Modal Analysis - Determine The Dynamic Characteristics
Of The Structure
Dynamic properties of structures are very important. For some
products, such as large rotating machines, the life of the bearings
and the whole machine are dependent on a smooth running system.
In cases such as a paper machine, product uniformity is dependent
upon a smooth running machine. In other cases such as automobiles
and handheld power tools, such as chainsaws, the userís perception
of quality is highly dependent upon how smooth the item operates.
Finally, in some cases, catastrophic failure can occur if a resonant
frequency coincides with a major excitation frequency.
In any critical structure the dynamic properties are calculated
with finite element analysis. The product design is iterated until
it is acceptable. The design then needs to be validated with a
modal test. This test will confirm that the analytical finite
element analysis was appropriate, or give specific information
as to how the analyst can improve the model.
Modal testing is performed by inputting a measured excitation
force into the structure, and measuring its vibration. The excitation
source can be an instrumented hammer, a shaker or a number of
shakers. The excitation type for the shaker can be random, burst
random, or stepped sine signals. The random signal type is optimum
for speed of testing, the burst random gives a better coherence,
while the stepped sine input is used to maximize the energy input.
The data collected is then fitted with a modal model. This will
give the modal properties of the structure, including the mode
shape and the damping.