What is Random Vibration ?
What you experience when driving over rubbles in a country road can be called ‘random vibration’. The important characteristic is that at any speed there is some vibration. Smaller, lighter cars experience it more, while heavier, bigger cars experience is less. Instead of a single cyclic frequency, there a continuous bandwidth of frequencies. Depending on the road conditions, and the structure of the car, a certain range of frequency would cause the most hardship on the structure. More importantly, from the design viewpoint, it would make no sense to attempt to provide a ‘factor of separation’ on the highest encountered frequency. The established practice is to perform a “Three-Sigma” statistical estimate of maximum stress in the component. The input for such problems is typically a “Power Spectral Density” (PSD) curve, which is obtained from tests. For example, an instrumented automobile can provide the frequencies encountered in good to rough road conditions, or over rubbles and rock
Random vibration is any vibration that does not follow a pattern. It is present to some extent in a wide variety of mechanical and electrical systems. Though random vibration cannot be predicted exactly, statistics can generate useful information for vibration environments. Cars on the highway and rockets launching are two situations that can face intense random vibration. Engineers use statistical data to simulate this vibration in the laboratory. Certain probabilities of random vibration behavior often can be predicted. For example, if a car on the highway is randomly vibrating in the vertical direction, its future positions above the ground cannot be known exactly. The probability that the car will be above a certain height, however, can be predicted. This is possible because random behavior follows a normal distribution or “bell curve.” The behavior of such a system can be analyzed with the tools of statistics.
Random vibration is exactly what the name describes: vibrations that occur randomly. Driving down the road makes your car vibrate. You never know when you will hit a bump in the road because they occur randomly. This is different from the vibration caused by your engine or tires rotating in regular cycles. The measurement of vibration is frequency or cycles per second, which is given in Hertz (Hz). In regular, cyclic vibrational analysis, only one frequency at a time is of interest. We may sweep through a range of frequencies, as in Frequency Response Analysis, but we are really only interested in the response at one frequency, such as at a natural frequency. In random vibration however, you can have all frequencies occuring simultaneously. Because of this, random vibration analysis is usually performed over a large range of frequencies, say from 20 to 2000 Hz (see the chart below). We are not looking at a specific frequency, specific moment in time or specific anything else; we are st
