before first flight of an aircraft prototype the ground vibration test is performed to deliver a structural dynamic identification of the whole aircraft on ground. The results cover modal parameters such as eigenfrequencies, damping ratios and mode shapes in the frequency range of interest. This test explicitly excludes aerodynamic effects to update the structural model for further clearance of the first flight. An example of a ground vibration test is given in the following blog article ( ISTAR Ground Vibration Test ).

Since the vibration behavior of an aircraft changes with varying flight conditions, the flight envelope is successively expanded during the flight test campaign of an aircraft prototype. This campaign is used to verify the predicted aeroelastic stability in terms of eigenfrequencies and damping ratios with varying flight conditions. Typically, artificial excitation is used with dedicated control surface inputs or other external excitation devices which induce aircraft vibrations in order to analyze the aeroelastic behavior. The DLR Institute of Aeroelasticity is propagating a method that uses the natural turbulence as the only source of excitation during flight.

Output-only modal analysis

The identification of aircraft modal parameters during GVT is a core competence of the Institute of Aeroelasticity. In recent years, the experimental identification methods have been extended in such a way that the modal parameters of an aircraft can also be identified in-flight [1-4] without further artificial excitation. Natural excitation by turbulent flow is sufficient in most cases. However, since the excitation forces cannot be measured, the methods are based solely on the measured response signals of the aircraft ("output-only modal analysis"). Modal analysis is a complex identification procedure and is therefore applied by trained engineers. The underlying mathematical algorithms have been accelerated by efficient programming that modal parameters are available within a few seconds. However, operation by an engineer would not be possible in this short time, so in this work autonomous algorithms take over the engineer's tasks which is described by the three following enhancements for the standard methods.

1. Autonomous analysis using artificial intelligence

The core task of modal analysis is to extract the physically correct solution from many possible mathematical solutions. This is solved manually via a so-called stabilization diagram (Fig. 1a). For the autonomous analysis, multilayer clustering is applied (Fig. 1b). This is a method from the field of unsupervised machine learning.

2. Optimization of hyperparameters

Clustering as well as the identification methods require setting parameters in order to achieve optimal results. In the field of machine learning those parameters are referred to as hyperparameters. In order to avoid time-consuming manual adjustment of the hyperparameters, a system was developed that optimizes them semi-autonomously using Gaussian processes. The intelligent system can thus teach itself an optimal way of analyzing, e.g. for a new prototype [5].

3. Data fusion – Combination of different analysis methods

Since during a flight the boundary conditions are different from those in the laboratory, the uncertainties of the modal identification methods are larger than, for example, in a ground vibration test. In order to reduce these uncertainties, the analysis system has been extended to include various identification methods. Now, different methods (e.g. time-domain methods and frequency-domain methods) with different strengths can be used in parallel. The results of these methods are fused automatically so that the advantages of both approaches can be combined [6].

Successful demonstration of the methods

The developed methods have been successfully demonstrated during a ground vibration test in Brunswick [7], a wind tunnel test campaign in the European transonic wind tunnel (ETW) in Cologne, a UAV flight test campaign in Cochstedt [5, 8] and finally on board the DLR research aircraft ISTAR.

Future research outlook

In the future, further reduction of uncertainty bounds for damping estimates is envisaged to provide real-time methods not only for flight vibration testing but also for further applications where online identification is necessary.

• Jelicic, Goran and Schwochow, Jan and Govers, Yves and Hebler, Anne and Böswald, Marc (2014) Real-time assessment of flutter stability based on automated output-only modal analysis . International Conference on Noise and Vibration Engineering - ISMA, 15.-17. Sept. 2014, Leuven, Belgien.       
• Jelicic, Goran and Schwochow, Jan and Govers, Yves and Böswald, Marc (2016) Automatische Schwingungsüberwachung von aeroelastischen Systemen . VDI-Berichte, 2259, Seiten 211-222. VDI-Verlag. VDI-Fachtagung Schwingungsanalyse und Identifikation, 15.-16. Mrz. 2016, Fulda, Deutschland. ISBN 978-3-18-092259-1. ISSN 0083-5560.
• Böswald, Marc and Schwochow, Jan and Jelicic, Goran and Govers, Yves (2017) New concepts for ground and flight vibration testing of aircraft based on output-only modal analysis . International Operational Modal Analysis Conference - IOMAC, 10.-12. Mai 2017, Ingolstadt, Deutschland. ISBN 978 3 8440 5247 3. ISSN 1610-4773.
• Jelicic, Goran and Schwochow, Jan and Govers, Yves and Sinske, Julian and Buchbach, Ralf and Springer, Julian (2017) Online monitoring of aircraft modal parameters during flight test based on permanent output-only modal analysis . 58th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference - AIAA SciTech, 09.-13. Jan. 2017, Grapevine, Texas, USA. doi: 10.2514/6.2017-1825.
• Volkmar, Robin and Soal, Keith Ian and Govers, Yves and Böswald, Marc (2022) Experimental and operational modal analysis: Automated system identification for safety-critical applications . Mechanical Systems and Signal Processing (MSSP), 183 (109658). Elsevier. doi: 10.1016/j.ymssp.2022.109658. ISSN 0888-3270.
• Volkmar, Robin and Soal, Keith Ian and Govers, Yves and Böswald, Marc (2022) Optimization of time and frequency domain methods for real-time modal parameter identification of aircraft . International Conference on Noise and Vibration Engineering - ISMA, 12.-14. Sep. 2022, Leuven, Belgium.
• Volkmar, Robin and Soal, Keith Ian and Buchbach, Ralf and Sinske, Julian and Govers, Yves and Böswald, Marc (2022) Semi-autonomous analysis of large aircraft ground vibration tests . International Forum on Aeroelasticity and Structural Dynamics - IFASD, 13.-17. Jun. 2022, Madrid, Spanien.
• Soal, Keith Ian and Thiem, Carsten and Meier, Tobias and Volkmar, Robin and Sinske, Julian and Govers, Yves and Böswald, Marc (2022) Embedded flight vibration testing system for online flutter monitoring of UAVs . International Forum on Aeroelasticity and Structural Dynamics - IFASD, 13.-17. Jun. 2022, Madrid, Spanien.

