Vibration Fixture Orientation

[SlipperyPete]

If I am designing a vibration fixture for testing in the vertical axis, does it matter if the test item is mounted upside down?
Some opinions say yes, since gravity is adding a 1g downward acceleration.
My opinion is that the feedback loop in the vibration controller will neutralise this and give an even acceleration in the up and downward directions.

 

[IRstuff]

Your shaker can only correct for what it can measure, which is typically the frequency dependent motion, not the static load. Moreover, the control accel is typically attached to the table or the fixture, not the UUT. Additionally, the inversion potentially alters the resonant frequencies within the UUT, and possibly adds new ones.

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[SlipperyPete]

Thanks for the response, but if the shaker table is being controlled such that the positive and negative accelerations are identical does it matter which way up the UUT is mounted? I would have thought that the modes and frequencies of the UUT would be the same either way.

 

[IRstuff]

"I would have thought that the modes and frequencies of the UUT would be the same either way."

Sure, if it's a solid block of tungsten. Since you've provided zero information about the UUT, no one can give anything but a generalized answer. If there's anything that can decouple under test, then it might make a difference which way it's oriented, since you have no ability to invertgravity.

TTFN
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