Vibiratlion Response Investigation during Automotive Parts Testing
The first step atter mounting, the test object is to make a vibration response Investigation with the purpose of checking its function and to examine the Influence of' resonances throughout the frequency range. For all types of tests the resonances are found by a sine sweep. The resonance frequencies are measured and the behaviour of the structure studied In detail by manually controlling the frequency. At the end of the test procedure a slmilar Investigation Is carrled out for comparison.
The behaviour of the structure is most easily studied by means of a stroboscopic lamp, triggered by the exciter control to follow the excitation frequency. Better, however, is to use a trigger signal which differs slightly from the excitation frequency. giving a slow-motion like ,image. This slow-motion frequency, normally 3 -5 Hz, can be set on the stroboscope to follow the excitation. A -further study of the behaviour can be made by ms'nually delaying the trigger signal to move the Image through one or more cycles or by using dual flashes to give a picture of the extreme positions of the resonating part.
Endurance Condiitionilng
During the endurance conditioning the specimen Is subjected to a vibration, which in severity, I.e. frequency range, level and time, should ensure that It can survive in the real environment. Dependent on these, the conditioning Is performed by sine sweeping, sine testing at the resonance frequencies or at other pre-determined frequenoles, or by random vibration.
Swept Sine Testing
In the swept sine test the signal to the exciter Is continuously swept back and forth over the appropriate frequency range. The main control parameter is the acceleration level, but below a certain frequency (the crossover point) a constant displacement is chosen. In the lEC-test the cross-over frequency is 60 Hz and the levels of displacement and acceleration are chosen to change continuously from one parameter to another. Other standards, e.g. the military standards, may demand further changes of level or of vibration parameter. Therefore, the eXCiter control used for environmental sine testing, has at least two measuring channels with integrators to calculate the displacement and velocity
levels from the acceleration level measured by the control accelerometer. There must also be a switching facility, to change the measuring channel at the cross-over frequency.
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