Vibration signal processing (Windowing) :

Windowing
After the signal was digitized using an A/D converter, the next step in the process (before it can be subjected to the FFT algorithm) is called windowing. A ‘window’ must be applied to the data to minimize signal ‘leakage’ effects. Windowing is the equivalent of multiplying the signal sample by a window function of the same length.

When an analog signal is captured, it is sampled with fixed time intervals. Sampling fixed time intervals can cause the actual waveform to get truncated at its start and end. The results obtained can vary with the location of the sample with respect to the waveform’s period.

This results in discontinuities in the continuous waveform. Windowing fills the discontinuities in the data by forcing the sampled data to zero at the beginning and at the end of the sampling period.

Windows can be thought of as a way to fill in the discontinuities in the data by forcing the sampled data to zero at the beginning and end of the sampling period (or time window), thereby making the sampled period appear to be continuous. When the signal is not windowed and is discontinuous, a ‘leakage error’ occurs when the FFT algorithm is applied.

The FFT algorithm sees the discontinuities as modulating (varying) frequencies and it shows as sidebands in the spectrum when none of these frequencies are actually present in the signal. The usage of windows also affects the ability to resolve closely spaced frequencies while attempting to maintain amplitude accuracy. However, it is possible to optimize one at the expense of the other.

There are many window functions. Some used in vibration signal processing are:
1. Rectangular & Uniform (basically no window)
2. Flat top
3. Hanning
4. Hamming
5. Kaiser Bessel
6. Blackman
7. Barlett.

Generally, only the first three window functions mentioned above are available in most
analyzers.

Rectangular (basically no window) When conducting a bump test for resonance or when trying to measure a single event or transient, use the “rectangular” window, which is the same as no window. This gives a good frequency reading but errs on the amplitude side of things. (Bump = Rectangular) .

Uniform
A Uniform window has a value of 1.0 across the entire measurement time . In reality, a Uniform window could be called “no window”.  Depending on the data acquisition system used, sometimes the term “Rectangular” window is also used.

Hanning or Hamming When collecting continuous vibration, say on a machine that is running at steady state; use the “Hanning” or “Hamming” window. These provide a good compromise between amplitude and frequency accuracy. (Continuous = Hanning, Hamming) .
When a Hanning window is applied to a non-periodic signal, the leakage is greatly reduced and the amplitude is higher.

Flattop The Flattop window has a better amplitude accuracy in frequency domain compared to the Hanning window . When calibrating a sensor, or when in need of very accurate amplitude readings, use the “flat top” window as this gives the most accurate amplitude reading but the worst frequency reading. (Accurate Amplitude = Flat Top) .