Vibration metrology

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.