TABLE OF CONTENTS The vibration of a machine is a physical motion. Vibration transducers convert this motion into an electrical signal. The electrical signal is then passed on to data collectors or analyzers. The analyzers then process this signal to give the FFTs and other parameters. We will take a brief look at the processing of the signals, which finally provide us with the necessary information for condition monitoring. To achieve the final relevant output, the signal is processed with the following steps:
Analog signal input
Anti-alias filter
A/D converter
Overlap
Windows
FFT
Averaging
Display/storage.
Before we can discuss the above-mentioned digital signal processing steps, we need to take note of a few more terms and concepts.
A vibration or a system response can be represented by displacement, velocity and acceleration amplitudes in both time and frequency domains (Figure 4.1).
Time domain consists of amplitude that varies with time. This is commonly referred to as filter-out or overall reading.
Frequency domain is the domain where amplitudes are shown as series of sine and cosine waves. These waves have a magnitude and a phase, which vary with frequency.
The measured vibrations are always in analog form (time domain), and need to be transformed to the frequency domain. This is the purpose of the fast Fourier transform (FFT). The FFT is thus a calculation on a sampled signal. If FFT is a calculation on a sampled signal, the first question that...
