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We have outlined the main features of the Sine test in our previous article, and also mentioned some of its key limitations – Sine can only cover one wave frequency at a time. This makes it different to imitate real vibration conditions. In comparison, Random is a mathematical model, which successfully compensates for this drawback.
Random vibration testing involves subjecting a product or system to a broad spectrum of vibration frequencies simultaneously, mimicking real-world conditions. It allows engineers to evaluate the response and durability of the device under unpredictable vibrations encountered during transportation, or general use. As the amplitude and phase of such a signal are, by definition, random, just as the vibrations surrounding us, this test type does a much better job of simulating real-life noise impacts.
Figure 1 - Example of Random Signal
As opposed to the Sine test, with the Random test, we get a representation of all frequencies of the object in the specified range. This representation is often given in the form of a PSD (power spectral density) vs. frequency plot.
Figure 2 - Spectrum density vs. Frequency graph in TestUp software for vibration testing
PSD is a key characteristic of Random tests. It represents the distribution of the signal in the given frequencies. The units of PSD can be, for instance, G squared per Hertz (if acceleration is measured in G). To obtain the necessary PSD parameter, the algorithms of vibration control use Fast Fourier Transformation (or FFT), transforming the received time domain data in Gaussian distribution into frequency domain data.
In Figure 2, the area under the green task line is the signal mean square. If we calculate this area, we will receive the value of the signal acceleration RMS. The method of calculation is described in detail in our article “Common Formulas of Sine and Random”.
A typical setup for Random vibration testing is no different than the standard vibration testing set, which involves a controller, a shaker table or electrodynamic shaker, a power amplifier, and the device under test (DUT) mounted on it. The shaker generates random vibrations with controllable intensity and frequency spectrum. The DUT's response is measured using accelerometers or other sensing devices strategically placed to capture critical points of interest.
A test engineer controlling the Random test should follow the common sense guidelines, applicable for every vibration test, as in:
To enter a Random test profile, you need to have the appropriate test specification, according to a relevant standard or DUT technical requirements. Here are the key parameters of a Random test:
Figure 3 - Random Test Profile window in TestUp vibration testing software
In our software package TestUp you can either enter the Random profile in lines of frequency, similar to the Sine test, or change the approach and enter the profile point by point. You can freely switch between two modes by changing the default settings. Each point will be characterized by a value of frequency and amplitude; each of them will be part of the resulting spectrum.
Figure 4 - Creating Random test profile by points in TestUp
Regardless of which method the engineer chooses (profile by points or profile in lines), having entered the Random test profile, the user can save it, and then run the same Random test with one button click.
It becomes even easier if you apply a dedicated test standard for your random testing application. There are a number of international standards providing guidelines and specifications for conducting such tests in different industries. Here are some for them:
RULA keeps integrating various test templates with the most common testing standards, including those for Random tests. It means that you can select the dedicated standard from our database, and the Random test profile will be entered automatically.
Figure 5 - Test Templates in TestUp
Sometimes test procedures require more advanced functions than the basic Random test. The functionality of Random in RULA software is extended beyond the basic parameters, and we can offer a number of options to considerably improve test experience and results, such as:
Figure 6 - Kurtosis settings in Test Up software for vibration testing
Figure 7 - Example of test profile with Notching Limits in place
Figure 8 - Spectrum Import in TestUp
Figure 9 - Fatigue Damage Spectrum module of TestUp
In addition to the listed Random test features, you might want to explore the capabilities of superimposed tests - SRoR (Sine-on-Random, Random-on-Random, Sine, and Random-on-Random). In this case, we deal with Sine tones or Random bands superimposed over the base Random spectrum. To learn the details, specifics, pros, and cons of superimposed tests, please follow our next articles.
Pros:
Cons:
Random vibration testing plays a vital role in ensuring the reliability, durability, and performance of products across various industries. Understanding the pros and cons, international standards, testing tips, and setup considerations will enable engineers to conduct effective random vibration testing, improve product quality to increase product reliability, and guarantee long-term customer satisfaction.