Aerospace Vibration Testing – What’s really happening?
A random vibration test is one of the best ways to validate Aerospace reliability. It generates a PSD graph that identifies resonances across a frequency range and shows if they’re within a defined tolerance. But is the PSD graph showing what is really happening? Maybe not, especially with the short duration tests commonly used for expensive launch systems and complex payloads.
The underlying problem is that a PSD graph is created by separating time-domain data into a series of frames, calculating the FFT for each frame, and then averaging the power values of the FFTS. The inherent nature of randomness means that there are very large variations in the PSD, called estimation error, until a sufficient number of data frames are captured and contribute to the calculation. This takes extra time, something engineers don’t have in a short duration test.
Aerospace test engineers want a test that starts at a low level but ramps up quickly, highlights resonances, and ends before the DUT is seriously stressed. Visually that translates into a smooth line on the PSD graph with some resonance peaks but staying within an established tolerance range.
Low-Level Data Multiplication
A common approach to achieving an attractive PSD graph is data multiplication. It involves starting the test at a low level and then, as the test ramps up, multiplying the low-level data by a scaling factor and presenting it as full level data. Sounds straight forward but its based on the assumption that a DUT’s mechanical behavior at the high-level mimics the low-level behavior – and that is simply not true.
Multiplying low level data is not just inaccurate but potentially dangerous. Resonances will typically shift in both frequency and amplitude when signal levels change. Data multiplication masks changes occurring at the product resonances, so the test engineer is unaware of potentially damaging vibration energy levels.
Fortunately, there is a statistically valid approach to quickly creating an accurate PSD graph. In a random test, the variance is based on a chi-squared distribution. That variance can be calculated and taken out of the PSD plot, essentially removing the estimation error. This approach delivers a smooth but accurate PSD graph.
iDOF (meaning ‘Instant Degrees of Freedom®’) is an optional module in the VibrationVIEW software suite. It uses an advanced, patented algorithm to deliver accurate, low variance PSD estimates, calculated much more quickly than those using traditional averaging. iDOF removes the estimation error while displaying the control error, which is the true difference between the PSD of the signal and the demand PSD.
With iDOF, a test engineer can verify that the control PSD is maintained within tolerance throughout the test and make an informed decision on how a test is affecting the DUT. Most importantly, the engineer can stop a test before the energy from a vibration resonance inflicts damage.
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