Chapter 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.

  • Summary
  • Acronyms
  • Nomenclature
  • 13.1 Introduction
  • 13.2 Representation of a Continuous Wave by a Series of Discrete Points
  • Aliasing
  • Apparent Frequencies
  • Example 13.1: Motor Gear Meshing Frequency
  • 13.3 Spectral Analysis
  • Unit of Power Spectral Density (PSD)
  • 13.4 The Fourier Transform
  • Discrete Fourier Transform
  • The Fast Fourier Transform
  • Resolution in Time and Frequency Domains
  • Example 13.3
  • 13.5 Windows
  • 13.6 Digital Filtering
  • Example 13.4
  • 13.7 Statistical Accuracy
  • Example 13.5: Sampling Rate and Length of Data Record Requirement to Resolve a Spectral Peak
  • 13.8 Beyond the Accelerometer and Fast Fourier Transform
  • The Ultrasonic Transducer as a Dynamic Sensor
  • The Eddy Current Sensor
  • The Hall Effect Sensor
  • Motor Current Signature Analysis
  • Beyond the Fast Fourier Transform
  • References
Topics: Signals

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