13 Signal Analysis and Diagnostic Techniques

Although present-day frequency-domain signal analysis (spectral analysis) usually involves the use of the Fast Fourier Transform together with digital computers, neither of these are absolute requirements for spectral analysis of vibration data. However, the digital format offers much higher dynamic ranges with great reduction in the weight of the data acquisition and analysis equipment. Care must be exercised when one converts the continuous time domain data into a discrete time series. If an insufficient number of data points is selected, that is, if the sampling rate is not high enough, a phenomenon called aliasing will occur, whereby data higher than one half of the sampling frequency will fold back into the lower frequency range and contaminate the useful data. The following table outlines the inter-relationship between the sampling frequency, the maximum frequency of interest, the resolution in the time and frequency domains, the number of data blocks to analyze to ensure a certain statistical accuracy and the total length of time record required. This applies to steady vibration data. In analyzing single-event transient data, such as in impact tests, one should not include data outside of the transient in averaging the data.