Robin Volkmar, Abteilung Strukturdynamik und Systemidentifikation, DLR-Institut für Aeroelastik

Structural Dynamics and System Identification

The Department of Structural Dynamics and System Identification deals with the dynamics and vibrations of structures in aeronautics and space travel. Through numerical analyzes and experimental investigations, essential mechanisms of the vibrations are recognized and cause-effect relationships are understood. Efficient measures for damping and influencing vibrations are developed with the help of numerical simulations. The system identification is used for the construction of mathematical substitute models of real technical systems.

Dr.-Ing. Marc Böswald

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Research on modal analysis method of CNC machine tool based on operational impact excitation

• ORIGINAL ARTICLE
• Published: 02 April 2019
• Volume 103 , pages 1155–1174, ( 2019 )

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• Liangjie Li 1 ,
• Huanbin He 1 ,
• Xinyong Mao 1 ,
• Xuchu Jiang 1 &
• Yili Peng 1  

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With the rapid development of modern industry and industrial upgrading, machine tool performances such as high machining speed, high machining accuracy, and high reliability have gradually become the development trend of high-end CNC machine tools. The vibration characteristics of the machine structure become an important factor affecting the machining quality and processing efficiency of the machine tool. Since the movable parts of the CNC machine tool will impact the machine structure itself during the acceleration and deceleration process, by identifying the structural vibration response signal, the modal parameters of the machine tool in the running state can be obtained, called active excitation mode analysis. However, this method still has certain defects in theory and experiment. For example, the excitation principle is not theoretically explained, and the excitation energy and the frequency band of the method are limited. To solve these problems, this paper conducts more comprehensive research on the modal analysis method of CNC machine tools based on operational impact excitation. The main research contents are as follows: considering the rigid body motion of the table and the elastic collision of the screw nut pair, the impact excitation is modeled and analyzed. We have introduced a response-based modal identification algorithm. The influence factors of impact excitation energy and frequency band were studied from both theoretical and experimental aspects. Design three excitation sequences for workbench excitation and spindle excitation, respectively, analyze the vibration response and modal identification of the machine tool structure under different excitation sequences. It is verified that the method does not depend on the specific excitation sequence, as long as it satisfies the random characteristics. Aiming at the problem of insufficient excitation energy of the machine tool structure for single-component impact excitation, a multi-component joint impact excitation method based on the principle of multi-point excitation technology is proposed. The design experiment is carried out to verify the impact excitation method, which shows that the joint excitation has better excitation effect than single-axis excitation.

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Acknowledgements

The authors are grateful to the other participants of the project for their cooperation.

The research is supported by the National Natural Science Foundation of China under Grants No. 51775212 and 51625502.

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National NC System Engineering Research Center, School of Mechanical Science and Engineering, Huazhong University of Science and Technology (HUST), 1037 Luoyu Road, Hongshan District, Wuhan, 430074, Hubei Province, People’s Republic of China

Bin Li, Liangjie Li, Huanbin He, Xinyong Mao, Xuchu Jiang & Yili Peng

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Li, B., Li, L., He, H. et al. Research on modal analysis method of CNC machine tool based on operational impact excitation. Int J Adv Manuf Technol 103 , 1155–1174 (2019). https://doi.org/10.1007/s00170-019-03510-x

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Received : 09 October 2018

Accepted : 19 February 2019

Published : 02 April 2019

Issue Date : 19 July 2019

DOI : https://doi.org/10.1007/s00170-019-03510-x

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