The "inherent" vibration behavior of machines or structures results essentially from their geometry, the materials used, and transition as well as boundary conditions. Mathematically, this "inherent" vibration behavior can be described by so-called "Eigenmodes". Every Eigenmode is characterized by three parameters:
- Natural frequency,
- Eigenform and
- modal damping ratio.
If the relevant Eigenmodes of a machine or structure are known, this can contribute significantly to the understanding of vibration effects (e.g. resonance effects). In addition, possibilities for a targeted optimization of the vibration behavior open up.
The objective of modal analysis is to determine the above-mentioned parameters. It can be performed on a digital model (e.g. FE model) or on a real machine (by measurement). Two procedures can be distinguished:
In a classical EMA, the vibration behavior of the test specimen is analyzed in installed condition or under "free-free boundary conditions". For this purpose, it is equipped with a sufficient number of usually acceleration sensors – alternatively, we can also use a Scanning Laser Doppler Vibrometer (SLDV). During the test, the specimen is typically excited with a (modal) hammer or a shaker. The excitation signal (force) is recorded with a force sensor. Special software is used to determine the modal parameters from the measured excitation force and the vibration responses.
One of the main differences of an OMA is that no defined external excitation is applied during such a measurement. The vibration excitation is provided by the environment – ideally broadband. The method of OMA originally comes from the field of building dynamics, where, for example, broadband excitation can be caused by the wind. In the analysis of mechanical engineering structures, any additional broadband excitation that may be required can be artificially generated by a shaker. Again, special software is used to extract the modal parameters.
- Determining modal parameters,
- that are difficult to model (damping)
- e.g. for model updating (MAC values etc.) of a digital twin
- Detailed analysis to gain insight into the vibration behavior of a machine or structure
While some tasks – for example, making production more efficient or reliable and complying with occupational health and safety regulations – are similar for all machine types, there are also individual challenges in each industry. Over the past decades, in countless projects for manufacturers and operators of paper machines and machine tools, we have been able to build up valuable specialist knowledge in these industries:
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We will be happy to advise you on questions relating to vibrations and noise in mechanical engineering.
Mechanical Engineering, TU Darmstadt
Dipl.-Ing. / Dr.-Ing