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level of the table tends to increase, which would cause the output voltage
from the control pick-up to increase, the input power to the vibration
exciter is automatically decreased until the status quo is regained.
The time interval from the increase in vibration level (felt by the control
pick-up) to the decrease in the input power to the vibration exciter is,
although very small, not negligible. This time constant, or in other words
"regulation speed", should be selected according to the expected Q-values
of the system resonances and the scanning speed chosen for the frequency
sweep, i. e. the regulation speed must be greater than the speed with which
the system resonances are built up.
Regarding which of the three vibrational quantities, acceleration
,
velocity,
or displacement, is the most important, with respect to structural
damage,
environmental tests show that at the lower frequencies the occurance of
structural damage is most likely to be proportional to the displacement
level of the vibrations, whereas at the higher frequencies the damage depends
upon the acceleration level.
Vibration tests are therefore often carried cut with a constant displacement
level at low frequencies, and at a constant acceleration level at higher
frequencies. The exact frequency of the cross-over from constant dis-
placement to constant acceleration operation is stated in the different
vibration test
specifications,
e. g. MIL-E-5272 A a. o., and depends o:Ll the
conditions under which the test object is supposed to operate in practice.
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