Sometimes no matter how well your vibration fixture is designed, you absolutely MUST use two or more control accelerometers to get all the way through your test spectrum. Now I'm NOT talking about using multiple control accelerometers to make a bad fixture work, there's no excuse for that, I'm talking about a large, complex fixture that is designed optimally, but you can't get around the laws of physics. If you decide to control on the average of the accelerometers, fine, but understand what is really happening. When you look at that beautiful post test plot that shows you ran the test right in the middle of the spec, it's an average of what really happening. Now look at the individual plots of each accelerometer. That's the truth, and what it will show is that at some frequencies in some …
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