Frequency analysis
Shock metrology
Piezoelectric transducers
Signal conditioning

Vibration metrology

Why do we measure vibrations?

  • To verify that frequencies and amplitudes do not exceed the material limits (e.g. as described by the Wöhler curves)
  • To avoid excitation of resonances in certain parts of a machine
  • To be able to dampen or isolate vibration sources
  • To make conditional maintenance on machines
  • To construct or verify computer models of structures (system analysis)

    The measurement chain

  • Transducer
  • Preamplifier
  • Detector/Averager
  • Filter(s) Output

    See the transducer and signal conditioning sections for more info.

    We will discuss here the ways of analysing the output signals of the preamplifier.

    Frequency analysis

    The frequency spectrum gives in many cases a detailed information about the signal sources which cannot be obtained from the time signal. The example shows measurement and frequency analysis of the vibration signal measured on a gearbox. The frequency spectrum gives information on the vibration level caused by rotating parts and tooth meshing. It hereby becomes a valuable aid in locating sources of increased or undesirable vibration from these and other sources.

    The process of Frequency Analysis is as follows: By sending a signal through a filter and at the same time sweeping the filter over the frequency range of interest (or having a bank of filters) it is possible to get a measure of the signal level at different frequencies. The result is called a Frequency Spectrum.

    System vs Signal analysis

    During development of new structures, and in some cases to analyse in detail existing structures, it is a requirement to try to make a model of the structure, in such a way that if input forces are given the output vibration can be calculated.

    The illustration shows such an application measuring the mobility by introducing forces at different positions and measuring the input force together with the output vibration. These types of measurements are used to make a modal model of the structure, which can then be used to predict the behaviour of the structure under given circumstances. The model can also be used to predict the effect of changes in the structure, especially if it is combined with Finite Element Modelling (FEM). This type of analysis is called System Analysis, but it is beyond the scope of this lecture to cover this in more detail.