Ashish Dhiman. A short summary of this paper. Download Download PDF. Translate PDF. What is Random Vibration? There is a degree of confusion about the different kinds of vibration tests available to the vibration testing engineer. Difficulties encountered usually center on the difference between sinusoidal vibration sine testing and random vibration testing. Sinusoidal Vibration Sine : Strike a tuning fork or pluck a guitar string and the sound you hear is the result of a single sinusoidal wave produced at a particular frequency Figure 1.
Simple musical tones are sine waves simple, repetitive, oscillating motion of the air at a particular frequency. More complicated musical sounds arise from overlaying a number of sine waves of different frequencies at the same time. Sine waves are important in more areas than music. Every substance vibrates and has particular frequencies resonant frequencies in which it vibrates with the greatest amplitude.
Therefore sine wave vibration is important to help understand how any substance vibrates naturally. Sine Wave: Displacement vs. Time 2 1. Note its repeatability and predictability. The vibration testing industry has made good use of sine vibrations to help assess the frequencies at which a particular device under test DUT resonates.
These resonant frequencies are important to the vibration testing engineer because these resonant frequencies are the frequencies at which the DUT vibrates with the greatest amplitude; and therefore, are the frequencies that are most harmful to the DUT.
Part of the usefulness of sine testing is its simplicity, and therefore, it is a good point of entry into the study of vibrations. Sine testing is used primarily to determine damage to equipment or product. Study modal responses; 2. Determine fatigue life in each mode. A sine sweep prior to any shock or random test will identify any resonances of the equipment.
Running a sine test after testing a product should produce the same data graphs. Any differences in the sweeps indicates damage to the equipment — perhaps something as simple as a shift in the natural resonant frequencies, possibly suggesting a few loose bolts that need to be tightened. Random Vibration: Vibrations found in everyday life scenarios vehicle on common roadway; rocket in take-off, or an airplane wing in turbulent airflow are not repetitive or predictable like the sinusoidal wave.
Consider the acceleration waveform for dashboard vibration found in a vehicle traveling on Chicago Drive near Hudsonville, MI Figure 2. Note that the vibrations are by no means repetitive. Random testing accomplishes this. Random vs. Sine: Sinusoidal vibration tests are not as helpful as random testing is, because a sine test essentially consists of a single frequency in time.
A random vibration test, on the other hand, consists of all the frequencies in the defined spectrum being sent to the shaker at any given time. That is close, but not quite correct. S ine waves have constant amplitude, cycle after cycle. Suppose that there were of them constant amplitude sine waves, VRC. Would the totality be random? For the totality to be random, the amplitude of each slice would have to vary randomly, unpredictably. Unpredictable variations are what we mean by random. Broad-spectrum random vibration contains not sinusoids but rather a continuum of vibrations with different 2 amplitudes, VRC.
If one has a spotlight in the backyard it will illuminate brightly a small area of the backyard. If one has a floodlamp it will illuminate a wide range of the backyard but more dimly than the spotlight. A random vibration test, on the other hand, similar to the floodlamp, will test a wide range of frequencies. Therefore, in order to accomplish the same degree of testing, multitudes of sine tests would need to be administered, where a simple random test accomplishes all of this in one test.
Random vibration testing is, therefore, much more efficient and precise. Advantages of Random Vibration Testing: One of the main goals or uses of random vibration testing in industry is to bring a DUT to failure.
For example, a company may desire to find out how a particular product may fail because of various environmental vibrations it may be faced with. The company will simulate those vibrations on a shaker and place their product under those conditions.
Random testing is the key testing method for this kind of application. Computer software collects real-time data over a time period and combines the data using a spectrum averaging method to produce a statistical approximation of the vibration spectrum. Generally the random vibration spectrum profile is displayed as a power spectrum — a plot of acceleration spectral density acceleration squared per Hertz versus frequency Figure 3.
This is important to remember. Since the PSD is the result of an averaging method that produced the statistical approximation of the spectrum, an infinite number of real-time waveforms could have generated such a PSD.
Thus, at any time during a test, it is impossible to know specifically from the PSD what forces the DUT is experiencing. The need for the PSD is that it aids the tester in making an appropriate test profile for the shaker that will come close to real-life vibrations that the DUT will experience.
The idea that an infinite number of real-time waveforms could generate a particular PSD can be seen from the following graphs Figures 4 through 7 produced from data collected at VRC on June 28 and 30, Note that the PSD spectra formed from the data in both cases is exactly the same, yet generated from different waveforms.
However, some of the acceleration values are quite large compared to the normal values. To help illustrate the range of acceleration values, the Power Spectrum Density is converted into an amplitude probability density graph PDF similar to Figure 8. Notice how much of the acceleration values fall near the average acceleration value represented by 0 Sigma.
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