Vibration shakers are most commonly used in laboratories for random vibration tests, but most laboratory users do not understand what random vibration is and how it is calculated. Here we will explain.
Random vibration refers to a vibration waveform that is chaotic, where the instantaneous value at any given time cannot be predetermined, and there is no definite pattern in the waveform as it changes over time. Examples include the vibration of vehicles traveling on uneven surfaces, high-rise buildings experiencing wind gusts or earthquakes, vibrations of airplanes during flight, and vibrations of ships in waves. Random vibration test results are uncertain, unpredictable, and non-repeatable, but multiple tests under the same conditions have inherent statistical regularity. Thus, probability and statistical methods are needed to describe their motion laws quantitatively.
Random vibration is also composed of sine vibrations, but the frequencies of these sine vibrations are not discrete, but continuously distributed within a certain range. The amplitudes and displacement changes of each sine vibration are unpredictable and change with time. Therefore, random vibration signals are expressed by the average value of the random vibration signal at a certain moment, the root mean square value, the probability density function, and power spectral density.
In the random vibration test standards of vibration shakers, the acceleration spectral density variation curve is often given, and this is used as a reference spectrum shape for random vibration control tests. The acceleration density spectrum PSD represents the root mean square value of the random signal passing through the center frequency and has no practical significance. The total acceleration root mean square value represents the total vibration level, that is, the total energy.
In actual random vibration tests, we can easily choose different vibration magnitudes for vibration based on different product use environments. However, we don't know much about which vibration level is higher for two spectrum lines or which is more severe for the product. We know that the total acceleration root mean square value represents the total vibration level given to the sample, which is the total energy transmitted to the sample. Therefore, we can judge the level of vibration by calculating the size of the total acceleration root mean square value to determine the vibration energy size.
Testing force plays a decisive role in the test. If the required force exceeds the rated force, the test cannot be performed, but if the force is far below the rated force, it may result in wasted resources.
In random vibration tests, the maximum displacement of random vibration cannot be determined from the vibration conditions, and its value is also uncertain. Therefore, it is necessary to estimate the maximum displacement before the test to avoid damaging the vibration shaker due to exceeding the stroke.
Accelerated root mean square value
This is a statistical parameter that characterizes the total energy of random vibration.
Currently, the frequency of electromagnetic vibration shakers can reach 3000Hz to 5000Hz, and can basically meet the requirements of most tests. CME mainly provides hydraulic vibration shakers with a random vibration frequency range of 0-300Hz.
Currently, the most widely used random vibration test on vibration shakers is broadband random vibration. Based on the vibration requirements of test samples, the test value is calculated and compared with the limit value of the vibration shaker to estimate whether the test can be performed.
Random vibration tests need to take into account random vibration force and the maximum random vibration displacement. Random vibration generally gives a relationship curve between power spectral density and frequency. Based on this curve and relevant calculation formulas, the root mean square value of acceleration can be calculated.