Inaccurate PSD Estimation Methods Give Smooth Lines but Mask Underlying Issues
It’s human nature to want life to go smoothly. When it comes to vibration testing, lab technicians and managers are generally happy when smooth lines are centered within the tolerance bands, signaling that the test meets its requirements. A raggedness on the random PSD plot can cause concern, and reasonably so. After all, equipment such as satellites and airplanes can be expensive, and the team cannot accept an out-of-tolerance condition. However, there are PSD estimation methods that are misleading and can lead to greater issues down the line.
Don’t Fear the Ragged Line
A random vibration test produces a power spectral density (PSD) that displays resonances within the frequency range of interest. The software applies the fast Fourier transform to frames of data and then averages the power values of the FFTs to estimate the PSD.
The process of PSD estimation.
The technician sets the degrees of freedom (DOF) value, which determines the amount of averaging the system uses to estimate the PSD. The test will appear out of tolerance until the system has enough data to estimate the PSD with sufficient confidence.
Some might not realize that some raggedness, especially at the beginning of a test, is normal for PSD estimation. For example, say a technician runs a vibration test at 120 DOF. After the test starts and the data begins to average, there will be an estimation error that causes the line to bounce in and out of tolerance. When the desired DOF is reached, a smooth line should appear unless there is an actual control error. This process requires time and patience.
Forcing a Smooth Line is Not Too Smooth
Some lab technicians and managers don’t distinguish an estimation error from a control error and treat any raggedness as bad news. They have picked up methods to circumvent estimation errors, putting their test item at risk of over- or under-testing.
One popular—but inaccurate—PSD estimation method uses low-level data to obtain a smooth line and then applies a multiplier to show product behavior at a higher level. The system generates a smooth line almost instantly at a high level at the cost of an inaccurate test. The technician or lab manager might be over- or under-testing the product unknowingly. Products behave differently when there is a change in level, and using a multiplier masks what is going on.
Learn More: An Exploration of Power Spectral Density Estimation
iDOF – An Accurate PSD Estimation Method
Vibration Research (VR) understands how important it is for lab technicians and managers to have a smooth day at the office. We are also committed to using accurate testing methods. As the innovator in vibration control, our team of engineers developed a statistically accurate way to remove estimation error while displaying control error. Our advanced test module, Instant Degrees of Freedom (iDOF), provides smooth lines and an in-tolerance PSD in a brief testing period.
An acceleration spectral density with (left) and without (right) iDOF.
If a fast and accurate smooth line is important for your vibration testing, download our technical paper to learn more about iDOF.
