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Vibration Modal Analysis


Modal analysis is the field of studying the dynamic properties of mechanical structures when subjected to vibrational excitation. It consists of measuring and analyzing the dynamic response of structures when excited by an input. Vibration modal analysis uses the mass and stiffness distributions to determine natural frequencies, damping, and mode shapes at the various natural frequencies. The excitation input signals can be impulse, broadband, sine, swept sine, etc., using an impact hammer or a vibration shaker.

  • Impulse - narrow, spike waveform
  • Broadband - wide waveform
  • Sine - sine waveform
  • Swept sine - sine waveform sweeping over a range of frequencies.

Testing and data acquisition can be done using the following methods:

  • Single excitation point and multiple output points (SIMO)
  • Multiple input points and a single output point (MISO)
  • Multiple input points and multiple output points (MIMO)

The SIMO technique was the classical approach to obtaining vibration data. Subsequently MISO became popular. MISO and SIMO are identical due to the theory of reciprocity. MIMO has currently become popular where partial coherence analyses can identify which part of the response came from which excitation source.

Data is acquired by the use of accelerometers, load cells, or non-contact laser vibrometers.

Fourier analysis is used to analyze the output vibration signals to yield information regarding resonant frequencies, damping, and mode shapes. The resonant frequencies represent the discrete frequencies at which the structure will vibrate as exhibited by high displacements. Damping is a measure of how readily the structure dissipates the vibrational energy and subsequently returns to rest upon removal of the excitation. The mode shapes represent the actual shapes taken by the structure at each of the natural frequencies.

The results of vibration modal analyses can be correlated with the results of finite element stress analyses to provide a complete description of the behavior of a structure under static and dynamic loading conditions.

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