Short Duration Random Tests
A random vibration test can be used to validate the reliability of a device under test (DUT). The test generates a power spectral density (PSD) that identifies the resonances across a frequency range.
Testing facilities in industries such as aerospace often test high-value equipment at extremely high levels of random vibration for a brief period. The goal is to run a realistic test that will identify any reliability issues without damaging the equipment. Ideally, the test engineers want to see a smooth control trace on the PSD that remains within an established tolerance range.
Short duration random vibration tests typically have very tight tolerances (+/- 1.5dB). The statistics of averaging FFT power values limits the possibility that all lines will be within tolerance in a short timeframe. Instant Degrees of Freedom® (iDOF) calculations are a statistically valid way to provide smooth lines and an in-tolerance PSD in a brief testing period.
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Statistically Valid Calculations
Time constraints and the statistical nature of PSD calculation make it difficult to create a clear PSD that accurately reflects test results in a short duration. Two methods used to address this challenge are Blank Display and Low-level Data Multiplication, but both hide the true test response of a DUT.
However, there is a statistically valid approach to quickly creating an accurate PSD. At each change in the test level, iDOF reduces the variance of the PSD estimate and exposes any lines out of tolerance without masking resonances. With iDOF, a test engineer can verify that the control PSD is maintained within tolerance and decide how the test affects the DUT. Most importantly, the engineer can stop the test before the energy from a resonance inflicts damage on the DUT.
Bridging the Gap Between Lab and Reality
Vibration test technicians and lab managers appreciate the “perfect world scenario” that iDOF offers. It is the innovation that the vibration control industry has needed for some time. As innovators in vibration control, Vibration Research listened and responded to customer needs.
With iDOF, your PSD plots will:
- Display incoming test data at all times of the test;
- Abide by the degrees of freedom determined by the user; and
- Reset averaging during the changes in a test level
Test engineers can expect to quickly see a smooth line except when real-life resonances and vibration patterns dictate that a raggedness should appear. In such cases, you can decide to abort the test before any potential product damage.
Too often, software “force fits” a smooth line by running the test at a low level (below demand), taking the averaged PSD, and multiplying it by a factor to bring it to full level. This method is invalid because it falsely assumes that the behaviors of a product at a high level mimic that of a low level. This vibration testing protocol is unfortunately common and is why high-value products are over and under-tested.
Harness the exactness, efficiencies, and peace of mind that iDOF offers to vibration control engineers.
Faster than Traditional Averaging
Control errors are visible much sooner with iDOF than traditional averaging. For example, the iDOF algorithm can process ten frames of data to produce a PSD estimate comparable to traditional averaging with 100 frames. This processing time enables fast detection of changing responses such as shifting resonances without using long averaging times: a critical capability for short duration tests.